Diamante -a discrete medium power opamp

[Luke]

Ive just read 3.13 Amplifier stability ? Preventing oscillation

Its a great read and reminds of things long forgotten.

Firstly, I have discovered my X1 X10 scope probe is stuck in X10 mode so the amp is oscillating at around 5V P-P. I can't believe it hasn't melted my cans or still sounds like a great amp, not that I want to plug them in any more.

There are no oscillations on my Power supply btw which is a copy of the LCDPS, its +-15V. On the amp boards I use good quality caps, wima and silmics for decoupling so thats no problem.

it says on P114 paragraph 2 that the feedback bypass cap should be around 22pf are correct and above 47pf tend to cause oscillations. I have changed it on one channel from 10pf to 20pf, it made no difference.

I have 20 turns magnetic wire around a 22R resistor as my zobel.

I just tried adding a Boucherot cell Zobel (RC 10 ohm 0.068uF from ground to earth ) and its reduced the oscillations to 5V P-P, yay Im enjoying this

Capacitors 5 are not as effective as layout and base stoppers on bjts are not really neccescary, so theres no point in going there.

There are several options for HF feedback capacitors (2) and (3) but I don't know where to put them on the Diamante.

I have tidied up the wiring so Im happy thats not the problem.

So where does that leave me? I am hoping that I can fit a miller cap somewhere with guidance but I have concerns that the layout may not be optimum, is this possible, how well tested is it?

I really want to resolve this cos its a great amp. It should be a keeper and not go in my parts bin

One other thing why does it sound so good in its current state?

EDIT: I forgot to mention that I checked the scope/probe against my other amps and there are no oscillations, I wondered if they are dodgy.

[Luke]

I have another question regarding C7 on diamante board, I used polystyrene cos I have one, are they effective at these frequencies or could that be a problem?

[Luke]

tried another power supply, it was 18 0 18 and when I hooked it up, one rail dropped to 10v and the led didnt light up. On the scope it oscillated but only the lower half of the signal as the top wasnt powered?
I replaced all caps with ceramic as per BOM. I checked the pots and they seem fine. I really dont see what else I can check. The other strange thing is that one oscillates at around 4v p-p and the other 500mV p-p. Same parts on each one. I cant see how I have a bad semi as it works as an amp, if a semi was dodgy it would not work, correct?
I haven't hooked up a pot or input cap yet, tests are done with the bias set to 50mV across the emitter resistors.