What is DC offset on the AC power line?

My SS amp stopped humming (note this is a physical transformer hum not audible in the speakers) on its own even before the Humbuster was added…DC gremlins taking a break? Some amp designer do put DC filtering of some sort in their stuff as my addled brain seems to remember reading about that in reviews…but hey…they certainly all should do some damn thing. I use a couple of "after market" power supplies for my DAC and phono pre that claim to have that feature.

NP nails it. It’s a little unusual to have DC on the AC line but it does happen, and as he points out, it happens from a device that draws current only when the voltage is going one direction, but not the other.

Dimmers, but also some digital power supplies may cause this. Any oscilloscope will show this problem, Usually you won’t just see a shift int he AC signal, but you’ll also see some nasty noise along with it.

The traditional fix is an isolation transformer, which are usually not toroids and present an AC at the output with little or no DC. DC may occur in cheap isolation transformers as a result of uneven winding if they are center tapped at the secondaries. One approach that is sometimes cheapest is to put the offending device itself on an isolation transformer.

Other fixes are to move to another circuit, or especially to a circuit that’s on a different phase than the one causing the problem. More expensive solutions MIGHT be to improve the wiring to the service transformer, allowing the device (dimmer) that is causing the problem to draw more current without a voltage sag.  If the house is old, uses fuses, etc. it's not a surprising thing to have, there may be other reasons to seek a wiring upgrade from a qualified professional.

Best,

Erik

DC off set is a much bigger problem than most audiophile's realize ... the humming transformers are merely the symptom ... the real deliberating disease is the loss of transformer efficiency 

DC off set is caused by the half wave bridge rectifiers used in all switching power supplies that draw current in uneven pulses .. this drives the sine wave off it's proper zero degree crossing point when the phase turns from positive to negative in the cycle

The humming in the transformers is caused by the transformer drawing an extra cycle and the plates snapping  back ... but the real issue is the loss of efficiency your equipment experiences  

Just about every appliance in your home including that flat screen TV ... cable box all your computer devices and some of your audio equipment uses switching power supplies and cause this problems with the harmonics they create  

The issue is that the loss of efficiency is not very audible like noisy RF can be ... your equipment just doesn't play with the authority or robustness it could or should  

I have a Emotiva CMX2... which also has some  common mode filtering ... on my wife's plasma  

I never A/B-ed it just bought it and installed it when I bought her a new 60 plasma .. along with the Oyaide R1 outlet and matching carbon fiber plate .... happy wife happy life 

I also use two PS Audio Humbuster III which I wouldn't be without in my 2 channel set up ... they had a profound impact  

Just today someone on Audio Circle put a used  Emotiva  CMX2 up for sale for $70 

I would have posted the link or sent it to the OP but the last time I tried to be helpful I was blocked  

Enter your text ...

My amp activates its protective system, and shuts down if it sees DC offset.

thanks to all who have responded.

davehrab, so here is a question perhaps you can provide an answer for. Your explanation is very good. Suppose that one does NOT have any electronic device with half wave bridge rectifiers and therefore would not have a DC offset problem. What is the disadvantage (if any) in that case if one were to use a DC offset filter (i.e. use the filter in a scenario where there is no DC offset to correct)?