[ARC5] Carbon cartridge replacement circuit

[Richard Knoppow]

    A crystal is interesting because it is a tuned circuit. Its virtue 
is that it has very low losses, i.e. very high Q. When used as you 
indicate with a source of power amplification it becomes an oscillator. 
The high Q makes it electrically very stable and its mechanical 
properties can add to that stability. So its possible (and done all the 
time) to make a crystal that will oscillate at a determined frequency 
with considerable accuracy.
     Its odd speaking of oscillators that positive feedback was 
discovered considerably earlier than negative feedback. The actual 
inventor of positive feedback is a matter of some controversy but I 
think Lee Deforest wound up with it (its late and my memory may not be 
working). Negative feedback was invented by H.S.Black, of Bell Labs, 
around 1934, maybe twenty years after the idea of positive feedback as 
the principle of an oscillator. Well, this is official attribution based 
on patents but who knows who actually had the idea first.
     The "singing" of a telephone circuit consisting of a carbon mic and 
magnetic earpiece was knows very early on and in fact was a problem with 
some early telephone circuits. Lots in the literature.
     In terms of class of operation I am not sure this applies to a 
carbon microphone is the same way it does to a tube or transistor. The 
linearity of the microphone is dependent on its mechanical properties. 
As long as the diaphragm moves approximately in relationship to the 
sound pressure acting on it the output will be linear regardless of the 
DC flowing through the carbon granules. If the current is increased the 
output will be increased but the modulation will stay linear.  I am 
speaking of an ideal device here because practical carbon microphones 
are not too linear. However, a carefully made double-button mic is 
capable of quite surprisingly good quality. The double-button 
arrangement has a push-pull effect which cancels even order distortion 
(assuming everything is balanced). Some early broadcast carbon 
microphones were made this way. I have a recording of the Tonight Show 
band recorded with a Western Electric 375 microphone sitting on the 
piano. It sounds nearly like a condenser mic with surprising bass but it 
still blasts a little on fortissimo passages.

On 4/28/2017 10:46 PM, Tom Lee wrote:

>
> Your attempt at an analogy with a crystal, whether an FT-243 or not,
> fails because it is only /almost /an oscillator. By itself, it cannot
> produce a sustained oscillation. An "almost oscillator" is as different
> from an actual oscillator in the same way as "almost dead" differs from
> actually dead (ref: The Princess Bride).


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