Electret Condensor Microphone Question......?

Question

So, I want to make an amplifier for watching TV so a hard-of-hearing person can watch without blasting the rest of the family out. I've got two electret condensor microphones and I know that one end needs to have a supply voltage, and the second pin has an audio output/ground connection. What I don't know is where this goes in a simple amplifier. The amplifier can be OTL, transformer or direct coupled, but can anybody point me in the direction of some good schematics please?

Answer 1

Electret microphones work by using a charged polyfilm. They are charged once then retain this charge for many years. Because of this, they need an isolation device between the capacitor and the outside world. Usually this is a FET. It is this fet that requires power (the electret condenser needs no power). That the fet also provides some gain is a bonus. There are 2-pin and 3-pin versions. You will need to provide this power to the capsule. You can either build the circuit into the microphone or use the amplifier to provide the power. A small battery will have a very long life as the drain is very small. It has the advantage that it will not add noise to the signal.

Output levels are somewhat similar to Dynamic microphones (unlike condenser microphones that have infinitesimal levels IMHO).

<>http://www.epanorama.net/circuits/microp...

Answer 2

The electrets will be ok for this.  I would suggest you have a look at the data sheet for something like a TDA 1020,, you will need to put a small pre-amp in the form of a couple of transistors (BC108 or something like this) connected as a differential input. (necessary as an input for the electrets). Alternatively, a small amp can be made using a few bipolar transistors.  Will you be running it on batteries or from mains supply?,

As far as schematics, drop me an email, I will have more idea on your needs as far as output etc, (Mono/stereo - headphones/small speakers etc).