# Understanding slew rate and damping factor



## my89_928gt (Aug 22, 2006)

Any one care to go into detail on explain slew rate and damping factor for amplifiers. What are good SQ numbers.
Did a quick search and did not see anything. So, let dig in and get educated.
I know someone on here knows 

Richard


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## 14642 (May 19, 2008)

How about this?

Neither matter much. Slew rate is basically a measure of distortion at high frequencies--how fast can the amp swing from - to +. It's taken into account for any power measurement over the useable band of frequencies at some distortion level. dampiing factor is nearly useless. It's an erroneous iindication of how muh control an amplifier's output impedance (or lack of imedance) has over the speed at which the woofer returns to rest after a signal stops. The formula fails to take into account the primary EMF generator in the speaker and its relationship to the speaker's DCR. Basically, the speaker controls its damping and the amp doesn't.

Amplifier output impedance can alter the sound of a speaker by attenuating frequencies where the speaker's impedance is low and attenuating frequencies where the speaker's impedance is high to a lesser extent. Tube amps with high output Z do this and output filters in Class D amplifiers do this as well. Most modern amps that aren't tube amps are good enough in this regard.

Basically, don't sweat either of these specs.


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## my89_928gt (Aug 22, 2006)

I would think they would matter to a point, then no noticed benfit. For example I would think if all the same except amp A had a slew rate of 3 and amp B had a slew rate of 30, it would have to show.
And damping factor; so, you say their is no befit from say one amp at 80 and one at 800?
I know specs do not tell the whole story on how an amp will sound but, it's can offten times be a starting point in narrowing down a list.


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## t3sn4f2 (Jan 3, 2007)

my89_928gt said:


> I would think they would matter to a point, then no noticed benfit. For example I would think if all the same except amp A had a slew rate of 3 and amp B had a slew rate of 30, it would have to show.
> And damping factor; so, you say their is no befit from say one amp at 80 and one at 800?
> I know specs do not tell the whole story on how an amp will sound but, it's can offten times be a starting point in narrowing down a list.


"Basically, don't sweat either of these specs.
__________________
*Global Product Line Manager, JBL Car Audio
Harman Consumer *"

Why would this person tell you this? To bring his company down or because it's the way it is. Hard call huh.


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## [email protected] (Jun 12, 2008)

I dont agree with that, years ago Kenwood exelon had made some amps with a sigma servo feedback system, basically the dampning factor was like 2000 or something really high like that. 

Now I am talking years ago, I had a Kicker zr600 hooked up to them, I swapped out the Kenwood exelon amp woth sigma servo, and the detail of the bass was way better, just more controlled. 

I believe from my personal testing that dampning factor has alot to do with how the speakers plays, Do you think that channel seperation has no effect or how about s/n ratio or any other spec for that matter?

I am also ones that doesnt believe all amps sound the same. Maybe the people that say these specs sont mean anything feel all amps sound the same?


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## t3sn4f2 (Jan 3, 2007)

BeatsDownLow said:


> I dont agree with that, years ago Kenwood exelon had made some amps with a sigma servo feedback system, basically the dampning factor was like 2000 or something really high like that.
> 
> Now I am talking years ago, I had a Kicker zr600 hooked up to them, I swapped out the Kenwood exelon amp woth sigma servo, and the detail of the bass was way better, just more controlled.
> 
> ...


It always going to come down to did yo compare the two amps in a proper blind test. If you did not then discussing this topic is pointless.


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## [email protected] (Jun 12, 2008)

t3sn4f2 said:


> It always going to come down to did yo compare the two amps in a proper blind test. If you did not then discussing this topic is pointless.


What your version of a proper test?


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## t3sn4f2 (Jan 3, 2007)

BeatsDownLow said:


> What your version of a proper test?


Anyone that doesn't involve comparing clipping characteristics since that would negate the whole point in paying more money for better specs that only matter with the clean power spectrum. One that I have no knowledge of which one of the two being compared I am listening to. One with many tries to make sure I didn't have a lucky guess. On and on.....


