neat little telephone carbon mic i just finished.

[pixeltarian]

so I just made this:
http://www.youtube.com/watch?v=4KVo3s-K6oI

first: whatdaya think? it's all just parts I had laying around. all I had to buy was a hose clamp for the grill.


second: i'm ultra paranoid that I'm going to forget to turn phantom power off when I switch mics and blow it up. does anyone know how I could utilize phantom but get it to go down to 1.2v (I think that is the optimal voltage. correct me if I'm wrong. that way I could ditch the battery too. otherwise, I'm sure there is a way to just make sure the phantom power doesn't do anything. someone mentioned a 1:1 transformer. any part recommendations would be greatly appreciated!

 

[mich]

Nice

 

[lernith]

That looks and sounds great!

You could use a 12-volt zener followed by a MCP1702-1202E/TO, which is a 1.2v regulator that wouldn't like a full 48v on its input.  Maybe you can use a neon instead of the zener, that would look cool.  Maybe noisy.
I'm guessing you actually need 1.5v, 1.2v just being what was measured on the battery?  S-817B15AMC-CWET2G then?  I'm not acquainted with a "maximizer".  What's normal?  What's home?  What are cows?
I think you could tap the phantom with two resistors like this schematic (R7 & R8):
http://www.pfarrell.com/music/shanghaimic.html
...only you would need a cap between where the resistors tee off and the rest of your mic circuitry.

Or rather than wasting that tasty 48v you could try using that entire schematic, or this one:
http://www.stefanavalos.com/microphones/smallcardioids/diymic5.jpg

Or chase around in the Mic META and find something.

Hope this helps.  Again, great job on that mic!

 

[PRR]

There isn't enough power available in P48 to burn the carbon mike.

I can't know how your other parts will react.

I don't think there's enough current, legally, in P48 to give "good" power for carbon mike (without some kinda current conversion). But heck, this isn't about good.

Find a chunky 600:600 transformer. Not a PC modem tranny.

EDIT  "1.5K" should be 3.3K.

 

[phishman13]

yeah i've really been wondering how to phantom power a telephone mic too.  I've seen the ones that that one guy makes, but haven't had much luck finding the info to make my own. 

 

[pixeltarian]

There isn't enough power available in P48 to burn the carbon mike.

I can't know how your other parts will react.

I don't think there's enough current, legally, in P48 to give "good" power for carbon mike (without some kinda current conversion). But heck, this isn't about good.

Find a chunky 600:600 transformer. Not a PC modem tranny.

really? so I can just wire it right it and use +48 to power the thing? or do they need 1.5v DC?
can you recommend a model of trafo?

 

[emrr]

My memory tells me carbon mics like 1.5V at something like 0.5 amps, traditionally.  If so, you can forget about deriving it from 48V phantom. 

I think the question of 'good' is still applicable.  You need the right current to get the noise figure and performance into the best (still crappy) range. 

 

[RuudNL]

0,5 Amps through a carbon microphone???
There is a carbon microphone in an old telephone set I have here, but I am sure the telephone company is not running 0,5 amps through the telephone line! At 0.5 Amps it would be impossible to operate the microphone from a small battery, because the lifetime would be very short. Also there is a chance that the carbon particles will 'bake' together if the current is too high.

 

[PRR]

I think 0.5 Amps is about 10 times too high. Probably a slippery decimal point in memory.

Output is almost directly proportional to current.

Telco needs a strong output to wiggle miles of wire and directly wiggle a receiver. "We" have relatively shorter wires and nearby preamps. I'm guessing that 10mA will be ample.

> the right current to get the noise figure and performance into the best (still crappy) range.

I don't have the basic data. But my guess is that both noise and output rise with current in similar ways (up until welding). So the signal/crap ratio is roughly constant. We only need to get the hiss-crap up above the hiss of our preamp. And considering the huge output at nominal Telco current, and the poor S/C ratio, nominal crap approaches a milliVolt. We have microVolt preamps. We could work at much-much-much less current.

