# Modding my amp to put out more power at 8 ohms



## MarkZ (Dec 5, 2005)

I've got an a/d/s/ P640 that I'm modding to run my 8 ohm mids and 6 ohm (?) tweeters. It puts out I think 40 x 6 channels, and I'm going to double it. I'm going all out and replacing the transformer. Then I'm upgrading the secondary caps from 35v to 50v. And I'm replacing the primary side caps just because it's an old amp (slight upgrade in capacitance too). Ordered new sil-pads and thermal grease. Altogether, the parts are costing me about $30 including shipping. 

This thing has the Toshiba 2SC3281's output devices in it, so the output stage should be fairly robust in the presence of the increased Vce. I checked the SOA on the datasheet, and it looks good to go as long as I don't run a low impedance. All the other circuit elements are immune to the higher rail voltage (A06/A56 drivers and small signal transistors), though I might have to replace two resistors in the accessory power supply. Big woop.

After this one, I'll do the same thing to my DEI 600d sub amp. Some day.


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## dodgerblue (Jul 14, 2005)

This is a fun - break from the norm - read. Some pics and specs in the future would be a cool thread  Are you re-winding or a new replacement trans ?


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## wheelieking71 (Dec 20, 2006)

sounds awsome! i wish i understood this stuff!


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## basshead (Sep 12, 2008)

Don't forget to take some measurement before and after. One thing that i would like to know is if this mod will change the temperature of the amp.



dodgerblue said:


> Are you re-winding or a new replacement trans ?


asking myself the same q


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## Oliver (Jun 25, 2007)

MarkZ said:


> I'm going all out and replacing the transformer.



It will have the potential to generate more heat, kinda like putting a bigger motor in a car


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## kyheng (Jan 31, 2007)

Do post the results... I may want to modify my 6.9 also......


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## basshead (Sep 12, 2008)

a$$hole said:


> It will have the potential to generate more heat, kinda like putting a bigger motor in a car


If he's beefing it up yup, but will rewinding the same core to get a higher voltage and less current generate more heat?


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## MarkZ (Dec 5, 2005)

Rewinding. I don't expect it to generate a whole lot more heat. The primary side will be identical. The secondary side will generate a higher voltage but proportionally less current. So I don't think the supply will generate more heat at all. The higher rails though mean a less efficient output stage. Those output devices will be taxed more than they currently are. But the amp is 2 ohm stable, the mod should make it close to 4 ohm stable, and I'll be running it at 8 ohms. So I'm not too worried about heat. As it stands now, it doesn't even get warm to the touch. 

When the parts come, I'll take some pictures. Unfortunately, it's already disassembled so I can't do any "before" measurements. But I have a P840 that I could measure... it's the same amp but with two extra channels.


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## wheelieking71 (Dec 20, 2006)

is there a book i can buy that explains how this **** works??!! it fascinates me. ive done a little searching, but havnt come up with anything "good".


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## Oliver (Jun 25, 2007)

Amazon.com: High-Power Audio Amplifier Construction Manual…

High-Power Audio Amplifier Construction Manual [Paperback]
G. Randy Slone


http://another-electronics.blogspot.com/2008/09/design-of-car-audio-power-amplifier.html


References

* Electronics Tutorials: Class B Amplifiers
* "Electronic Devices"; Thomas Floyd; 1996


Read more: How to Build Your Own Power Amp | eHow.com http://www.ehow.com/how_6108826_build-own-power-amp.html#ixzz0vHrpdXdP


http://www.ehow.com/how_6108826_build-own-power-amp.html


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## wheelieking71 (Dec 20, 2006)

thank you good sir. im terrible at searching (i do try) back to the amp, cant wait to follow this.


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## ChrisB (Jul 3, 2008)

Perry Babin's repair tutorial is also something worth having: Learn To Repair Car Audio Power Amplifiers

You can also learn a lot just by going to his website: Basic Car Audio Electronics


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## MarkZ (Dec 5, 2005)

Yeah, the Slone book is very good. Doug Self also wrote a very good amplifier book. But before you read any of them, you need to have a pretty good understanding about electronics -- particularly transistor circuits. Any analog electronics text should help you with that. Really, if you're good at analog electronics, you won't even need those amplifier books.


