[jtwrace]

In my previous post I've tried the Jensens and they haven't done what I was hoping. 

So the question is how can I check to see if there is DC on my amp line? 

[HAL]

Which one, DC on the AC power line or DC on the I/O signal lines?

[roscoeiii]

For DC on the AC line, check out something like the PS Audio Humbuster or CI Audio XDC-2. Both are about the same price new and both have a trial period of 30 days or so. I had some nasty toroid hum on my amp that the Humbuster cleared right up. As I understand it (and I hardly consider myself an expert), DC on the AC line manifests more in hum coming from the chassis, not through the speakers, which was the problem I was faced with.

Hope this helps. Good luck. I've got a new hum/hiss problem with a tube amp in another room, which is coming through the speakers not at the chassis. "Cheater plug" fix didn't work, now need to figure out what step two should be...

[Speedskater]

DC on the power line?  I don't recall any of the authorities on EMI/RFI (like Henry W. Ott, Jim Brown and Bill Whitlock) ever writing about DC on the power line.  In any case, measuring it will be tricky. AC power signal asymmetry is one thing and true DC current is another.

[HAL]

Had the issue here due to power meter connection problems due to corrosion.  The toroidal transformers were mechanically vibrating causing chassis hum.  Best explanation I could find was DC offset causing the toroidal core to saturate and hum.  Once the power company came and checked the power meter, found and cleaned the corrosion off the contacts.  Problem stopped after that.   

I measured the AC line distortion with a Fluke 43B power line analyzer and the distortion before contact cleaning was in the 6% THD range.  After cleaning in the 1-2% THD range.  The meter only measures the odd harmonics, so no direct measurement of the DC component.   

[ctviggen]

DC causes transformer hum.  See, for instance:

http://www.psaudio.com/ps/knowledge/why-do-transformers-hum

Theoretically, transformers should not pass DC.   They will, however, hum because of this.  Is the hum a physical hum (that is, you put your ear next to your amp and there's a hum in the amp)?

[jtwrace]

Thanks for the replies.  Yes, there is some transformer hum.  The speakers have a hiss / slight hum.  It's not a pin 1 issue or ground loop though...

That's why I'm asking this question.  I'm thinking it's DC on the line.   

[Speedskater]

So where is the DC coming from?  I don't think that any modern home appliances create DC and if they did it would be intermittent (like a hot water tank turning on/off). I look out by back window and see a power company transformer that reduces 8000V to 240V no DC is going to make it through that baby. Your AC power wiring system has a resistance of well under 1 Ohm to DC (that's pretty much a dead short).  So -I don't know on DC could get into your power system (unless you live over a factory) and whatever is making that DC will be unhappy feeding a dead short.

[roscoeiii]

Well, see HAL's post above for one way DC might enter. I am sure there are others. It has been a while since I looked into this, but I believe some people have said that the replacement of the power company's transformers eliminated the DC from the line. For those of us with this problem Humbusters and the XDC-2s are a blessing.

[jtwrace]

Humbusters and the XDC-2s are a blessing.

Is one better then the other?

[roscoeiii]

I went with the best deal I could get, but I think that Channel Islands offers two versions, one of which is better suited for amps. I forget the details, but if I was going to have to pay around the same price for them I thought at the time that I'd go with CI. Worth giving them a call to discuss. For both PS Audio and CI, since the issue may not be DC on the line for some buyers, it seems fairly common for there to be demo units available.

[NagysAudio]

Hiss and slight hum is perfectly normal in most systems. So is slight transformer hum/noise. What exactly are you trying to accomplish here? What does your system consist of? Just as Speedskater said, checking DC on AC line is very complicated and not very likely to begin with.

Do you have cable TV in the same room??

[Speedskater]

It well may have stopped HAL's problem. But HAL's problem wasn't DC.

[jtwrace]

I called my power company and explained the issue.  They're coming out to record the line and see if they see anything.  They were extremely helpful and interested.

**fingers crossed**

[roscoeiii]

Kevin,

Since we thus far lack an expert on DC on the line in this circle, perhaps you should look at other threads about DC on the line for an explanation of where the DC comes from. There are a number of threads on this subject. Or you could give PS Audio or CI Audio a call and find out from them. I was mainly concerned that the problem get fixed, rather than where the problem came from. Maybe if I wasn't renting I'd be more interested in the DC's origin...

[Speedskater]

I think that HAL's problem was a "lost neutral" (or at least a high resistance neutral).  That means, all the current going through any power device has to find it's own way back to the neighborhood power transformer.  The noise may chose the hi-fi as the best path. More correctly 2 hot (120V) wires and 1 neutral wire come into your house.  If the neutral is poor the two hot wires won't divide evenly 120/120, so moment by moment the 120V lines will drift all over the place.

[Speedskater]

I think that HAL's problem was a "lost neutral" (or at least a high resistance neutral).  That means, all the current going through any power device has to find it's own way back to the neighborhood power transformer.  The noise may chose the hi-fi as the best path. More correctly 2 hot (120V) wires and 1 neutral wire come into your house.  If the neutral is poor the two hot wires won't divide evenly 120/120, so moment by moment the 120V lines will drift all over the place.

I don't think that the DC problem exists! 
I have never seen a reasonable explanation for how DC could get into residential AC power. I think that the problem is elsewhere and the problem gets fixed by accident.

[roscoeiii]

Well, as I suggested before, check with PS Audio or CI Audio. Their products work. As they are designed to eliminate DC from the power line, then it would seem reasonable that DC is the problem. Sure there could be a lurking variable that the devices designed to eliminate DC also fix but my research on the problem when I was trying to diagnose my chassis hum all pointed to DC as the likely source. Look at other forums and other circles here. diyaudio might be a more technically oriented place to look.

Regardless. For the purposes of listening, all I need to know is "Device X fixes my problem with chassis hum." Humbuster did that for me and others.