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Old 4th April 2008, 07:46 PM   #1
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Join Date: Aug 2002
Location: Eire
Default Weird Input transformer hum.

Hi all,
I am reaching the end of the development of my latest amp. After a few ups and downs it seems to be panning out well.
However I have a weird hum issue. Let me describe the input circuit a little. I have input transformers with the input floating on the primaries. The secondaries are biased at -300V via a negative rail, and drive both inputs of a LTP. The problem is I have some buzzing hum. This goes away if I touch the sleeve of the input phono plugs. It doesn't respond to grounding the input sockets to the case at the case. I haven't yet attempted to ground the sockets to the main star ground. This seems to be the opposite of the usual situation where touching the screen induces hum.

Has anyone got any ideas whats going on.

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Old 4th April 2008, 09:13 PM   #2
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Join Date: May 2005
Location: Dallas (but I am not a Texan!)
Is the buzzing there with a source connected, or without?

I've had some similar issues using input xfmrs. With the input floating, if I connected a source (CD player) that had a 2-wire AC line cord, I got buzzing. It went away if I grounded one side of the primary. I think the source was the EMI filter on the (ungrounded) CD player inducing common-mode AC onto the floating "ground" and output of the CD player, which was then getting electrostatically coupled across the input transformer.

A different CD player, which had a real ground connection, didn't cause this type of noise.

Also had buzzing issues with nothing connected. I believe this is just that the input transformer, with an open primary, is an incredibly sensitive electromagnetic transducer. It was picking up noise from the power supply, and also anything else nearby. Terminating the primary in any reasonable source resistance eliminated it.

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Old 4th April 2008, 09:13 PM   #3
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Join Date: Aug 2002
Location: Eire
Problem solved - the earth connection wasn't good enough. Absolutely quiet now.

The only remaining problem I have is that the gain is to high. I could do with losing about half the gain, but I already have the transformer configured as a 2:1 step down.

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