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-   -   The Look of Preamp Hum (http://www.diyaudio.com/forums/tubes-valves/119876-look-preamp-hum.html)

tgorham3 22nd March 2008 03:10 PM

The Look of Preamp Hum
 
My formerly healthy Audio Note Preamp (kit) developed a hum.

Playing around with a scope I can see that there is a ~120 hz signal on my B+. When I look at the hum at my outputs it appears to be the same signal... which makes sense. It also explains why the hum on the line level inputs is relatively insensitive to the volume control and why, when the phono input is selected, the hum increases more dramatically as the volume is turned up.

And now the questions:

* Would I see the hum from a ground loop on my B+?

* Wouldn't a ground loop typically be ~60hz in the US?

* If its not a ground loop, does it sound like a PSU filtering problem?

* Would I expect to see problems with one of my gainstages "reflected" in my B+?

It is really interesting to be able to see my problems. Now I just need to get knowledgeable enough to fix them!

Cheers...TG

SY 22nd March 2008 04:22 PM

If it's a ground loop involving the power supply, then it will be 120, the ripple frequency. Same if the power supply caps deteriorated or a ground made by mechanical contact started coming loose.


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