[Ian Iveson]

Peter Wieck wrote in

I have a 1987 MFA Magus A2 preamp that a few weeks ago
started making
a buzzing/humming sound from both channels on any source
setting. It
is strongest in the right channel but audible from both.
I have
swapped out the line stage tube (6DJ8) to no effect. I
have verified
that it is the Magus because if the amps are on (MFA M75
monoblocks)
and the preamp off there is no noise from the speakers. I
have also
tried removing the phono stage tubes to eliminate them as
the problem
but again to no effect. The noise seems to start 20 or 30
seconds
after the preamp relay clicks on and after the amps warm
up - in other
words, it doesn't start as soon as the amps warm up but
maybe 20
seconds later. Anyone have any ideas regarding what the
problem could
be and what I should do to fix it? Local technicians have
little or no
knowledge about tube gear so taking it to a shop isn't an
option. It
seems to be slowly getting louder over time so I need to
fix it. The
noise level does increase as the volume or gains are
turned up if that
helps. Any ideas?

Filter caps.


Why would it be stronger in one channel, Peter? I was
thinking either filter or cathode resistor bypass cap but
figured one would be on both channels, and the other on just
one.

Would it be wise for the OP to renew all the electrolytic
caps he can find? Same values and voltage please, OP, high
temperature, low ESR.

Ian