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## [email protected] (Jun 12, 2008)

t3sn4f2 said:


> Anyone that doesn't involve comparing clipping characteristics since that would negate the whole point in paying more money for better specs that only matter with the clean power spectrum. One that I have no knowledge of which one of the two being compared I am listening to. One with many tries to make sure I didn't have a lucky guess. On and on.....


I am not talking about how much power it puts out before clipping, thats besides the point, clipping or not the sub exerted more cone control with that amp, to my logical thinking and understanding of what I hear and see, the amp controlled the movement of the sub better, which to me corelates that the sigma servo feedback sytem with the way higher dampning factor was what did it. Both amps set with a DMM, no scope since this was close to 10 years ago, I didnt have access to a scope.

I dont get how people seem to deny that specs relate to anything??


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## t3sn4f2 (Jan 3, 2007)

BeatsDownLow said:


> I am not talking about how much power it puts out before clipping, thats besides the point, clipping or not the sub exerted more cone control with that amp, to my logical thinking and understanding of what I hear and see, the amp controlled the movement of the sub better, which to me corelates that the sigma servo feedback sytem with the way higher dampning factor was what did it. Both amps set with a DMM, no scope since this was close to 10 years ago, I didnt have access to a scope.
> 
> *I dont get how people seem to deny that specs relate to anything??*


I don't know what to tell you then. Pay for what _you know_ matters then and enjoy.


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## 14642 (May 19, 2008)

All amplifiers DO NOT sound the same. 

There are plenty of things one can do to an amplifier to change the way bass sounds and those things may change the "damping factor" but that doesn't mean damping factor is important--that a low output impedance has anything to do with the speaker coming to rest more quickly. If the amplifier has a circuit that actually drives the cone to rest rather than allowing it to come to rest more quickly, then that would improve the performance, but that isn't damping factor amd searching for an amp that has a low output impedance and the same damping faactor spec as your Kenwood is no guarantee of the same performance. 

Sorry folks, wishing doesn't make it true and unbridled love for your favorite brand doesn't make their technology better. If you like it, fine. There's nothing wrong with being in love with a pig, but a pig is still a pig.

Oh, and to the person who can't figure out why I would say this stuff...

I post this stuff for your benefit. I'd rather see you spend money and time on things that will improve your systems than on specs designed by marketing departments that aren't relevant. Not all specs are crap, but the ones that are crap are crap.


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## vecc205 (Nov 18, 2007)

A slew rate of 2V/microsecond is good enough to pass a full amplitude sine wave at 20Khz. The only op amp that has problems with slew rate limiting is the old 741 which no amps use in their input section. So slew rate like has been said can be ignored. Damping Factor in solid state amps can be ignored also, with all solid states having a low enough output impedance to not be effected by a difficult load. Tube amps with transformers on the output may have a harder time driving a difficult load compared to a solid state amp because they will have a higher output impedance. So in most cases these topics can be ignored as I find build quality, options the amp offer and power output should be more of what is looked at.


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## MarkZ (Dec 5, 2005)

BeatsDownLow said:


> I dont agree with that, years ago Kenwood exelon had made some amps with a sigma servo feedback system, basically the dampning factor was like 2000 or something really high like that.
> 
> Now I am talking years ago, I had a Kicker zr600 hooked up to them, I swapped out the Kenwood exelon amp woth sigma servo, and the detail of the bass was way better, just more controlled.
> 
> I believe from my personal testing that dampning factor has alot to do with how the speakers plays, Do you think that channel seperation has no effect or how about s/n ratio or any other spec for that matter?


I do. I can't think of an amplifier spec that's worthwhile aside from...

1) power output

That's all. 

s/n is a horrible spec because it doesn't predict how difficult it is to exorcise the noise ghosts. That's quite a different matter entirely. Channel separation is useless in a reflection-heavy car!

