Odd Ground Hum Issue

[bellchilton]

Hey all,

Got a bit of a weird situation here...

I noticed one the channels on my UA 4-710D unit has a noticeable ground hum (ch 4). I tried different cables different mics, different power supplies but the hum was still there. It wasn't until I turned off the 6176 unit sitting beneath it that the hum went away. With the 6176 off, the channel is as quiet as all the other channels but as soon as I turn the unit back on, the hum returns.

My questions are - why would only one channel be affected by this? AND is there anything I can do to stop/reduce it?

BTW ch 3 on the 4-710D has a slight hum but it's nowhere near as prominent as ch 4. Channels 1 & 2 are dead quiet.

Thanks in advance.

Darren

[Ben M]

My hunch is toroidal transformer interference.. from the 6176.
It may be as simple as channels 3 and 4 on the UA 4-710D are situated directly above the power supply or other transformer of the 6176.
or
Maybe some power leakage from the 6176?

What happens to the signal when you remove the 6176 from the rack but still have its power in the same socket and powered on? i.e. moving the two units far away from each other.

It may be a job for a sheet of mu-metal?

Cleverer peoples suggestions to follow

[bellchilton]

Exactly what it was Ben.

I moved the 6176 1u below where it was and channel 4 has cleaned up considerably. There's still a very slight hum but it's barely noticeable. If I had a larger rack I'd move the 6176 even lower.

Thanks mate!

Darren

[Ben M]

Cool!

If you want to experiment more and possibly get rid of more hum..buy a sheet of mu metal and tape it to the bottom of the UA 4-710D case.
..or remove 6176 to another location.

*note: don't tape mu metal sheet to top of 6176 as tubes are aerating.

[rob]

All mains transformers radiate "hum". All audio transformers pick up "hum"

Some more, some less. In an ideal world they'd live in different worlds.