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Old 23rd February 2014, 01:30 AM   #1
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Default Parasound P/PH-100 hum

I have this phono preamp and it's got hum issues. About 4-5 mv of 60 cycle at the output. It's worse in one channel, but present in both. If you look at the schematic at plexus PPH-100 mods (which has an error - C35 is not in series with signal ground, it goes from signal ground to chassis ground), it's not present at the input lines of the last op-amp, but is on the output lines. I've replaced all the electrolytics, but the hum persists. I'm not a hardware guy, but can solder and wield a scope. Any ideas where to look? Power transformer? Other caps?

Appreciate any help rendered...

Bob

Last edited by lacanadio; 23rd February 2014 at 02:56 PM.
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Old 26th February 2014, 12:34 AM   #2
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No ideas at all? I guess I'm surprised. What other information can I provide that might help?
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Old 1st March 2014, 06:23 PM   #3
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A 60 Hz hum of different amplitude on both channels suggests a grounding-related (or cabling) problem. But just to be sure, are the supplies looking clean?

Last edited by sgrossklass; 1st March 2014 at 06:28 PM.
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Old 1st March 2014, 06:44 PM   #4
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Thanks for the reply...

Yes, both supply rails look clean. No 60hz there. The + rail measures 14.72v and the - rail measures 15.01v. The output op amp is a OPA2604. Inputs look clean, it's just the outputs that have the 60hz.
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