Buffered LM3886 + overthetop regulated power supply

[random007]

This is partly an exercise in learning to use PCB CAD software so it's more complex than strictly necessary but I'd still like to get a good quality, good sounding end product. Any comments welcome.



Notes:

VAC A & B are the two secondary windings from a 160VA 30v toroidal transformer.

Power supply and main regulator circuit are from Roman Black's gainclone amp modified to be a symmetrical design.

Buffer/preamp pre-regulator design from Pre-Regulator

Buffer/preamp regulator from LM7815/LM7915 data sheet.

Buffer/preamp from ESP - A Better Volume Control (part 2) with a linear potentiometer used at input.

LM3886 circuits are pretty much generic.

Power switch and transformer circuitry not in the schematic.

Some of the odd component choices are because I have them in my parts collection already (eg lots of 2200uf 105degree capacitors), various power transistors and so on.

The links/jumpers are there so that it can be powered up and tested in stages.

I'll have to redraw the star ground, it's confusing the way it is with a shaded box.

[random007]

Things to do:

Reduce LM3886 decoupling caps from 2200uf to 470uF and add a low value non-polar cap.

Having separate stars for signal and power grounds may be a good idea.

[AndrewT]

Hi,
the 3886 does not need a buffer at it's input.
Your inadequate source may need a buffer at it's output.

This is a very complicated PCB project. Why can't you start with something much simpler?

[theAnonymous1]

Just out of curiosity, why the use of two op-amps (non-inverting buffer -> inverting op-amp with gain), as opposed to a single non-inverting op-amp with gain?

[random007]

Originally Posted by AndrewT

Hi,
the 3886 does not need a buffer at it's input.
Your inadequate source may need a buffer at it's output.

This is a very complicated PCB project. Why can't you start with something much simpler?

See explanation below. It's probably misleading for me to call the op-amp a buffer.

Inadequate source? Can you explain?

All together complicated, granted, but each section is simple and easily understood. It's an exercise in learning that suits my level of experience.

Originally Posted by theAnonymous1

Just out of curiosity, why the use of two op-amps (non-inverting buffer -> inverting op-amp with gain), as opposed to a single non-inverting op-amp with gain?

The op amps are there to change the response of a linear potentiometer at the input to reproduce a log response. See the ESP link. Seems like an interesting idea.

[nightanole]

I really never did like that power supply design. Hey lets make a poor lm317 regulate till it dies and then have an external part shunt the current. Why Not just get a 5amp rated lm338

http://diyaudioprojects.com/Technica...Data-Sheet.pdf

[Andy5112405]

As everyone is saying. Why the op-amps at the input at all. The Gain Clone has enough gain on its own to amplify a 100mV line input. Why add extra noise, distortion and complication with the op-amps.

[theAnonymous1]

Originally Posted by random007

The op amps are there to change the response of a linear potentiometer at the input to reproduce a log response. See the ESP link. Seems like an interesting idea.

Oh, I didn't see a pot connected to the feedback loop in your schematic.

[Andy5112405]

The PSRR (Power Supply Rejection Ratio) of the Gain Clone is excellent. No need to overdo the PSU. LM317 / LM337 with good capacitors is more than adequate. A High Power Transformer with GOOD capacitors is probably just as good.

[Andy5112405]

NOT a good idea to use a potentiometer in the feedback loop of an op-amp. If the pot fails, the op-amp will go into maximum gain which would be horrible if not disastorous.