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## turbo5upra (Oct 3, 2008)

can I give it a listen when its done? lol

On a side note,.... ChrisB nice new avatar.


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## MarkZ (Dec 5, 2005)

Well, since there's some interest here... Here's the amp in question.










The power supply is below the dotted line, the amplifier sections are above the dotted line. There are six channels... one channel is circled. You should be able to visually pick out the other five. On the right side of the amp, where all the knobs and switches are, is the preamp section with the crossover and gain controls, etc. I removed the transformer (it goes right next to the arrow). The boxes outline the rail capacitors. These are what "stiffen" the +25v and -25v rails. They're currently 1000uF 35v capacitors ... they're being replaced by 1000uF 50v capacitors. The new rail voltage will probably be close to +/-40v, we'll see.

The arrow points to a 1000uF 35v capacitor on the primary side of the transformer. What's weird is that the other three capacitors in the bank are 2200uF 25v caps. I'm going to make this one also a 2200uF 25v cap and replace the other three with new ones. The input choke, right next to the power terminals, sequesters these primary side capacitors from the rest of the car's electrical system. The 1000uF capacitor directly northwest of it is on the other side of the choke, and so it's like a capacitor that you'd hook up to your system -- only about 1000 times smaller. 

I'll probably also replace the two smaller capacitors on the bottom right hand side of the board just to say I've done it. They're 470uF caps that "stiffen" the accessory supply. [The accessory supply is what supplies power to the op amps... it's a low current +/-15v supply derived from the secondary side of the transformer.]

PS - Where are all the big ass output transistors, rectifiers, and switching MOSFETs? They're on the underside of the board.


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## MarkZ (Dec 5, 2005)

While I'm waiting for the parts, I decided to do another thing. I bypassed the entire input circuitry (gain, crossover, input buffer) with a single jumper per channel. Had to remove one capacitor per channel to do it.  The crossovers in this amp are defeatable (truly defeatable...it jumps the crossover op amp), but this mod eliminates two more op amps from the signal chain. So now it goes straight from source to amplifier input stage.

How did I get around the gain stage? Well, the gain stage in this amp is a 10x amplifier. The potentiometer where at the gain setting I was using was dividing the voltage by about 10 first. So it was already a gain of 1 going into the amp, so bypassing the gain stage turns out to be inconsequential. I can fine-tune at the source.

Edit: This dropped the input impedance of the amp from 51k to about 20k ohms (5k ohms during startup). Not a problem. Still high enough. In fact, a lower input impedance should mean lower Johnson noise, right?


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## thehatedguy (May 4, 2007)

Hehehe you said Johnson.


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## MarkZ (Dec 5, 2005)

Just sayin


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## Oliver (Jun 25, 2007)

Someone's having fun


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## MarkZ (Dec 5, 2005)

Yup. Wound the transformer today. Still have to run some tests on it before I solder it in place (saturation, symmetry, etc). Hopefully will put the caps in Friday and finish jumpering the inputs. Bench test before Saturday for a weekend install? We'll see...


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## bumpnzx3 (Apr 6, 2007)

Would you be interested in doing this for a fee? I've got a mint P840 that I would like to have a couple channels worked over to your new specs. I've got my eye on a set of mids....only problem being that they are 8ohm. I don't think the ~60watts that a bridged pair of channels on this amp would provide them would be enough.


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## MarkZ (Dec 5, 2005)

Yeah, but I'm swamped for the next couple months. Part of the reason why this didn't get done last weekend. Snail's pace for now, but I'll keep you guys updated when I do something new to it. Then in a couple months maybe I can offer this service. In the meantime, I'll have a couple months worth of "burn-in" on this one.  I've also got a P840 that I'll also do various things to and set it for sale afterwards.


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## funkalicious (Oct 8, 2007)

What's a rough ballpark figure on what something like this would cost to have done professionally? I have 8ohm and 16ohm speakers for my front stage and would love to be able to make full rated power into those loads. Please forgive if my ignorance of the subject is showing:blush:.


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## MarkZ (Dec 5, 2005)

Honestly, I have no clue what people charge for this sort of thing. It's kind of involved because there are a lot of caps to replace and transformers are kind of pricey if you don't wind your own. I decided to do it with this amp because 1) the power supply is VERY simple in this thing... and 2) I know these a/d/s/ amps pretty well. I'd be hesitant to do it on most other amps, unless maybe I had schematics.


