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In Reply to: Telephone Mic Element for Vocal Mic posted by Randy Thatcher on September 2, 2004 at 15:19:20:
Mics from old POTS telephone sets are carbon mics. They don't produce current by themselves, but, inserted in serial with a DC source (the 48V used by the phone system), modulate the current flowing from the DC source. In other words, they act as variable resistors. You get the signal via a condenser connected to a 600 ohm resistor between the bottom of the carbon mic and the DC return ground.
So what? Without the DC source in serial, you'll get no signal at all.
(1) You may build the set up with the DC source, the 600ohm resistor and the cap. Maybe you can get the DC source from a phantom power, but (1a) make sure it can flow 100mA and (1b) decouple it with 10uF right at the mic's.
(2) however the carbon mic is very noisy. I doubt that you can use it connected to a P.A and get more than 30 or 40 dB of max S/N ratio
(3) the output condenser would be nice at 1uF, 63V. No impedance concern on a regular mic input. Be careful to settle the gain low, since the signal is likely to be much stronger than electrodynamic mics
(4) Why not cut off at 18dB/oct before 330hz and over 3,3KHz with a parametric EQ on a regular mic? An old "tired" electrodynamic mic will be fine for the job.
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Follow Ups
- Re: Telephone Mic Element for Vocal Mic - Jacques 08:27:07 09/03/04 (0)
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