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Is there a method to test for the presence of DC on my AC mains supply? There is considerable hum coming from the toroidal transformers of my monoblock power amps. The hum is barely audible when they're first unmuted in the morning (they're always powered), then increases throughout the day while playing music. One amp hums louder than the other. Both are on the same dedicated line with Flavor 4 Power Cords. At night, I can hear the hum quite distinctly from 18 feet away! Next morning, it's almost gone again. I can live with a low level of hum, but not this.Do I need AC conditioning or new transformers? Or new amps? I've added vibration damping to the insides of the cases and tightened the clamp plates on the transformers without improvement. No other amps in my home (every price, every quality level) have ever hummed like these babies do. I've contacted the manufacturer, and he says it's DC that's galling me.
Follow Ups:
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Thanks, all, for your suggestions; although, I must say, some of them were not entirely sensible and prove that the moniker of this organization is aptly given. I am not about to go sticking a volt meter or anthing else, other than a plug, into a live circuit. I will not install a cumbersome and probably expensive isolation transformer.Because the manufacturer, who will remain unnamed here unless they absolutely fail to support a potentially defective product (and then, all hell will break loose!), has admitted that they use a transformer with the fewest possible turns so as to make it more efficient (although I suspect it is just as likely that fewest turns makes it less expensive for them), and they have informed me that such a configuration of minimum turns is the most susceptible to DC saturation and associated humming, I am willing to purchase for trial the CI Audio gizmo to filter whatever DC may be on my mains, according to their and your recommendations. I only wish I had known when purchasing the amplifiers that I would need to spend another $300 to get them to work properly; that is, quietly.
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"Thanks, all, for your suggestions; although, I must say, some of them were not entirely sensible and prove that the moniker of this organization is aptly given. I am not about to go sticking a volt meter or anthing else, other than a plug, into a live circuit. I will not install a cumbersome and probably expensive isolation transformer."Why did you ask here if you feel that the advice isn't sound?
Poindexter told you exactly what to do to troubleshoot the problem and now you want to cure something you don't even know you have. If your not comfortable with a meter get an electrician or tech that knows his way around test equipment.
"If your not comfortable with a meter get an electrician or tech that knows his way around test equipment. "If you read the comments in the AA archives, you would have seen the ones from real engineers (Jon Ricsh, John Curl, etc)who have measured this. They report that most hand held DVMs cannot measure this (John Curl tried three different Fluke models with mixed results).
If you really want to try and measure the DC off-set on your AC line with a cheap hand-held DVM, try the following:
Put a 100K resistor in series with a 100µF cap (this is called an integrator). This now goes in parallel with the AC line. Measure the DC voltage across the cap.
Even a few tens of mV DC off-set can make a toroid buzz, especially low priced ones.
The image here is a PA Audio DC blocker. Note the two series connected bridges, this gives four forward diode drops vs the two of the Bryston circuit. Note also that the caps in parallel with the diodes are very small, just for RF suppression. The original LC Audio filter was similar to this PA Audio one as well, only they used three forward diode drops, and only small RF caps.
link
Tom, the problem may not be the manufacture's fault... Did you try to find the electrical device causing the problem?
http://www.audiocircle.com/circles/index.php?topic=35659.0
I don't think you DC on your mains. Don't see how it would get there or if it did, how it would increase throughout the day. On the other hand, the fact that it doesn't make sense doesn't mean it isn't happening. So try using an isolating transformer. DC can't get through that.
There is a very REAL possibility U have DC riding on your AC line and it can vary in intensity throughout the day and this very well could be your flucuation of transformer hum intensity.There are products on the market that specifically address removing DC from the AC line and it just might be worth a shot for U. I have included links if U want to peruse it further.
Of course, as others have mentioned, it might be the transformers themselves. I have one vintage amp that hums like hell when all others are dead silent when powered by the same Balanced Power source
just as an example.The PS Audio unit has a 30 day money back policy and this would be a good first step in troubleshooting your problem.
Cheers,
~kenster
itself is the culprit, and/or it is mounted in such a way as
to audibly transmit vibration to the chassis. Also, the circuit may be demanding more current from the transformers than they are really
rated to deliver. I'm not suprised to hear that the manufacurer
denies cupability.Seriously, this is almost 100% certainly NOT a problem with your
AC mains.
Link
http://forums.audioholics.com/forums/archive/index.php/t-3879.htmlhttp://sound.westhost.com/xfmr.htm#7-noise
I could enable seeing the image with right-click mouse but where is the lower end of the bridge connected?
"where is the lower end of the bridge connected?"Look at the four lines going off to the left. The top one comes from the 'hot' of the IEC inlet and would go to the fuse holder, then to the power switch, and the 'hot' lead of the transformer. The return lead of the transformer would go to the lower end of the bridge. The other end of the bridge returns to the neutral of the IEC inlet. The bridge is just a standard 25A bridge, the voltage rating is not important.
You may have to register to view images, takes two minutes.
Set your voltmeter, of whatever type, to DC volts; lowest range.Put the positive probe on the neutral (large slot) of the outlet, and the negative probe in the safety/earth slot (the roundish one in the middle). Take your measurement.
If you are in a normal residential situation, there will be no (or less than a volt) difference. Where this comes into play is in large-scale (like apartment buildings) predicaments where there is a long path between neutral and earth ground, back at the supply. It's always prudent to test, though. At my single cottage, there's not a millivolt's difference.
Live long and prosper,
"Set your voltmeter, of whatever type, to DC volts; lowest range."Most meters will not work for this test. I do not recommend it.
"Where this comes into play is in large-scale (like apartment buildings) predicaments where there is a long path between neutral and earth ground, back at the supply."
Absolutely wrong.
The DC off-set is caused by asymetrical power devices such as a crock-pot, halogen floor lamp, bulb-saver, and similar devices. These have a single diode that is switched in on the 'low' setting. All houses on the same transformer will interact (in otherwords, your neighbor turning on his crock-pot will cause your toroid transformer to buzz).
From a Bryston schematic (http://www.diyaudio.com/forums/attachment.php?s=92313b72885f4f2608e2716ac182707e&postid=328821&stamp=1077146064):
Thanks for your response, but I cannot see the image you sent, and I cannot get access to the web address you sent.Can you explain what you sent, or offer it in a different format?
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