LM3875 kit troubleshooting - diyAudio
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Old 16th April 2012, 03:31 AM   #1
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Default LM3875 kit troubleshooting

I am having some issues with my amplifier and was wondering if I could get some advice. I have been getting some high frequency hums/noises when I hook it up to my computer. I plan to use my PC as my source. Hopefully this is not too much of an issue.

Unplugged from the PC-source, it is dead quiet. When I attach it to my Ipod Touch, it is also very quiet with only a small hum when I press my ear to the speaker. When I hook it up to my computer, the hum/noises is almost as loud as the music (it does not get louder if I adjust the volume through windows). I have no pot or volume control on the amp.

Using a single transformer and rectifier board to power two amplifier boards.

Currently I am using your power star ground configuration (I think):
Earth ground
PG+ from rectifier board
PG- from rectifier board
PG+ from both amplifier boards
PG- from both amplifier boards
CHG from both amplifier boards with a 8-10 Ohm resistor (although its shorted now, so I don't think the resistor is doing anything. I read somewhere that it was supposed to help...)

-----

I also read somewhere that you want to put a capacitor on each RCA connector. So I believe I have a 1 uF (not sure of value) under the heatshrink. If this is an issue I can verify by cutting the heatshrink.
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Old 16th April 2012, 04:52 AM   #2
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You have a ground loop problem due to Earth connection of the PC.

Gajanan Phadte
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Old 16th April 2012, 05:00 AM   #3
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It could be ground loop related,to an extent.

PC's are full of noise. I would add a volume control to the amp,and then turn up the output of the PC,then use the pot on the amp to adjust the volume. Otherwise the signal is just going to be buried in the noise from the PC.
The idea here is to try and increase the signal to noise ratio coming out of the PC.
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