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Old 17th December 2012, 03:04 AM   #1
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Default Nailing Hum in my Phono Stage

OK, so, after a couple weeks of tinkering, I've managed to reduce the amount of hum coming from my phono stage, I re-grounded the whole signal circuit and it is all "star" at one common point in my audio chassis. I have a separate PSU chassis which is also common at one point and is earth-grounded. My heaters are DC and my B+ is filtered using one choke and one two-section filter cap, and then there is a twisted umbilical running to my audio chassis.

The hum is 100% 120Hz, and constant regardless of voltages and lifting my earth ground or reversing polarity. Shorting inputs to audio chassis resulted in no change, shorting inputs together resulted in some 60Hz being injected into the hum.

A little test I did– attach an alligator to the signal input and move it around the PSU. I notice a little hum around the transformer, but when I hovered the clip near the filter capacitor, the hum increased DRAMATICALLY. Like, to the point of where I had to turn the volume down because it was so loud and uncomfortable. Specifically the side with the negative terminal seemed to be worst.

This is concern because I believe this could be my issue– my umbilical stems from my PSU chassis near the cap, and thus I feel like I'm getting hum induced there.

Any ideas or tips on how to proceed?

SK
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Old 17th December 2012, 04:16 AM   #2
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Is there at least one final B+ filter capacitor in the amplifier chassis proper? If not, there should be. I would also investigate the capacitor that seems to be radiating hum by replacement. Also consider rerouting umbilical leads or shielding them. Although this should not normally be necessary.

Also, the PS chassis should be earth grounded for safety reasons, as you have done, because that's where the power transformer is. But the B- could/should be lifted from the PS chassis and only connected to the amplifier chassis through the umbilical. Some of the preamps that I've serviced, like the CAT SL-1, do it that way.
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Old 17th December 2012, 04:25 AM   #3
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I think you will find you don't have nearly enough filtering on the B+ to your phono stage, you probably need an additional choke and capacitor.

My latest design uses complex voltage regulation (see muscovite thread in analog source) in order to get the noise down enough so it is not audible.

You might consider the Salas SSHV2 shunt regulator to power your phono stage - very quiet power and minimal voltage loss.
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Old 17th December 2012, 04:05 PM   #4
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Thanks for the suggestions. I've suspected the filter cap as being the problem, I will try and replace it and also possibly add an additional filtering stage. I will try lifting the B- from the PSU chassis and see if that helps today.

Thanks!
SK
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