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In Reply to: Testing for DC presence on AC mains posted by madisonears on January 12, 2007 at 17:46:17:
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.
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Tom, the problem may not be the manufacture's fault... Did you try to find the electrical device causing the problem?
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