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Carbon microphone adapted to condenser mic input?

V

Vince Bafetti

Jan 1, 1970
0
I'm working on a project that involves switching an amplifier input
between a condenser mic and a carbon mic (telephone handset). The
amplifier is designed for the condenser mic: it has a 5vdc bias, etc.

I think it shouldn't be too hard to adapt this amp to work with a
carbon mic...but are we talking about a purely passive mod, or do I
need to make a preamp?

Interstingly, I've found just about nothing on the web regarding
carbon microphone amplifiers. Any help in this area will be much
appreciated.

Vince Bafetti
 
R

Rich Grise

Jan 1, 1970
0
I'm working on a project that involves switching an amplifier input
between a condenser mic and a carbon mic (telephone handset). The
amplifier is designed for the condenser mic: it has a 5vdc bias, etc.

I think it shouldn't be too hard to adapt this amp to work with a
carbon mic...but are we talking about a purely passive mod, or do I
need to make a preamp?

Interstingly, I've found just about nothing on the web regarding
carbon microphone amplifiers. Any help in this area will be much
appreciated.

Vince Bafetti

Do a little searching on their specs - I can't quote carbon mic
specs off the top of my head, but remember they used to use
them in telephones, with no amplifiers. You'll probably need
an attenuator. :)
 
B

Ban

Jan 1, 1970
0
Vince said:
I'm working on a project that involves switching an amplifier input
between a condenser mic and a carbon mic (telephone handset). The
amplifier is designed for the condenser mic: it has a 5vdc bias, etc.

I think it shouldn't be too hard to adapt this amp to work with a
carbon mic...but are we talking about a purely passive mod, or do I
need to make a preamp?

Interstingly, I've found just about nothing on the web regarding
carbon microphone amplifiers. Any help in this area will be much
appreciated.

Vince Bafetti

A carbon mike has pellets of graphite being squeezed by a diaphragm. This is
equivalent to a variable resistor. The value should be in the k-ohm range.
you can measure it with the ohmmeter. You can connect it exactly as a
two-terminal electret capsule with bias voltage.. Probably the amplitude
will be bigger, so you can just use a series resistor with the capsule to
get the same level as the other mike. There will be a lot of distortion and
granular noise since the capsules are non linear and the compression of the
carbon will cause little steps and discontinueties in the transfer function,
but for telephone the quality was adequate.
 
V

Vince Bafetti

Jan 1, 1970
0
Thanks for the info, guys.

I measured the resistance of the mic in the 150K-ohm range. Does this
make sense?

Then, I connected it to the input...and got nothin. Tried with and
without series resistors, still nothing.

The mic casing has + and - markings--these things aren't polarized,
right?

anyway, I'll keep you posted on further developments.

thanks,
vince
 
J

Jim Thompson

Jan 1, 1970
0
Thanks for the info, guys.

I measured the resistance of the mic in the 150K-ohm range. Does this
make sense?

Then, I connected it to the input...and got nothin. Tried with and
without series resistors, still nothing.

The mic casing has + and - markings--these things aren't polarized,
right?

anyway, I'll keep you posted on further developments.

thanks,
vince

A carbon microphone is just that, carbon particles in a cannister,
arranged so that it's effectively a variable resistor between two
terminals.

So your amplifier needs to run some current thru the variable resistor
so that you get a variable voltage (or conversely, apply a voltage and
see a varying current).

It's been so long since I've used one in a design (try 300 baud modem
:), so someone else will need to provide specifics.

...Jim Thompson
 
T

Tim Wescott

Jan 1, 1970
0
Jim said:
A carbon microphone is just that, carbon particles in a cannister,
arranged so that it's effectively a variable resistor between two
terminals.

So your amplifier needs to run some current thru the variable resistor
so that you get a variable voltage (or conversely, apply a voltage and
see a varying current).

It's been so long since I've used one in a design (try 300 baud modem
:), so someone else will need to provide specifics.

...Jim Thompson

150K sounds way too high.

As far as I know they're not polarized -- but it wouldn't hurt to check.

The carbon granules can get packed -- on a telephone receiver you whack
the receiver _hard_ against a wood surface and it shakes them loose. On
just about anything else this would probably leave you with little bits
of microphone, so maybe you don't want to try.
 
J

Jim Thompson

Jan 1, 1970
0
Jim Thompson wrote: [snip]
A carbon microphone is just that, carbon particles in a cannister,
arranged so that it's effectively a variable resistor between two
terminals.

So your amplifier needs to run some current thru the variable resistor
so that you get a variable voltage (or conversely, apply a voltage and
see a varying current).

It's been so long since I've used one in a design (try 300 baud modem
:), so someone else will need to provide specifics.

...Jim Thompson

150K sounds way too high.

As far as I know they're not polarized -- but it wouldn't hurt to check.

The carbon granules can get packed -- on a telephone receiver you whack
the receiver _hard_ against a wood surface and it shakes them loose. On
just about anything else this would probably leave you with little bits
of microphone, so maybe you don't want to try.

I remember that trick!

They're also good for **serious** harmonic distortion. That's why, as
you may recall, my mention of driving signal into the earpiece
**inductively**, instead of acoustically into the carbon mike, of a
phone set; 300 baud "muff" modems ;-)

...Jim Thompson
 
E

Externet

Jan 1, 1970
0
Hi Vince.
I would bet a six pack you took the first step wrong assumming the
carbon.
Telephones before the sixties may have a carbon microphone. I doubt
yours has it. Shaking it you will confirm this as you would hear the
carbon granules.
What you see marked with a + - is a condenser electret microphone,
from a modern handset. The - is connected to its body, about 10mm Ø.
Then, try the conections on the RJ22 connector for the handset this
way:
Outer two terminals are microphone. Reverse connection if the
microphone does not work... Inner two are the earphone.
Miguel
 
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