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It is currently 20 Jun 2018, 12:21

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 Post subject: 60 Cycle hum- monoblock
PostPosted: 31 May 2018, 19:41 
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Joined: 07 Jan 2018, 07:13
Posts: 10
Hello,
I’m hoping someone would be able to help me narrow down a 60 cycle hum I’m getting in a monoblock I rebuilt.

I am in the process of rebuilding two monoblocks I built a few years back. I’m basically putting them in nicer chassis but the tubes and boards remain the same. They worked no problem in the last chassis which was a basic Hammond steel chassis. However I did the grounding scheme differently than before which I believe to be far better and which works perfectly in my Forewatt.
I basically created a buss at the inputs and outputs, ran a single wire from the PS and active circuitry, to the buss and then the chassis to buss through an X2 cap and resistor. Trans center taps also go to the buss.

I believe I have a ground loop somewhere just not sure where.

I now get a 60 cycle hum that doesn’t change with volume and is always there. If I unplug the preamp the hum is reduced quite a bit but is still there. If I place my hand on the output transformer which is on top of the chassis it is reduced somewhat. If I place my hand on the power trans it gets a little louder. BTW, the top bottom and rear steel chassis’s panel are all bonded together with copperbraid, the sides are now wood.

The only thing I can think of that might be the problem is that I had the chassis panels and transformer covers powder coated. The chassis panels and transformer frames are basically electrically isolated from each other to to the powder coating. I’m not sure if I should remove some of the powder coating where the transformers bolt to the chassis to allow continuity, as I did with the chassis braids.

The preamp I am using is a Forewatt that I just built. When the Forewatt is connected to a known good solid-state amp it operates perfectly with a zero noise. So I know the preamp is good. I slap swapped preamp tubes and outlets in which the amp was connected to.

Any help will be greatly appreciated,

Thanks!!!


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