 

[joro]

I just picked up a telephone handset I would like to turn into a carbon mic. I was thinking about using phantom to power it, but I see now it might be a little under powered. What current does the carbon element like to see. I have seen various values stated on different websites. It seems that 10-20mA might be a common answer, and 5mA seems to be the lowest current it likes to see. What would be the highest it could handle?

I have seen many different schematics with all kinds of different values of components and I'm a little unsure of the best way to design the circuit. Please pardon my complete ignorance on this subject. I have a wall-wart that is rated 9V 500mA. I can use this as the power supply for the mic, correct? What other components would I need to give the mic the correct power? Do I need a transformer (what value?) or can I just use a cap to block DC? What is the difference in using a transformer vs. cap?

I greatly appreciate any help I can get!

 

[Mike Cleaver]

Here's a circuit from the excellent Vintage Broadcast Microphones website.

 

[joro]

I found this

http://shure.custhelp.com/app/answers/detail/a_id/3999/~/external-circuit-to-power-a-carbon-microphone

and this

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

It looks like 50mA is typical operating current and it's not recommended to go over 100mA.

 

[PRR]

Just FYI: telephone systems and classic carbon mikes ARE powered by 48V. There's a lot of resistance in the CO drop relay and the miles of line. We only have to approximate that.

> phantom to power it, but I see now it might be a little under powered.

Why do you say that?

> What current does the carbon element like to see.

How much load does your truck like to see?

Too much, it burns-out. Too little, the truck won't mind, but the mike won't vibrate a telephone ear-piece loud enough. We are not running a telephone company, we have HIGH gain mike preamps. If 10mA gives 1/5th the output of 50mA, that's still far more than we need. I suspect 1mA is plenty: Dynamic gives 2mV at sound level which would yield 200mV from a telephone, we could use 1/100th the current. Telco uses 10mA-50mA, we could use 0.1mA-5mA.

> I have seen many different schematics with all kinds of different values of components

Many ways to skin cats.

> I have a wall-wart

Do you know how to fix hum/buzz? Stick with a battery for starters: a mistake won't put 4 WATTS of power in the carbon-can, it doesn't hum. When proven-good, then you can tackle the very secondary problem of running cost. (Unless you use the telephone mike all day, the cost of battery is trivial.)

> a circuit from the excellent Vintage Broadcast Microphones website

Double-button mike. Same basic principle (twice), but made for very different goals and probably different operating parameters.

> and this http://www.shure.com/............

Yup, that'll work too. It does give HIGH output, which we may not need. How big a cat-skin do we want?

 

[joro]

Thanks for the response PRR. I understand now what you are saying about the amount of current needed. I was thinking it needed more current because of how I read what you wrote earlier.

"I don't think there's enough current, legally, in P48 to give "good" power for carbon mike (without some kinda current conversion). But heck, this isn't about good."

So, I would like to try your circuit for using phantom power. I'm curious how you got the 3.3K resistor value? Also, I have a transformer from a cheap LDC mic. I'm not sure of the turns ratio, though. Is there a way I can incorporate this or should I get a 600:600?

 

[Diaos]

I'm making a Phantom powered telephone mic

I found this page:

http://www.epanorama.net/circuits/microphone_powering.html

Since a telephone is crappy sounding anyway, the simple unbalanced circuit might suffice:

  HOT (2) ---------|_____|------+
                    47 kohm        |+
                                        MIC
  COLD (3) ----+                CAPSULE
                      |                  |-
GROUND (1) --+-----------------+

 

[Whoops]

Hi PPR,
I would like to power my carbon capsules with phantom power,  and would like to try your circuit ,
could you please help me out with your scehamtic?

Is -48v the 0V reference, connected to Pin1 on the XLR?

Also what's the value for the 2 resistors after the +48V?

thank you so much

 

[Whoops]

I don't think there's enough current, legally, in P48 to give "good" power for carbon mike (without some kinda current conversion). But heck, this isn't about good.
EDIT  "1.5K" should be 3.3K.

Telco needs a strong output to wiggle miles of wire and directly wiggle a receiver. "We" have relatively shorter wires and nearby preamps. I'm guessing that 10mA will be ample.

 

[Whoops]

Anyone?