[ARC5] Carbon cartridge replacement circuit

[Tom Lee]

I agree that defining the class of operation for a carbon mic amplifier 
isn't without its problems, not the least of which might be uselessness. 
:) But if we go with the comparison between the bias current and the 
signal current as the criterion (conduction angle of the 
control/amplifying device is what ordinarily determines amplifier 
class), then a carbon mic amplifier is class A, as current always flows, 
except in quite extreme circumstances that are far beyond the onset of 
significant distortion.

Your memory re: de Forest is spot on. Armstrong got the original patent 
for regeneration, but de Forest managed to challenge it successfully. 
Black invented electronic negative feedback in 1927, but patent offices 
in the US and UK took quite a long time to issue the patent because they 
could not understand the concept. J.C. Maxwell beat them all, having 
written the first paper on negative feedback ("On Governors") in the 
19th century, describing how speed governors for steam trains worked, 
and describing the conditions under which such systems could go unstable.

The "singing" of early phone circuits was due to two different problems. 
One was the proximity of the earpiece and microphone, the other was 
instability of the early valve/vacuum tube circuits themselves. De 
Forest worked hard to overcome the latter, while Armstrong exploited and 
patented it. De Forest then decided retroactively that working to 
eliminate the phenomenon was equivalent to inventing it, and managed to 
prevail over Armstrong after a vicious, protracted battle.

--Cheers,
Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu
650-725-3383 (public fax; no confidential information, please)

On 4/29/2017 12:21 AM, Richard Knoppow wrote:
>    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).
>
>