I think the point about damping factor and slew rate isn't that they don't matter -- it's that every solid state amp under the sun is going to measure well beyond our ears' capability of detecting.


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## MarkZ (Dec 5, 2005)

BeatsDownLow said:


> I am not talking about how much power it puts out before clipping, thats besides the point, clipping or not the sub exerted more cone control with that amp, to my logical thinking and understanding of what I hear and see, the amp controlled the movement of the sub better, which to me corelates that the sigma servo feedback sytem with the way higher dampning factor was what did it. Both amps set with a DMM, no scope since this was close to 10 years ago, I didnt have access to a scope.
> 
> I dont get how people seem to deny that specs relate to anything??


Because if you take the specs and you run the numbers through the appropriate equations (as Richard Pierce has done), you'll find that the output is roughly the same.

What does "more cone control" sound like anyway? And how do you know that damping factor is contributing to it? There are lots of reasons one sub will sound more "controlled" than another, and it usually has a lot more to do with its interaction with the enclosure and the immediate environment than it does with the amplifier.


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## TXwrxWagon (Sep 26, 2008)

Andy & T3 from JBL/Harmon... can you guide the group, especially the noobies, as to a general outline, as to what one SHOULD look at when comparing amps of similar power output @ the same voltage & same general (yet worthless) THD spec?

I mean, there has to be SOMETHING on the general spec sheet that can help get the ball rolling. 

Sure, ideally we would all be able to walk into the local audio shop, have them pick up a pair of comparable amps, set them up in a realistic listening environment, match them for output & sensitivity to the source & do a true blind A/B. But we all know sound boards are all configured to simply play loud, & no shop will swap amps out of cars keeping everything else the same.

So there has to be SOME outline/guideline to follow.

thanks!

Rob


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## ca90ss (Jul 2, 2005)

TXwrxWagon said:


> Andy & T3 from JBL/Harmon... can you guide the group, especially the noobies, as to a general outline, as to what one SHOULD look at when comparing amps of similar power output @ the same voltage & same general (yet worthless) THD spec?


Features, build quality, aesthetics, price etc.


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## t3sn4f2 (Jan 3, 2007)

TXwrxWagon said:


> Andy & T3 from JBL/Harmon... can you guide the group, especially the noobies, as to a general outline, as to what one SHOULD look at when comparing amps of similar power output @ the same voltage & same general (yet worthless) THD spec?
> 
> I mean, there has to be SOMETHING on the general spec sheet that can help get the ball rolling.
> 
> ...


x2 on the above

Anything more then that you'll need to know what your system will need and where it will be getting it from. It all comes down to are you pushing the amp into a dirty range at the output levels you need.

I'm not from JBL by the way.


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## TXwrxWagon (Sep 26, 2008)

oops it was the way you quoted Andy... I thought you both were from JBL/Harmon...

cool... I've always lived by "if it weighs more its gotta be built better"... since none of the specs mean ****... cool... 

Rob


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## robspeed325i (Nov 8, 2008)

Andy Wehmeyer said:


> How about this?
> 
> Neither matter much. Slew rate is basically a measure of distortion at high frequencies--how fast can the amp swing from - to +. It's taken into account for any power measurement over the useable band of frequencies at some distortion level. dampiing factor is nearly useless. It's an erroneous iindication of how muh control an amplifier's output impedance (or lack of imedance) has over the speed at which the woofer returns to rest after a signal stops. The formula fails to take into account the primary EMF generator in the speaker and its relationship to the speaker's DCR. Basically, the speaker controls its damping and the amp doesn't.
> 
> ...


LOL!!! Leave it to the marketing department to ignore and gloss over these FACTS! google the terms, and LEARN for yourselves! I for one remember the "golden age" when amp mfgrs actually marketed their products based on these *REAL* terms... RIP the golden age!


