[DSK]

For ages I've been meaning to find a multimeter (I've never used one) hidden in the garage somewhere and test my AC mains voltage and the DCV level on it. I've done it just now and my wall outlet measures a rock steady 245V (I'm in Melbourne, Australia - nominal 240V/50Hz) and DCV constantly jumps around between 0 and 0.77VDC (on the 20V DC setting - I think I've measured it correctly  ).

I do have xfmr hum in a couple of components, not major but it would be nice to reduce or eliminate it. I just wanted to ask those more experienced in this area, at what DCV level xfmrs typically start to hum? Some AC conditioners address line noise (not DC on lines) but can make the system sound worse. So, I'm wondering if units aimed at reducing DC (eg. PS Audio HumBusters, CI Audio XDC-2 etc) are always or only sometimes effective, and whether (in addition to reducing DC) their insertion in a system will make it sound better, worse or no different?

[ctviggen]

PS audio also has a description of what causes hum:

http://www.psaudio.com/articles/hum.asp

DC does cause hum, but it's only one culprit.  They offer tips regarding how to minimize the non-DC culprits.

I was going to measure my 220V, to see whether I had DC on my lines, but I gave my multimeter away (to prompt me to buy a new one).  I do think it's odd that you measure a varying amount of DC.  If it were me, I'd find another meter to use to verify before spending money on a PS audio device.  As for whether these devices work, I don't know, although I do know it's not hard to block DC. 

[JoshK]

There was discussion somewhere else, I can't remember, that if you were measuring DC on an AC line with a multimeter, unless it was a hi-end fluke, most likely what you are measuring is incorrect.  This is because the meter rectifies a tiny bit of the AC when measuring DC.  The real way to measure the DC on an AC line is with a scope.  The offset (around zero) of the trace is the measurable amount of DC.

[DSK]

What are the typical ways of blocking DC?  .... a mains rated DC blocking cap? .... an isolation xfmr?

[Speedskater]

How does DC get on an AC power line in a residential area?