# Amplifier Mods or "upgrades"



## 2167 (Dec 5, 2007)

I got my hands on my final third Autotek Mean Machine amplifier. I had heard that zed (not only designed them) but offers upgrades and or Mods. So I emailed them/him and after identifying the particular models (MM 99,88,44). He mailed back,

"We can upgrade capacitors, preamplifier IC chips and power supply Mosfets

The Charge for this would be 125.00 + plus return freight"

Now my question is...Can someone explain in doofus terms what these upgrades do? 
Perhaps even a personal opinion price vs benefit?

Thanks Guys


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## AAAAAAA (Oct 5, 2007)

They allow the purchaser to reap the benefits of added psycho-accoustical SQ increases.


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## ChrisB (Jul 3, 2008)

It is amazing what psychoacoustics can do to make one justify being separated from their money. I know, because I fell for the hype and BS associated with modifying another brand of amplifiers because a certain repair tech had me convinced that it would make a HUGE improvement while bringing my amplifier "up to date". Man, was I ever stupid to believe him.

Long story short, the modifications netted me an inaudible power increase and decreased the useful life of my amplifier. The amplifier was initially sent in for repair, and I had it modified while it was there. Sadly, four months later, the other channel started doing the same dang thing that I had initially sent the amplifier in for.

It's your money, but most mods are just snake oil to reduce the money in your bank account.


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## [email protected] (Jun 12, 2008)

Upgrading mosfets can do alot, like changing to "N" mosfets, I dont know what is in there now but things change over time. Bigger caps store more juice for musical peaks, cant be a bad thing.

To say its a waste is ********, its not about changing the sound with those 2 mods, has nothing to do with it.


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## [email protected] (Jun 12, 2008)

ChrisB said:


> It is amazing what psychoacoustics can do to make one justify being separated from their money. I know, because I fell for the hype and BS associated with modifying another brand of amplifiers because a certain repair tech had me convinced that it would make a HUGE improvement while bringing my amplifier "up to date". Man, was I ever stupid to believe him.
> 
> Long story short, the modifications netted me an inaudible power increase and decreased the useful life of my amplifier. The amplifier was initially sent in for repair, and I had it modified while it was there. Sadly, four months later, the other channel started doing the same dang thing that I had initially sent the amplifier in for.
> 
> It's your money, but most mods are just snake oil to reduce the money in your bank account.


None of the "mods" you had done to your amp, could or can fix a bad channel. I think it is time you stop talking about your "Linear Power Gripe" we have all heard it and read it numerous times. Not trying to be a huge dick, but it has nothing to do with the OP


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## bose301s (Dec 8, 2008)

BeatsDownLow said:


> Upgrading mosfets can do alot, like changing to "N" mosfets, I dont know what is in there now but things change over time. Bigger caps store more juice for musical peaks, cant be a bad thing.
> 
> To say its a waste is ********, its not about changing the sound with those 2 mods, has nothing to do with it.


Ok, you go and add those bigger caps and MOSFETs in and when they reach a peak and burn the traces on the PCB because it wasn't designed to handle the added current you'll have an even better amp. Sorry, upgrades likes this ARE ********.


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## razholio (Apr 15, 2008)

I'm gonna go out on a limb here and suggest that Steven Mantz of zed audio just might have a point. Caps don't last forever (ever seen the tops of electrolytics bulging?) and if your amps are old enough, just may well be worth replacing. Cheaper components can also run out-of-spec significantly enough to affect the output of your amplifier. If you read through zedaudiocorp.com a bit you'll see that zed is no fan of audiophile snake-oil either.


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## [email protected] (Jun 12, 2008)

bose301s said:


> Ok, you go and add those bigger caps and MOSFETs in and when they reach a peak and burn the traces on the PCB because it wasn't designed to handle the added current you'll have an even better amp. Sorry, upgrades likes this ARE ********.


2 oz traces wont burn by adding a bigger cap, how do you know they will? I have changed out several amps now with bigger caps, not a single problem with any of them. I can talk from experience can you?


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## bose301s (Dec 8, 2008)

razholio said:


> I'm gonna go out on a limb here and suggest that Steven Mantz of zed audio just might have a point. Caps don't last forever (ever seen the tops of electrolytics bulging?) and if your amps are old enough, just may well be worth replacing. Cheaper components can also run out-of-spec significantly enough to affect the output of your amplifier. If you read through zedaudiocorp.com a bit you'll see that zed is no fan of audiophile snake-oil either.


Yea, he also wasn't a fan of Class D topology saying it could NEVER sound good and guess what all his new amps are, Class D. He is a great talker but he never backs it up.


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## bose301s (Dec 8, 2008)

BeatsDownLow said:


> 2 oz traces wont burn by adding a bigger cap, how do you know they will? I have changed out several amps now with bigger caps, not a single problem with any of them. I can talk from experience can you?


If all you're doing is replacing the caps to larger ones you won't change the amp at all, it already had in it what it needed, unless it was a cheap amp to begin with and then it doesn't matter what you do.


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## [email protected] (Jun 12, 2008)

bose301s said:


> If all you're doing is replacing the caps to larger ones you won't change the amp at all, it already had in it what it needed, unless it was a cheap amp to begin with and then it doesn't matter what you do.


 so it goes from me burning the amp up to now it wont do anything, ya ok

That class d thing has nothing to do with it also, you are taking a statement from years ago and trying to make it relevant to this discussion. Technology changes, when that statement was made, can you tell me 1 good class d car audio amplifier?


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## bose301s (Dec 8, 2008)

BeatsDownLow said:


> so it goes from me burning the amp up to now it wont do anything, ya ok
> 
> That class d thing has nothing to do with it also, you are taking a statement from years ago and trying to make it relevant to this discussion. Technology changes, when that statement was made, can you tell me 1 good class d car audio amplifier?


No, I'm going with what you said. In your second post all you said you replaced was caps, that would do pretty much nothing. However if you replace caps and PSU FETs like was originally stated you will either not see an increase in power because the output stage is the same or you will burn the output FETs and/or traces.


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## razholio (Apr 15, 2008)

bose301s said:


> If all you're doing is replacing the caps to larger ones you won't change the amp at all, it already had in it what it needed, unless it was a cheap amp to begin with and then it doesn't matter what you do.


and we're down to ad hominem in 2 posts. wow. bravo.


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## ChrisB (Jul 3, 2008)

Hmm, they had decent pro audio Class D amplifiers as far back as 1998. 

Regardless, take what I say with a grain of salt, because as I stated, it's the OP's money and he should feel free to do whatever he wants with it. 

When I recap amplifiers, I generally try to go up one tick on the voltage or one level on the capacitance, provided it fits in the allotted space. I've heard everything from my replacement will be fine to more damage being done to the amplifier if something in the power supply lets go. 

When it comes to upgrading the FETs... My philosophy is if it ain't broke, don't try to fix it! Besides, are the power supply FETs going to net any gains in power output without doing something with the transformer? Granted, I know FETs have come a long way, and it is quite possible that newer models will be more efficient and possibly run cooler than the older ones, so I could envision a justification to upgrade.

As for OPAMP replacements, do they really make an audible difference? Really? I've seen some people "upgrade" their OPAMPS on other forums only to go with something that was considered inferior to what the amplifier originally came equipped with. To boot, they brag about this "openness in sound, better clarity, etc." with their inferior OPAMPS.


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## MarkZ (Dec 5, 2005)

As others have indicated, these mods aren't going to make it sound any better. But, if you have the money to spend, I can think of far worse things in the world than replacing old capacitors! If it's an older amplifier, it could save you headaches down the line. I just had to replace two caps in my Zed-built 10 year old amplifier a few months ago because they were generating a turn on pop.

