Very simple quasi complimentary MOSFET amplifier

I found something interesting while testing the boards tonight.

I set the bias to 100mA (33mV R20) but observed it was fluctuating some.

I realised that if I put a finger near or touched C5 (33P Mica), the bias current would increase.

Just bringing a finger near and touching will get the bias to about 140mA, but if I bring my finger close quickly and hold it there, the bias can sit at around 240mA.

Wonder if anyone has come across that kind of behaviour / if it is something normal. It won't really matter in the end because the amp will be boxed up, but I'm curious anyway.
 
Got the first 2 boards mounted as a module in preparation for boxing up.

Will probably not use the 14+ year old computer fan for the real deal though...
 

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Strangely, the smoke came out of board 2 last night :-/

I put a 200mV 1KHz signal in but the output across the 8ohm dummy load looked horribly distorted. In under a minute, the PSU went into protection mode and the dreaded smell appeared soon after...

Appears the IRFP250 oscillated to death :-/ Although the signal generator is new to me so perhaps it was that..

Nothing else seems to have been harmed but I'll check through the current path to the FET to be sure.
 
I’m putting my order in for parts, couple of questions please.
The Zenors, are the 0.5, 1.3 or 5 watt?
33p mica? Is this a silver mica capacitor?
What are the metal connections for the PSU on the pcb called?
What size trim pots 5 x 10?
C8 OPTN what is this?

Thanks gang :)

Edit:
Also the 220p cap, is plain ceramic?
 
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These were the parts I used from E14:

180351-2 TERMINAL, FEMALE DISCONNECT, 6.35MM

726386-2 TAB, STRAIGHT, 6.3X0.8MM

1N4742ATR ZENER DIODE, 1W, 12V, DO-41-2

The pots were 5x10. Note the PCB value of 200 ohm for the bias (as opposed to 500R on the schematic).

33p I used was a silver mica.

Search this thread for C8 as there are a few posts regarding that one.
 
Ive got mail, yay. Feeling very grateful to Ermine22 and Avtech23 for there generosity tonight. I asked if anybody had a spare board a week ago and tonight I open the letterbox and bam! There’s two waiting for me. My plan is to build both, do a side by side comparison and then give an amplifier away as a present to a friend.
I’ve ordered two set of fets and transistors and will order the rest of the missing pieces this weekend.
Big shout out to Hugh for helping me along with this. I’ve been slowly reading through this thread and have to say that there is an amazing amount of knowledge and experience that has been shared here. I’m learning so much and haven’t even started the build.
Thanks gang :)
 

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I replaced the FET on board 2 today and all seems well.

I think I identified the problem... one of the test leads in the set I got had been factory assembled red-black :eek::mad: I'm guessing that lead perhaps shorted the output.

Anyway, board fired up with +/-36VDC and 1KHz sine wave looks pretty good! There is some noise on the output waveform, looks like switching noise from the SMPS.

I realised I only have 4 Ohm test speakers, so apart from a brief no-input check on the noise floor, I've yet to hear if the switching noise is audible.

The output devices put out quite a lot of heat when the current is flowing! Definitely need some good heatsinking and cooling when they get boxed up!
 

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