"William Sommerwerck" wrote in message
news
"Arfa Daily" wrote in message
...

I did actually say in my first post in this thread, that hum as a result
of a power supply problem -- for example a bad cap -- will not usually
be affected by the volume control, so by implication, will still be there
with the volume at minimum.

Yes, you did say that. But one might reasonably assume that the PS hum
ahead
of the volume control would add to the hum following the volume control.



As Don has said, it only works as a power supply problem, if the preamps
have their own supply rails, derived from their own transformer winding,
rectifier, and filter cap(s), in which case, a failed cap, or a leaky diode,
will place ripple on the preamp supplies, giving an audible hum which will
completely disappear, when the volume is turned to minimum. If the preamp
rails are derived from the main power amp rails, as is quite often the case,
then any cap or diode failure that produced ripple on the main rails, would
cause the audible hum to be present even with the volume at minimum.
Downstream decoupling on the preamp rails, would likely remove most, if not
all of the ripple from those rails, so when the volume was turned up, there
would be no additional amplified hum contribution from the preamps.

Arfa