My computer is attached to an old amplifier, which in turn is attached
to 4-foot speakers. The sound is awesome for games or for parties. I
just make a list of mp3s and let it play for hours. Lately, however,
I've been hearing through the speakers the faint signal of an AM
station. When the music is turned up there's no problem, but when
there's silence, i.e. between songs, I hear the AM station and it drives
me crazy.

When I disconnect the cable running to the computer the noise stops. At
first I thought it was the computer's fault, but then I touched the tip
of the cable (the one that plugs into the computer) to a metal cabinet
and the AM station came back on. It seems that the cable coming from the
amplifiers "aux in" is acting as an antennae of sorts. Switching the
cable to another "in" port yields the same results.

Can anyone suggest any remedy for this? Is there a special type of cable
I should get, or is my amplifier just a piece of junk?

Any help is appreciated.

The cable you are using is acting like a front end resonant circuit to the
amplifier, and the amplifier input stage is acting like a detector to it.
We've seen this before. The total resonance of your combination of things
is at a close frequency to that of the radio station, and the station's
radiation is strong enough in your area to drive this type of circuit that
you've created, in combination with the characteristics of the amplifier
that you are using.

Sometimes I have cured this by putting a small ceramic disk capacitor of
about 100 pF to about 200 pF across the input of the amplifier. The voltage
of the cap will not be critical. Most these small disk caps are rated at
1kV by standard. This small a value of capacitor should not effect the
performance of the amplifier at the audio frequency range.

The cap must be located right on the amplifier's input, not on the opposite
end of the cable. The idea is for the cap to detune the cable from acting
like a resonant circuit. If it is resonating through the ground side, which
you will find out when you try this, then the situation will be different.

Another method that also can work if the cap method does not, is to put
about a 470 ohm to about 1 kohm 1/4 to 1/2 Watt rated resistor in series
with the amplifier input, on the side of the amplifier (not on the end of
the cable). The resistor should normally isolate the length of the cable at
the high frequencies, such at where a radio station would be in the
spectrum. In testing this, you can probably go to about as high as about 10
kohm in the resistor value before making any measurable performance change
in the amplifier's input specs. Normally the input impedance of a standard
audio amplifier would be about 50 kohm and up.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"the unknown" wrote in message
...
My computer is attached to an old amplifier, which in turn is attached
to 4-foot speakers. The sound is awesome for games or for parties. I
just make a list of mp3s and let it play for hours. Lately, however,
I've been hearing through the speakers the faint signal of an AM
station. When the music is turned up there's no problem, but when
there's silence, i.e. between songs, I hear the AM station and it drives
me crazy.

When I disconnect the cable running to the computer the noise stops. At
first I thought it was the computer's fault, but then I touched the tip
of the cable (the one that plugs into the computer) to a metal cabinet
and the AM station came back on. It seems that the cable coming from the
amplifiers "aux in" is acting as an antennae of sorts. Switching the
cable to another "in" port yields the same results.

Can anyone suggest any remedy for this? Is there a special type of cable
I should get, or is my amplifier just a piece of junk?

Any help is appreciated.

the unknown wrote:
My computer is attached to an old amplifier, which in turn is attached
to 4-foot speakers. The sound is awesome for games or for parties. I
just make a list of mp3s and let it play for hours. Lately, however,
I've been hearing through the speakers the faint signal of an AM
station. When the music is turned up there's no problem, but when
there's silence, i.e. between songs, I hear the AM station and it drives
me crazy.

When I disconnect the cable running to the computer the noise stops. At
first I thought it was the computer's fault, but then I touched the tip
of the cable (the one that plugs into the computer) to a metal cabinet
and the AM station came back on. It seems that the cable coming from the
amplifiers "aux in" is acting as an antennae of sorts. Switching the
cable to another "in" port yields the same results.

Can anyone suggest any remedy for this? Is there a special type of cable
I should get, or is my amplifier just a piece of junk?

Any help is appreciated.

Many times the shield of audio cables is spiral wrapped around the
center wire insulation. This is very poor when it comes to rejecting
RFI. Try replacing the cable between the Amp and Computer with a good
quality shielded cable. I'd suggest using RG-58 Coax cable since it has
a braided wire shield and nearly 100% shield quality. If that doesn't do
the job, try some of Jerry G's suggestions. Good luck.

--
Art Jackson W4TOY Owensboro, KY USA
Life is God's open book test. In order to pass,
you must open His book to find the answers.

Hi there,

This used to be a common fault and is due to the base-emitter junction of
the first transistor acting as an AM detector. If you can, trace the input
circuit through and put a small cap of about 47pF to 100pF across the
base-emitter junction of the first transistor. I am assuming that the 'old
amp' is old enough to discrete and not a bag of chips.

Hope this help, Mike


"the unknown" wrote in message
...
My computer is attached to an old amplifier, which in turn is attached
to 4-foot speakers. The sound is awesome for games or for parties. I
just make a list of mp3s and let it play for hours. Lately, however,
I've been hearing through the speakers the faint signal of an AM
station. When the music is turned up there's no problem, but when
there's silence, i.e. between songs, I hear the AM station and it drives
me crazy.

When I disconnect the cable running to the computer the noise stops. At
first I thought it was the computer's fault, but then I touched the tip
of the cable (the one that plugs into the computer) to a metal cabinet
and the AM station came back on. It seems that the cable coming from the
amplifiers "aux in" is acting as an antennae of sorts. Switching the
cable to another "in" port yields the same results.

Can anyone suggest any remedy for this? Is there a special type of cable
I should get, or is my amplifier just a piece of junk?

Any help is appreciated.