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-   -   grounding issue with F1 (http://www.diyaudio.com/forums/pass-labs/165869-grounding-issue-f1.html)

recstar24 27th April 2010 01:26 PM

grounding issue with F1
 
I am having a pretty bad grounding issue with my Firstwatt F1 and my preamp. My preamp is built by dsavitsk, a spud 71A parafeed autoformer coupled headphone/preamp, singled ended, but with input transformers to take in a balanced signal, and the autoformer has a phase splitter output that is the preamp out.

When fed into the F1, we are getting serious hum and noise/hiss. Doug deduced that it may be an issue with the F1, as the signal goes cap coupled directly into the mosfets. If anyone has any ideas that would be great.

recstar24 27th April 2010 02:49 PM

I should add that the phase splitter out on the autoformer was designed because we wanted to be able to feet that signal into the balanced in on the F1. Doug recommended using input transformer coupling in lieu of C1 and C2 on the F1 input but if possible I would like to avoid that.

Nelson Pass 27th April 2010 05:46 PM

Probably you should use a secondary winding which has a
center tap to ground so that the input is well referenced to
ground. You cannot count on the CMRR of the F1 to reject
all noise if the signal has poor communication to ground.

:cool:

recstar24 27th April 2010 07:39 PM

Thanks Nelson, I was fearful of that response, as it would require using completely different outputs for the amp. Ah well, I am sure Doug and I will figure something out. He did recommend using input transformers on the F1...


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