To connect or not to connect..........

G · 12th October 2011, 02:29 AM

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.

G

tomchr · 12th October 2011, 03:18 AM

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.

~Tom

G · 12th October 2011, 11:04 PM

Quote:

Originally Posted by tomchr

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.

~Tom

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

G

Tom Bavis · 13th October 2011, 01:49 AM

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.

Vinylsavor · 13th October 2011, 08:55 AM

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

12th October 2011, 02:29 AM	#1
G diyAudio Member Join Date: Apr 2002 Location: Belleville, IL.	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. G __________________ Gavin

12th October 2011, 03:18 AM	#2
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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. ~Tom __________________ 21st Century Maida Regulator, Universal Filament Regulator, 300B Driver PCBs, and more... Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

13th October 2011, 08:55 AM	#5
Vinylsavor diyAudio Member Join Date: Feb 2002 Location: Lindau	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 __________________ http://vinylsavor.blogspot.com/

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13th October 2011, 01:49 AM	#4
Tom Bavis diyAudio Member Join Date: Jun 2002 Location: Macedon NY	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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