[ARC5] Reverse engineeing the BC 456 - carbon microphones

[Michael Hanz]

On 3/19/2017 1:12 PM, J Mcvey via ARC5 wrote:
> Meanwhile, I've been doing some baseline tests on what amplitude is 
> produced by a carbon mike.
> Here's what  i found with a "whistle test"  ( whistling a tone in the 
> mike)
> power supply 18VDC and a load resistor of 300 ohms . This simulates 
> the 60ma current limit supplied by the BC456.
>
> After several tries, the best amplitude from the T-17 was about 1.12 
> volts p-p.
> Doing the same test with an old telephone carbon element yielded over 
> 3 volts p-p.

A whistle test seems a rather unrepeatable way to go about it.  Make a 
continuous loop of your voice on your computer singing a steady 2kHz 
tone frequency into the mike and you'll have a source that you can raise 
and lower in amplitude with a better degree of repetition.

> I suspect that the T-17 is supposed to be more like the telephone 
> element and that's why the modulation is so weak.

I'm not so sure.  If you look at the Cruft Laboratory report on several 
WWII carbon microphones, none of them exceeds the 1 volt zero dB output 
reference using their standard of 103.8dB SPL in a calibrated test 
fixture and the usual range of carbon currents used at the time.  The 
curves are interesting.  See 
http://aafradio.org/docs/NDRC_Interphone_Equipment-carbon_microphone_characteristics.pdf

> looks like this is going to need an amp with an emitter follower to work.

Sounds like a very practical solution.

> I'm working on the premise that 4v-pp will definitely over-drive the 
> modulator. What say you folks?

I'd say that was a good bet.  Easy to check with a sine wave generator.

73,
Mike  KC4TOS

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