[Boatanchors] Carbon Mic Rejuvenation (& related)

[J. Forster]

In fact, ALL the generating type microphones are 'reversible
electro-mechanical transducers'...  if an AC voltage is applied to them,
they will produce sound (although some may not be very loud :) )

And, before you say that ndoes not apply to condenser microphones,
remember they are the dual of earphones and speakers, which also require a
bias.

-John

===============


>
> ----- 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
>
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