See if he'll offer a discount if you only replace the caps. Replacing the op amps is probably unnecessary. Not sure what, exactly, the point is in replacing the MOSFETs in the power supply. There may be no point to it. But it's possible it's a design upgrade. Get the service manual of just about any amp and it usually has attached to it circuit upgrades to work out bugs and whatnot. Upgrades do exist. Just don't expect it to sound any different. 

Whether or not it's worth $125 is up to you and you alone. It probably wouldn't be worth it to me, but maybe you have more money than I do.


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## jimbno1 (Apr 14, 2008)

I talked to Steve about my ESX amps a while back and he discouraged me from sending them in. He did say he "Could" change out the CAPs but unless I was having a specific problem with the amp he did not recommend it. 

So I would ask what is wrong with your Autotek amps that you think needs to be improved or fixed?


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## 2167 (Dec 5, 2007)

jimbno1 said:


> I talked to Steve about my ESX amps a while back and he discouraged me from sending them in. He did say he "Could" change out the CAPs but unless I was having a specific problem with the amp he did not recommend it.
> 
> So I would ask what is wrong with your Autotek amps that you think needs to be improved or fixed?


ok ok I'll add some details... I got three mean machine amps (99 88 44) all second hand. Soon after arrival I popped open the cases and did basic visual inspection no leaks, bulges or burns looks good ( why? To make sure of course). So after visual inspection hooked them up to 12 volts and iPod and woo hoo no problems and I actually received the original owners manual 
The question of upgrades is nothing more than just that. I really value opinions good or bad, the learning process NEVER ends. The only real lingering question for me is what is the actual power output


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## chad (Jun 30, 2005)

BeatsDownLow said:


> 2 oz traces wont burn by adding a bigger cap, how do you know they will? I have changed out several amps now with bigger caps, not a single problem with any of them. I can talk from experience can you?


the added inrush of larger caps can cause issues if you go MUCH larger... which is kinda possible now with the amazing shrinking capacitor we have going on currently.


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## MarkZ (Dec 5, 2005)

chad said:


> the added inrush of larger caps can cause issues if you go MUCH larger... which is kinda possible now with the amazing shrinking capacitor we have going on currently.


From nuts to blue balls.. Those are usually some big ass traces or bars on the primary side, at least until you get to the transistors. Inrush limiting or soft start (or whatever the hell you call it when the switcher does it) would further make it less of an issue, although I don't know if those MM's have that. When I've seen traces go, it's usually either output stage or Vrail. Or mechanical.


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## chad (Jun 30, 2005)

I'm not talking traces, I'm talking FETS, it's gonna take A LOT to eat up traces on an inrush.


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## Matt R (Aug 29, 2007)

I've modded several amps and changing the right caps CAN make a sonic improvement. There are some very high quality caps out there for coupling, Black Gate's and Elna Silmics, they can make a marked improvement in the sound. 

Add to that some high quality feedback caps like some silver mica's and you just improved all the caps that are directly in the audio signal path.

Not all opamps are created equal either. I've done hours and hours of head to head listening comparisons of different opamps. Im my experience price is a fairly good indicator of quality. The best opamps will make a considerable improvement in sound quality. 

These are some of the more mild mods I'm doing to the Zapco amps. None of these mods will make any difference in reliability at all. I'm not sure what parts are being selected for the mods or if their going to do the coupling and feedback caps but it can be well worth it. I would inquire about this if I were you. The $125 doesnt sound like alot to me if they are doing what I would do.

Good luck


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## MarkZ (Dec 5, 2005)

Next time you do a mod, it would be neat if you could take before/after measurements and post them here. I'd really be interested in seeing what sorts of differences you're finding.

My assumption, and this may have been a bad one on my part, is that Stephen would replace the electrolytics, which are mostly in the power supply section and shouldn't yield an audible difference (except maybe when you're overdriving the amplifier, or if they're going bad and not filtering noise well). I can't really see that changing caps in the signal path could make much of a difference, unless you significantly change the capacitance value itself (eg. changing the feedback attenuation frequency might give better high freq distortion performance at the expense of stability). I can't really think of other caps that could have an impact. I'm not talking about the filters or other preprocessing stages though. And even then, it's more about precision in higher order filters than name brand.

Besides, black gates are too expensive. 

I disagree with you about the op amps though. The FR and distortion performance of even a low priced op amp is usually orders of magnitude lower than what's produced by the rest of the amp. Upgrading the op amps always seems to me to be attacking the wrong side of the problem. 

Besides, if you're really a purist, just bypass the op amps altogether.  [Except for the manufacturers who use op amps instead of bipolars in the input differential stage... no such luck there.]


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## lycan (Dec 20, 2009)

It's _conceivable_ that:

1. Lower quality coupling caps could have a larger _voltage coefficient_, which means the cap value changes significantly with voltage, which means harmonic distortion  You might not see it in a 1kHz measurement, but if it's real ... you WILL see it in a distortion measurement at significantly _lower_ frequencies.

2. It can't be discounted that the replacement cap _could be_ much closer in value to it's target value. Maybe the original cap is +/- 20%, and the replacement is +/- 5%. But any artifacts of this, would show up in a _frequency response_ measurement of the amp.

I agree with Mark  the right measurements will reveal _all_ ... or _nothing_.


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## kyheng (Jan 31, 2007)

Well, I think it otherwise. Engineers when designing a circuit board(heart of an amp or other equipments like HU, DSPs) have already calculated all the important factors, starting from power input's current consumptions, charge needs to be stored on the capacitors till output transistors. Even opamp's output voltage is considered. Just that we won't know the actual scenario. And each of these equipments do have their tolerence value, with cap's tolerence of +/- 20%, we change to +/- 5%, for sure we will see some "improvements". Will it be significant? I don't know. 

That's why certain companies offers the so-called "upgrading" components inside. Because they know the best. they can always give you some crap type of opamps and capacitors and then offer you some additional cost to upgrade. Maybe the "upgrade" is the original circuit design just that they downgrade the components and can make more profits....


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## chad (Jun 30, 2005)

people that listen to high end headphones, specifically ones that tend to frequent that one forum can be convinced that you can get a penguin to speak Portuguese.


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## MarkZ (Dec 5, 2005)

cajunner said:


> I think of it like this:
> 
> Opamps, generate the first order of distortion in an amp. What follows, is subject to that first line component, the analogy is that of the source in a system.
> 
> Upgrade the source, the sound at the terminus can only be bettered. Now, does it matter if the opamp you have now, is a 43 cent component produced in a Chinese plant that makes no distinction between it and something used for industry? IF it works as advertised, (and what does, really?) then the .04 distortion rating on a great day, becomes magnified by the various up and down tolerances of other cheap components, and your amp- that in a lab setting filled with perfect pieces, and generates .04 distortion, is now a 1.4% bomb. Additive losses with a loudspeaker that has distortional spikes in the same places, and you could be in the 5% category, when all the ratings would have you under 1%.


Distortion (percentages) does not work the way you describe. It's not multiplicative. If it generates .04% THD and you pass it through an amplification stage, the result is still .04% THD.


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## envisionelec (Dec 14, 2005)

A lot of good and correct information here.

Something that needs to be mentioned is that the quality of the designer (not the manufacturer) determines much of what kind of changes will make a difference. The cheap Korean and Chinese amplifiers are built to a price, not a spec. They have enough trouble building a stable topology, let alone getting the layout right and using good parts. 

