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scottr923 12th January 2013 09:53 PM

The Old Hum question
Hi all,
i recently got a Belles 150A amp, used. i keep it running and it sounds great. just a quick question;today i noticed what sounded a lot like a ground loop hum, and moving closer it was coming from the amp. i went away and about an hour or so later noticed it was gone, back to silent. does this mean or portend anything?
thanks for any input,
Go Seahawks!!!

nigelwright7557 12th January 2013 10:10 PM

Do you mean hum from the amp or from the speakers ?

scottr923 12th January 2013 11:29 PM

it was coming from the speakers and the amp. no other symptoms. then, it just sort of went away. almost like noise from the xformer. but i'm not sure exactly what was causing the sound.

nigelwright7557 12th January 2013 11:32 PM

A noisy transformer is usually a sign of a heavy load.
I would check your speaker leads for poor connections/shorts.

DUG 13th January 2013 12:33 AM

If this happens with increased heat for output devices hook up a scope and check for oscillations...might be drawing lots of power.

Just another possibility.

scottr923 13th January 2013 02:19 AM

yeah, that was what i was thinking, but why would the load increase when i'm just listening to FM tuner at really low volumes? and then it goes back to normal. just kinda puzzling really. can relative humidity do this? it's unusually dry and cool here lately.....
hmmm, (not to be confuse with Hummmmmm) :)

Ian Finch 14th January 2013 04:21 AM

We occasionally see threads referring to periodic low level hum coming from transformers and sometimes also audible through the speakers.
This is often put down to mains-borne noise and DC appearing on the mains due to various local loads and load sharing problems that are in
the hands of the power company. Unfortunately, you have to get some fancy measurements of your supply to be sure of that.

Some amps and particularly big transformers are sensitive to DC components in the supply, for some reason, but it does happen. If there is
no sign of deterioration in the normal power capability and sound quality of your amplifier, I would be looking there for trouble.

However, if your amplifier is old and constantly on, I would be considering some maintenance like replacement of the electrolytic capacitors
- just as a reassurance that the power supply caps, for example, aren't failing. This is not uncommon.

scottr923 14th January 2013 12:42 PM

Hi Ian,
thanks for the reply, very helpful. i'll look into replacing the caps here soon, i've been thinking about that since i got the amp.

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