[ARC5] [Boatanchors] mike current

[mac]

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