Not strictly stompbox related question about electret microphone capsules....

[MicFarlow77]

Hi All,

Been doing some research along these lines since it has been too cold to finish building enclosures   

What is the significance of the 1G ohm resistor in the various circuits on the page below?

http://www.mp3forkidz.com/mic/tsb-165a.html

I'm sure it has something to do with the electret capsules, but I can't wrap my head around it yet...

Thanks in advance!

Mick

[PRR]

> What is the significance of the 1G

Whole books have been written.

Start by finding out how a conventional Condenser Mike works.

The Electret form does not need the 60VDC bias, but still needs the connections.

Can you actually BUY that type capsule? I have not seen one for 25+ years. What you get today are externally biased or self-contained electret with JFET buffer and gate-leak.

[sean k]

They look pretty cool don't they? I like them and may have picked up some of the earlier electret capsules myself recently. I'm not sure as yet because they also look like ultronsonic mics but anyways.

Okay, as we all know I blunder through all this stuff, but my understanding is that there are two functions served with a resistor way up in the megaohm range, though I might add that sometimes people use them this high, as in 1G... because they have a stash of them and smaller values would still work.

The capsule is basically a capacitor with one plate free to move and the other loaded with a voltage so when the movable one moves the capacitance changes and creates a changing voltage on the other plate which is the output kinda like a volume added to a volume changes the overall volume and theres a surplus. Throw a car in a swimming pool and the excess water from the pool goes over the edges... thats your output. So you want almost pure voltage on the plate and almost 0 current... this is why the resistors are so high in value because you just want the plate charged but not storing as it were. The other reason is that because the electret is a capacitor it exhibits capacitance and if it's followed by a resistor you have a high pass filter and because the capacitance is tiny, picofarads, then you need a high ohmage resister to get lows accross the threshold... just like a piezo. More modern electrets already have the j-fet in them which you can isolate somewhat by cutting the trace between the case and cold and have a little more sway on how you bias the fet.