[Boatanchors] Carbon Mic Rejuvenation (& related)

[Richard Knoppow]

----- Original Message ----- 
From: "jmfranke" <jmfranke at cox.net>
To: "rbethman" <rbethman at comcast.net>; 
<boatanchors at mailman.qth.net>
Sent: Saturday, March 16, 2013 1:32 PM
Subject: Re: [Boatanchors] Carbon Mic Rejuvenation (& 
related)


> The carbon microphone was used with a transformer having a 
> high turns ratio as an amplifier and oscillator. In such a 
> configuration, gain was achieved. The amplifiers were used 
> as early telephone line amplifiers and as amplifiers for 
> crystal radios. General radio used carbon mike amplifiers 
> in its microphone hummers and many models of tuning fork 
> oscillators. Some texts referred to microphone amplifiers 
> as relay amplifiers.
>
> John  WA4WDL
>
    The famous General Radio 650-A Impedance Bridge uses a 
"microphone hummer" as the source of AC for impedance 
measurements. This consists of a magnetic buzzer 
mechanically coupled to a carbon microphone button. The 
resonant frequency is determined by the mass and stiffness 
of the parts and can be adjusted over a small range.  These 
little units are very reliable although the output has 
rather high distortion.  Again, the carbon microphone 
element is used as an amplifier since one can not have an 
oscillator without some gain somewhere.   The transformers 
used for many carbon microphone applications does not supply 
the gain but is only for impedance matching.  Typical carbon 
microphone elements have a steady state resistance on the 
order of 25 to 50 ohms although some higher resistance units 
can be found.  For telephone use the typical current is on 
the order of 15 ma.  Since carbon microphones are composed 
of resistive elements with rather poor connections to each 
other the noise level tends to be high.  Bell Labs worked on 
this for the single-button mics used in telephone sets and 
managed to reduce the noise but it can never be eliminated.
    In general, microphones fall into two categories: 
generators, and modulators.  Piezo-electric (crystal and 
ceramic), moving coil dynamic, ribbon dynamic, magnetic or 
controlled reluctance microphones are all generators. The 
electrical power can not exceed the actuating acoustical 
power and, usually, is a fraction of it.  Modulators include 
carbon and condenser microphones and some other types such 
as hot wire microphones.  The carbon microphone is of 
particular interest because it is an amplifier. A condenser 
microphone is a variable condenser, it can be used to 
produce a varying voltage when a charge is placed upon it or 
can be used to vary the frequency of an oscillator. Both 
methods have been applied in commercial equipment.  The 
electret microphone is a condenser microphone with a special 
dielectric that will hold a permanent charge so it does not 
need a source of bias voltage. Electret microphones have 
exceedingly high impedance so require some sort of active 
device to translate that to a usable level. The matching 
device can also be an amplifier but the microphone itself 
does not provide amplification.  In fact, condenser 
microphones, if they are to have low distortion, are not 
very efficient in coupling acoustical power.  Condenser 
microphones and hot wire microphones can be designed to have 
response down to zero, that it, they can be made to respond 
to steady state variations in pressure between the front and 
back of the diaphragm.  Hot wire microphones were used for 
laboratory purposes.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com