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#1
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Help! Amplifier acting as radio
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. |
#2
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Help! Amplifier acting as radio
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. |
#3
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Help! Amplifier acting as radio
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. |
#4
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Help! Amplifier acting as radio
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. |
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