P3A 100W layout - comments?

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Hi guys,

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

Any tips or tricks?

:)

IzKBU.jpg


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

Schematic: http://sound.westhost.com/project3a.htm

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! :)
 
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Hi,

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.
 
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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?

Regards
Rick
 
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?
 
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.
 
Okay, so it should track Q5. I will make the adjustments in the PCB today.

I know the board can be a lot smaller, but I want to use heatsinks that I already have from an older amplifier wich already have screw holes 11cm from each other. Do you think the width of the board will decrease performance? (noise etc.)

An externally hosted image should be here but it was not working when we last tested it.
 
Hi,

The bias loop is Q9, Q5 and Q6. Mounting Q9 on the heat sink wont help
at all in matching Q5 and Q6. Q7 and Q8 temperatures do not affect bias,
unless they are thermally linked to Q9.

Cheapest and simplest is to mount them all on the same heat sink (bracket).

Alternatively Q5 and Q6 have their own or shared small heatsink,
here Q9 should be linked to at least one of the devices, in theory.

This is all theoretical, and the P3A is very liberal in its claimed biasing
requirements, whilst in fact the optimum bias is a very precise affair.
(See Douglas Self and CFP output stage optimal bias.)

If your planning on a nominal 50mA to 100mA bias its already
overbiased and most of the above does not matter. Overbias
can wander around without becoming any sort of real issue.

Just make sure Q5 and Q6 have heatsinks.

rgds, sreten.
 
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1. Are scattered electronics on the PCB. You must place components on PCB more economical.
2. You want to build one or two channels? If you want to build two channels or even one, it would be good to include the PCB and power supply, so that you can manage better and correct way PCB routing traces.
3. Some transistors should be mounted on the heatsink, like Q4, Q5 and Q6, not only Q7 and Q8.
4. Emitter of Q1 and Q2 connected directly to the collector of the transistor Q3 is not the best choice. Why? Eg. when you connect the input signal, a simple mishandled it, can cause a pretty annoying noise in the speaker. To avoid this, we need some resistors there, perhaps a trimmer to help dc offset adjustment's..., even if you want to build a simple amplifier. So, we should not overlook that part.
 
Thank you for your reply Donpertru.

I have some questions about your recommendations.
1. Wich components should I group?

2. Do you mean that I should put the layout of the power supply in this topic?

3. Q4, Q5, Q6 will be equipped with a heatsink.

4. What value resistor do you advise there?
 
1. Almost all electronic components must be re-placed on the circuit board. Look - for example - make a comparison between one of the PCB's designed by me and attached below, and your PCB, to see how well you placed components.

2. Yeah, I do not mean only power supply electronic diagram, but the inclusion of that diagram in the PCB power amplifier.

3. Yes, you will need to attach a small heatsink those transistors.

4. Two resitors 22 Ohm 1% and one resistor about 4.7k.
 

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I have made some adjustments to the PCB. At first, the PCB its much smaller now. It was difficult to reposition the component because the two power transistors have a fixed position due the heatsink.

Also I have added two 100uF caps at the +35V and -35V (C9, C10)

pVQy7.jpg
 
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