slight hum still haunting me

[Carlp]

I sent out a question about humm or buzz recently on a SE amp (Fred Nachbaur design) and thanks to replies, was able to get rid of most of the hum and buzz. But I still have some hum on one channel. Seems like there's a dead zone on the potentiometer on one of the monos that also causes the hum. Does this sound familiar? Any ideas where to start if it isn't a bad pot?

Thanks,
Carl

 

[Jeb-D.]

Sounds like a ground loop on the input stage. What is your signal source? Do you know if it is referanced to ground or floating?

 

[Sheldon]

If the hum is related to the pot, never mind the following.

If the hum is on the channel with the opt closest to the power transformer, you might be getting some inductive coupling. If so, try a piece of steel plate between them and see if it diminishes.

Sheldon

 

[Carlp]

Thanks for the suggestions. I'll try the steel plate to see if it changes anything (note I DID mount the power and opt at 90 degrees, and the transformer layout on both sides is essentially the same, so no obvious difference in the distance between power and opts).

Input is tied to a star ground but both monoblocks are done the same way. At about 1/2 volume, the right side seems to fade in volume a little, and it has some faint scratching for a few degrees, then it's normal. That's the side that has the most hum (both sides have a little background hum, but that is bearable - it's the extra hum on the right side and the odd "dead" spot that bothers me). I seem to recall different readings on the two pots, with the right side topping out at about 90k ohms and the left side topping out just over 100k (they are 100k taper pots).

Thanks for any more thoughts.

Carl

 

[Shoog]

Is the case of the pot grounded to the chase or does it have its own ground leg to the star. My understanding is that it is bad to ground a pot to the chase.

Shoog

 

[Carlp]

Thanks, shoog. It has it's own terminal that's connected to the ground, but I believe the pot itself may be grounded at the chassis since the shaft is metal and so is the chassis (and there's no isolation between the two). But it could be the mounting shaft is isolated from the mechanics of the pot. In any case, both monoblocks are constructed the same way, yet only one has this problem.

Carl