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## katodevin (Feb 14, 2008)

Hey Mark, seeing as you're familiar with the a/d/s amps, what do you think of the PH15's?


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## MarkZ (Dec 5, 2005)

All these a/d/s/ amps are funny, because they've gained this weird ass backwards reputation and I don't know where it came from. The general impression that people have of these amps is that they're very clean low distortion cleverly designed amps, but they lack the balls that some of the other brands of the time had. I have the opposite impression. Their design is very _simple_ (at least the amplifier section...not the crossover section). Not quite cookie cutter, but certainly cost cutting. The input stage is for the most part not discrete. Without actually making measurements, I suspect that the input stage is a high-ish distortion design, I think the PSRR is probably low, which also leads to crappy channel separation. But they use these ballsy output transistors (I think most of them the Toshiba 2SC3281/2SAwhatever), more than adequate protection circuitry, and IMO a larger-than-necessary heatsink. In short, I'd describe them as high-ish distortion amps but with the nuts to be driven hard. Kinda flies in the face of the general impression that they're wussy amps designed for people with discriminating tastes who drink tea with their pinkies sticking out.


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## fertigaudio (Jul 18, 2010)

Any updates?


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## MarkZ (Dec 5, 2005)

Sadly, no. I haven't gotten a chance to sit down and work on it yet. In fact, I'll be very busy right through to mid October. But it's definitely one of the top things on my agenda after that!


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## fertigaudio (Jul 18, 2010)

Its funny you mention mid October. That is when I get back from my "business trip". I have a few projects planned as well. Looking forward to your updates. 
Brandon.


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## envisionelec (Dec 14, 2005)

MarkZ said:


> All these a/d/s/ amps are funny, because they've gained this weird ass backwards reputation and I don't know where it came from. The general impression that people have of these amps is that they're very clean low distortion cleverly designed amps, but they lack the balls that some of the other brands of the time had. I have the opposite impression. Their design is very _simple_ (at least the amplifier section...not the crossover section). Not quite cookie cutter, but certainly cost cutting. The input stage is for the most part not discrete. Without actually making measurements, I suspect that the input stage is a high-ish distortion design, I think the PSRR is probably low, which also leads to crappy channel separation. But they use these ballsy output transistors (I think most of them the Toshiba 2SC3281/2SAwhatever), more than adequate protection circuitry, and IMO a larger-than-necessary heatsink. In short, I'd describe them as high-ish distortion amps but with the nuts to be driven hard. Kinda flies in the face of the general impression that they're wussy amps designed for people with discriminating tastes who drink tea with their pinkies sticking out.


No No NOOOOO. Don't tell us the TRUTH!

Hope you bought the right magnetics! I didn't think Digikey had 'em. If you run into trouble, PM me.


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## MarkZ (Dec 5, 2005)

no, the digikey box was full of capacitors and fresh new sparkly clean silpads.


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## Allan74 (Jun 17, 2010)

Any chance of doing something like this to an Audison SRx4 to beef up the output @ 4ohms from it's current 60w x 4 RMS, atleast to 2 of the channels ? to better suit driving a pair of Mids on a pair of stronger channels, then tweeters on the other 2 ? (_with midbass duties on another amplifier all together, in a 4-way active system_).

I am having a difficult time placing my SRx4 in my current system, because as a 4 channel @ 4ohms, it's a bit weak.....and at 4 ohms bridged as a 2 channel, it's a bit overwhelming for a pair of 3" mids @ 170w x 2 RMS.

Is there any hope for me or this amplifier ?


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## GibTG (Mar 11, 2010)

The "truth" on these a/d/s/ amps really intrigues me. I use a P640 on a set of 8-ohm mids, 8-ohm tweets, and 4-ohm midbass and even though I loved the crossover and signal routing I wasn't as impressed with the sound as I had hoped with it after hearing about how they are the supposedly super-fantastic SQ amps...

I've only seen inside my P640 and P440, so... Does the PH15/PQ10 family of amps differ very much in design? Did they earn the reputation for utlra-low distortion? What about the MX family of amps?


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## sqshoestring (Jun 19, 2007)

ADS changed designs at times, I have a black plate and it has IC outputs just like an old blaupunkt I also have....three of, lol.


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