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## t3sn4f2 (Jan 3, 2007)

http://www.klippel-listeningtest.de/lt/default.html

-40dB = 1% THD
-60dB = .1% THD


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## ca90ss (Jul 2, 2005)

robspeed325i said:


> LOL!!! Leave it to the marketing department to ignore and gloss over these FACTS! google the terms, and LEARN for yourselves! I for one remember the "golden age" when amp mfgrs actually marketed their products based on these *REAL* terms... RIP the golden age!


Since you seem to think you know so much more than Andy why don't you give us some examples of solid state amplifiers that are audibly different form one another due to differences in slew rate and/or damping factor.


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## norcalsfinest (Aug 30, 2008)

Rod Elliot said:


> _Another aspect of amplifier design is slew rate. This term is not well understood, and the possible effects even less so.
> 
> Slew Rate Nomograph
> It has been claimed by many writers on the subject that a slew-rate limited amplifier will introduce transient intermodulation distortion, or TIM. In theory, this is perfectly true, provided that the slew rate is sufficiently low as to be within the audible spectrum (i.e. below 20 kHz), and the program material has sufficient power at such high frequencies to cause the amplifier to limit in this fashion.
> ...



Large-signal maximum slew-rate is usually a non-linear input stage limiting effect, and the input signal amplitude needed to achieve this can be widely different for different input stage designs. Input stages using fets, or using bjts with emitter degeneration resistors, may need 1V or more differential input voltage to cause input stage limiting and achieve the maximum slew-rate specified. Used as a unity gain buffer the difference between input and output voltages is equal to the input stage differential input, and should ideally be zero, but close to maximum slew-rate the difference could be around 1V, which is then an error voltage.
The actual output slew-rate achieved with a fast input step at a lower voltage may be far below the large-signal value. This is no longer a non-linear effect, but is determined by falling high frequency open-loop gain. A high value of maximum large-signal slew rate is no guarantee of a good small-signal value. Usually it is the gain-bandwidth product which determines the maximum small-signal output slew-rate.


As for damping factor, too little is bad and too much is bad.

Too little, changes in frequency, mostly with woofers, will not be properly controlled, and the amplifier likely has a very high output impedance.

Too much, and the amplifier has a very low output impedance, and without careful design, can have a lot of negative feedback. Too much negative feedback, again without a carefully designed amplifier, will cause phase shift of the signal, which in turn shows up as distortion.

While not OVERLY important, still specs to look at. If the numbers are WAY overinflated, look at other factors to determine whether or not the amp is quality designed and built.

Robert Zeff's amps usually have high damping factor and slew rate numbers, but also very low distortion, as do Stephen Mantz's amps. However, these are two very special cases of engineers, and take the time to design an amp properly so big damping factor doesn't add distortion via phase shift. For example, in Zed's last line of amps, they were spec'd as only lagging 10 degrees in phase at 20kHz. Impressive.

/end]

Teh interwebz vill get joos


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## my89_928gt (Aug 22, 2006)

Nice article norcalsfinest.
Thanks for the input guys.


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## TXwrxWagon (Sep 26, 2008)

ca90ss said:


> Since you seem to think you know so much more than Andy why don't you give us some examples of solid state amplifiers that are audibly different form one another due to differences in slew rate and/or damping factor.


*edit* smart azz comment removed

I myself, was seriously asking on be half of the many newer audio enthusiasts, if there are ANY specs worthy of consideration.

Aesthetics, features, size, footprint etc are just BS answers. You guys know that... with the advent of EIAJ, in theory  the power ratings "war" has all but been eradicated. HOWEVER as I keep seeing, everyone talks about clipping, distorted outputs etc... there has GOT to be some spec or combination of specs that can help guide a bewbie in his/her learning curve.

Andy said something about (paraphrasing) "blind brand loyalty" *edit* "*a pig is a pig*"& that is EXACTLY why there is a need, a genuine need, for the non-engineers of the audio world to have something to depend on. The whole "overrated/underrated" thread even more confuses the issue, since some hate Zapco & others hold it as the Holy grail. Others think Zed was just an "average" designer/engineer with a great following. 

