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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.

Step 1: The Schematic

Here is the final circuit I used.  Not shown on the circuit are the a) LED -- green high efficiency with a 3k or 3.5k resistor (can't remember which I used) and b) a switch to cut power when the mic isn't in use.  

The LED resistor was picked to minimize current.  The mic didn't draw much and I didn't like the resistor being 50% of the power use!

With my resistors shown and a 9V power source, the mic draws around 6 mA when idle and up to around 8 mA when singing into it.  The LED was another 2.5 mA.

R1=600 ohm (ok ok, in reality it was 2 x 270 ohm resistors, what I had in my drawer)
R2=120 ohm
R3=2k ohm.  Not necessary but for me it quieted hiss and was worth it.  Too much and if affected the sound and output.
C1=10 uF electrolytic (what I had in my drawer).

The transformer was an audio 600 ohm : 600 ohm from ebay ($3).  Mine was centre tapped but I didn't use them,
 

Step 2: Breadboard Time

For me this step was really important.  I set it all up and played with resistors until found the optimal values (very rough use of the word!).  In reality most values worked, some just better than others.  I tried R1 from 1k to 10k, and R2 to 10k to 100k (roughly). Compared to R1=600 ohm and R2=120 ohm, they had much higher hiss and didn't sound good.

I also tried R1=300 ohm, and it had higher hiss than R1=600 ohm.  So for me, R1 was roughly 600 ohm for my mic to sound it's "best".  Remember this is a lo-fi mic for effects, not for crystal clear vocals.

I didn't vary C1, but should have.  I'll always wonder if C1 as 20 or 50 uF would have been better.

Step 3: Case Work

I wanted to keep everything self contained, so I hacked the phone apart and fit it all into a Hammond 1591 case with the brass inserts for the screws.  The brass was important since the 9V battery will need replacing (every 10-15 hours of use based on my current draw).

1. I cut the original phone handle but kept the end intact.  The mic cartridge fit nicely into the handle and it made sense to re-use the threads / mouth piece.
2. I used a hole saw bit very slightly larger than the threaded portion of the original handset.  The threads first through the hole, and the mouth piece could thread on from the opposite side.
3. The original handset (now just the very end) was trimmed using a coping saw.  There wasn't much room left in the case and this is where I had to put the battery in the end
4. A circuit was made on strip board and crammed in, along with LED, switch, XLR jack and mic clip.
5. The mic clip was the female threaded part from normal mic clip with a bolt and washers.  The photo explains it better.
6. Everything is bolted on and closed up.  

Step 4: Fitting It Together

After the case was done, I got the electronics inside, closed it up and rocked out.  I'm super happy with it.   See step 1  for some sound samples using this mic.

<p>You could use a separation transformer, that is inside the scavenged phone. It is 600 Ohm and free. </p>
Hey that's a great idea thanks. I would also like to convert the phone headset speaker to a mic. I'll use the phone transformer if possible.
<p>I loved the old phones I remeber people putting them down and walking away while I would quietly listen, you could hear a whole house. Weird, but while no hi-fo to be sure they were very sensative. Or maybe my ears were just better!!!</p>
<p>I used to repair these telephones and they did the best with only 3 (Tree) volts! The battery then nearly last its shelf life and the mike becomes noisy with frying noise at higher voltages The line impedance and the exchange circuits limited the current as well as the anti side tone induction coil! I did talk to another guy about 500 miles from me also only with 3 volts on what we called a &quot;Portable telephone&quot; If used all day every day two Eveready 950 torch cells (today size D) lasted up to four years they started leaking but still had sufficient power to speak to somebody nearby and supply current if the telephone was an automatic exchange type of phone, Two alkaline penlight cells should last a very long time!! R1 R2 and the capacitor can be left out and the current sent straight through the transformer primary without any ill effect</p>
<p>Exactly how did you talk to him? Was this over the net, over wire or ham radio, perhaps into your cell phone. </p>
Hey thanks for the info! I didn't spend the time to try different voltages. I guess with the configuration I have the distortion is neat and what I was going for. Your comment makes sense, as a kid I don't remember phones sounding distorted.
<p>This is awesome! I actually have an old handset laying around. I think I'm gonna try to build this one! :)</p>
<p>I did it!!! </p>
Wow looks great! How does it sound?
<p>Sounds great! I was very surprised to how nice and lo fi its sounding! Can't wait to use it on the next record I'm working on. Thanks so much for the design. </p>
<p>Cool idea...maybe your post will drive up the cost of old, analog dial phones :-)</p><p>Anyway.....what is the purpose/advantage of this kind of mic over anything from Sure Heil etc.?</p><p>That is to say, is this a vocal mic, or for something else.?</p><p>Thank You</p>
I'm not sure how the mic compares to other mics. My reading across various forums and such is that it's very hard to get the same sound with digital effects, mostly due to extreme frequency response / jumps in the carbon mic (old microphone technology). We are hoping to perform with this mic as an effect, but definitely not use it as a primary vocal mic. It's lofi, it sounds cool, we'll work it in somehow. If you haven't listened to the audio samples it would give you an idea of it's sound.
<p>10-4</p><p>Gotcha.....!</p><p>Thanks</p>
<p>I'm more than a little obsessed with old telephones; one of the things I like about them is the sound quality. Congratulations on the beautiful sound you've achieved in those samples!</p>
<p>lol, looking at your profile picture after reading this post made me laugh.</p>
<p>Thanks! I also took the headset speaker from the phone too and it sounds great as a lofi mic, similar but zero distortion. Right now the speaker mic is unbalanced and you can hear the humm. Once I get a signal transformer, I'll finish the other mic and put it up as another instructable once done.</p>
<p>Nice project! Very tidy, think i'll have a go at it, thanks!</p>
<p>Enjoy the build! Post photos / improvements if you get a chance.</p>
<p>Nice instructable, but the first sound clip &quot;carbon mic&quot; sounds like my dog....</p>
<p>gives a new meaning to heavy breathing on the line !</p>
yup. Agreed. Sounds more out of breath than my dog does after running! If I sounded like that, call the morgue, I am probably about dead :P
<p>Very Cool. A chrome art-deco version would be good for stage use ! T</p><p>Thanks -Lee</p>
Wow this sounds awesome! I'm a performer and always looking for something different in the studio or stage. Could you possibly build me one of these? Thanks and great job!
<p>Yeah, we are borrowing my buddies PA setup and didn't want to break it.... I found quite a few phantom powered circuits on groupdiy.com. I'm sure you'll find a resource there if interested.</p>
<p>That looks fantastic - really clean :)</p>
Thanks very much, I'm happy for sure.

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