Tweakers' Asylum

Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

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Re: Not trying to start a flame but....

Posted by djk on January 15, 2007 at 04:00:50:
"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.