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Stijnio 9th October 2012 05:21 PM

P3A 100W layout - comments?
Hi guys,

I would like to receive comments about my designed P3A layout.

Any tips or tricks?


rsavas 9th October 2012 07:00 PM

Should you not mount the biasing transistor on the heatsink with the output devices? Use a BD139 for this!!
Add a current mirror instead of R6?
Matched/selected transistors for Q1/Q2. You have no means of offset voltage nulling? Some people put the TO-92 faces together for thermal tracking purposes
Compensation? Cap across R5.
Take a look at Douglas Self's Blameless Amp, similar to what you are showing, but he has some nice proven circuits to use.

Good Luck

Stijnio 9th October 2012 07:11 PM

Thank you for your reply!
I use the circuit from Elliot.


To be honest I want the amp to be as simple as possible with the best performance. I dont want to make it too complex.
In other designs I dont see the the biasing transistor on the same heatsink as the power transistors. Is it really needed?

I use matching transistors for Q1 and Q2.

What is the benefit of putting a C across R5? Can you explain?

Thanks! :)

Mooly 9th October 2012 07:16 PM

:cop: Copyright images from Rod Elliot removed.

Please link to Rods site and circuit :)

Mooly 9th October 2012 07:21 PM

Thanks :)

sreten 9th October 2012 07:36 PM


Q9 should be thermally coupled to Q5 and Q6, they both need heatsinks.
Small capacitor across R5 can be used to create "phase advance" around
the unity gain point of the amplifier improving stability if C6 is marginal.

rgds, sreten.

rsavas 9th October 2012 08:01 PM

Looks as if C4 acts as the compensation capacitor in this circuit, so putting one across C5 is not required.
Putting the bias transistor on the heatsink allows you to temperature compensate the bias voltage generator and it will adjust the ouput bias current for different heatsink temperatures. Transistor thermal run-away comes to mind?


Stijnio 9th October 2012 08:07 PM

Thank you!

I understand that putting Q9 to the heatsink will compensate the the run-away.

At this moment I have only the two power-transistors on the heatsink. I will put Q9 on the heatsink. But what about sreten's post? You are saying that I need to couple Q9 to Q5 and Q6?

rsavas 9th October 2012 08:26 PM

I do not believe that the driver transistors,Q5/6 need to be on the the same heatsink as the output transistors, Q7/8, but they probably will require their own heatsinks!!
I agree, that putting a place holder for a cap across R5 would be a good idea.

jaycee 9th October 2012 08:35 PM

The output stage is a CFP (sziklai pair) design so the bias transistor Q9 should track one of the driver transistors. On Rod's design, it is mounted near to Q5.

If you manage to get the MJL4281/4302 pairs, use MJE15032/3 as drivers. They are superior to the BD139/140.

I would add some 100uF decoupling capacitors to the circuit too, these are not on Rod's original design but they can help with stability. Mount them as close the the output transistors as you can.

You should be able to squish the board's width quite a bit.

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