In article , Serge Auckland
writes

"Arny Krueger" wrote in message
...
"Jo" wrote in
message

In ,
Arny Krueger typed:

The most likely way that RF enters power amps is thorugh
the input terminals. Most of the time there is a simple
small series resistor and parallel cap at the input that
deals with this issue.

RF can also enter via the speaker leads if this isn't
taken care of in the design.

I keep hearing about this, but have never seen the problem up front and
personal. I have had RF problems but they were in the front end, not the
back end.

A while back I had an amp
that would pick up VHF taxi and police transmissions
quite clearly.

Brand - model?

It even picked up Radio Moscow on
occasions when global MF radio propagation conditions
were favourable. Home made longitudinal chokes on the
speaker leads fixed most of this problem.

Most power amps already have chokes in series with their output terminals.

In 1972 I had a friend who lived in South London, half way between Crystal
Palace and Croydon. In those days, there was 405 line TV, which had AM
sound. He had a Ferrograph F307 amplifier which picked up TV sound through
the 'speaker leads and/or through the mains lead, probably both. TV sound
came through at normal listening level regardless of volume control position
and source selected, and indeed, even whether there was a source plugged in
or not. Shorting the inputs made no difference.

We tried ferrite filters in the 'speaker leads, in the mains leads,
capacitors across the mains, and across the 'speakers. Putting the whole
amplifier in a screened box, using coax for the 'speaker leads, (of course,
the leads inside the speakers remained unsceened) nothing worked. After a
few weeks of not being able to use his audio system, my friend moved!

The only thing you need to have done is stick a few hundred PF across
the base emitter junctions of the input stage transistors and thats what
I had to do to lotsa equipment many years ago, when that bloody AM cb
craze was all the go!..

Those junctions are very good demodulators!....

I used to have a home-built power amplifier which ocasionally picked up
Radio Moscow. Couldn't find a reason until one day I put a 200MHz scope on
it. The amp was hooting at around 20MHz, never saw it on my 5M scope.

There are lots of reasons why rf interferes, and getting rid of it is the
very devil.

Beg to differ guv!, but RF is a black art and should not be spoken of in
these forums, its much too high for the likes of uk.rec.audio;-).

Takes quite some understanding and its much best left to only those who
"need to know"..know..nuff said;-)..

--
Tony Sayer