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B&K Power Amplifier - Fluctuating DC Offset

Hi, I have a B&K EX4420 power amplifier with balanced and non-balanced inputs. I tryed to get rid of the DC offset, each channel has a pot for correction but it keeps fluctuating. When the amplifier starts I have around 200mV then after warming up it's fluctuating between -10mV and 70mV. I changed all the electrolitics inside the amp with no luck. The amp has 5 pairs of mosfets on each channel, dual mono.

With no source and no loudspeakers connected to the amp I get fluctuating DC offset when it's switched to the non-balanced inputs. If I switch to balanced inputs I get 0mV but when I connect a source it's fluctuating too. Anyone has a clue about this? or schematics?

Thanks

Reply to
Tonic
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Guess is that one of the parallel MOSFETS is open circuit or you have a few dud caps.

You are setting the offset with a load on ?

for example, HH amps hunt up/down if the quiescent current is set without a load on.

Reply to
nospam

Hi, I checked the voltage drop on the emitter resitor at each mosfet and all seems to ok, 12-16mV on 0.47ohm resistors. There are still a few old small green caps around. I'm trying to set the offset without any source or speakers on it. There is a separate pot for the quiescent current.

Thanks!

Reply to
abuimistriuc

Hi, I checked the voltage drop on the emitter resitor at each mosfet and all seems to ok, 12-16mV on 0.47ohm resistors. There are still a few old small green caps around. I'm trying to set the offset without any source or speakers on it. There is a separate pot for the quiescent current.

Thanks!

Reply to
Tonic

Good morning,

While I don't know this amp specifically, I would suspect the input differential pair. Since this amp uses FET output devices, I'd guess that it uses an FET differential pair, though bi-polar is possible. The differential pair could be discrete transistors or some form of multi-leaded device. If replacing individual transistors, matched ones are a good idea.

If you find this device, try blowing on it, and you'll likely see the offset drift around. Lastly, I'd suggest monitoring the offset with the covers on the unit, so that everything gets up to temperature the way it normally would. Some drift in offset is normal on some amps.

Regards, Tim Schwartz Bristol Electronics

Reply to
Tim Schwartz

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