So the question is: Are there any specs that are a reliable indication of a better built product? Yes or No? if Yes: what are they? if No: bummer.

thanks...

Rob


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## MarkZ (Dec 5, 2005)

TXwrxWagon said:


> *edit* smart azz comment removed
> 
> I myself, was seriously asking on be half of the many newer audio enthusiasts, if there are ANY specs worthy of consideration.
> 
> ...


I really don't see why you're considering the answers you've been given "BS answers".


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## 14642 (May 19, 2008)

The point is that after nearly 80 years, amplifier design is well understood. One has to do a really bad job to design and build an amp that sucks. All of this effort in understanding the pros and cons of amplifiers is basically a waste of time and if you pay an average price for an average amplifier from a reputable manufacturer, that's all there is to worry about. Choose an amplifier based on the featuers that make it perform well in your system and spend your time learning about things that make a much bigger difference--like analysis tools and tuning. 

My car is filled with chip amps--23 of them and a basic amplifier that I use to drive a pair of IB subs--and these are no esoteric subs, either. I guarantee you that the signal processing in the car and the painstaking attention to tuning makes a much bigger difference than some gold-plated amp PC boards would. 

You guys are spending too much time trying to understand the molecular makeup of the lawnmower blade and not enough time on the landscaping.


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## chad (Jun 30, 2005)

Damping factor has little to nothing to do with the amplifier and everything to do with wire resistance for longer runs and final impedance on the amplifier.


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## norcalsfinest (Aug 30, 2008)

chad said:


> Damping factor has little to nothing to do with the amplifier and everything to do with wire resistance for longer runs and final impedance on the amplifier.


pretty much was discussed earlier. but yes to an extent


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## TXwrxWagon (Sep 26, 2008)

MarkZ said:


> I really don't see why you're considering the answers you've been given "BS answers".


Hmmm... let's see... 

BS: aesthetics: WTF? they all should match right? 
BS: Features: DUH! If I need additional crossovers or a Sub-sonic filter... double ****ING duh!

BS: Foot print: Don't you think if I had a freaking Neon I would know my square footage available?

for the love of ****ING Zeus... give the newbies SOMETHING... 

I personally have all the amps I need, I am trying to be the advocate here... I have my own biases & I accept that...

Geez... come on "grand masters" give the flock something to chew on... You are all the self professed "experts"....

Rob


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## MarkZ (Dec 5, 2005)

TXwrxWagon said:


> Hmmm... let's see...
> 
> BS: aesthetics: WTF? they all should match right?
> BS: Features: DUH! If I need additional crossovers or a Sub-sonic filter... double ****ING duh!
> ...


Well, I think you pretty much got it all there. 

Nah, seriously though, you obviously want to buy an amp that will deliver the power you need into the impedance you're using. And you'll want to get a handle on how reliably it can do that without overheating. There's no readily available spec that will tell you _that_.

Other things to keep in mind are efficiency and how much ventilation it needs (in case you're installing it in a tight spot). Efficiency numbers are sometimes available. But this is a spec that depends on things like supply voltage and output amplitude/duration, so a single number doesn't exactly encapsulate it. But I guess it's a good starting point.

As for reliability...well, like most things, there's no easy measure. Forums, shops, and word of mouth are ok sources, as long as you take it all with a grain of salt.

I bought one of my amps back in '99 or so. I'll be installing it into my newish car in a week or two. That'll be like the 5th car it's going into. It's lasted a long time for me. So I bought a 4 channel version of the same thing, and a class D subwoofer amp by the same manufacturer. The only specs I used were #channels, rated power, and price. 

Edit: Also, better than the spec sheet is the weight test. The one that's heavier is usually the better amp (assuming they're the same class). You may laugh, but it's a more reliable test than any spec I know of.


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## ca90ss (Jul 2, 2005)

TXwrxWagon said:


> Hmmm... let's see...
> 
> BS: aesthetics: WTF? they all should match right?
> BS: Features: DUH! If I need additional crossovers or a Sub-sonic filter... double ****ING duh!
> ...



