A couple of people have mentioned the possibility that the speaker leads
could act as an antenna, and pick up unwanted radio signals. If you
consider that the output of an audio amp is very low impedance (8 ohms), it
would take a hugely strong radio field to generate any appreciable voltage
in that circuit.

It's not the voltage but the signal that is the problem. The signal will
'ride' on whatever is available in the cables. The field does not need
to be 'hugely strong' either, even passing cars with radio transmitters
(CB, Ham, taxis, emergency services etc.)can affect audio systems with
long speaker wires.

Secondly, it is hard for me to see how a radio frequency,
applied to the output circuit of an audio system could have any effect.


Believe me it happens and I have had to deal with it several times. The
signal may be applied to the 'output' in terms of the speaker wires but
it is a 'closed loop' system and that signal can and *does* get back
into the amp where it 'can' get into the amplification circuit and be
amplified and sent out to the speakers as part of the 'mix'. A lot has
to do with the design of the amp and how good it's rejection circuits
are on the 'front end'. Most 'cheap' modern amps have very poor
rejection circuits and are prone to picking up stray signals.

A few years ago there was a big scare up here in Canada when Industry
Canada proposed some new rules with regard to domestic amplification
equipment. The European Community had recently introduced much higher
standards for equipment in terms of the ability to reject unwanted
signals while Canada's proposals virtually eliminated the need for
filtering by 'effectively' saying that the equipment in question had to
'accept' any extraneous signals. This raised huge problems for the Ham
Radio community as it meant that Canada could be 'flooded out' with
cheap Asian amps that could no longer be sold in Europe and which would
mean that Hams would suddenly be 'causing' interference problems with
neighbours who bought said amps despite the fact that they had changed
nothing in their transmitting equipment!

I am not sure if the proposals ever went through but I do know of
several cases of Hams suddenly getting into cheap 'home theatre' systems
and having to help cover the cost of fitting ferrite loops to the
systems in an attempt to cure the 'problem'. In most cases the Hams
either paid for the loops themselves or helped to pay for them despite
the fact that it was not really 'their' problem simply because the
alternative was to face being 'taken' off the air for 'supposed or
implied' interference problems!

--
Larry Green