- 34 posts total
- 34 posts total
cleeds, I was not aware that DC on the AC power line could cause a transformer to hum. What would be the mechanism of that, I wonder? In any case, even if DC is the cause, the end result is a hum caused by mechanical vibration of the transformer the energy of which is transmitted to the chassis resulting in its amplification to the point of being audible. So, it is not necessary to search for an electrical grounding problem per se.
I did a very quick websearch. Apparently there are products on the market that can block DC on the AC line. One was made by PS Audio and is now discontinued, the "Humbuster III". One other is still made by Emotiva. I suppose that one transformer can be more sensitive to this phenomenon than another, because rarely does anyone with a multi-component system report that ALL his or her transformers have a mechanical hum. For example, me. Each monoblock in one of my systems has two massive power transformers, one for the input stage and the other for the output stage and tube filaments. Only one of the total of 4 transformers has a faint mechanical hum, which is not audible at my listening position. Yet both amplifiers, my phono stage, and my preamplifiers, turntables, etc, are supplied by the same dedicated AC line. This has been going on for 20 years, at least. I've never measured DC offset.
There was a dissenting opinion stemming from a thread here on Audiogon Forums. One guy claimed that DC on the AC does not cause hum but IS not a good thing, can cause the transformer to overheat.
If DC is on your AC, it is probably coming from some contaminating source in your house. You can identify the source by closing circuit breakers until you find the circuit that is producing DC on the line. The hum will vanish when that breaker is thrown. This cannot be applicable to my system, because it runs on a single dedicated AC line. Nothing else is plugged into it.
Still, you don't NEED to have DC on your AC if you have transformer hum. It can be simply a consequence of poor mechanical isolation or loose transformer laminations or loose coil, in the case of a toroid.