Re: weird(?) DC voltages at output of mic preamp

From: John Popelish (jpopelish_at_rica.net)
Date: 11/29/04 

Date: Sun, 28 Nov 2004 22:20:34 -0500

tempus fugit wrote:
>
> Hey all;
>
> I posted a message on this topic a few days ago (pops problems when preamp
> is turned on). I've done a little more experimenting, and found these
> results:
>
> 1. When I switch the mic preamp on, I get a little (1 volt or so) pop. I
> assume this is pretty normal, and not much of a problem. Someone answered
> with some great ideas for a mute circuit, which I'm sure will solve the
> problem.
>
> 2. (this is the biggy) When I turn the phantom power switch on, I get a big
> (10 - 15v) DC blast at the output. If I probe the actual output pin (i.e.
> before the DC blocking cap) this DC voltage remains for several seconds.
>
> 3. When I turn the phantom power switch off, I get another big (-15v) DC
> blast at the output. Again, it remains there for several seconds at the
> output pin of the IC I'm using, but drops away fairly quickly at the output
> jack (post DC blocking cap).
>
> 4. Both 2 and 3 only happen with a mic connected - there is virtually no DC
> pop (<1v) with no mic connected. Also, there is a brief DC pulse at the
> inputs (with a mic connected) of similar size, but it disappears quickly.
>
> There are DC blocking caps at the inputs. Where is all this DC voltage
> coming from? Any suggestions as to how to get rid of it?
>
> Thanks
You are seeing the amplified blocking capacitor charging and
discharging currents. Any time the DC voltage across a coupling cap
changes, that is a form of AC, so it passes through the capacitor with
an RC time constant of the capacitance times the resistance that the
current has to pass through. If this pop takes several seconds, a
coupling cap is way larger than needed to pass frequencies no lower
than about 20 Hz. 

-- 
John Popelish

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