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Old 3rd March 2005, 07:35 PM   #1
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Default Help - Preamp Humming

I have a ML38 preamp board I acquired a while back without chassis. It has been working well and sounded great until 2 weeks ago when it started generating loud humming sound. I checked the output DC offset and both channels are below 0.5mV. I also checked the voltage on each opamp and all seem to run at spec'd of +/- 14.89V. The preamp is functioning as normal. The weird part is when only 1 output channel is connected, either right or left, there's no hum. But only when both output channels are connected, loud humming occurs. I used XLR for inputs and RCA for output. The rectifiers (MUR860s), capacitors and regulators were replaced about 2 months ago. Help, what can be wrong? (or maybe it's complaining to me it's time to get the preamp a new chassis)

thanks,
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Old 4th March 2005, 01:20 AM   #2
anatech is offline anatech  Canada
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Sounds like a ground loop through the input RCA grounds. Try a 10R resistor in each RCA jack ground, or find out what changed (possibly broke off).
-Chris
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