Use a Electret Mic on a system designed for a Dynamic Mic

I have 2 parts of this problem that I cannot change:
Mic being used is a electret mic
Amplifier unit is designed to be used with a Dynamic Mic.

So right out of the gate I know that will have to build a power supply (because the mic will need some bias power for the FET). But what is catching me out is how to replicate the signal a Dynamic mic puts out with the Electret mic and protect the amp.

Oh and I am a hobby electronics guy, I know enough to make my fair share of "factory smoke".

Can someone nudge me in the right direction? Thanks

Thanks to Google I think I found my answer. if I used a small watch battery or a single 1.5v AAA battery would I need to adjust the resistor? or is it almost any voltage to bias the FET? I would like to use as small a battery with the longest life as possible.


Now I suspect I will have a gain problem. What would be the easiest way to adjust the output on this circuit to ensure that the mic is not TOO loud? would it be as easy as wiring a Pot in series with the Amp + input and the mic +?

Am assuming that you have a High Impedance input in your Amplifier.
You could wire a 22k preset pot across your 'Hot' (red) and 'Ground' (black) wires of the circuit you have given, with the wiper then going to your Amplifier Input. This will then provide you with a control range from zero to full output.
Placing a resistance simply in series as you ask is far less effective.
Electret mic's generally have a higher output than moving coil dynamic ones for the same sound pressure level, so the pot wired as suggested will be able to compensate for this for you.

For the battery, use a switch in series to extend life when not in use and with the circuit shown AAA cell(s) should be your better option.

hope this helps
Mik

Well that did not work... BAD feedback with and without the battery installed. Will have to re-think this one.

Any thoughts on how to use a Electrec mic on a system designed to use a Dynamic Mic?

Now that's a very interesting reply; because the pot when connected as described is able to control the Mic output signal between Zero and Maximum.
You must have not wired or set it correctly.
Mik

Okay I had some time in the shop tonight to break this wiring down. The WHOLE story. This circuit is part of an intercom for rally racing (I have a similar post about this sort of thing). Each of us in the car have a mic and headset in our helmets. Both of us plug into an amp that enables us to talk to each other in the car. In an ideal world the driver and co-driver have matching intercom and helmet systems. But this is what I am trying to make is a work around.
Here is the ideal setup for the system I am working with. It is designed to be used with Dynamic mics.


Here is what I have to work with and it will not work since the electret mic needs to have its FET biased. I cannot change the common mic and speaker -, I know its not ideal but its just the way these helmets are wired:


I do not know the internal wiring of these amps.

Now If I wire in the electret mic like this the mic works just fine (if I leave the speaker disconnected):


As soon as I touch either the - speaker wire from the amp to the - mic (common) I get major feedback.:



Ideas?

Looks like you will have to use a 4 wire connection with the Electret Mic Capsule (screened cable from the mic) as what you describe is the the very high impedance microphone picking up the output of your amplifier from the (small) resistance of the common ground wire or capacitive coupling when you touch other wires. Good screening of the capsule and all external circuitry as well is needed.

Consider the very high impedance of the Electret element of the microphone with (usually) an FET amplifier powered from your battery and how this will be affected by stray signals.

Suggest with this particular application that you try to find a Dynamic mic capsule without the very High impedance amplifier stage in the mic, but still use a screened cable for the mic separate from the pair to the headphones. Two core individually screened cable for Hi-Fi signals is useful for this as a stock item or for a 'coiled' cable the ones used for Comms microphones which have a single screened core and seperate single wires for PTT switching which you could use for your headphones.
You may well also have reliability issues with Electret capsules from moisture within the enclosed environment of the helmet.

hope this assists
Mik

Edit: Amend suitable cable types

Thanks Mik. I wish I could break the wiring out and use a proper 4 wire setup. But alas I cant. What I was hoping for is to make an adapter to plug the helmets already wired 3-wire setup into. Then plug the adapter into the amp breaking it out to the proper 4 wire setup and enabling the use of the helmets electret mic.

The helmet is wired as a 3 wire setup with the mic and speaker sharing a common negative.

So, is this dead? Can this be done or is it an impossibility?