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

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Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
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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

No I said it has a battery. This is the powe it needs. There is no power
from the mixer at all, simply a resistor from the end of the input
electrolytic to ground, though I think its not a small one.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"Chris Morriss" wrote in message
...
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

"Brian Gaff" wrote in message
...

No I said it has a battery. This is the powe it needs. There is no power
from the mixer at all, simply a resistor from the end of the input
electrolytic to ground, though I think its not a small one.


If the battery is connected between the drain of the FET and the cable inner
then the signal would be developed across a resistor connected between the
cable inner and ground. Normally I would expect that resistor to be included
in the mic, but if these mics didn't have that and the resistor in the mixer
were isolated via the bass-cut capacitor then there'd be no DC path and the
mic wouldn't work.

David.

Well it is indeed odd as the other microphones I have are in fact AC coupled
and thus do not need this as you say. I'll have to await some sighted help
to ascertain what is going on.
Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"David Looser" wrote in message
...
"Brian Gaff" wrote in message
...

No I said it has a battery. This is the powe it needs. There is no power
from the mixer at all, simply a resistor from the end of the input
electrolytic to ground, though I think its not a small one.


If the battery is connected between the drain of the FET and the cable
inner then the signal would be developed across a resistor connected
between the cable inner and ground. Normally I would expect that resistor
to be included in the mic, but if these mics didn't have that and the
resistor in the mixer were isolated via the bass-cut capacitor then
there'd be no DC path and the mic wouldn't work.

David.