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Old 2nd March 2009, 08:30 PM   #1
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Join Date: Jun 2008
Default Need some advise with a hum problem

I have an audible hum coming from a HK 6V6 PP integrated amp. It measures about 4 to 7 mv on the OT secondaries at the 8 ohm tap. It's a kit build and looks to be of very good build quality. The previous owner has recaped the electrolytics on the power supply and replaced the coupling caps with Orange Drops and has also replaced all the resistors. The amp sounds wonderful but for the hum.

Here is a scope photo of the signal at the coupling caps between the second preamp triode and the phase splitter. The frequency is 60 hz. This is with no source and the input jacks grounded. All four caps look the same on the scope.

[IMG]Click the image to open in full size.[/IMG]

The original builder must not have read the chapter on twisting AC filament wires so I rewired with some noticeable improvement but the hum is still a bit much. The hum circuit is wired as drawn and the resistor values are spot on. I connected a pot to see if an improvement was possible and I found the factory numbers to be optimal.

When I connect a regulated DC power supply, the problem goes away. I'm sure it is a heater hum.

Here is a Schematic

Any ideas where I go from here. I'm out of ideas.
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Old 2nd March 2009, 09:56 PM   #2
cerrem is offline cerrem  United States
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Join Date: Jun 2005
Location: San Diego, CA
I would suggest getting rid of R54 and R55.... They are dissimilair values...
Put in a 100 Ohm hum pot with center wiper to ground.....

Also....

This amp shows no grounding at the mains.....
You need somewhere in the system to have a ground.....
Typically the power-amps will have no ground and the pre-amp would be the central and only grounding point for all associated units in the sytem... Sort of like star grounding at the pre...
Instead, if all units were grounded then there could be a good chance that AC ground loop currents would be flowing through all inter-connects creating additional hum levels...

Chris
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Old 3rd March 2009, 12:07 AM   #3
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Join Date: Jun 2008
Chris,

Thanks for the response. I did try the 100 ohm pot and found that no improvement could be made over the resistors.

The ground is an interesting idea but how can you ground the mains with a voltage doubler? I don't know of a way. The ground must be made through the lower 100 uf cap.

Dave
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