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kriss99 6th January 2011 06:41 PM

Crescendo question
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Hi everyone.
I have built 2 times the Elektor mini-crescendo.Then one I am still using was built in 1986 and is still working.I use it very frequently but I was not aware for his instabilities because my interest turned to tubes.

After reading here about oscillations I checked out my amp. I attach a pic of my oscilloscope at 1khz.This is happening to channel (a) only and at mid-levels power output.At his maximum output the waveform is clean.The boucherot resistor is mildly hot but nothing of severe.

Is this the oscillation of crescendo you are are talking about?

Many thanks.Kris

wahab 6th January 2011 08:25 PM

Awsome picture...
That s a typical oscillation burst..
Have you the schematic of your amp ?..
Seems to me that the Crescendo was badly compensated.

kriss99 6th January 2011 09:49 PM

Crescendo question
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Thank you very much.I scanned from the magazine.As i remember after so many years, I followed the instructions exactly.The other channel is free from this malfunction.By the way I had never had any damage like burned resistors,mosfets,fuses etc.

audiofan 6th January 2011 10:00 PM

i would also check power supply rail decoupling ... and maybe add a few capacitors ......

wahab 6th January 2011 10:15 PM

This amp rely on the mosfets input capacitance as a shunt compensation,
but since there s less output pairs than in the 140W crescendo, the circuit
is not well damped.
You should try to add two 10 to 22pF compensation caps, one from T9 base
to T10 collector and the other from T7 base to T8 collector.
Use preferably caps rated at least 300V, 500V being even better.

kriss99 6th January 2011 10:25 PM

Seems logical.I put the probe at both ends of R27 with no change to the oscillation picture but some wobble in rails altered his form.I'll try with some capasitors or resoldering maybe.

kriss99 9th January 2011 08:08 AM

Before starting putting caps i checked the quiescent current and it was too high.After so many years I had never adjusted it.Now its clean from oscillations at all.The sine waveform is perfect but there is some rounding of square wave at 10 Khz though but I think many amps have. Anyway I want to thak you for your kind help.Regards.Kris

AndrewT 9th January 2011 12:40 PM

changing the output bias is not curing the stability issue. You are just hiding it.
If you investigate more thoroughly you will find that the oscillation still exists.

I can only see the 330pf and the output Zobel as added stabilisation components.

I think Wahab could be right.
The stability needs to be improved. The easy way is adding Cdom.

AndrewT 9th January 2011 01:13 PM

just noticed the typical voltages around the input are wrong.
0V at the bases should be Rin * Ibupper-Iblower. Expect +-10mV to +-30mV for an imbalance of +-0.3uA to +-1uA of upper and lower base currents.
The +0V7 and -0V7 have been transposed. Expect +-0V8. R7/8 to T6 to be -0V8

kriss99 9th January 2011 07:35 PM

To be honest after the adjustment I didnt get better sound.In a tube amp in the past I had had similar oscillation but when I adjusted the bias that produced better sound.I'll check again.I have a question.Some people say here that mosfets oscillate in the MHz band.How do they investigate it? With a wider band oscilloscope? With FFT? I would like to learn more.This Crescendo is a good amp to learn things, a better name of his could be Tormento :))

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