In message , Brian Gaff
writes
On a similar note to the lat thread, ie strange microphone happenings..
Last night we tested some tie clip mics given to us as surplus to
requirements. I'm sure you know the sort, they have a little battery in them
and a slide switch on the side. Unbalanced with 3.5mm plugs on the end of a
long wire.
We have a mixer with a home made bass cut switch in the input. this simply
puts a capacitor in series with the mic so the bass rolls off. Works fine
with most of the other mica used, but when you switch it in here, the mics
stop working altogether, as if they need that dc path to the input of the
mixer. Now as far as I'm aware, inside the mixer is a resistor just to stop
the input just being high impedance, but I've never come across microphones
that can be affected by this not being there, so to speak. Anyone any ideas.
Have I missed something obvious?

Brian


They will be electret mics that need a bias supply. I guess your mixer
has a resistor (1k to 10k) that is connected between the audio feed from
the mic and a +5V supply. This is used to power the (open-drain) output
of an N-channnel MOSFET. This MOSFET is used as a buffer stage to
provide the very high impedance load that the electret capsule requires.

There's plenty of webpages showing this if you search a bit.
--
Chris Morriss