[ARC5] Carbon mic replacements

[Richard Knoppow]

    I am late adding to this thread.  Electret mics are related to 
condenser microphones. They carry a permanent charge so do not need a DC 
source but still need an impedance matching amplifier.  Carbon 
microphones are a peculiar case because they are not generators as are 
electromagnetic and piezoelectric microphones. The carbon microphone 
controls a DC source as a variable resistor. It needs a DC source. 
Carbon microphones are actually amplifiers and were popular because they 
often did not need additional amplification, as in telephone service.
    Old carbon microphones often become inactive or noisy because the 
carbon particles have absorbed moisture and stick together. Sometimes 
they can be reactivated by baking them. The temperature must be low 
enough not to damage the mounting but something like 150F is probably 
safe.  It may take baking for a few days to get the carbon back in 
working condition.  This does not always work but is worth trying.  
Carbon microphones are also subject to burning from excessive current. 
This welds the carbon particles together and is not in general fixable. 
Sometimes just banging the capsule several times will get them going again.
     Some carbon mics have a very long life, some don't, depends on the 
design. The most thorough design was the microphone used in the WE 500 
series telephone.  It was designed to have good voice response, long 
life, insensitivity to position and high output. Some early carbon mics 
were designed with two carbon buttons, one on each side of the 
diaphragm. When carefully adjusted this cancels the inherent distortion 
of the microphone. Western Electric made some quite high fidelity carbon 
mics which were used in early broadcasting and sound reinforcement but 
never for sound recording. They sound very good when they work right but 
were still noisy and not very reliable.

On 3/25/2016 9:25 AM, Glen Zook via ARC5 wrote:
> The old Motorola MK-301 microphone are of the rounded military style 
> and are actually called "military style" on the Motorola schematics.
>
>
>
> Glen, K9STH
>
> Website: http://k9sth.net
>
>
> ------------------------------------------------------------------------
> *From:* Dennis DuValll via ARC5 <arc5 at mailman.qth.net>
> *To:* W9RAN at oneradio.net
> *Cc:* arc5 at mailman.qth.net
> *Sent:* Friday, March 25, 2016 10:55 AM
> *Subject:* Re: [ARC5] Carbon mic replacements
>
> Yes, the Motorola and other microphones mentioned work very well.  But 
> they don’t look military….  :^(
>
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20160325/2e4725ff/attachment-0001.html>