I've recently built an MP566 and since a few weeks, I#m getting strong hum at 60-70hz.

The hum is also there if there's no input connected.
Without the module, there is no hum coming from the single slot housing.
The gain and input knobs do not change the volume of the hum.
the output pot changes it.
it's not there during the first minutes when I power on the device.
when directly connected to my AD converter (SSL Alphalink) this hum-noisefloor is at -60dBFS.

Any help is very appreciated!
Merci beaucoup!

Christoph