active cable shield driver in a phono amp -- does it make sense (noise- or sonicswise - diyAudio
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Old 1st June 2008, 10:53 AM   #1
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Default active cable shield driver in a phono amp -- does it make sense (noise- or sonicswise

Hi,
in some Neumann phono pres there are active shield drivers that reroute a part of signal back to the cable shield (between pickup and pre). Has someone actually tried this with good results?
Rüdiger
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Old 25th November 2016, 04:51 PM   #2
Bigun is offline Bigun  Canada
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Bump - I'm also curious about this ?
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Old 26th November 2016, 01:09 AM   #3
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It's something that they do in instrumentation when dealing with very small signals in harsh electrical environments.
But it's in extremely high impedance circuits and phono stages are low to very low impedance.

Keithley Instruments might still have some notes on 'guarding'.
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Old 26th November 2016, 03:20 AM   #4
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In theory you can negate some of the cable capacitance which loads-down the top end of the cartridge response. However such carts are *wound* assuming some capacitance. Typically 3 feet of cable and the grid of a 12AX7. Blindly nulling the expected cable cap may not be best. The 3 feet was very standard in days of tubes, had to reach from the turntable to the adjacent amp chassis. But with transistors we can put the preamp almost anywhere, and then drive any length of line.

You would think it "could" improve shielding. I have doubts. But the cartridge itself is always going to pick-up crap from the room. It can't be 101% shielded, the needle and pins have to stick out. Happy hi-fi means finding a happy (quiet, solid) place for the turntable.

I have done something like that for an in-turntable preamp. Mostly for compactness and circuit simplicity. (There was DC on the cart, but not through it.) I did not find any sonic revelation I could pin on this trick.
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Old 26th November 2016, 02:37 PM   #5
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The Keithley 'Low Level Measurement' handbook. Start with Guarding on page 2-15.
http://www.tek.com/sites/tek.com/fil...ndbook_7Ed.pdf
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