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sangram 23rd January 2010 04:04 PM

Passive preamp signal ground
I have to put together a passive preamp for a friend, and am looking for an input on signal ground.

Obviously this is not a problem with a single source, but the proposed project requires three sources. I was planning on using relays to switch the input signal and a stepped attenuator for volume control. Relays would be switched through a standard industrial 3-way rotary switch, a single relay for each channel so six relays total.

Normally, I would lump all the signal grounds together, but since this is reasonably high quality project I was wondering if using a small (~0.1 ohm) resistor on the signal ground inputs would help avoid ground loops. The second potential method would be to use a third relay for each source, for the express purpose of switching the grounds alone (so 9 relays total).

Obviously I would need to take precautions to ensure there was always one signal ground connection through the attenuator to the power amp, I'm thinking some large caps across the relay terminals to delay the relay disengaging for a make-before-break system.

What would be the better system or should I combine both methods? Or just lump them all together?

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