Replacing capacitors is nearly a fruitless endeavor. The capacitors between the power input terminal and the center tap on the switching transformer are there to reduce perturbations on the power cable - it does nothing for the amplifier, sonically. The capacitors after the rectifier diodes in the amp play a much bigger role in the power supply's regulation. But it's not as big as you'd think. The switching frequency of a typical car amp is pretty high - going bigger on the filter capacitor will net you a couple things, but not reserve capacity. Both are negatives: higher peak current and higher ESR. In regulated designs, the filter capacitor value is carefully calculated. Just making it bigger will throw off the loop compensation, possibly to the point that the amplifier will self-destruct. 

Upgrading FETs is a pretty good thing - especially if the amp has some IRFZ24s or some other low current junk. However, bigger FETs have a higher Ciss (equivalent gate-source input capacitance) which means they are slower at turning on, all things similar. If the amp drives the FETs directly from the controller IC, the upgrade is not recommended. If the amp has a pair of totem (push pull) drivers, the likelihood of success (however you might measure that) is higher. A better candidate will have a "gate transformer" or even a little bootstrapping power supply for the MosFet gate drive (I think Zed does this in his house brand). Gate resistors can be as high as 150 Ohms. The bigger FETs operate more happily with 10-22 ohm.

OpAmp swaps. Well, that's just a big argument waiting to happen. The basics are this: good power supply noise rejection (PSRR), proper gain structure, not messing with the phase response and utilizing the correct type of opamp for the application. Unfortunately, there are so many horribe, terrible implementations out there in the front end of many amplifiers, that it's no wonder that a swap makes an audible difference. The question remains, however - was it a positive change or just different? An unstable opamp will sound awful - high distortion, limited bandwidth - you name it. Most car amplifiers these days use a Zener-shunt regulator. It gets the voltage down to 15V, but does it compensate for noise? No. 

As you can see, it's not just a matter of a simple part swap. Is it worth it? Only rarely.


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## MarkZ (Dec 5, 2005)

cajunner said:


> I did not state it was multiplicative. I said it was magnified, but point taken. I was trying to illustrate the difference between a low-grade component and something better. If an op-amp is rated for .04% and you can replace it with something that has .0004%, then even if the correction in response is inaudible, you're starting with something that will finish cleaner than what you had before. What that's worth, is not much to me but to some it's a big deal.
> 
> The thing that I do see, is that same .0004% op-amp is usually something that isn't easily dropped into an older amp, because more times than not it runs on higher specs, or it will oscillate, whatever, and even if it is a 10 dollar piece upgrade, it isn't something that can be considered without intimate knowledge of the amplifier's capability in accommodating it. Granted, I am just going off of what I've read and unlike the fine gentlemen responding with hands-on experience, you can take my consideration with a grain of salt.


Sorry, I misinterpreted what you said. My point was that, as far as distortion is concerned, it doesn't really matter a whole lot whether it's at the source or at the output. .04% is .04%. It won't be magnified by virtue of where it appears in the circuit.

Point taken on making sure that your upgrades fit within the parameters of the design. Op amps are pretty plug n' play though. They have infinite input impedances and very low output impedances. Usually, your only concern is whether or not the power supply is right (and there's usually a HUGE range on supply voltages for most op amps) and whether its output voltage swing is capable of doing what the circuit wants it to do. That's why it's so easy for people to pop in new op amps without much modification.


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## kyheng (Jan 31, 2007)

envisionelec said:


> A lot of good and correct information here.
> 
> Something that needs to be mentioned is that the quality of the designer (not the manufacturer) determines much of what kind of changes will make a difference. *The cheap Korean and Chinese amplifiers are built to a price, not a spec. *They have enough trouble building a stable topology, let alone getting the layout right and using good parts.
> 
> .


I just cannot find a way to deny this.....


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## chad (Jun 30, 2005)

kyheng said:


> I just cannot find a way to deny this.....


The whole thing bears repeating.. I baited them with the inrush thing, he baited them with switching frequency and regulation....... nobody has bit yet.


It's Fukin great to see Aaron back.


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## MarkZ (Dec 5, 2005)

kyheng said:


> I just cannot find a way to deny this.....


Neither can I. But I'd love to know which manufacturers AREN'T building things to price. Maybe a few of the top tier (read: thousands of dollars) companies are sparing no expense, but everyone else has contraints not only on part quality but also on design quality. All of the lowest distortion designs have parts counts that contain 5 times (or more!) as many parts as conventional amplifiers. 

For example, I have two amplifier books that I really like because the authors provide design examples with real life measurements. Both of them offer progressively "better" designs as you move through the book. In each book, they provide examples of improving distortion performance from .004% to .0004% by providing essentially the same circuit but adding a second mirror image to it (mirror topology in the input stage and a push-pull VA stage nearly doubles the parts count). 

But manufacturers just aren't implementing this sort of thing! Can you imagine going to your boss and saying "hey, I can improve distortion figures from .004% to .0004% by adding twice as many parts." They'll tell you to go **** in your hat.

Anyway, the moral of the story is that price is _always_ a constraint. There's always a price point in mind when designing a product. It's just more of a constraint with some lines than with others.


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## chad (Jun 30, 2005)

new sig.


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## ChrisB (Jul 3, 2008)

I know beatsdownlow said that I need to stop bitching about my negative experience with a certain repair tech, but I do have a question. Can someone provide a real world example where sending a near 20 year old relic off for "modification" would actually help? For the fee of $125 or more I just can't see the justification to do so. Keep in mind, I am not talking about just replacing electrolytic caps because I can do that myself. 

I am sure you see where I am going with this... IMHO, it would be cost prohibitive to "modify" or redesign a 20 year old amplifier versus purchasing the diamonds in the rough, like the Cerwin Vega EXL-400.4, Stealth, or the Clarion APX/DPX series when they were on closeout. For near the price of the modifications that others including Mantz charge, I can have a new product with a warranty that is more powerful and has less noise issues than a 20 year old relic from the get go.

ETA: I guess my problem with electronics is I don't feel that they can be modified like cars for performance. Even when modifying cars, one must take in other variables in order to gain the performance aspects otherwise one ends up with a rod through the hood.


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## kyheng (Jan 31, 2007)

Mark : That's the conflict between market and technical department. They don't have a same goal. 

I always dreaming to have a company that makes an amp with BB's opamp, Blackgate's capacitors and other high quality components. But then when I'm awake, I know this won't be the fact, as such amp will cost a human's life....


Chiris : I don't have a 20 year old amp yet, but a 9 years old amp, it still standing strong(I use it for midbass sometimes compare to last time subwoofers) with repaired once, changing a cap and some resistors. The cost are quite cheap(considering my country's cost of living is low). For me, such an old amp, I will change whatever are needed, rather than spend a chunk of money to change or "upgrade" it.


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## ChrisB (Jul 3, 2008)

This is an interesting modification: Modified Punch 45

I personally wouldn't do it even to run a Punch 45 at low impedance, but it is interesting to say the least.

ETA: Even Perry Babin comments on which caps to use in the link above. QFT: "Also choose a capacitor rated for 105C (rated to operate at 105 Celcius) instead of a capacitor rated to operate only up to 85C. The Panasonic FC series capacitors (available from Digi-Key) are a good choice (until something better comes along)."


