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Static sound from computer connected to amplifier

Last response: in Home Audio
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November 12, 2013 8:11:49 PM

In the store where I work, we play music through a computer connected to an amplifier. It's worked fine for years (though you sometimes have to adjust the different knobs on the amp to improve the sound quality), but in the last couple of months it's become practically unusable. Even with all the proper adjustments, there is a kind of staticky hum or buzz drowning out the music--we hear it even when the music's not playing, simply as soon as I hook in the wire connecting the computer with the amp. And the green lights showing how much sound is coming out of the speaker remains high even when no music is playing.

It doesn't always happen--there have been short periods in which the music works fine, then suddenly without warning the buzz comes on again. I tried hooking the amp to another computer, and it worked fine, suggesting the problem is with the computer, not the amp. But even on this computer, the music sounds fine through a headphone. I tried a new wire, to no avail. I tried hooking it to a port in back rather than the usual one in front--the problem still remained.

I'd like some advice on how I can go about fixing this problem. Thank you.
November 12, 2013 8:31:20 PM

It could be a bad/intermittent ground connection in the electrical wiring between the amplifier and computer.

To verify if that might be the issue, you can connect a #16 gauge wire from the computer case to the amplifier (chassis screws should work well for that assuming the PC and amplifier enclosures are metallic - some amplifiers also have a ground terminal, use that if present) and see if that fixes your buzz problems. If it does, you might want to ask an electrician to check for bad grounding to find and fix the actual source of the bad ground connection... there is ground current flowing though you audio cables that shouldn't be there.
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November 12, 2013 8:37:17 PM

It could be many different things causing your problem.
It could be a source of radio frequency inside the shop is inducing voltage onto the cable. It could be a magnetic device (electromagnet or earth maget) near the amplifier or the wire connecting the headphone jack to the amplifier. It could be an impedance mismatch between the amplifier and the headphone jack. It could be frayed cabling (but you said you replaced the cable).
Where is the volume on your output (of the headphone jack of the computer) set? If it’s turned up all the way then you are pumping 2.4 vrms (+8 db) into an input jack of a stereo amplifier that is designed to accept a -20db signal (commonly referred to as line level). Overdriving the input could cause considerable distortion. This would vary according to the level the digital files were recorded at. That might explain why sometimes it happens and sometimes it doesn’t.

Good point on the grounding.
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Best solution

November 12, 2013 9:42:10 PM

groundrat said:
It could be many different things causing your problem.
It could be a source of radio frequency inside the shop is inducing voltage onto the cable. It could be a magnetic device (electromagnet or earth maget) near the amplifier or the wire connecting the headphone jack to the amplifier.

Inducing voltages in wires is actually a lot more difficult to do than it sounds since the amount of EMI that can be successfully emitted, received or coupled into pseudo-differential wiring is proportional to the surface area between the signal wire and ground, which is usually only a millimeter or so for low-voltage cables and 3-4mm live-neutral for AC wiring. At audio frequencies, the EMI coupling under most circumstances is practically nonexistent. For magnets and electromagnetic fields, whatever voltage gets induced in the signal cable also gets induced in the ground wire that goes with it so a large chunk of whatever voltages get induced cancel each other.

Where small-signal hell breaks loose on otherwise remotely good wiring is when you have bad electrical ground between equipments so leakage currents start causing significant voltages (couple of mV for ~1Vrms audio signals) across signal wires. All that crap would normally follow the path of least impedance (mainly the power ground wiring) but when the primary ground is out of service, it goes everywhere and gets magnified by orders of magnitude.
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