"Arny Krueger"
"Trevor Wilson"

**First off: The "Mains hum" is not mains hum.

Well, it is still mains-related.

** The term " mains hum " includes 50/60 Hz AC supply frequencies plus ALL
audible harmonics, either originating from AC supply voltage and current
itself, magnetic fields associated with transformers connected to same AND
harmonic frequencies produced in various rectification processes whenever
these frequencies enter a signal carrying circuit.


It is the sound made by an amplifier when so
much current is consumed from the power supply, that
ripple appears on the outputs.

Agreed.


** There is generally no audible hum from the un-driven channel of a
correctly functioning stereo amplifier, even when the other channel is
driven to full output. This is true despite the fact that in the vast
majority of cases, the two channels share a common and unregulated PSU.


Agreed. An open bias network component, possibly a pot is a common cause.


** Even a very high bias setting in a channel also does not result in a "
loud" audible hum from the attached speaker - as was described by the OP.

Any functioning hi-fi amplifier has very high rejection of its own supply
ripple.


If the output devices run cool when no load is connected,
then it is likely you have a large DC Voltage at the
output. If so, the fault could lie anywhere in the amp.

IME large DC voltage on the output is often very detrimental to the load,
unless the load is a stout bench resistor bank.

** A large DC offset at the output of an amp will cause the voice coil and
cone of the bass driver to be move well away from its normal, central
position and ( if sustained ) for it to emit smoke signals.



...... Phil