[low.pfile]

Guys,

I was recently told by a vendor that a small issue I am having with their gear could be caused by having DC on my dedicated 20 amp line.  How would I go about trying to validate this?

Is there a way to measure DC using simple tools (such as my volt meter)?

Thanks,

George

George,
I would be curious to find a way to measure how much DC is present. I have  standard house wiring (rental) and experienced a light hum/buzz too.

I am not electrically inclinded so I tried out a CI Audio XDC-2 AC Filter--with a 30-day money back guarantee. It took away about 95% of the light hum in the transformer of my modwright preamp.  Guess I have DC in the lines. the pre-amp checked out on its own from the manufacturer. Maybe you tried one of these or a similar device already?

good luck, ed

[jtwrace]

I tried them from CIA and it didn't work on my amps.  YMMV

[Mike B.]

Aussie Rod Elliott has a simple circuit to block DC but you would need to implement it at the  AC input of the equipment or power strip. I took a quick look but couldn't find it among the many papers and projects at his sight. Here is a link    http://sound.westhost.com/

[rajacat]

Use a Felix! http://www.audiocircle.com/index.php?topic=25757.0

-Roy

[Bob in St. Louis]

Interestingly enough, I've got a "OneAC" and a "PowerVar" unit.
Both raised the DC voltage by .050volts {over the .900VDC found in 'unconditioned' outlets}

For what it's worth.

Bob

Edit for clarity (twice) 

[JoshK]

Let's hope you don't have 900vdc on your line. 

[Bob in St. Louis]

  That little dot makes quite the difference doesn't it? 

Bob - dork

[BradJudy]

Interestingly enough, I've got a "OneAC" and a "PowerVar" unit.
Both raised the DC voltage by .050volts {over the .900VDC found in 'unconditioned' outlets}

Odd - an isolation transformer should stop DC from propagating (from my understanding). 

[BradJudy]

Use a Felix! http://www.audiocircle.com/index.php?topic=25757.0

The Felix is designed to filter high frequency noise, not block DC. 

[rajacat]

Thanks.

[mgalusha]

Interestingly enough, I've got a "OneAC" and a "PowerVar" unit.
Both raised the DC voltage by .050volts {over the .900VDC found in 'unconditioned' outlets}

Odd - an isolation transformer should stop DC from propagating (from my understanding). 

Yep, transformers can't pass DC but they will happily pass an asymmetric distorted waveform.

[sts9fan]

In one amp that I built I used an Antek transformer that when I first put it together was dead quiet.  I had to take the whole ting apart for some reworking and when I put it back together the transformer had a slight buzz.  I don't think its DC because I doubt I suddenly got DC on the line.  No answer here just another story.  The buzz is real small and with the top on it cannot be heard.  I am done F'n with this amp.   

[Kevin Haskins]

Aussie Rod Elliott has a simple circuit to block DC but you would need to implement it at the  AC input of the equipment or power strip. I took a quick look but couldn't find it among the many papers and projects at his sight. Here is a link    http://sound.westhost.com/

Two electrolytic caps, back to back.   - (minus)  +  (plus)   + (plus) - (minus) on one leg of the power (neutral). 

I use a pair of 16V Panasonic FM Series, 2200uF each.   They cost like $1 each from Digikey.    I also use a pair of  zener diodes (http://www.centralsemi.com/PDFs/products/CZ5333B_SERIES.PDF) back to back in parallel bypassing the caps for safety. 

The concept is simple.  The caps are virtually zero impedance @ 60Hz, and an exceedingly high impedance to DC.   I've sold a simple PCB for it for years and I've troubleshooted a couple situations on my own amplifiers when the "buzzing transformers" issues comes up.   

[jtwrace]

Aussie Rod Elliott has a simple circuit to block DC but you would need to implement it at the  AC input of the equipment or power strip. I took a quick look but couldn't find it among the many papers and projects at his sight. Here is a link    http://sound.westhost.com/

Two electrolytic caps, back to back.   - (minus)  +  (plus)   + (plus) - (minus) on one leg of the power (neutral). 

I use a pair of 16V Panasonic FM Series, 2200uF each.   They cost like $1 each from Digikey.    I also use a pair of  zener diodes (http://www.centralsemi.com/PDFs/products/CZ5333B_SERIES.PDF) back to back in parallel bypassing the caps for safety. 

