HOW DO YOU STOP INTERFERENCE COMING THROUGH STEREO AMP?

[fleis...@comcast.net]

Hello,

Any suggestions on how to prevent my Radio Shack Stereo amp (1969
solid state ) from picking up public service bands? It acts as a radio
reciver, as well as an amp. It's driving me nuts.

Please advise.

Thanks Barry F.

[Michael Black]

Googlegroups is back, and bringing some variety out of the woodwork.

If it's really getting into the audio amplifier (rather than some receiver
ahead of it), then you have a strange situation. The public service bands
are FM, and while it might interfere with the operation of the amplifier
by providing odd bias to it, the modulation won't be recovered. With an
AM signal, the amplifier's first stage would act like a crystal receiver
and recover the modulation, which you'd hear. But FM varies frequency,
not amplitude, so all it would do is provide a constant DC voltage.

So something else is going on.

Michael

[dave]

Aircraft perhaps. Some public service on Low Band VHF and HF might still
use AM or maybe he's hearing somebody using CB 'cause their town is too
broke to afford radios.

[Brenda Ann]

"dave" wrote in message
news:2fidnRLonMCSISzS...@earthlink.com...

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In theory (and ONLY in theory!), FM modulation will not be detected
(properly) on an AM receiver. HOWEVER, it can be, and often IS detected and
sometimes very well over a broadband audio amplifier. This is because of
slope detection, which can sound quite good if the amplifier frequency
response curve is kept below 70KHz or so. I personally had a Teac tube reel
to reel tape machine that would irritate the hell out of me because it would
"hear" (then) KPAM-FM 97.1 MHz any time it was in record mode. The station
was 95KW and in direct line of sight from my apartment, about 2.5 miles
away.

By the way.. AM aircraft band frequencies can be heard on many FM radios on
the "mirror" of the 10.7MHz IF... i.e. the LO frequency for a station on
98.1 would be 108.8 on a standard FM radio. 108.8 + 10.7 would be 119.5MHz,
near the middle of the aircraft band. You would think that an AM signal
would not be heard through an FM detector, but this doesn't necessarily hold
true. Using AFC can stop most of it, but if you have that off, or don't have
AFC at all, it's difficult to keep the frequency centered solidly enough to
keep the AM signal out of phase.

[Brenda Ann]

"Brenda Ann" wrote in message
news:RqKdnSIhL9Vygy_S...@giganews.com...

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Oh... and my bad. You can cut down substantially on this type of ingress
using snap-on ferrite beads on all input and speaker connections, and by
grounding both sides of speaker connections (at the amp) to earth ground
through small capacitors (say, .001uF 630V axial lead caps).

[William Sommerwerck]

> Oh... and my bad. You can cut down substantially on this type
> of ingress using snap-on ferrite beads on all input and speaker

> connections...

And the power cord.

Something larger than a "bead" is desirable. Full-size ferrite chokes are
about an inch in diameter and an inch to two inches long. "Looping" the
cable before snapping on the choke increases the choke's effectiveness,

It's also a good idea to do this with FM tuners, to make sure the only thing
that gets into the tuner is the signal //from the antenna//. With the
antenna (but not the coax lead-in) disconnected, you should hear absolutely
nothing from the tuner.