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Old 3rd April 2003, 06:47 PM   #1
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Default Gainclone hum and digital multimeters.

I have just got one channel of my gainclone working. I have built it as a single supply so it has input and output blocking caps. I have a loud hum problem. I have tried single star grounding and I have tried double star grounding. In the double configeration I have all the grounds attached to one point, excepting the input grounds and the feedback loop ground.
Here is the strange thing, I attached my digital multimeter on continuity test between the output side of the output blocking cap and ground and the hum goes away. Is this a pointer to the source of the problem and if so what is it.

Shoog
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Old 3rd April 2003, 06:49 PM   #2
Bricolo is offline Bricolo  France
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can you post your amp's shematic?

I don't exactly see how you builded it, and why you use an output cap
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Old 3rd April 2003, 06:55 PM   #3
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Default It is the standard single supply circuit.

It is the standard single power supply circuit given on the LM1875 data sheet.

Shoog
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Old 3rd April 2003, 07:02 PM   #4
Bricolo is offline Bricolo  France
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maybe you were simply doing a short circuit with your multimeter
that's why you had no more hum
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Old 3rd April 2003, 07:06 PM   #5
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Default The signal was still there

The signal was still there so I dont think a short circuit would explain it.

Shoog
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Old 3rd April 2003, 07:19 PM   #6
tiroth is offline tiroth  United States
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Hmm...what happens if you tie the output side of the output capacitor to ground via ~100k-1M resistor? This shouldn't make a difference, but maybe a DC ground point at the amplifier side is helping for some reason.
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Old 3rd April 2003, 08:53 PM   #7
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Shoog

Is the continuity test on your meter also a diode test? Try reversing the leads and do the test the test again. If you return a hum, you defintely have a ground loop problem.
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