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## Dryseals (Sep 7, 2008)

MarkZ said:


> Neither can I. But I'd love to know which manufacturers AREN'T building things to price. Maybe a few of the top tier (read: thousands of dollars) companies are sparing no expense, but everyone else has contraints not only on part quality but also on design quality. All of the lowest distortion designs have parts counts that contain 5 times (or more!) as many parts as conventional amplifiers.
> 
> For example, I have two amplifier books that I really like because the authors provide design examples with real life measurements. Both of them offer progressively "better" designs as you move through the book. In each book, they provide examples of improving distortion performance from .004% to .0004% by providing essentially the same circuit but adding a second mirror image to it (mirror topology in the input stage and a push-pull VA stage nearly doubles the parts count).
> 
> ...


Price is a constraint when designing. But after the design, it goes out the window. The new way of purchasing is reverse auction, it's the killer of industry, but it's very popular to lower costs. The engineer hands me a design, now I need to purchase the parts to build his design. In that design will be the tolerances for each component. I'm buying in bulk, so say I need 10K gizmos within the stated spec. I post it in a reverse auction and the suppliers do a reverse bid for the sale. Bob can supply them at 0.012 cents per gizmo. Tom can supply them at 0.011 cent per gizmo.

Add in time lines and reject rates for the gizmo and it can get hairy. But the savings for production are seen at several stages of building a product. Every thing is completed as a "just in time" manufacturing. No one wants to warehouse an item, that costs money.

So add in that you have a few folks that make circuit boards, a few that make each component and a few that do the assembly and each one is has windows for production. All this had to be timed to make one product.

Think about it. I can buy the metal case for the amp from George in July cheaper than I can in August, because George has other manufacturer to supply. But what do I do with 10K cases that I don't need until October.

Understanding each step can save a company money. It takes carbon black to make the electrodes for aluminum production, carbon black is the final waste of oil production. A change in oil production can change the price of your aluminum.

Oh well back to amps...................


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## MarkZ (Dec 5, 2005)

If one component costs the majority of the price, then it makes sense to build around that component. For amplifiers, that component tends to be the heatsink. It's hard for me to imagine that teams regularly approach the issue with a certain design in mind without any regard for the cost of potential alternatives. That doesn't seem like an efficient way of going about things. Small signal parts, though? Yeah, it makes more sense.


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## Dryseals (Sep 7, 2008)

MarkZ said:


> If one component costs the majority of the price, then it makes sense to build around that component. For amplifiers, that component tends to be the heatsink. It's hard for me to imagine that teams regularly approach the issue with a certain design in mind without any regard for the cost of potential alternatives. That doesn't seem like an efficient way of going about things. Small signal parts, though? Yeah, it makes more sense.


I've sat in on a ton of design meetings, I don't build amps, but I work in plants, petro-chemical. The design gets compromised by time factors. We may have several designs on the table, each with and end result, but each differing based on components, much larger scale, but the same principal is involved.

You never finalize a design until you've checked all the time tables for the components. In many cases, the time tables are built based on one or two main components. Then comes in delivery dates vs production dates.

Most industries operate the same way, the key word is "just in time". And those words will compromise a design with ease. I've seen many engineers curse and throw things because their design gets compromised by some ones time table.


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## TrickyRicky (Apr 5, 2009)

BWM's also have mods available (such as turbo stage 1-4), that doesnt mean something is wrong with them if not upgraded/modded. It just means the owner wants more from something already good.


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## envisionelec (Dec 14, 2005)

cajunner said:


> I look at ampguts on occasion, and I do find that the amps with the most 'guts' are typically not american in design origin. When I compare old Linear Power to say, Luxman, I feel like there should be a terrific increase in sound quality from one to the next, considering the more obvious build, and the parts used. I look through all my processors, and find TL07X all over the place, or the 741's, and not until I get to the Clarion ADSC1 do I find the 5532, itself considered to be a mid to low grade choice compared to the 627 or 4582 or whatevers, and I think it's the same. A 25 year old amp design can be indistinguishable to something made today, proved by blind testing.


Japanese amplifier designs are remarkably similar. They use precision current sources and go heavy on the parts count. At least, that's what they did in the 1990s-early 2000s. I haven't looked at them lately, but Pioneer, Alpine, Kenwood and Sony were remarkably similar in design with their A/B amps. They also shared a common trait - weak power supplies. The Japanese just don't test their amplifiers with as much bass-heavy music as we Westerners like. The spec on their toroids was "good enough" for steady state power output, but heavy currents saturated the cores often enough to smoke the switching FETs. 

A good design is systemic and not something achieved with a couple cheap mods. Want to make a measurable difference? Redesign the power supply. Build another toroid. It's not hard - and I've done it many times to get rated power at 8 ohms for some nice SEAS Revelators. Now THAT is a custom install. 

The TL072 is a great OpAmp when used within its limits. Often times there is more Johnson (yes, I said Johnson...) noise in the amplifier-proper's differential stage than the lowly TL072. Ever wonder how often amplifier manufacturers electrically match (let alone thermally bond) those two critical parts? Not very often!!


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## MarkZ (Dec 5, 2005)

envisionelec said:


> The TL072 is a great OpAmp when used within its limits. Often times there is more Johnson (yes, I said Johnson...) noise in the amplifier-proper's differential stage than the lowly TL072. Ever wonder how often amplifier manufacturers electrically match (let alone thermally bond) those two critical parts? Not very often!!


Unless you make them the same part. Input buffer AND differential amplifier. Can't get more matched than that.


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## envisionelec (Dec 14, 2005)

Dryseals said:


> I've sat in on a ton of design meetings, I don't build amps, but I work in plants, petro-chemical. The design gets compromised by time factors. We may have several designs on the table, each with and end result, but each differing based on components, much larger scale, but the same principal is involved.
> 
> You never finalize a design until you've checked all the time tables for the components. In many cases, the time tables are built based on one or two main components. Then comes in delivery dates vs production dates.
> 
> Most industries operate the same way, the key word is "just in time". And those words will compromise a design with ease. I've seen many engineers curse and throw things because their design gets compromised by some ones time table.


My day job is in manufacturing/electrical engineering. We have to build test fixtures and jigs and I sit in those meetings, too. There is a distinct difference between a system design and a mass produced product and it could be construed as unfair to link them as one in the same.

I've covered this in other threads, but the majority of the companies that spring up every five years are simply someone that figured out that they could buy a design from AudioPower or DAESUNG, get someone to design a logo and buy 200 of each model for $40-100 each plus shipping. But the designs are cookie-cutter. The manufacturers will work with you if you have a design, but the majority of business owners have little to no engineering experience and even less experience handling the specific quirks of car amplifier design.


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## envisionelec (Dec 14, 2005)

MarkZ said:


> Unless you make them the same part. Input buffer AND differential amplifier. Can't get more matched than that.


I KNEW someone would bring this up.

Linear Power, US AMPS, Alphasonik, Rockford Fosgate (old Power 600,1000) and many more used the bootstrapped front end principle. The trouble with making the OpAmp do double duty is that you have increased even order harmonic distortion. It's a cool concept that works, but - like everything - has its compromises...

I should have clarified that. I meant that few manufacturers match the diff amp input transistors. Old Soundstream, PPI, Orion and RF used either the obsolete dual NPN (or FET) or bonded the physical components face-to-face. The reason for this (for our other readers) is that any thermal gradient in one transistor is equated in the second so that their bias currents are equaled under all operating circumstances. There is a whole line of thought on the front end design of an amp. I like the LTP with LED current sources. Phoenix Gold did this as did ESX and Audison. I'm sure I've left some oddballs out, but those are the prominent ones. Today, there are precision zener references that accomplish the task at an even greater level of precision. I just happen to like purty LEDs.


