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>>>crystal microphones

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Someone Electro

New Member
If in a circuit can i replace a crystal microphone whith a carbon tipe one?

If not whats the difrene in the output?
 

Sebi

Active Member
Not impossible. The crystal mic. give 50...100mV signal, but the carbon type just increase or decrease resistance, depending from voice-pressure.You need a resistor for biasing, and a serial condenser to amp.
 

panic mode

Well-Known Member
You cannot make direct replacement but with few more components sure you can. Take a look at following example. It's just a small FM transmitter
operated from 9V battery.
 

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pebe

Member
You are comparing chalk with cheese. The equivalent circuit of a crystal mic is a voltage generator in series with a capacitor, whereas a carbon mic is essentially a variable resistor needing a polarising voltage and probably a transformer as an interface.

Changes to the circuit will depend on the available output of the carbon mic - ie. whether it is button or transverse type, etc.

BTW, carbon mics tend to be museum pieces these days.
 

panic mode

Well-Known Member
Hi pebe,

I'm not quite sure which post was addressed in your comment.
You are absolutelly correct about crystal and carbon microphones.
One thing they have in common, both are pretty much obsolete
and hard to find nowdays.

It is not difficult to condition signal of one microphone type to
simulate other type (well, within limits of course. nobody should
expect studio quality recording done with carbon mic).

I guessed that some newbe was having hard time to get his first
microphone work. Typical first time project using microphone is a
small transmitter and many old circuits relied on chrystal microphones.
So I tried to help with something robust enough to work with just
about any microphone. As a matter of fact, carbon mic can be
used as well in the provided schematic. To do so, R1 can be removed
(it doesn't have to). Replace Mic on schematic with a jumper and
insert carbon mic between T1 emitter and negative pole of the battery.
That's all it takes. Chrystal mic was often used for simplicity of the circuit because it provides pretty high signal and it has high impedance. To
connect it to my circuit, everything on the left side of C3 can be removed including C3. Chrystal mic would be connected directly to base of T2.
 

pebe

Member
panic mode said:
Hi pebe,

I'm not quite sure which post was addressed in your comment.
You are absolutelly correct about crystal and carbon microphones.
One thing they have in common, both are pretty much obsolete
and hard to find nowdays........
Hi panic mode,
My reply was to the original enquirer. I should have made that clear. My fault - I didn't scroll far enough down the page so I missed your input.

I think we are in agreement that it's not simply a question of just changing the mike. Circuit changes are required as well.

I doubt if you would get enough gain from inputting a crystal mic direct into T2 though. The equivalent series capacity of a crystal mic is about 1nF and it requires a minimum of a 2megohm load. The 10nF and 2K2 as shown would kill the output.
 
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