zen rev. 2 feep-back?

[dieringe]

Hi,
I have a Zen rev. 2, I had it running on 1.3Amps because with the Aleph-feedback I get 2A which is the limit for my heatsinks.
Because this sounded somewhat weak I decided to rise the current to 2A again, making 3A with feedback.
(I made a switch for the feedback via relais and a fan)
So without feedback it's fine, but when switching feedback on, I get a very high feeping noise (inside the amp!).
So I guess this is not the usual oscillation, because it is in the audible range?
What can I do about this?

thanks
martin

 

[Nelson Pass]

It's probably oscillation with mild feep syndrome

Want to post a schematic?

 

[dieringe]

It's probably oscillation with mild feep syndrome

yes. ;-)

Want to post a schematic?

It's your revisited ZEN, part 2
(with the aleph current source).
It doesn't matter if I wire the current feedback directly or via relais. Relais and fan have a separate supply.

somewhere else in this forum I read about small capacitors parallel to the big supply capacitors could cause oscillations. I have such (I think 1µF) just at the connectors on the PCB. Is this a very probable cause?
Supply voltage is about 35V (2x15V transformer).
AFAIR I used IRF150s and some other type of ZTX instead of the 450 (6xx I think).
(It's quite a big thing to open it, very badly packed)

I also added the voltage regulation of revision 3 (without z-diodes), but it didn't help (well, at least it sounds better in no-feedback-mode).

martin

 

[Nelson Pass]

Try placing a high value resistor across the switch, say
10K ohm or so, to keep the capacitor charged.

 

[dieringe]

that must be a misunderstanding.
the feep is there when the switch is on (_with_ feedback-connection)