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## BigAl205 (May 20, 2009)

Bigger tires on my Honda made it go faster


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## lycan (Dec 20, 2009)

envisionelec said:


> I KNEW someone would bring this up.
> 
> Linear Power, US AMPS, Alphasonik, Rockford Fosgate (old Power 600,1000) and many more used the bootstrapped front end principle. The trouble with making the OpAmp do double duty is that you have increased even order harmonic distortion. It's a cool concept that works, but - like everything - has its compromises...
> 
> I should have clarified that. I meant that few manufacturers match the diff amp input transistors. Old Soundstream, PPI, Orion and RF used either the obsolete dual NPN (or FET) or bonded the physical components face-to-face. The reason for this (for our other readers) is that any thermal gradient in one transistor is equated in the second so that their bias currents are equaled under all operating circumstances. There is a whole line of thought on the front end design of an amp. I like the LTP with LED current sources. Phoenix Gold did this as did ESX and Audison. I'm sure I've left some oddballs out, but those are the prominent ones. Today, there are precision zener references that accomplish the task at an even greater level of precision. I just happen to like purty LEDs.


In a well-designed amplifier, the bias current "split" between the input diff-pair transistors will _not_ be determined by how well those diff-pair transistors match. It sounds counter-intuitive, but that's the magic of feedback: either local, or global 

Typically, the bias current "split" between input stage transistors will be determined in one of two ways :

1. Local feedback. Here, there's a current mirror _load_ on the input diff-pair. Typically, this current mirror load uses emitter degeneration to help the load "match". This technique is _very_ typical of integrated circuit opamps. If the current mirror load is behaving properly (with the help of emitter degeneration) , the bias current in the two legs is identical ... independent of input diff-pair matching.

2. Global feedback. Here, the overall negative feedback of the amp (at DC) determines the bias current "split". One leg of the diff-pair may be loaded with a resistor, for example, that must develop a Vbe (or two) to effectively drive the output stage of the amp. The bias current in this leg is then Vbe/Rload ... and subtracting this from the _total_ diff-pair bias, will determine bias current in the other leg. Again, bias current "split" is independent of input diff-pair matching.

So does it matter, to match the input diff-pair? YES ... given that feedback somewhere (local or global, as above) will be forcing the bias current in the two legs to be equal, how well the input diff-pair matches will determine the INPUT REFERRED OFFSET voltage. This will ultimately impact DC offset at the amplifier output (but hopefully, there will be some caps in the feedback network to make sure that input referred DC offset is _not_ "gained-up" by the audio-band voltage gain). So YES, you do want them to match  Including, of course, temperature drift (Vbe drifts at abouve -2mV per degree C).

Don't believe me? Post up an amplifier schematic, and let's determine how the input stage bias current "splits"


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## ChrisB (Jul 3, 2008)

Attached are a few of sets of schematics that I happen to have laying around.


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## SQ_Blaze (Sep 29, 2008)

I have a cheap Chinese tube headphone amplifier that I bought years ago, then decided to upgrade just about every component in it. Because it is a very cheap amp, it also has very cheap Chinese components with very lose tolerances. 

First mods were replacing the stock preamp tubes, driver tubes and output tubes with quality but inexpensive NOS tubes. That helped some, but the amp was still lacking bass so I upgraded the stock cheap volume pot with a quality Alps unit along with bypassing the input caps. Bypassing the input caps gave me all the bass that was in the original signal and the Alps pot gave me even balance between both channels throughout the entire pot's range. The cheap stock pot would shift a little from left to right throughout the range.

A while after that, I decided to replace every resistor, diode and capacitor in the amp with equal value pieces with 1-2% tolerances. None of these components "changed" the sound, but they did improve the sound as far as giving the amp a bit more headroom, greater dynamics and better transients.

However, considering how much all those quality resistors, diodes and capacitors cost me, I wouldn't have done it again. 

If I had to do it all over again, I would have just stuck with replacing the volume pot, the tubes and bypassing the input caps. Those has the biggest impact in the overall sound and performance of the amp. All of that only cost me about 70 bucks. All the other stuff cost three times more than the entire amp and was not worth the time or money.


You may be asking, what's my point? LOL

Well, if you want to run an old school amp that's also second hand, it may be a good idea to at least replace the caps as they do run out of spec over the years and some could be bad even though it doesn't show on the outside. 

The only thing I would do is replace the caps with equal value, but step up some on the tolerances. That kind of upgrade never hurts and could possibly help a little... possibly.


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## lycan (Dec 20, 2009)

ChrisB said:


> Attached are a few of sets of schematics that I happen to have laying around.


Okee dokee ... let's look at that Reference 5.0 (just picked at random), top half:

Left leg bias current is the DC current though R104 (1.3k).

Right leg bias current (after emitter-degenerated D7/Q10 mirror) is the DC current through R103 (1.33k).

If the DC current "split" in the diff pair tried to "move" ... let's say, _less_ current through R104 and _more_ current through R103 ... the amplifier's output voltage would _drop_ (output stage being driven by driver transistors Q19 and Q20). This _drop_ in output voltage would be fed-back to the gate of the right transistor in the Q9 diff-pair, to re-establish an equal split in diff-pair bias current 

Note that nowhere in this discussion, does the "matching" of the Q9 diff-pair establish the bias current "split" ... instead, global feedback (and matching elsewhere) performs the function.

Like I said .. .input diff-pair matching IS still important, for low input-referred offset voltage (including drift over temperature). The currents are established to be equal elsewhere, but the Vbe's that "result" from an equal current split must "match" for low input referred offset voltage.

EDIT : Short version : Let's assume that the input diff-pair Q9 mismatches badly, but everything else in this amp behaves very well. The amplifier will develop _whatever_ DC offset is required at its output, to make sure that the currents through the input diff-pair are identical  That's the magic of feedback


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## envisionelec (Dec 14, 2005)

lycan said:


> Don't believe me?


Nope, you're right. I was thinking of something entirely different.


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## lycan (Dec 20, 2009)

envisionelec said:


> Nope, you're right. I was thinking of something entirely different.


no problem 

It is a little "upside down", if you will, to think that a mismatch in the input stage diff-pair _won't_ translate to an unbalance in bias current. But the feedback amplifier is really "monitoring" that _current difference_ ... this is, in fact, the "quantity" that the amplifier is "amplifying", after the input transconductance stage. Essentially, the feedback action will do whatever it possibly can, to keep those currents equal ... which means, apply the appropriate _offset voltage_, if the input stage mis-matches.

Your conclusion is, of course, still valid ... it's quite important to match those devices, even over temperature! But that match is important to keep offset voltage low, not to keep the bias currents balanced.


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## chad (Jun 30, 2005)

SQ_Blaze said:


> You may be asking, what's my point? LOL


The point is that you enjoyed doing it, learned something, and got good results, and to that I say cheers!

If you decide to mod you own amp, get A result and don't nuke it then hell, it was worth the time saying you did it.... 

Look up, top left hand corner, see the big DIY?


That's what it's all about man.


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## SQ_Blaze (Sep 29, 2008)

chad said:


> The point is that you enjoyed doing it, learned something, and got good results, and to that I say cheers!
> 
> If you decide to mod you own amp, get A result and don't nuke it then hell, it was worth the time saying you did it....
> 
> ...


Quite true. I did learn a bit and the results were very good. And considering the cans I used with it, it was worth the work... AGK K601's and Sennheiser HD600's.


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## Oliver (Jun 25, 2007)

Very NICE ! *Lycan. envisionelec and Chris* !!


