[ARC5] [Boatanchors] mike current

[Richard Knoppow]

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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