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Old 13th June 2009, 06:26 PM   #1
phrarod is offline phrarod  United States
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Default hum on pre-amp when volume is turned up

I'm getting closer to optimizing my pre-amp project. One problem is hum.

I don't think its ground loop and I have split the grounds - floating for amp board and ps and input/output jacks (OT)/pots. Chassis for IEC, mains transformer.

The hum gets louder as I turn up the volume. It also doesn't sound like RF. I put a 100R grid stopped in and a 220K to ground to see if that would help. Not really.

I'm stuck. I tried moving the transformers around but that didn't help either. Unfortunately I don't have a scope.

I have a grounded steel shield between the mains trans/ power supply and the OTs/amp board so I don't think the ps is radiating into the volume.
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Old 14th June 2009, 06:42 AM   #2
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Have you tried shielded input wires?
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Old 14th June 2009, 03:33 PM   #3
phrarod is offline phrarod  United States
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They're litz braided but I'm considering it.
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Old 14th June 2009, 05:51 PM   #4
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I actually built a preamp recently and had a similar problem. I star grounded it and added some shielded wire i butchered off a computer cable. I grounded the wire sheild to the chassis at a point different to the star an had veery good results. I dont know what this computer cable was from but it has 4 small gauge pvc covered stranded wires surrounded by a metalized mylar type of coating that was then surrounded by a copper wire mesh. The mesh was what i of course grounded to the chassis at a different point than the star. Initially I grounded it to the star but it didn't help. Grounding it off the star helped considerably. Still have been unable to get all the hum out but it is almost imperceptable now. Let me know if you come up with an interesting solution, I would like all of mine gone too! But I can live with it the way it is now.
Jeff M.
Lawton, USA
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Old 14th June 2009, 05:58 PM   #5
phrarod is offline phrarod  United States
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Thanks Jeff. I'll let you know. I'm been doing a lot of reading about grounding. I'll keep pursuing it.
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Old 14th June 2009, 06:07 PM   #6
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What exactly did you build? I am curious? Do you have a schematic?
Jeff
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Old 14th June 2009, 06:11 PM   #7
phrarod is offline phrarod  United States
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Just a 5965 transformer output. Very simple. If I could fix a few problems it will be nice. No schematic. I'll make one eventually.
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