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## envisionelec (Dec 14, 2005)

lycan said:


> no problem
> 
> It is a little "upside down", if you will, to think that a mismatch in the input stage diff-pair _won't_ translate to an unbalance in bias current. But the feedback amplifier is really "monitoring" that _current difference_ ... this is, in fact, the "quantity" that the amplifier is "amplifying", after the input transconductance stage. Essentially, the feedback action will do whatever it possibly can, to keep those currents equal ... which means, apply the appropriate _offset voltage_, if the input stage mis-matches.
> 
> Your conclusion is, of course, still valid ... it's quite important to match those devices, even over temperature! But that match is important to keep offset voltage low, not to keep the bias currents balanced.


I design similar stuff with current mirrors so I know how they work. I simply misspoke. Thank you for setting the record straight. 

These days, however, it's not at the forefront of my mind because I'm working on a lot of switching designs. They're pretty much a black box at the front end and require intensive firefighting (sometimes literally!) at the outputs to be emissions-compliant. A guy can only remember the exact words so often... :blush:


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## chad (Jun 30, 2005)

envisionelec said:


> They're pretty much a black box at the front end and require intensive firefighting (sometimes literally!)


LOL nice, done a bit of that myself


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## Pulse-R (Jan 14, 2007)

Our "Just In Time" system is often "just too late" and customers will go away.

on the subject of amp mods: 'good quality' amps have little benefit to be modded, and cheaper ones aren't worth the time usually.

on the subject of distortion: If you decrease THD from .004 to .0004% - that's an improvement of 10dB. If you use low noise parts as well, even better. One of the traps some people seem to fall into is that they use low distortion parts, but don't account for the increase in noise floor. Nothing worse than having your tweeters hissing at you when there's no music.


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## lycan (Dec 20, 2009)

Pulse-R said:


> Our "Just In Time" system is often "just too late" and customers will go away.
> 
> on the subject of amp mods: 'good quality' amps have little benefit to be modded, and cheaper ones aren't worth the time usually.
> 
> on the subject of distortion: If you decrease THD from .004 to .0004% - that's an improvement of 10dB. If you use low noise parts as well, even better. One of the traps some people seem to fall into is that they use low distortion parts, but don't account for the increase in noise floor. Nothing worse than having your tweeters hissing at you when there's no music.


If you can't hear the 0.004% distortion to begin with (and you can't) ... then the improvement to 0.0004% means exactly squat.

Remember that distortion drops _dramatically_ as signal level drops, unlike the background noise floor.


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## Luke352 (Jul 24, 2006)

Pulse-R said:


> Our "Just In Time" system is often "just too late" and customers will go away.


Sounds like you have the same experience with the "Just In Time" process as I did Simon. Ours was normally "just too late" or "**** it, i'll get it myself".


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## kyheng (Jan 31, 2007)

Sometimes following Jap's way of doing things is not the best concept..... Toyota for example.


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## Dryseals (Sep 7, 2008)

envisionelec said:


> My day job is in manufacturing/electrical engineering. We have to build test fixtures and jigs and I sit in those meetings, too. There is a distinct difference between a system design and a mass produced product and it could be construed as unfair to link them as one in the same.
> 
> I've covered this in other threads, but the majority of the companies that spring up every five years are simply someone that figured out that they could buy a design from AudioPower or DAESUNG, get someone to design a logo and buy 200 of each model for $40-100 each plus shipping. But the designs are cookie-cutter. The manufacturers will work with you if you have a design, but the majority of business owners have little to no engineering experience and even less experience handling the specific quirks of car amplifier design.


You miss understood, the point was not between mass produced and system design. The point was time tables, markets, investments and profits.

Bringing a product to market is far more complex than most think, more so in this day of investors who'll jump ship on you in less than a heart beat. Everyone want's a huge return and wants it quickly. Corners are cut to meet time tables, time tables are profits.

It's sad, but we've allowed these things to rule, so a company trying to build the best has to compete with one that is skirting the wire. Sure they'll get a loyal following with a few, but producing in mass is what makes profits. Profits is what attracts investors, investors are where the next gizmo comes from.

And FYI, if I had the mind set of profits, I could retire today. I invested in too many of the quality players, they've come to roost, but it's taken well over twenty years.


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## Dryseals (Sep 7, 2008)

Luke352 said:


> Sounds like you have the same experience with the "Just In Time" process as I did Simon. Ours was normally "just too late" or "**** it, i'll get it myself".


That's why they pay good marketing folks well, they know when and how. Your purchasing folks need to be on the ball and working closely with marketing. Scheduling is key, but it all falls apart if the components aren't there or the markets been sucked up by some one else.

Small business generates the ideas, problem is they normally don't have the what it takes to bring it to market at the right time. Look how many big names in the auto aftermarket players have lost their arse. They end up selling their name to some other manufacturer....


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## Pulse-R (Jan 14, 2007)

Yeh, we're delayed 12 to 16 weeks to get parts out of UK.. we're seriously considering going to the Chinese fab, because they can meet delivery and cost. quality is lower, but customer's want product and support, not excuses.

THD+N usually goes up as signal level goes down.


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## lycan (Dec 20, 2009)

Pulse-R said:


> Yeh, we're delayed 12 to 16 weeks to get parts out of UK.. we're seriously considering going to the Chinese fab, because they can meet delivery and cost. quality is lower, but customer's want product and support, not excuses.
> 
> THD+N usually goes up as signal level goes down.


Only because the "N" part stays constant, as signal level drops 

The ratio of signal-to-noise _decreases_ as signal drops, because "noise" is uncorrelated with signal ... in other words, "noise" stays constant as signal drops. Numerator gets smaller, while the "N" part of the denominator stays constant.

Distortion, though ... which forms the "THD" part of "THD+N" ... behaves _dramatically_ different. Distortion is tightly correlated to the signal. As signal level drops, "THD" drops _faster_. If the signal drops by 6dB, for example, the 2nd harmonic will typically drop by 12dB ... so signal-to-distortion _improves_ as signal level drops. Higher order harmonics drop even _faster_ (there's one or two exceptions to this rule for distortion, but that's an exercise left to the reader ).

It's fine to lump "N" and "THD" together ... but they behave _very_ differently, as signal level drops.


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## MarkZ (Dec 5, 2005)

lycan said:


> As signal level drops, "THD" drops _faster_. If the signal drops by 6dB, for example, the 2nd harmonic will typically drop by 12dB ... so signal-to-distortion _improves_ as signal level drops. Higher order harmonics drop even _faster_ (there's one or two exceptions to this rule for distortion, but that's an exercise left to the reader ).


I don't understand why this is the case. I would think that the influence of crossover distortion or gm-doubling distortion or anything generated by imperfect bias tracking would be more prominent with lower signals.


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## lycan (Dec 20, 2009)

MarkZ said:


> I don't understand why this is the case. I would think that the influence of crossover distortion or gm-doubling distortion or anything generated by imperfect bias tracking would be more prominent with lower signals.


Mark is a pretty smart cookie  crossover distortion, is _one_ of the precious few examples of a _distortion_ component that does NOT drop with signal level.

But alas, crossover distortion is not "commonplace", or representative of larger classes and categories of distortion. MOST analog distortion mechanisms can be approximated by a polynomial relationship :

*Vout = a0 + a1*Vin + a2*Vin^2 + a3*Vin^3 + a4*Vin^4 + ...*

where we hope that any coefficients _other than_ *a1* are small. In audio, we even hope that the "DC offset" term, *a0*, is small too (did you know that, strictly speaking, the DC offset makes the system nonlinear? I digress ...). We expect this relationship to "hold" over a large range of input signal levels. It may "hold" for different frequencies, too ... or it may be that different frequencies require a different set of "approximation" or "curve fitting" coefficients.

