[LAU] DIY messurement-mic

[fons at kokkinizita.net]

On Thu, Dec 23, 2010 at 01:47:59AM -0200, Fabio wrote:

> but since I have a preamp I'm asking myself if it wouldn't be
> sufficiant to use a t-power circuit. what would be the disatvantage?
> sure, it would come with less gain to the pre, but would that mean
> more/too-much noise?

> (circtuit diagram)

Assuming the mic needs 9V/2k2 this will work, but it's not really
ideal. 

This uses the FET in the mic as a gain stage. The output Z is then
the drain load resistor (2k2), and since you balance the Z of the
passive line, the total impedance seen by your preamp is more than
4K, which will make it noisy. 

The alternative is to include active circuitry, or use the FET in
the capsule as a source follower. The latter requires some surgical
work on the capsule itself but it's perfectly possible to do it.
Directions for it are out on the web. 

Ciao,

-- 
FA

There are three of them, and Alleline.