I'm trying to find out how to completely get rid of the AC hum. The device I'm trying to "fix" is an instrument preamp with two 5670 tubes. It works fine and it's very quiet but there's that slight HUM even with the volumes down, with everything on zero etc. It's definitely not from the sound path and I'm trying to find out where it comes from. It's very, very deliberate - you do not pay attention to it untill you turn the power off and notice there's no background humming. I've got many other devices that make the same type of noise - for example, my AIWA stereo amp is also humming with everything down.
There's no change in the hum level when you adjust the volume, it's a constant noise.
I understand it should come from the device's ground, a ground loop maybe? Or is this some other known bug?
I'm not sure the schem is needed to answer this, but still here's the schematics and the layout.
There's no change in the hum level when you adjust the volume, it's a constant noise.
I understand it should come from the device's ground, a ground loop maybe? Or is this some other known bug?
I'm not sure the schem is needed to answer this, but still here's the schematics and the layout.

