Audible low level humm from Vincent st-70

If it's 120 hz hum, look to the power supply. If you're into mods larger value filter capacitors might be the ticket. I have no idea of the circuit topology of this amp, so it's hard to say much more....

Did a quick search on your amp and came up with this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1341061762&openfrom&1&4#1

I just had a similar problem on new Conrad Johnson Amp/Preamp. Here is the response I got from Spearit Sound eliminated the problem. Hope this helps you.

"I’m 99% positive you have a ground loop between the preamp and the power amp. Thank the Underwriting Laboratories for requiring equipment to have the third ground plug. This isn’t a problem with less expensive gear because they use all sorts of filters so you don’t have any hum. The problem is they tend to kill the sound of the components. cj prefers to do a clean design and solve the problem at the source, the wall outlet. The usual fix is cheater plugs. Try one on either the amp or the preamp and see if the hum goes away. If not, try a cheater on both plugs.

It’s possible that the hum is being induced. i.e. a strong emi field near the amp or preamp. It that is the case, and this is rare, it may be necessary to move the components slightly to reduce their susceptibility to emi and rf. Again, I do think the problem is a ground loop."

Yikes! The third prong (ground) on the power cord should never be a problem. If it is, this indicates that the manufacturer has not done their homework and figured out how to ground their equipment properly.

Using a cheater to look for ground loops is OK, but it should not be used on an on-going basis. There are certain types of failures that could result in fire or shock hazard. The equipment should be fixed! So if you have an amp or preamp that hums when the ground prong is plugged into the wall (and this will usually only manifest when connected to other gear) it needs to be returned to the factory and reworked.

If they tell you that is not possible (for example: "thank the UL Laboratory" or some nonsense, like that 'hum' switch on the other thread) sell it and buy something else that is properly engineered! Seriously, there is no excuse for this.

Dimmer switch?