So what exactly are you looking for? There are no other reasons other than what's been listed for choosing one amp over another.


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## 14642 (May 19, 2008)

So Rob, what would you like to chew on? An exhaustive explanation of specs that don't matter? What's the point?


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## br85 (May 2, 2008)

http://sound.westhost.com/biamp-vs-passive.htm

Another one by Rod Elliot. Basically explains that output impedance of an amplifier (a spec which IS worth looking at, even though damping factor might SEEM like the right one to look at) is of no benefit at all if a passive crossover is used. A good read if you can keep up with it.


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## 14642 (May 19, 2008)

br85 said:


> http://sound.westhost.com/biamp-vs-passive.htm
> 
> Another one by Rod Elliot. Basically explains that output impedance of an amplifier (a spec which IS worth looking at, even though damping factor might SEEM like the right one to look at) is of no benefit at all if a passive crossover is used. A good read if you can keep up with it.


Yes, in terms of the ability of the amplifier's output to dissipate the energy stored in the EMF generator, this is correct. The output impedance of the amplifier, if great enough, does change the frequency response of the system by attenuating output by an amount inversely proportional to the impedance of the speaker, whether there's a passive crossover or not.

Unless the amplifier's output impedance is damn close to the DCR of the speaker, it doesn't affect circuit damping.


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## chad (Jun 30, 2005)

Lets throw a monkey wrench into this sucka......

Many believe that tubes are the holy grail to SQ, they like the way they look, the way they sound and the way they can make a speaker "sing"

What is this "singing?"

In my corner of the world we try to shoot for a DF of 20 or greater at the speaker, including wire, that's a damn good number with the runs we make. It has been explained over and over that DF is the ability to restore the cone to rest position or "control it," this prevents "ringing" in subwoofer systems and even mid-bass systems (for car folk). Our wire lengths really don't constitute us worry too much in an automobile unless we are running VERY thin wire.

Lets look at DF, we know it's a ratio involving amplifier output impedance and combined load impedance right? You have to MATCH output impedance of a tube amplifier to the load in order for it to work properly, a 35W tube amp will put out 35W regardless of load because it must be matched. This means THAT IF THE AMPLIFIER OUTPUT TERMINALS WERE WELDED TO THE SPEAKER then we would be seeing a DF of around ONE. Ths number falls to less than one due to speaker wire and passive crossovers. This is why people like Kipinis say that things need to be "properly matched" in the tube world. You can literally "voice" the low end "ring" by changing out things such as OP transformers, crossover topology, drivers and enclosures.

That warm sound? It's ring, due to the lack of amplifier damping.

Chad


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## chad (Jun 30, 2005)

DIYMA automatic dupe feature.


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## t3sn4f2 (Jan 3, 2007)

One more time for good luck


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## TXwrxWagon (Sep 26, 2008)

First off, I do apologize for the "rant" & outburst... 

I guess a mythical unicorn in the current world of electronics. I read questions by fledgling audio-buff's everyday & wish I could provide them with "something" to use in making their decisions, besides being mislead by the current trend/fad or the sales-spiff of the day on the sales floor. 

Having sold & installed car audio going on 22 years, I'm just jaded I guess. Andy, you're absolutely right about amplifier design not changing significantly in forever. What has changed is the expectations of the consumers, they have polarized. Now you have a very small, elite contingent who want true SQ/performance & the rest want price/size/features with little regard for SQ.

So, I guess.. its just like I said... a search for a mythical unicorn.

Rob


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## Oliver (Jun 25, 2007)

I once thought i slew a unicorn but his DF saved his ass !


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## robspeed325i (Nov 8, 2008)

if you like distorted, "ringing" sound, thats fine. to each his own.

slew rate = speed. the faster the better.

damping = control. the more the better.

any questions?


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## ca90ss (Jul 2, 2005)

robspeed325i said:


> any questions?


Yeah, can you name any solid state amps that are aduibly different due to differences in either of these 2 factors?


