[ARC5] Carbon cartridge replacement circuit

[Brian]

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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