pignose 30/60

[nickb]

Across D4 & 5?
.7v across each.

When I bias it harder lowering the value of R23 & 24 it gets rid of the nasty oscillations on both halves of the sine wave.

I would leave the bias so that it runs cool and increase the 470pF C7. But I really should ask, about what frequency are these oscillations?

[drewl]

Hard to tell, looks to be about 800khz Burst's in the middle of each peak, then fuzzing out the whole wave as the master is dimed.

[drewl]

I think you are on to something there with the 470pf C7.

While probing it the oscillation is all but gone.
Seems to be an intermittent connection as there were others I fixed.
I'll re-flow the IC socket and clean the connections also.

Output devices still running hot, increase the 4.7K's R23 & 24 as you suggested?

[Enzo]

In my mind I would have paralleled those diodes with resistors, like 100 ohms or less.

Increasing current might stop oscillation, but won't cool the amp.

[drewl]

It seems the amp is very particular about what opamp you use.
With a 4558 it had 120hz buzz with the master over 3/4 up.
That seems to be the point where it oscillates even with a good 353/TLO72.
I'll try lowering the bias and see if that cools it down and gets rid of the oscillation.

Thanks.

[drewl]

In my mind I would have paralleled those diodes with resistors, like 100 ohms or less.

Increasing current might stop oscillation, but won't cool the amp.

Resistors across D4 & 5?
I'll try it.

Bumping up the 4.7K resistors has the amp cool at idle.

Still getting that burst oscillation around 8vac out.


I've been testing it with a 3055/2955 pair. now that it's idling cool I'll see how it responds with 15022/15023.

[nickb]

I think you are on to something there with the 470pf C7.

While probing it the oscillation is all but gone.
Seems to be an intermittent connection as there were others I fixed.
I'll re-flow the IC socket and clean the connections also.

Output devices still running hot, increase the 4.7K's R23 & 24 as you suggested?

Not intermittent. Your probe adds capacitance and that is what fixes the problem. Increase C7 gradually and see where it gets you.

[drewl]

Well I put 100 resistors across D4 &5 and upped the cap to .001uf
Amp is much more stable up to 20v out where it has some oscillation
It's basically a screaming square wave at that point.

[drewl]

So with 100 ohm resistors across D4 & 5, amp had nasty crossover distortion. 220 Ohm seems like the sweet spot.

After installing MJ15022/23's was still getting oscillation on top of sinewave even with C7 up to .01uf.

Putting an 82pf from master volume wiper to ground killed the oscillation, signal looks good up to full volume and bandwidth.

Will try it with guitar and speaker when I get home.

[Enzo]

Did adding those resistors make the amp run cooler?

[drewl]

Yes, it ran much cooler and put the feed resistors back to 4.7K.
Thanks.