[ARC5] Carbon cartridge replacement circuit

[Tom Lee]

Brian, if you are truly interested in learning, please pay attention to 
this supplement to what I wrote earlier:

A persistent oscillation is only possible if there is power gain (I am 
precluding superconducting circuits). Period. If you have a sustained 
oscillation, then there must be power gain, for there is always loss in 
ordinary circuits. A continuous output means that this loss has been 
overcome. This in turn implies that you have power gain (not voltage 
gain or current gain suffices; you need power gain, which is why a 
simple transformer cannot be made to oscillate despite its capability to 
step up voltages or currents).

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).

When you replaced the carbon mic with the potentiometer, you actually 
came close to getting it right, but then drifted off the rails. The 
current through the varying potentiometer is a sum of dc and ac. The ac 
power delivered to the load is proportional to the square of the ac 
current and the product of the load resistance into which that current 
flows. That output power can be orders of magnitude greater than the 
acoustic power incident on the original carbon mic, or the mechanical 
power expended in rotating the potentiometer shaft.

This amplifier is class A for the same reason that a valve or transistor 
amplifier can be class A: The ac signal current never exceeds the dc 
current, so the latter always flows. It can certainly operate class B or 
C (just shout loudly enough), but distortion necessarily results. The 
normal operating mode of a carbon mic is therefore class A.

--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 4/28/2017 8:59 PM, Brian wrote:
> Let me see if I have understood all the posturing.
> Clearly, some of you didn’t get my water tap or hydro-electricity 
> generation analogies. So, let’s try an electrical analogy.
> Let’s say a carbon mic element has 200 Ohm static resistance; and, as 
> a result of incoming, varying sound pressure waves, that resistance 
> changes by plus and minus 50 Ohm. So, the mic element resistance 
> varies between 150 and 250 Ohm. Now let’s imagine that to get an 
> electrical analog of those resistance changes, we put a 1 k Ohm 
> resistor in series to simulate the mic transformer primary. To get, 
> say 30 mA to flow as the static current, we need a stable DC source of 
> 36 Vdc. Tethered telephones were supplied with off-hook pressure of 48 
> Vdc from the exchange; by the end of your telephone line, the Voltage 
> in your telephone desktop set would have been between 36 and 40 Vdc. 
> So, we’re in the right ball park. In order to sense the electrical 
> analog of those sounds waves, we could put a CRO across the 1 K Ohm 
> resistor. A DC Voltmeter won’t respond fast enough.
> Now, let’s replace the mic element by a 250 Ohm pot, and put a big 
> knob on the shaft so it’s easy to vary the pot. better still, let’s 
> attach a light platter and spring load it centre the pot shaft at 200 
> Ohm. If I waggle that big knob or shout at the light platter, I have 
> now effectively simulated the mic element. Please tell me where the 
> Class A amplifier has gotten to.
> Someone will pick up on the water tap analogy and think about water 
> reticulation in the street – and because the water is flowing 
> continuously, that is also a Class A amplifier.
> The story about getting howl-around between a dynamic earpiece and a 
> carbon mic all connected in series with a battery, as a demonstration 
> of feedback does not stand up in the electrical domain – it is 
> acoustic, ie, mechanical feedback, modulated by 3D radiation and 
> reception patterns and the varying impedance of the air path between 
> the mic element and the earpiece.
> Next, someone will claim that a crystal in say an FT-243 holder is an 
> amplifier because it can be made to oscillate – but there’s no feedback.
> In general, two-lead devices cannot be considered as amplifiers. Come 
> back Nyquist, Terman and Weiner – all is forgiven.
> 73 de Brian, VK2GCE.
> *On* Saturday, April 29, 2017 3:14 AM, Warne almost got sucked in:
> >Just as with an ordinary valve, a carbon microphone amplifies by
> mediating the flow of power from a source to a load.<
>
> First time I heard someone assert that a carbon mike was an amplifier, 
> I thought "Whaaaaat???" Until I thought of the mike button as the 
> analog of a grid controlling a current flow.  Of course, we could then 
> philosophize over whether is it absolutely correct to call a tube an 
> amplifier, or is it only a key element to an amplifier circuit?  :-)
>
> Wayne
> WB4OGM
>
>
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