The concept is simple.  The caps are virtually zero impedance @ 60Hz, and an exceedingly high impedance to DC.   I've sold a simple PCB for it for years and I've troubleshooted a couple situations on my own amplifiers when the "buzzing transformers" issues comes up.   

Can you give more details on the PCB that you have for this?

[Glen B]

Odd - an isolation transformer should stop DC from propagating (from my understanding). 

In my experience, isolation transformers themselves are subject to saturation by DC and will also buzz.  Therefore, using one to try to stop DC would be jumping from the frying pan into the fire.  Devices like light dimmers, ceiling fan speed controls, hair dryers, and fluorescent ballasts, are notorious for superimposing DC on the power line.  A DC blocking circuit like described above is the best solution.  I recently built one using 4 diodes and electrolytic caps as a favor to a member at another forum who lacked technical DIY skills.
 
http://img.villagephotos.com/p/2003-5/191557/2020464

[RPM123]

I had a buzzing or humming problem as well with just about every amp that I had owned or tried and plugged into a dedicated 20 amp line. I was also told by the amp manufacturer that it was likely DC in the line, so I tried an Audiophile APS power regenerator and voila... no more transformer hum! Unfortunately, the fan runs all of the time and it bugged me so much that I eventually sold it. (I have heard that the newer versions are supposed to run quieter.) Of course the hum returned when I purchased a PLC that did not regenerate power, but at least there was no fan noise. I then tried the Silver Circle Audio Vesuvius dcB power cord that is designed to reduce DC hum and now my amp (transformer) is totally quiet once again and without any annoying fan noise.    The power cord has a black box, probably a DC trap. It is not cheap, but I have not had any DC issues since I connected it to my RSA Jaco over 8 months ago. The company does offer a 30 or 45 trial, so you can try one to find out if it works for you. The PC itself is pretty good as well. A cheaper way to go may be the PS Audio Humbuster, if it is still offered.

Glenn

[Glen B]

The Silver Circle product, like similar others that block DC, at $950 and $900 is very expensive for what the active components are.  The Wattgate plugs and power cable probably contribute to a large percentage of the cost. The company website mentions "heatsinking" which is probably in reference to a high-current rectifier bridge, using the enclosure as a heatsink. 

[Audiovista]

How about a low pass R-C-R-C filter across the line? Then any DMM can measure resulting DC, if any. Staying safe while performing the test would be a paramount though.

Few other notes:
- Asymmetrical w-form passing through an isolation transformer would still be asymmetrical, but DC component would be zero.

- Quick look at the transformer construction may uncover sources of buzzing. A transformer does not need to be close to saturation to make noise. High input line may also cause it to buzz (say 125-130V).

[Poindexter]

I will offer a model of this problem with which I have operated for many yonks, and been challenged on as well.  I will assume that we are dealing for the moment with the standard US power standard.

There will be three terminals in the outlet; a small slot (the AC hot), a large slot (the AC neutral), and a round or 'D' shaped terminal below and between the other two (the earth ground).

The AC hot terminal delivers a nominal 117Vac sine wave which is unreferenced to any other voltage level.  The AC neutral terminal provides a 0Vac signal which is the primary reference to the connection above.
The earth ground is a safety terminal directly connected to an earthing rod sunk into the actual planet surface at the closest possible point, and with a surface area that can safely ground any current level (200 amps in a residential service) that can be provided.  The above AC neutral s connected to this  terminal at the breaker box in all cases.

Thus, there is no question of misreferencing of the AC hot signal (different voltage levels on the positive and negative swings of the signal), since there is no reference to be miscentered.  The only issue is the (possible) voltage difference between the AC neutral terminal and the earth ground terminal to which every durn thing is ultimately referenced. 

Set your meter on lowest DC volts, put one probe in the AC neutral terminal, the other in the earth ground terminal, and measure whatever (if any) difference there is.  This is your DC offset.  If any, which seems unlikely to me in a residential environment. 

There are some small complications if you live in a big apartment building or something, but otherwise I do not understand why you guyz are getting so elaborate about this (non)problem.

Comments?

Aloha,

Poinz
AudioTropic

[JakeJ]

... many yonks... 

Comments?

Aloha,

Poinz
AudioTropic

Quote edited for content.

What is a "yonk"?