Hum when amp is turned on

[Dinesh G Dutt]

Hi,

I have a Bel Canto Evo 200.2 amp that had a bad relay which caused no sound to
come out of the right speaker. I sent it in to get it fixed. It was a cheap fix
(~$100). But now when I power on the amp, there is a loud audible click through
the speakers and there is a hum. This is independent of whether the preamp is
powered on and I'm playing something or not. I have the RCA/XLR input in the
correct position and I'm using it as a stereo amp (not bridged mono).

I contacted Bel Canto about it and they're unable to determine why the hum is
there now as opposed to before. I wonder if any of you would have some
hypotheses that I can test. I tried putting in a Monster Cable outlet
(Powercenter HT800) and tried moving the outlet to other places in the
house. But the hum persists.

A related question is whether this hum can damage the speakers in some way.

Thanks,

Dinesh
--
Much unhappiness results from our inability to remember the nice things that
happen to us. - W. N. Rieger

[Gene Poon]

Dinesh G Dutt wrote:

> Hi,
>
> I have a Bel Canto Evo 200.2 amp that had a bad relay which caused no sound to
> come out of the right speaker. I sent it in to get it fixed. It was a cheap fix
> (~$100). But now when I power on the amp, there is a loud audible click through
> the speakers and there is a hum. This is independent of whether the preamp is
> powered on and I'm playing something or not. I have the RCA/XLR input in the
> correct position and I'm using it as a stereo amp (not bridged mono).
>
> I contacted Bel Canto about it and they're unable to determine why the hum is
> there now as opposed to before. I wonder if any of you would have some
> hypotheses that I can test. I tried putting in a Monster Cable outlet
> (Powercenter HT800) and tried moving the outlet to other places in the
> house. But the hum persists.
>
> A related question is whether this hum can damage the speakers in some way.
>
======================

Do you have a DC volt-ohm meter? Any cheap one will do, as long as it
is functioning. You can buy one for a pittance at Radio Shack, and it
comes in handy to check continuity on interconnects, speaker cables,
opaque ceramic fuses, etc. Measure the DC voltage across the speaker
output terminals of the amp, with speakers DISCONNECTED. After the amp
stabilizes, whcih should take a minute or so, it should measure zero, or
within perhaps a small fraction of a volt of zero. If it's not,
something is wrong with the amplifier circuit.

My hypothesis is that the relay wasn't bad, but had interrupted the
signal because of an amplifier fault. This would mean the relay and the
DC sensor circuitry that drives it were working as designed to protect
your speakers against unwanted DC, which will cause undesired heating of
the voice coil; and that the root cause isn't completely cured yet.
Many amplifiers will "pop" when first turned on, because the circuitry
has not completely stabilized as yet, but the constant hum is improper
and is characteristic of a fault in the output stage.

-Gene Poon