In any case, the second harmonic is dominated by the *a2*, or "square" term. And the simple consequence of the "squaring" operation that generates the second-harmonic is this : drop the signal by 6dB, and the second harmonic will drop by 12dB 

Similar relationships can be developed, of course, for the higher-order terms.


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## MarkZ (Dec 5, 2005)

Why is the second harmonic squared (and subsequent harmonics raised by exponents)?


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## lycan (Dec 20, 2009)

MarkZ said:


> Why is the second harmonic squared (and subsequent harmonics raised by exponents)?


The second harmonic isn't squared ... the input signal is squared, as part of the "polynomial curve fitting". It's that squaring operation of the input signal that _creates_ the second harmonic.

Example : voltage input, current output realtionship of a bipolar transistor:

*Ic = Is*exp(Vbe/Vt)*

Where *Is* is called the "saturation current" and *Vt* is the thermal voltage, about 25mV. *Ic* is the collector (output) current, and *Vbe* is the base-emitter (input) voltage.

It's a classic, simple _exponential_ relationship. Time to research the _Taylor-series expansion_, for the exponential relationship  You'll find that this non-linear relationship (like almost all of them ) can be approximated as a "power series" expansion, with a unique set of coefficients (that do NOT depend on signal level). In other words :

*exp(x) = e^x = 1 + x + x^2/2! + x^3/3! + x^4/4! + ...*

It's the "squaring" of the input signal in the power series expansion (for almost _any_ nonlinearity), that generates (or at least, dominates) the second harmonic. And it's a consequence of the squaring operation on the input, that the second harmonic will drop by 12dB if the signal drops by 6dB


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## Sex Cells (Jul 21, 2007)

I'd think if it was that big of an issue you'd just sell the amp and use that extra money towards a new one.


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## ChrisB (Jul 3, 2008)

Sex Cells said:


> I'd think if it was that big of an issue you'd just sell the amp and use that extra money towards a new one.


Or in my case, sell the modderiffic, overpriced, old school relics; buy more amps; then still have money left over for other stuff. Besides making enough money to replace the amplifiers, I also purchased a Alpine CDA-9887, a HO alternator, and a reprogrammed ODB1 ECU with ODB2a to ODB1 jumper harness. After all those purchases, I still had money leftover!


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## lycan (Dec 20, 2009)

lycan said:


> The second harmonic isn't squared ... the input signal is squared, as part of the "polynomial curve fitting". It's that squaring operation of the input signal that _creates_ the second harmonic.
> 
> Example : voltage input, current output realtionship of a bipolar transistor:
> 
> ...


an elaboration ...

Mother nature has given us a wonderful world  where the coefficients of the power series expansion for most non-linearities _decrease_, as the coefficient order (or subscript number above) _increases_. In other words, _most_ naturally occurring non-linearities can be accurately approximated by only the first few terms 

For example, in the power series example above, the coefficients decrease like this :

*a0 = 1
a1 = 1
a2 = 1/2
a3 = 1/6
a4 = 1/24
.
.
.*

A natural consequence of this wonderful gift of nature is that _many_ natural (and most un-natural) non-linear systems can be _approximated_ as _linear_ systems  That makes the math ... and therefore the understanding, by limited minds of **** sapiens ... _a whole lot_ easier.


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## MarkZ (Dec 5, 2005)

lycan said:


> The second harmonic isn't squared ... the input signal is squared, as part of the "polynomial curve fitting". It's that squaring operation of the input signal that _creates_ the second harmonic.
> 
> Example : voltage input, current output realtionship of a bipolar transistor:
> 
> ...


I know what the taylor series expansion is. I don't understand how you're arriving at this polynomial equation as a representation of harmonic distortion. There are several sources of harmonic distortion in an amplifier. How are they all being represented by a singular Ic equation?

You've completely lost me here. It's not in the math. It's in the explanation. I don't know what you're trying to say.


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## MarkZ (Dec 5, 2005)

Quick matlab script to model crossover distortion...



> function xover_dist
> 
> xover = .5;
> t = 0:0.001:10;
> ...


I measure the ratio of the 2nd harmonic amplitude to the fundamental and plot it.










My point is that some distortions, like crossover distortion, seem to increase with decreasing signal levels.


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## lycan (Dec 20, 2009)

MarkZ said:


> I know what the taylor series expansion is. I don't understand how you're arriving at this polynomial equation as a representation of harmonic distortion. There are several sources of harmonic distortion in an amplifier. How are they all being represented by a singular Ic equation?
> 
> You've completely lost me here. It's not in the math. It's in the explanation. I don't know what you're trying to say.


All sources of distortion are NOT represented by a single Ic vs Vbe equation ... that was just an example. I'll develop it further.

Let's start with a SINGLE bipolar transistor. The relationship between Iout versus Vin is the exponential, as we have seen (there's a power series expansion for this function). A "small signal" analysis of this single transistor will yield the classic "linear" terms, such as gm, rpi, etc. These are _linear approximations_ for the _small signal_ behavior of the single transistor. But our first conclusion is this :

*There's 'some' power series expansion for the operation of the single bipolar transistor, that's valid for large signals as well as small*.

Now let's put TWO bipolar transistors together to form a differential pair. No need for the details, but it turns out that the relationship for the differential output current versus differential input voltage is a hyperbolic tangent (there's a power series expansion for this function). A "small signal" analysis of this diff-pair will also yield the classic "linear" terms of the diff-pair. These are also _linear approximations_ for the _small signal_ behavior of the diff-pair. But our second conlcusion is this :

*There's 'some' power series expansion for the operation of a two-transistor differential pair, that's valid for large signals as well as small*.

The natural extension of this discussion leads to a similar conclusion for almost any complex, non-linear system :

*There's 'some' power series expansion for the operation of the non-linear system. It won't have the same coefficients as the description of any single part, but the power series expansion EXISTS. And for most non-linearities, the coefficients decrease as the order increases.*

Mathematically, this is identical to saying : draw me any single-valued curve on a piece of paper, and i''ll give you a polynomial approximation to any desired level of accuracy 

We don't need to know the value of each coefficient, to draw some significant conclusions, when we excite our complex system with a *SINEWAVE*:

1. Whatever the *a0* coefficient for our system is, it will be the dominant contributor to the description of the *DC offset* of the system's output.

2. Whatever the *a1* coefficient for our system is, it will be the dominant contributor to the description of the *gain* of the system.

3. Since *[sin(x)]^2 = 0.5 - 0.5cos(2x)*, the *a2* coefficient (whatever it is) will be the dominant contributor to the *second harmonic* at the system's output (the trig identity shows that squaring a sinewave creates a new sinewave at twice the frequency). In other words, *it's the very EXISTENCE of an a2 coefficient in the power series expansion that CREATES a second harmonic when the system is excited by a sinewave.*

4. Similarly, the *a3* coefficient will dominate the *third harmonic* of the output.

5. and so on ...

A few key points :

a. The coefficients of the power series expansion that describe our complex system MAY very well depend on frequency, and they MAY be complex (rather than purely real). That's OK  the key point is that the coefficients of the polynomial don't change as the input signal _level_ changes. And since they are constant as signal level changes, the trend of decreasing distortion, as level drops, remains the same.

b. The property of "decreasing coefficients as order increases" (displayed by _most_ nonlinear systems) is _also_ responsible for the observation that higher-order harmonics tend to be lower in amplitude than lower-order harmonics ... for _most_ non-linearities.


