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

Tonic wrote:
> 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


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.

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!

On Nov 19, 11:33*pm, nospam <"f825_677(nospam)"@ntlworld.com> wrote:
> Tonic wrote:
> > 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
>
> 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.

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!

On Nov 19, 11:33*pm, nospam <"f825_677(nospam)"@ntlworld.com> wrote:
> Tonic wrote:
> > 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
>
> 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.

Tonic wrote:
> Hi, I have a B&K EX4420 power amplifier with balanced and non-balanced
> inputs. I tried 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 electrolytics 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

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