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Old 6th April 2008, 10:05 PM   #1
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Default Shielding phono preamp input

OK, I am after that last bit of hum from my VSPS. I am pretty sure by now that the issue is is the input wiring. I just lazily used a selfmade loose twisted pair of wires and am paying the price. I built the case out of wood which doesn't help. The TT and preamp are unbalanced.

So, what is the best fix for wiring from the input RCA jack to the board: (1) a shielded single conductor wire, where the conductor is the signal wire and the shield is the ground, (2) a shielded twisted pair, where 1 conductor is the signal, the other conductor is the ground, and the shield is grounded at one end, or (3) a double shielded single conductor, where the conductor is the signal, the inner shield is the ground, and the outer shield is grounded at one end?
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Old 6th April 2008, 10:17 PM   #2
paulb is offline paulb  Canada
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I'd try #2, and you can always revert to #1 as an experiment. Does your turntable have a separate ground wire?
I'm using #1 in one I just built (battery powered), and noise is completely absent.

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Old 6th April 2008, 11:20 PM   #3
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IMO, it doesn't matter. What does matter is where the currents flow. If the ground point locations aren't perfect you'll get hum. Obviously you need a shielded enclosure to to be mostly rid of it. The connection point to the enclosure should be as close to the *isolated* input connectors as possible, and that's also where the tt ground lead should attach on the other side of the box- I wonder if there's some merit in using an insulated wire attachment and running it right to the single point ground? Always ask yourself, "Where is the ac current flowing and why? How can I reduce loop area?" Remember that once things are basically shielded most hum comes from current flow in wires having finite resistance, and inductive pickup of stray fields because of loop area that could have been reduced.
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Old 15th April 2008, 05:27 PM   #4
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Just wanted to post an epilogue:

I tried shielded coax, still kept playing with the grounding layout, still had annoying hum....

With all my trials, the LM4562 I was using finally succumbed. It was the last one I had, so I looked for whatever opamp I could find. I popped in an LF412, and WOW, the hum dropped to a barely audible level. I though I would pass on the info
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Old 15th April 2008, 05:51 PM   #5
paulb is offline paulb  Canada
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That's an unexpected result. Are you sure it wasn't something else? LF412 has less quiescent supply current...?
I probably have an LM4562 around somewhere if you're nearby and want to try rolling opamps again. If it's soldered in, and it sounds okay you probably want to leave it alone. I work up by Deerfoot Mall.
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Old 15th April 2008, 06:03 PM   #6
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I don't think it's unexpected at all. The input impedance of the LF412 is very high because it is a FET 1st stage, but the LM4562 only has a differential input Z of 30 K because it is a bipolar. The hum went down because the cartridge loading improved.

I'm sure it wasn't anything else, the circuit did not change between usage of the different opamps.
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