[Milsurplus] [Boatanchors] [ARC5] mike current

C.Whitaker whitaker at pa.net
Thu Sep 8 09:20:14 EDT 2011


de WB2CPN
Two things:
When I was USAF in Germany in 1946 there were two types
of mics I encountered.  The condenser mic, that had an amplifier
section, a tube about 12 inches long, at the top of the stand.
And, carbon mics which were made of a piece of ceramic about
3 by 8 inches, hollowed out to make a space for the carbon to
be poured in.  Electrodes at the back of the carbon, and over
most, but not all, of the front side.  The front side was then covered
by a very thin sheet of something that let some carbon show through.
Next, The old F1 carbon button could have its audio quality improved
by removing some of the shims from the inside of the button.  That
took some pressure off the carbon.  The best systems rotated the
polarity of the battery from time to time to keep the granules from
sticking together.  OK, Trivia.
Hue, is right, it ain't how much the granules get squeezed, it's how
many get squeezed.
Back in High School we could make a mic by laying the end of a
lead pencil at an angle on the top of a soup can, then yelling into it.
73  Clete




On 9/7/2011 10:17 PM, mac wrote:
> Richard,
>
> Bell System Technical Journal, that makes sense.  Seem to be available
> on line.
>
> Thanks.
>
> Dennis D.  W7QHO
> Glendale, CA
>
> ***********
> On Sep 7, 2011, at 3:49 PM, Richard Knoppow wrote:
>
>> ----- Original Message ----- From: "mac"<w7qho at aol.com>
>> To: "ARC-5 List"<arc5 at mailman.qth.net>; "Boat Anchors List"<boatanchors at mailman.qth.net
>>> ;<milsurplus at mailman.qth.net>;<TCS_Radios at yahoogroups.com>
>> Sent: Wednesday, September 07, 2011 12:55 PM
>> Subject: Re: [Boatanchors] [ARC5] mike current
>>
>>
>>> These things have been around for over 100 years now and there gotta
>>> be a large body of science and art out there in the technium on the
>>> composition, manufacture, etc. of just the carbon granules not to
>>> mention the microphone elements themselves.  I've observed a wide
>>> variation in the external characteristics (at least) of the elements
>>> found just in the venerable T-17 not to mention the many thousands of
>>> telephone, broadcast and other microphone elements that proceeded
>>> (and
>>> followed) it.   Browsed around on Google a bit and didn't find too
>>> much except for a reference to a 1934 paper that seemed to say that
>>> the interaction between granules in response to sound pressure is a
>>> simple make-break action, i.e., the element resistance overall
>>> decreases with increasing pressure because more granules come into
>>> contact with each other, not because increased pressure between
>>> individual granules lowers the resistance of the individual contacts.
>>> Anyone have a good reference(s) in this area?
>>>
>>> Dennis D.  W7QHO
>>> Glendale, CA
>>      Carbon as a variable resistance element for microphones goes
>> back considerably more than a century. One of the earliest attempts
>> was the Reis carbon block microphone, invented around the early
>> 1870s but it was not sucessful. Among the first loose granual
>> microphones was the one invented by Thomas Edison for the
>> competitors of the Bell System but Bell came up with a better one
>> invented by Blake and perfected by Emile Berliner both around 1881.
>> The Berliner design was the one used in telephones for many decades.
>>      The idea is that the pressure of the diaphragm on the loosely
>> packed carbon granules increases or decreases the number making
>> contact and thus results in a resistor whose variation in resistance
>> follows the energy in the acoustic wave actuating the diaphragm. The
>> important property of the carbon microphone is that it is an
>> _amplifier_. That is why it was used in the telephone system nearly
>> exclusively until the invention of low power solid state amplifiers
>> and electret microphones along around the 1970s.
>>     The carbon microphones used in telephone service and in
>> communications are known as single button types. That is, there is a
>> single container of carbon granuals acted upon by the diaphragm.
>> Such microphones can be made to have high efficiency but are not
>> very high in fidelity. Another form called the double-button carbon
>> microphone, has a carbon container on each side of a diaphragm. The
>> push-pull action tends to cancel even harmonic distortion resulting
>> in higher quality. Usually these microphones were made with
>> diaphragms which were stretched and damped so that the resonance was
>> above the range of interest and was well controlled. The most
>> sophisticated versions used damping grooves similar to those used
>> later for condenser microphones. The Western Electric type 387-W is
>> an example. These microphones are finicky about balance of current
>> in the two sides and are generally fairly position sensitive.
>>     All carbon microphones suffer from high noise. The reason is the
>> poor contact between granuals. The cause of the noise and methods of
>> reducing it were studied extensively especially by Bell Labs because
>> this type of microphone was so important to the telephone system.
>>     Probably the most advanced design of single-button carbon
>> microphone is the one used in the Western Electric 500 type
>> telephone. I believe is is described in detail in the Bell System
>> Technical Journal but I don't have the specific citation. Earlier
>> telephone microphones were described in the BSTJ Vol XI, No.2, p.245
>> (Jones and Inglis), and Vol X, No.1, P.46 (Jones) which details the
>> WE double-button microphone.
>>     In general, the current through the buttons should be the minimum
>> possible. Most Bell System phones had about 4.5 volts across the
>> microphone. They will operate with much less. Reducing the current
>> minimises burning of the edges of the granuals where they contact
>> each other. Excessive current results in internal arcing and a
>> resulting great increase in noise of a sort described as "frying".
>>     One of the important features of the last WE microphones was the
>> attempt to minimise the change in characteristics due to position.
>> This can have a very great effect on earlier microphones. Also, the
>> carbon granuals tend to "pack". This can be  due to moisture
>> absorption or welding due to high current or simply to settling.
>> Often the output of a microphone can be considerably increased by
>> shaking it.
>>     For common communications type carbon microphones a single D cell
>> provides enough voltage and current. Carbon microphones are often
>> used with a transformer such that the exciting current flows through
>> the primary winding. Usually, some means of adjusting the current is
>> provided, often no more than a rheostat in series with the battery.
>>     The output of these microphones should be quite high but often
>> old ones have carbon which has become degraded due to moisture or
>> other causes. Sometimes baking them in a slow oven (130F) for
>> several hours will bring them back. The sensitivity and noise of the
>> microphone is dependant on the condition of the surfaces of the
>> carbon granuals and sometimes nothing short of replacing them will
>> fix a poor performing microphone. Broadcast type double-button
>> microphones can sometimes be restored with carbon from telephone
>> capsules. They usually have a seal around the carbon formed of a
>> "book" of fine tissue paper which allows free movement of the
>> diaphragm.
>>     BTW, double-button carbon microphones were supplied as part of
>> some early broadcast equipment and were widely used for early public
>> address systems but were _never_ used for sound recording. The early
>> condenser microphone of E.C. Wente preceded the development of
>> electrical recording by a few years and was generally employed for
>> that purpose.
>>     FWIW, the technical literature abounds with papers on carbon
>> microphones. See beside the BSTJ, the Journal of the Acoustical
>> Society of America and other sources.
>>
>>
>> --
>> Richard Knoppow
>> Los Angeles
>> WB6KBL
>> dickburk at ix.netcom.com
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