Audible hum emanating from small power supplies is usually caused by loose
hardware somewhere in the supply that is vibrating in "tune" with the
60-cycle applied AC. Hum coming from the connected radio, however, is
commonly
due to poor filtering of the DC output of the supply and unless the power
supply is half-wave will be at 120 Hz, not 60 Hz.
Hum generated within a transformer itself - barring loose hardware -
can actually be caused by magnetostriction causing the transformer core
laminations to physically change shape in "tune" with the applied 60-cycle AC.
This phenomenon is usually confined to large transformers such as those
outside in cabinets fed by the power grid, although if the laminations of a
small transformer are a bit loose they may audibly vibrate. You can find a
detailed explanation in "The Electrical Engineer's Handbook". I researched
it once. If you like I'll dig out the reference and post it.
Paul, K4MSG
In a message dated 11/13/2010 7:34:14 P.M. Eastern Standard Time,
[email protected] writes:
Hi guys:
I've rebuilt my AC-4 using the Heathkit Shop Kit recently and all went
well. All the voltages read within range and I'm happy with the rebuild and
the kit.
I have the AC-4 mounted inside the MS-4 cabinet and it sits atop my MN2000
with brand new rubber feet, yet I can hear a very low AC hum coming from
the cabinet. It's not coming from the speaker because I can hear the same
low level AC hum with the speaker cable disconnected. It's not enough to
really bother me and I only hear it when the room is quiet and the audio from
the rig is turned down. I'm just wondering if this is normal or do I have an
issue that I need to take care of.
Any ideas certainly most welcome. Thanks.
Joe
WW8X
Miamisburg
Ohio
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