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Old 7th September 2005, 03:09 PM   #1
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Default 3875 GC Hum

I have searched but I can't find anything directly relating to my issue. The obvious bit is my amp hums slightly, but only at very high volume. So long as the volume is set slighty below full volume I can't hear anything.
Oddly with nothing connected to the input it is dead silent, perfect, not even a hiss. With my commercial amp (older Yamaha) acting as a switch/preamp the hum is there.
Directly connected to my DVD player as a source it's as silent as with nothing on the inputs.

So I know this is pretty cut and dried, just don't use the Yamaha. My question is if I get another commercial pre-amp as my input in future will I have similar issues?
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Old 7th September 2005, 03:29 PM   #2
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It depends on the gain your pre-amp gives.
For example if your source has noise and the
pre-amp gives 20dB gain then you may hear a
noise that you were not able to hear before.

It's also possible that the pre-amp introduces
the noise itself. I had noise in my own pre-amp
(Build myself). Proper grounding got rid of it.

The question is really does it matter? Are you
going to play it at full volume? If you do will
you still be able to listen to the noise above the
music?
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Old 7th September 2005, 03:30 PM   #3
Vikash is offline Vikash  United Kingdom
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maybe. maybe not. how long is a piece of string?
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Old 7th September 2005, 03:32 PM   #4
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Is it "hum" or "hiss" --you may be picking up some 60/120Hz from the power supply -- a photo would be nice.

Other stuff -- these chipamps require good supply bypassing (minimalists need not apply). Dressing of the power supply leads at right angles to signal paths is important also -- remember that the gain of the LM3875, 3886, 4780 is quite high and problems will migrate in various and sundry ways.
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Old 7th September 2005, 09:48 PM   #5
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My Chip amp has a gain of 42 (21 from the LM3875
x 2 from the DRV134). This is a lot. After proper
grounding no audible noise. My multi meter
shows <0.5mV noise.

Harry
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Old 7th September 2005, 10:12 PM   #6
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Try plugging the pre-amp into a different outlet, far away from where it is now. You could be picking up noise in many places.

Also, try grounding the chassises of both amps. Does your pre-amp include a grounding screw?

How about your signal ground? Isolated from the power ground except for the star joiner? Signal ground nice and close to your inputs?

Wes
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