Re: electrolytic mystery

From: Walter Harley (walterh_at_cafewalterNOSPAM.com)
Date: 08/23/04 

Date: 23 Aug 2004 16:42:04 GMT

"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
news:jlhji0l7mjq8efpk17fdo29tuivmp1t9e3@4ax.com...
> Here's my guess;
>
> It is probably easier to analyse if you look at asimpler mic amp
> circuit such as
> http://www.siliconchip.com.au/cms/A_102159/article.html
> In this design the blocking caps are 10uF 16V NP.
>
> Imagine that the balanced output is plugged into a mixer input
> sourcing 48V phantom power.
>
> The 48V phantom supply in the mixer desk will have a 6k8 resistor to
> each of the XLR input pins (pins 2 & 3) thus its output impedance is
> effectively 3k4 ohms. When fed down a cable to the mic amp output this
> voltage is recombined via the two 1K resistors then via a blocking
> diode and the 12V zener regulator to ground. [...]

Thanks, but I think that analysis does not apply to the situation I'm
describing. The reason is that the circuit you chose appears to in fact be
phantom powered, which means that at DC it has a steady current draw. A
non-phantom-powered output would just have DC blocking capacitors on the
output lines, which means that no DC current would flow (after startup)
except for leakage through the caps; thus the caps see all 48V.

If the circuit you showed weren't phantom-powerable, then the 1k resistors
would not exist and there would be nothing to draw down the voltage.

Relevant Pages

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