[Milsurplus] Carbon Mic Subs

[J. Forster]

That circuit is nuts, even after moving the + end of the capacitor.

Before doing any design, you really neet to know:

The OC voltage from the radio mike jack.
The SC current from the radio mike jack.

The circuit should look like:


<------------o----------|
             |          |
             R1         col
             |
       |-----o--mic--base
       |+    |
      C1    R2         emit
       |     |          |
       |     |         R3
       |     |          |
<------o-----o----------|

You really do not want the current for the bias divider running through
the Mic element.

Typical design:

Voc = 5  VDC
Isc = 15 mA


You want the no-signal base voltage to be roughly (1/2 supply+0.6) or 3.1 V.

So, for 1/2 of the Isc, you want that to corrwespond to about 7.5 mA; so
R3 will be V/I = (3.1-0.6)/7.5 mA = 333 Ohms. So R1 = 330 Ohms.

With decent transistor Beta, the Base current will be perhaps 75 to 150 uA.

At a guess, pick R1 + R2 to be about 5K Ohms, so the divider current will
be about 1 mA, which is >> than the base current.

So, if you pick R1=2.2k and R2 = 3.3k

C1 can be pretty much anything...  10 to 100 uF @ 6 VDC.

YMMV,

-John

Oh, if you want reverse polarity protection, you can use a FW diode bridge
with the mike circuit across the output. Schotty diodes will give a lower
voltage drop.

=============






> There is a circuit here but also read the comments lower
> down the page
>
> <http://www.electronicecircuits.com/electronic-circuits/carbon-mic-replacement-to-magnetic-mike-converter-circuit>
>
> Dave
> VK4KDP
>
> On 17/03/2013 1:52 AM, David Stinson wrote:
>> Have any diagrams?   Thanks!
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