Carbon Mic Build

[pittsburgh]

  I'm trying to repurpose a carbon mic from an old rotary telephone set. I found this schematic and am trying to find a transformer that will work but I can't find the exact values on Mouser. Is there something that will work in place of a 400 Ohm/ 40,000 Ohm transformer?

 

[emrr]

Sure, secondary just needs to be appropriate for the preamp you are using.  A 200:200 or 600:600 should work.  Note that schematic is for a double button carbon mic.  Offhand I don't recall if that's what you find in a handset. 

 

[pittsburgh]

Oh ok, does a double button carbon mic function like a figure of 8?

 

[RuudNL]

I think the whole idea behind it is not to have any DC current running through the transformer. But since carbon capsules vary a lot, it is a bit tricky...
I would try 6 volts (or so) with a resistor in series with the carbon capsule. Use a capacitor to separate the DC and forget the transformer. Or use a transformer after the capacitor, if you want a balanced output.

 

[pittsburgh]

Ok, I'll try it without a transformer then. What kind of Capacitor should I use? 6VDC? I found this on Shure's website.

http://www.shure.com/idc/groups/tech_pubs/@global_managed/documents/webcontent/us_pro_104c_ug.pdf

 

[joro]

Use a 9 volt battery for the supply and a 10K pot in series with it and you can adjust the resistance to give you whatever current you are looking for. (You might want to also put a 150 ohm resistor in series along with the pot so that you don't put the resistance at 0 ohms and send too much current to the mic.) I'd leave it that way, but you could find what resistance works for you and replace it with a resistor of that value. From what I have read, I think typical operating currents for carbon mic's may have been around 20mA or so. Instructions often say (as it does with the Shure mic) that the current to carbon mic's can be up to 100mA without damage. I have found that when the current is high like 50-100mA that the level is quite hot to go into a mic pre. I have also found that the sound does not really change weather you are operating at 50mA or .5mA, so you might as well choose a lower operating current that works well with your mic pre gain setup.

The capacitor I believe I used was a polypropylene 10uF.

Think of supplying the power first. Connect the battery to the resistor and the carbon mic terminals to get the current and then connect the output cable to the capacitor and the mic terminals.

There is some more info in this thread.
http://www.groupdiy.com/index.php?topic=43727.msg546124#msg546124

Hope this helps.