[ARC5] Carbon cartridge replacement circuit

[Richard Knoppow]

     A carbon microphone is an amplifier because the power out is 
greater than the power in. The controlling force is the acoustic energy 
available to the diaphragm, the diaphragm in turn converts the acoustic 
energy to mechanical energy which moves one of the contacts with the 
resistance element. This in turn varies the voltage across the resistive 
element and varies the current applied to it. The input energy, as 
absorbed from the air, is a small fraction of a watt but the output can 
be very large depending on the dissipation of the carbon element.  The 
action is rather like a vacuum tube which also acts as a variable 
resistor. The input power to the grid is very small compared to the 
output power from the plate the variation being in the effective plate 
resistance.
     One of the earliest attempts to make telephone repeaters, i.e. 
amplifiers, was done by coupling a magnetic speaker or headphone to a 
carbon microphone. These worked will enough to be used on experimental 
long distance circuits but were ultimately not very satisfactory due to 
excessive noise and distortion.
     Other types of microphones are not amplifiers but either generators 
or controllers.  Dynamic microphones, which includes moving coil, 
magnetic, variable reluctance, balanced armature, and ribbon types 
generate a voltage by moving a conductor through a magnetic field or, in 
some cases, by moving the magnetic field. The energy available to 
generate the output is only the amount that can be absorbed from the 
sound wave. Piezo electric microphones depend on the generating ability 
of crystals when deformed for their operation. Again, the power 
available to deform the crystal is only as much as can be absorbed from 
the sound wave.  Another type is the controller. This includes the 
condenser and the related electret microphone. These depend on the 
displacement of one plate of a capacitor. The output is taken either by 
inposing a bias voltage across the plates and taking the voltage 
difference as one plate moves with regard to the other.  The output is 
amplified by some sort of amplifier at the microphone. Another type uses 
the variable capacitor to change the frequency of an RF oscillator the 
FM output being detected and put out as an audio signal. Since neither 
form generates a voltage directly they are neither generators or 
amplifiers although they are used with some form of amplifier.  An 
electret is the same as a condenser mic but uses a dielectric which 
holds a permanent charge. Again, the mic must be associated with a very 
high impedance amplifier that does not absorb power from the electrodes.
    In a way the carbon microphone and condenser microphone are similar 
because both depend on an external source of power to operate but while 
the condenser or electret requires an electronic amplifier to work the 
carbon microphone stands alone as both the sound sensor and an amplifier.

On 4/27/2017 5:54 PM, Brian Clarke wrote:
> A carbon microphone is DEFINITELY NOT an amplifier. It is a variable
> resistor.
> 73 de Brian, VK2GCE.
>
> On  Friday, 28 April 2017 4:33 AM, Richard opined:
>
>     An ohm meter reading won't tell you much. Both a dynamic or carbon mic
> will have resistance of similar value.  Carbon microphones were used for
> many decades because they are fairly simple to make but mainly because a
> carbon microphone is also an amplifier so the mic has very high output and
> does not need as much amplification as other types do.
> <snip>
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL