Need help reviewing a design and layout.

akkudakku

Member
2014-11-26 7:34 pm
Hello. This is my first post ;-)

I need some help from more experienced people - is my design viable?
The thing is a mixed feedback power amp with ~10W power into 4-16 Ohms that I designed for a guitar amp for myself. I'm a complete DIY self-learned guy and this is the first time I decided to make anything that I could call completely my own.
First time making a layout too.

Design is like this:
- Input is a LTP with a current mirror and current sink
- VAS is a single transistor class A
- Output is complementary darlingtons (TIP122 & TIP127) biased through a bootstrap and a VBE multiplier (also TIP122 for ease of mounting on the heatsink)
- Powered from +/-25V.

I don't like the way I routed the bootstrap cap, but I couldn't find a better way.

Schematic and layout are in the attachments.

Would love some feedback and info how to attach Eagle files, so you can see the design better.
 

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akkudakku

Member
2014-11-26 7:34 pm
I always decouple front end from output transistors with 56R 1w and 470uF capacitors on both rails. I found without them I got a little hum with no input signal.

Use star grounding so you don't get the output modulating the front end power supply.

I can make only a single sided board DIY so a star gound is hard to do. Would a ground plane be acceptable instead then?

The decoupling seems like valid concern, thanks! Will try to cram that into the board.
 
Yes, the star ground comment is very valid. Ground plane is also fine, but in some cases it's more difficult to arrange than the star ;) Yes - some jumper wires are not a problem if they really optimise the layout.

One more important point - if you want a good-sounding amp - all the high-current traces have to be as wide as possible, including:
- collectors and emitters of the output transistors;
- emitter resistors;
- output traces;
- power rails traces, going to the output transistors.

See my example attached.
Also, the NFB trace has to go right from the output terminal trace.

Cheers,
Valery

P.S. One more thing - speaker return ground normally has to go directly to PSU main ground. In your case, you've got additional feedback network there, so you need to think carefully on how to arrange it the best way. In any case - the traces around 0.33R have to be wide again.
 

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akkudakku

Member
2014-11-26 7:34 pm
Yes, the star ground comment is very valid. Ground plane is also fine, but in some cases it's more difficult to arrange than the star ;) Yes - some jumper wires are not a problem if they really optimise the layout.

One more important point - if you want a good-sounding amp - all the high-current traces have to be as wide as possible, including:
- collectors and emitters of the output transistors;
- emitter resistors;
- output traces;
- power rails traces, going to the output transistors.

See my example attached.
Also, the NFB trace has to go right from the output terminal trace.

Cheers,
Valery

P.S. One more thing - speaker return ground normally has to go directly to PSU main ground. In your case, you've got additional feedback network there, so you need to think carefully on how to arrange it the best way. In any case - the traces around 0.33R have to be wide again.

How thick is thick enough? I expect 2 A current peaks (1.4RMS) and have used 40 mil tracks on current traces (and a calculator shows me 25 mil is enough). I could always tin them for lower resistance.

Fixed the voltage feedback trace.

And now working on the star ground, seems easier then I thought before.

After the power-amp I will try my hand at a pre-amp.

BTW very nice layout, I like the symmetry. Also I see some SMD in there? I can only hope I will get better soon :)