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Telephone Microphone Wiring Help

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Jack.Straw

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Hello. I'm wanting to convert an old telephone to a microphone. I found this schematic online, but have a couple questions. First, there seems to be what looks like a diode going between the terminals on the speaker:

**broken link removed**

Should i leave that in place?

Also, i'm not sure the component values are correct. I tested my speaker and it has a resistance of 155 ohms. Does this schematic look correct? I want it wired so that power is turned on when the jack is plugged in. Here is my schematic so far:

**broken link removed**

Does that look right?

Thanks for your time,
-Scott
 
Is it that the "earphone" that you want to use as a mic? If so, the thing wired across it is a "limiter", which is like a bi-directional Zener with very soft knees... It is only useful to prevent blowing your ear drums, so leave it out... If you yell into the earphone, it will work like a dynamic mic, and generate a small voltage without any batteries, resistors, or capacitors.

If it is the carbon mic that you want to use, the question is WHY? They sounded like crap when they were invented, and still do. Get an electret or dynamic.
 
Mike, thanks for the response. The part in the picture is the earphone. I did wire it directly to the jack and it worked, but I thought the battery was meant to bring it up to "line voltage".

Now that you mention it, I think i am supposed to be using the carbon mic with that schematic. I'm aware that it's going to sound lo-fi, but it's just for fun. I guess i could wire up both parts of the phone to a switch and be able to toggle between the carbon mic and the earphone speaker :)

Would that schematic be good with the carbon mic?

Also, you mentioned the electret and dynamic mics. Is that what the newer phones use?
 
The only thing that will "bring-up-the-level" of a low-impedance dynamic mic is an audio step-up transformer (150Ω to high-Z), or a low-level audio amp (op-amp or LM386)

A carbon mic requires about 10mA of "bias" current flowing through it. Sound pressure waves rattle the carbon granules, varying the resistance, causing a varying voltage to appear across the mic element. To get 10mA, it will take much higher voltage than a single 1.5V battery.

Here is the idea. Simulated voice varies the resistance of the element. Because the bias resistor and the mic element form a voltage divider, and the mic part is changing with audio, there is a varying voltage developed at M. That is capacitively coupled to a load such as the input of an amplifer...

10mA will kill a 9V battery pretty quickly, so dont leave it connected. Another plus for an electret, which draws almost no bias current.
 

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Mike, thanks for your help! I was looking at your schematic. R4 is the carbon mic? Or, do i need a resistor across the mic terminals?

I do have several LM386 chips around, as i've been building little practice amps for fun. The schematic I use incorporates a JFET as well:

**broken link removed**

Should i just run the earpiece speaker directly to the input of the low-level amp, and then output to my real amp?
 
Mike, thanks for your help! I was looking at your schematic. R4 is the carbon mic? Or, do i need a resistor across the mic terminals?

R4 is my crude Spice model of the carbon mic, modeled as a 500Ω fixed part, and a 100Ω variable part. You do not need a resistor wired across it.

I do have several LM386 chips around, as i've been building little practice amps for fun. The schematic I use incorporates a JFET as well:

Should i just run the earpiece speaker directly to the input of the low-level amp, and then output to my real amp?

The posted circuit will likely have too much gain...

What are you trying to drive with the earpiece mic? It might have enough level for most things. Like I said, a 150Ω to 5kΩ audio transformer will boost the level. Or a single transistor preamp, or an op-amp, or an LM386 will work.
 
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An LM386 is a power amplifier designed to drive an 8 ohm speaker, not as a preamp for a mic and drive earphones because its hiss level is too high. Use a low noise opamp like a TL071 opamp.
 
Thanks guys. The earpiece speaker works fine wired directly to an output jack. I tried running it through a little JFET preamp that I built for my piezo powered instruments, but it just made it a little quieter. Anyway, i'm fine with it just going straight to the amp.

So i've moved on to the carbon mic. Mike, is that bias resistor (R2) really 500M? I don't think i've ever seen a resistor with a value that high. Do i need to use several resistors in a row to get there?

Thanks for your help!
-Scott
 
The 500 ohm resistor that powers the carbon mic from 9V can be 470 ohms, 510 ohms or 560 ohms.
 
Thanks guys. The earpiece speaker works fine wired directly to an output jack. I tried running it through a little JFET preamp that I built for my piezo powered instruments, but it just made it a little quieter. Anyway, i'm fine with it just going straight to the amp.

So i've moved on to the carbon mic. Mike, is that bias resistor (R2) really 500M? I don't think i've ever seen a resistor with a value that high. Do i need to use several resistors in a row to get there?

Thanks for your help!
-Scott
The "M" on the diagram is probably the name of the signal. It's not associated with the resistor even though it is near.
 
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