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I have an immediate need (for a stage play) to take the audio from the microphone of a telephone handset and feed it to a Shure M67 audio mixer, familiar to anyone who's been in radio for a while (mixer PDF). Note that the schematic of the unit is on the last page.

I took a gamble and just soldered two wires to the two pins on the microphone and fed it directly to one of the inputs on the mixer to see if it might be "good enough". It moves the needle, and I can hear myself in headphones, but the audio controls are turned all the way up and there's a lot of noise, of course. Note that there's also a "line input".

I'm hoping the only problem is an impedance mismatch and someone can figure out a resistive matching circuit for me since it's been too long since I've done that and I need it by Thursday (and I'm panicing). Can someone help me out, please?

EDIT: Note that the schematic of the unit is on the last page.

Here's a picture of the phone with a line connected to the microphone and running to the XLR connector: http://i.imgur.com/DULOG.jpg

And here's the back of the mixer that it connects to: http://i.imgur.com/1GuPj.jpg

EDIT2: I just realized I hooked up to the earpiece and not the microphone but I wouldn't think that would matter but maybe I'm wrong.

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    \$\begingroup\$ Not sure if I understand what you're doing. Can you upload a (few) picture(s) and/or a wiring diagram? \$\endgroup\$ – jippie Aug 7 '12 at 20:54
  • \$\begingroup\$ @jippie - included links to images. \$\endgroup\$ – Rob Aug 7 '12 at 21:07
  • \$\begingroup\$ You are trying to connect an unbalaced probably high inpedance output (the microphone) to a low inpedance balanced input(the mixer). Best to try and get hold of a direct injection box \$\endgroup\$ – pault Aug 8 '12 at 3:40
  • \$\begingroup\$ If this is an old style carbon microphone, as commonly used in simple / passive POTS telephone handsets, it needs a bias current to work. Can you draw a circuit diagram of the telephone's internals you're using? \$\endgroup\$ – jippie Aug 8 '12 at 7:29
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This may work okay, depending on the impedance of the mic (which unless you have the capsule datasheet there is no way of knowing)
Make sure you are using the microphone rather than the earpiece though. The earpiece impedance will be different and will not be as sensitive.

In any case, let's make sure you have the connections correct. Connect pin 2 ("hot") XLR input to one pin of the mic, and pin 3 ("cold") to the other. Connect pin 1 (cable shield) to the capsule case. Use the mic input, not the line input.
Let us know how it goes, if it's still bad you will probably need some passive/active impedance matching, which is easy enough.

XLR pinouts

The above is assuming it is a dynamic capsule. If it is electret (or carbon as Jippie mentions, although I doubt this as it looks quite a modern handset) then it needs a bias voltage to work properly.
It should be possible to tell which it is by examining the capsule and surrounding circuitry (if any) If you post a couple of close up photos (front/back/surrounding circuit) we can hopefully let you know.

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  • \$\begingroup\$ This pointed me to getting this to work. I looked at your pin numbering of the connector and, even though I knew that's what it was, I still managed to connect to 1 & 2 instead of 2 & 3. So I fixed that this morning and everything works great. \$\endgroup\$ – Rob Aug 8 '12 at 13:47

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