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Old 12th October 2011, 03:29 AM   #1
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Default To connect or not to connect..........

Hi all. I'm getting ready to start wiring up a parafeed preamp with transformers on the output and my question is simple. Do I connect the negative output of the output transformers to the negative rail or do I take it straight to the negative of the output RCA jack and leave it isolated? I have seen schematics with it both ways.

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Old 12th October 2011, 04:18 AM   #2
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I'm not a fan of floating windings as you never know what voltage they end up settling at. The differential voltage is controlled by the flux in the transformer, but the common-mode voltage is not. With parafeed it would take the failure of two components (coupling cap, transformer isolation) to get B+ on the output jacks so I guess you could argue it's "safe". I'd still recommend grounding the secondary for safety.

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Old 13th October 2011, 12:04 AM   #3
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Quote:
Originally Posted by tomchr View Post
I'm not a fan of floating windings as you never know what voltage they end up settling at. The differential voltage is controlled by the flux in the transformer, but the common-mode voltage is not. With parafeed it would take the failure of two components (coupling cap, transformer isolation) to get B+ on the output jacks so I guess you could argue it's "safe". I'd still recommend grounding the secondary for safety.

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Thank you for your reply. I will do just that.

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Old 13th October 2011, 02:49 AM   #4
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On the other hand, it's going to feed a grounded unbalanced input on a power amp, right? Keep it isolated and avoid ground loops. You can use a resistor from shell to ground - 100 Ohms or so. Actual ground will be at power amp input.
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Old 13th October 2011, 09:55 AM   #5
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Hi!

There are advantages to both methods. grounding it at the peramp side can potentially cause a ground loop if preamp and power amp both have their grounds connected to safety earth.

On the other hand the secondary winding can develop a statuic charge when the peramp is unconnected. This will cause a loud plo if you connect the preamp to a power amp which is on.

A good compromise might be to connect the secondary through a resistor, say 47k to the ground in the preamp.

Best regards

Thomas
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