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## robspeed325i (Nov 8, 2008)

no, but that's only because I run/ listen to oldskool PPIs. (been out of the loop for a while...)


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## t3sn4f2 (Jan 3, 2007)

...


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## squeak9798 (Apr 20, 2005)

ca90ss said:


> Yeah, can you name any solid state amps that are aduibly different due to differences in either of these 2 factors?


Wasn't there a Phass amp that had a DF of .2 ?


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## chad (Jun 30, 2005)

robspeed325i said:


> if you like distorted, "ringing" sound, thats fine. to each his own.


Actually the topology of amplifiers that will exhibit a ''ringing sound'' in speaker systems point out the fact that the passive crossover is inferior for the most part, this is where most of the reactiveness comes into play  IT ALSO points out the fact that htese high numbers are worthless in the fact that there are MANY tube amplifiers out there that sound and measure JUST LIKE SS amps in FR, etc and have a resulting DF of less than one.



robspeed325i said:


> slew rate = speed. the faster the better.


Slew rate is a worthless spec in a modern SS amplifier, and even in tube amps that have good transformers.



robspeed325i said:


> damping = control. the more the better.


Not necessarily, I'll take an uber reliable and powerful amplifier with rated 200DF ANY DAY over a shotty design with a 2000 damping factor because, after wire resistance, the difference is.. NIL for the most part. Just do the math man.


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## chad (Jun 30, 2005)

I would also like to point out this thread as opposed to using the "you should have searched first feature"

http://www.diymobileaudio.com/forum/showthread.php?t=15011


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## chad (Jun 30, 2005)

I would also like to point out this thread as opposed to using the "you should have searched first feature"

http://www.diymobileaudio.com/forum/showthread.php?t=15011


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## Diru (May 23, 2006)

lets toss in a OTL amp in here

http://www.jogis-roehrenbude.de/Leserbriefe/Kunz-Amp/beschreibung.htm


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## 14642 (May 19, 2008)

This thread is ridiculous. Absolutely ridiculous. 

Damping factor is speaker's nominal impedance divided by amplifier output impedance. It was designed by marketers as a spec to "explain" the benefits of transistor amps vs. tube amps. The suggestion is that the higher the damping factor, the more control the amplifier has over the speaker's ability to stop moving when the signal goes away. The problem with the spec is that it taks into account only nominal impedance.

OK. Now for a speaker lesson. The motor moves the cone using magnetic force that's generated when current flows through the voice coil. Any time current flows through a wire, a magnetic field is generated. That magnetic field is generated at a right angle to the direction of current travel. If the current alternates, then so does the direction of the magnetic field. The fixed, or static, magnet doesn't move. Therefore, the field from the electromagnet pushes and pulls against the field of the static magnet, which is focused on the coil in the gap between the top plate and the polepiece. The opposiing and attracting causes the cone to move because it's attached to the voice coil. The DC resistance of the voice coil is CHOSEN by engineers to determine how much current will flow through the coil. 

The suspension's job is to keep the coil in the gap and to move the cone back to the resting position if the coil moves far enough outside the gap that the two magnets no longer interact well enough to control the motion of the cone. The spider is mostly responsible for this. Because the spider is a spring, it ALWAYS exerts some force in oppositon to the direction the cone is being driven. Since it's a spring, when the signal stops, it causes the cone to bounce back and forth a little bit. Mechanical Q, or Qms is the value that describes this behavior. A high value means more overshoot is allowed and a low value means little overshoot is allowed. 

This is measured at resonance. Resonance is the frequency at which the speaker (or any mechanical system) will continue to move if it's set in motion. 

Now...we know that when a current passes through a wire, a magnetic field is generated. The converse is also true. When a wre is moved inside a magnetic field, voltage (potential) is created and current flows through the wire if a closed circuit is present. When the signal stops, the speaker continues to move, thanks to the springiness of the spider. That movement at the frequency of resonance causes a voltage to be created and current to flow. Because that current flows through the voice coil, it causes the speaker to move, which creates more voltage, which causes the speaker to continue to move, which causes more voltage...and so on and so forth. This continues until all the energy has been dissipated. 

