It's been done before, but here is my take on converting an old rotary telephone carbon microphone into a lo-fi mic. I wanted something that I could hopefully use on stage so I wanted to make it balanced. I also wanted it battery powered as opposed to phantom powered on the chance that my DIY job fried someones mixer.



I didn't really invent anything here, just combed the internet.



1. I got some great resources from groupdiy.com a www.diystompboxes.com and found a circuit I liked. I used on a circuit by PRR (post here). The only difference was that I a) added a transformer (see point #2), b) used different resistor value (see point 3) and c) added a 2k resistor after the transformer (it really helped to reduce hiss). I also wanted to try this one on groupdiy by abbey road d enfer (post here), but didn't in the end. If you have, how did it sound?



2. After that I added an audio transformer to convert my signal to a balanced single as described in RaneNote 110 (http://www.rane.com/note110.html)



3.I bread boarded the circuit to tweak resistors and get the best sound I could from the mic. I found this step crucial as the resistor valves affected how much hiss / noise floor I had.



4. I put it together and hacked a mic clip to fit my enclosure



Sound samples at the bottom from this mic. It's all voice (in one case a little banjo), with some delay/reverb.



Parts

-Old phone (rotary / vintage). Needs to have an old carbon mic for the sound (free if you're lucky, $5 to $20 flea market, used, etc)

-resistors (I used 1 x 600 ohm, 1 x 120 ohm, 1 x 2000 ohm). Something close, but really doesn't matter too much (< $1)

-capacitor (I used 10 uF) (< $1)

-600 ohm 1:1 audio transformer ($3 ebay + shipping)

-XLR jack ($6, local supply store)

-on off switch ($5)

-LED (optional) ($1)

-some sort of case ($8, I used a hammond 1591 case)



I was lucky and able to get most stuff free or use spare parts. My build cost was $20, but I only had to buy the case, transformer and XLR Jack.



Quick Update: I made a mic using the speaker part of the phone. It's here if you're interested.



