Help - Preamp Humming

acliao · 3rd March 2005, 06:35 PM

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,

anatech · 4th March 2005, 12:20 AM

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

3rd March 2005, 06:35 PM	#1
acliao diyAudio Member Join Date: May 2002 Location: Los Angeles, CA	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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4th March 2005, 12:20 AM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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