I have an old telephone made in the sixties (FF-OB/ZB or FF53). Now, I was planning to make an external speaker for that telephone, but I failed miserably, so I've put that aside.
Now, the handset has a headphone, which accepts line levels and I had no problems connecting it to my sound mixer. The Microphone is a different story, though. I know it works correctly, as I can use the telephone without problems. However, when I connect it to the microphone leads (XLR connector) to the sound mixer, which has microphone amplification and phantom power (48V), the microphone appears to have a very weak pickup.
I need to turn up the gain almost to maximum, to get a signal out of it, and at that levels, it acquires huge amounts of static.
The microphone is a DKO48, here's a picture:
I don't know, whether I should construct a preamp, or if I'm doing something fundamentally wrong. In case someone has experience with that sort of thing, please respond.
Best Answer
IIRC almost all microphones used in telephones were carbon. If this is a carbon microphone (shake it and you should hear a noise of the carbon powder moving around) then you should connect it in series with a speaker and a DC power supply. That's it.
The microphone basically acts like a variable resistor.
If you want to use it to record something (that is, not just for connecting to a loudspeaker), you need to connect it in series with a resistor and a power supply, like this:
To protect your equipment from DC offset, connect a small capacitor (10uF or so) in series with the "+" wire.