Quiescent current drops when i touch the heatsink

[blu_line]

Hello All,

As the Subject says;
I protoyped my amp and it works , sort off !!!
When i started the quiescent current was way to high and i popped several power transistors.
Now, the quiescent current is fine, at least , when i hold the heatsink. When i don not , it raises.

The power transistors are mounted on the heatsink with Mica sheets.

Any one an idea !

[Bill Fitzpatrick]

Perhaps you are killing oscillation by touching the sinks.

[blu_line]

High frequent oscillations then ! Cause i do not hear any.

grtz

Simon

[Bill Fitzpatrick]

Yes, high frequency oscillations from an unstable amp.

[djQUAN]

did you connect the heatsink to ground?

[Graham Maynard]

Hi,

I trust you have a Zobel network fitted, if yes try an additional one;- series 10n + 2.2 ohms in parallel with the normal one.

Cheers .......... Graham.

[blu_line]

Zobel ?

No not yet ! Thought about that.
Does it make such a difference ?

No ground connection to the heatsink either !

grtz

SImon

[MikeB]

Strangly behaving quiscent current is in my experience a definitive
sign for highfrequency-oscillation, and it's not always audible.
Maybe your feedbackcompensation is not proper adjusted, or
mistakes with layout ?
Often the cap in feedbacknetwork is too big.

Can you show schematic ?

Mike

[Mr Evil]

Originally posted by blu_line
...Cause i do not hear any...

Oscillation usually occurs at at least 100s of kHz, if not MHz, well above what is audible or what your speakers could reproduce.

[jackinnj]

Originally posted by Mr Evil

Oscillation usually occurs at at least 100s of kHz, if not MHz, well above what is audible or what your speakers could reproduce.

the oscillations I have inadvertently created are in the high tens of kHz -- it is really helpful to have a scope with FFT for this --

to cure the problem experienced by blu_line I would recommend 100nF caps on each of the power supply rails as nearby to the output transistors as possible. Polypropylene is (purportedly) best of all, but polyester (mylar) is much less expensive.) I hope you have a ground plane since a bypass cap with a long lead to the ground is about as useful as teats on a bull.