Hum Balance Pot question

[JoshMorr]

I just got my crack + SB back from Bottlehead after a repair and I get loud hum over everything. Only occurs when RCA's are plugged in, gets louder as I turn the volume up. Tried swapping tubes, same problem. Reading the "some tips" section of the manual it sounds like the deep hum without any buzz - the 120Hz hum. Note says to adjust the hum balance pot. What is this? I do not see reference to this anywhere else in the manual.

Thanks,

Edit: After reading further and continuing the search I found another post saying that the is only for DHT amps, which the crack is not. I likely have a ground problem or cap problem. Currently running through the ground path. Will test the caps.

 

[Paul Birkeland]

If you have no hum when nothing is plugged in, then your Crack is working properly.  If you plug in the cables from a source, and you get hum, then you have a grounding issue between your components. 

The easiest thing to try is to take a scrap of wire and press it against the top plate of the Crack and a metal screw on your source to see if that removes the ground loop.

(What is the source by the way?)

 

[JoshMorr]

Thank you for the response! I tried the piece of wire to ground the amp from the source and it did eliminate this hum. I tried this on 2 difference sources - #1 a Schiit Bifrost Dac and #2 A Project Turn Table. I do not get the same ground loop hum with other amps. What could cause the issue between 2 sources.

 

[Paul Birkeland]

We are a little more nit picky about safety standards with our equipment, and we are unwilling to eliminate the safety ground.  Other components from other manufacturers may or may not observe those standards. 

On the turntable, you surely should have a phono preamp between the turntable and Crack, and it likely has a ground post.  This ground post should be connected to the shell of the RCA jacks on the inputs/outputs of the phono preamp, and should eliminate the ground loop 

 

[JoshMorr]

Thank you again for your quick response. I am surely glad that the consumer and safety precautions are at the forefront of your mind.

This is different than how the crack previously operated, I had the crack hooked up to the same dac without ground loop issues. And the phono preamp rcas are grounded, from the phono pre amp to the turntable. Does this also need grounding from the pre amp to the crack?

 

[JoshMorr]

After more testing today - I defenitly have a ground loop problem between the amp and the source. Oddly its with every source I can find. It even happens with my phone. I have to touch connections at both ends, and dead silence.

What in the amp could be causing this problem?

 

[Deluk]

Ground loops often occur when different items in your system are plugged into different power sockets. The newly introduced Crack might be on a new supply?

 

[JoshMorr]

I just tried plugging the source and the amp into the same outlet in several different locations. Hum does not go away. Hum also occurs if source is powered on or off.

I double checked that the ground to the chassis is a good connection and roughed up the aluminum a bit, still no results.

If I understand correctly, the amp must be getting some type of feedback through the RCA connections, but when a ground is connected between the amp and the source, this difference in potential is neutralized. I am still convinced something came loose through shipping back and forth across the country, but am having trouble locating the source of the bad connection.