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## Robin W. (Feb 18, 2010)

I will probably be laughed at and beat with a big stick, but under controlled conditions I believe it is 100% possible to hear the difference between some op-amps. I will refrain from saying better or worse, but different isn't a hard call to make. Now in the context of this thread where the question is more of is there a difference in the amplifier as a whole when you only change the op-amps or caps etc. That would probably come down to the basic design and how good the amp is in the first place. And unless the system as a whole is very very revealing then single op-amp changes or capacitor changes probably would not be audible. However if you start changing op-amps and capacitors in EVERY SINGLE component in the chain then you may start to see a change. 

Now to better describe the "controlled conditions" I mentioned at the beginning of this post, a headphone amplifier fed by a high quality source, and powering high quality headphones. Where 1 or 2 op-amps are the ONLY amplification stage within the amp and there are no capacitors in the signal path. Since those 1 or 2 op-amps are really the whole amplifier changing an op-amp can change the sound signature of the system as a whole. Better or worse is a matter of opinion so it's not a fair measure of performance in all cases, but I'm sure most of us (atleast the ones without hearing damage) could describe differences in the overall sound in these controlled conditions using very familiar music and headphones. 

Let me close this with a disclaimer, this is based on my extensive testing of a headphone setup and NOT a car stereo setup. I'm in the process of modifying an Arc KS300.4 as well as an Alpine PXA-H700 I will be happy to express my OPINION of these modifications when they are complete.


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## lycan (Dec 20, 2009)

MarkZ said:


> Quick matlab script to model crossover distortion...
> 
> 
> 
> ...


I've already said ... there are a precious FEW exceptions to the observation that signal-to-distortion ratio (not including noise!) _improves_ as signal level _drops_. Crossover distortion is one of them. In the whole world of possible distortion mechanisms, I only know of one other.

The power series expansion of crossover distortion (i don't know what it is, but i know that it exists) will NOT display the very typical tendency that higher-order coefficients are lower in amplitude than lower-order coefficients.

EDIT : the short version of what i'm saying is this : MOST nonlinear systems get "more linear" as the input signal gets SMALLER. Crossover distortion is an EXCEPTION (one, of two that i know), because by definition it displays a _discontinuity_ near the origin. MOST non-linear systems don't have this behavior.


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## Sex Cells (Jul 21, 2007)

ChrisB said:


> Or in my case, sell the modderiffic, overpriced, old school relics; buy more amps; then still have money left over for other stuff. Besides making enough money to replace the amplifiers, I also purchased a Alpine CDA-9887, a HO alternator, and a reprogrammed ODB1 ECU with ODB2a to ODB1 jumper harness. After all those purchases, I still had money leftover!


SEE!


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## Sex Cells (Jul 21, 2007)

Too many numbers and pocket protectors in this thread now. It's ruined.


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## MarkZ (Dec 5, 2005)

I understand what you're saying. However, I would argue (maybe not successfully, but I'd argue...  ) that the dominant forms of distortion in most typical amplifiers come from output stage nonlinearities. And _specifically_ from crossover distortion for underbiased designs, gm-doubling for overbiased designs, and those are both exacerbated by crappy thermal tracking strategies. Those distortions both behave as I described in my matlab model.


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## lycan (Dec 20, 2009)

MarkZ said:


> I understand what you're saying. However, I would argue (maybe not successfully, but I'd argue...  ) that the dominant forms of distortion in most typical amplifiers come from output stage nonlinearities. And _specifically_ from crossover distortion for underbiased designs, gm-doubling for overbiased designs, and those are both exacerbated by crappy thermal tracking strategies. Those distortions both behave as I described in my matlab model.


I don't necessarily disagree. Crossover-like distortions may very well dominate most Class B/Class AB amplifier designs. It's one of the reasons why i hate Class AB amplifiers for some categories of amplification (like driving high-efficiency drivers).

And I know this thread is about power amps, but in the interest of "sweeping generalities" , i would like to point out some nonlinear systems that _don't_ display the rather unique behavior of discontinuity near the origin :

- Class A amplifiers
- Class D amplifiers
- any preamp, active crossover or EQ (with well-behaved opamp output stages!)
- oversampled delta-sigma data converters (ADC's and DAC's)
- loudspeakers
- core non-linearities in inductors
- voltage-coefficient induced non-linearities in capacitors
- air compression & rarefaction

Even a Class AB amplifier, whose distortion components DROP (not including noise!) as the signal level drops, is not being dominated by crossover distortion.

But I really do agree that the "crossover output stage" may very well be the dominant issue in most Class AB designs (including drift over time & temp).


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## lycan (Dec 20, 2009)

one last thing, for "completeness" and just to avoid confusion ...

You can certainly draw an input/output "curve" for a system that has a discontinuity near the origin (like crossover distortion). And so it's certainly possible to "curve fit" a very high-order polynomial to that "transfer function". In other words, even this wacky crossover distortion (like most nonlinear systems) has a _power series expansion_.

What you'll find in that power series, though, is something i've called "uncommon" : the higher-order terms will _not_ diminish in size. And the polarities of the coefficients will be all over the place.

And so this power series expansion will _not_ exhibit the property that harmonics drop "nicely" ... or even, drop at all ... as signal level drops. Such a system does _not_ get "more linear" as the signal gets smaller.

Finally, what non-linear systems do _not_ have power series expansions, as i've described them? Well, one class is multi-valued non-linearities, like hysteresis.

OK, i'm done.


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## piyush7243 (Sep 9, 2009)

I am looking forward to replace all the OpAmps on my SS ref4.920 with Burr Brown's OPA627AP. What difference would it make and will it be VFM?


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## ChrisB (Jul 3, 2008)

cajunner said:


> Tons of people modding headphone amps, home audio equipment, even portable CD players, but in mobile audio it's considered a moot issue?


The home audio crowd doesn't have to deal with a 60 dB or higher noise floor compared to the car audio crowd. Sadly, aside from major power increases, any subtle differences made to make an amplifier "sound" better in a car will be lost between the engine, exhaust, road, and wind noise.


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## MarkZ (Dec 5, 2005)

Just because some people make modifications doesn't justify that it's a useful thing to do.


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## ChrisB (Jul 3, 2008)

cajunner said:


> this is a fair point, and make no mistake, I spend far too much time in my car with the engine off listening critically for errors in between enjoying the music, when I could just walk inside my home and enjoy the advantages that are in the home environment.
> 
> However, I would venture to say that the people discussing the relative benefits of 24bit processors, the ones who do head unit shoot-outs, the ones who roll into the judging lanes, aren't worried about the road noise as much as what they can get out of the system in the parked mode.
> 
> ...


It all depends on who is doing the modifications and what they are doing. There are some shysters out there who WILL sell you on something that you DO NOT need all under the pretenses of "bringing your amplifier up to date". I fell for that with a certain repair tech who pointed me to his web site when I asked him what services he performed in a mod. Well, let's just say what he did versus what he implied he would do are two COMPLETELY different things.

On the flip side, I have an amplifier that is capable of 1 ohm stereo operation. One of the designer's techs said that he could swap out some resistors and capacitors if I wanted to run it in 4 ohm stereo mode ONLY, citing it would REALLY up the power (as in double it), provided I would NEVER run it below 4 ohms stereo. IIRC, he said the transformer would not need to be rewound because it was overbuilt for high current mode and could easily sustain high voltage mode, provided I need the additional 4 ohm stereo power.

In all honesty, I think I would buy another amplifier before I would pay someone to modify one of mine. My first wife, may she rest in peace, liked buying shoes and purses whereas I like buying guns and amplifiers.


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