ok i found the message about a dummy impedenace. good i remembered.
this is taken from
http://groups.yahoo.com/group/micbuildersFrom: "dayVel" <dayvel@f...>
Date: Thu Nov 11, 2004 9:28 am
Subject: Re: basic battery box - what does the resistor do?
--- In micbuilders@yahoogroups.com, "thepompousdwarf" <sanaka@j...>
wrote:
>
> DayVel said, in part:
> > 1.8Kohms isn't all that low for a balanced mic preamp. Most
nominal
> > 600 ohm mics have an actual impedance of 150-300 ohms or so.
> > and most modern mic pres have an input impedance of about 1k-5k
> ohms.
>
> He's right. My mixer is a Spirit F1, fairly decent. Their better
> E-Series mic ins are 2.4k, Yamaha tends around 3k, and a Mackie
> 1202VLZ is 1.3K.
>
> I am studying up on transistors, thanks for all the info and
guidance.
> Bob, Hans, and DayVel seem to concur that if R1 is "in the drain
lead"
> Zo would follow R1, but if it's "in the source lead" Zo doesn't
follow
> R1 and is much lower, like a regular mic.
>
> Soooo...
>
> I'm not sure the question is settled of _which_ situation is in
effect
> given the WM61 capsule and the typical battery box.
>
> Is it known for sure how the WM61's internal FET is configured?
>
>
> Here's a schematic I did from my understanding of KenBob's
suggestion
> about buffering the signal with a transistor:
>
>
http://sanaka.smugmug.com/photos/11178253-M.jpg>
> Is it correct? I thought perhaps the collector should also get it's
> voltage through the 10K resistor...?
>
> DayVel said, in part:
> > Am I correct in assuming you want to run this into a balanced mic
> pre
> > and run the whole thing off a 9v battery? I'd be happy to suggest
a
> > few ways to do this, all of them simple, like me.
>
> Yes! Though the signal can just stay unbalanced. I've been using a
> basic battery box, see circuit in my first post in this thread, and
> it's been 'ok.' I unfortunately don't have much to compare to, but I
> suspect I can up the performance I'm getting now. I'd be very
grateful
> for your suggestions :-)
>
> Pece,
> Sanaka
That circuit looks correct as is, though you may want to use a
slightly lower value for R2, say 1.5k-4.7k Ohms. Use a 47uF
electrolytic for C1 with the positive end going to the transistor. If
you want to use it balanced, connect a dummy impedance (100 ohm
resistor in series with a 47uF cap) from the cold side (pin 3) to
ground on the mic end of the cable.