The impedance peak in a speaker at resonance is a result of this voltage. It causes a current to flow in opposition to the current from the amplifier and we express that as impedance (resistance)Remember, the spider contributes some force to the continued movement of the cone and the motor contributes some as well. THe electrical analogue for a spider (compliance) is a capacitor. It stores energy in the form of voltage. The part of the motor that contributes motion is the inductance of the voice coil--it creataes the magnetic field. Where the reactance of the capacitance and the reactance of the inductance meet (are equal) is resonance and the peak impedance is resistive--where the curve changes direction. We call that the "motional' impedance. The parameters for those three things are Ces, Res and Les. 

If you look at the impedance curve of a speaker, you'll find that the impedance of the speaker is offset from 0 ohms by the DC resistance of the voice coil. The voltage that's generated by the moving coil causes a current to flow if there's a closed circuit and the amount of current that flows determines how quickly all the stored energy is dissipated and, as a result, how quickly the speaker comes to rest. Remember, the voltage is ceated by the reactance of the "capacitor" (spider) and the inductor (inductance of the voice coil). These are the only two parts of the speaker that contribute motion. If I short the speaker terminals together, that closed circuit will allow current to flow THROUGH THE DC RESISTANCE OF THE VOICE COIL WHICH IS NOT PART OF THE EMF GENERATOR AT RESONANCE. 

Let's say I have a speaker with a 4-ohm nominal impedance, so the DCR is something like 3.2 ohms. Instead of shorting the speaker terminals to dissipate the stored energy, I put an amplifier's outputs in the circuit. Lte's say it's a perfect amplifier and it has NO output impedance. The rate at which the current will flow from the EMF generator (resonance) is determined by the DCR of the speaker. It's in series with the EMF generator. Hmmm...

Now let's say the amplifier is a damn good one with a super high damping factor of 4000 at 4 ohms. That means the output impedance of the amplifier is .004 ohms. Since the output impedance of the amp is in series with the speaker, the resistance through which the speaker's stored energy is dissipated is now 3.004 ohms. LESS CURRENT FLOWS SINCE THE TOTAL RESISTANCE IS HIGHER, SO THE ENERGY TAKES LONGER TO DISSIPATE.

Looking a little more closely at the numbers reveals that the speaker's DCR has 3/0.004 times the influence over how much current flows. That means that the speaker's DCR has 750 times the influence over how quickly the speaker comes to rest, which is also called damping. 

The damping factor spec doesn't take any of this into account and simply uses the nominal impedance of the driver to suggest that the amp controls the speaker as it comes to rest. The nominal impedance is so over simplified in this model that the spec is meaningless--in fact, it's miseading.

It's crap. Plain and simple. 

For another proof, simply add the amplifier's output impedance to the DCR in the formula for speaker Qes and then recalculate the Qts. You'll see that it makes very little difference in the spec that ACTUALLY DESCRIBES THE SPEAKER'S BEHAVIOR IN THIS REGARD.

Output impedance is important if it's high enough to change the frequency response of the speaker. There are things that can be done to amplifier circuits that CAN influence the behavior of the speaker after the signal stops but simply reducing the output impedance IS NOT ONE OF THEM.


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## Grim0013 (Nov 4, 2008)

Andy Wehmeyer said:


> This thread is ridiculous. Absolutely ridiculous.
> 
> [main body removed in the interest of space savings]
> 
> Output impedance is important if it's high enough to change the frequency response of the speaker. There are things that can be done to amplifier circuits that CAN influence the behavior of the speaker after the signal stops but simply reducing the output impedance IS NOT ONE OF THEM.


*Translation:* "This thread is OVER. Next time, raise your hand if you want to talk to me."


Oh, and thanks, Andy. I, like many others here, are the type who can't simply be told that the world is round. We needed to know why.


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