I’m trying to use a Cmoy headphone amp (built around an OPA2227P chip) as a hard-wired in-ear-monitor.
I bought a stereo electret condenser microphone from Jaycar this afternoon, but I can’t get any response from it when plugged into the amp.
The headphone amp works if I plug my iPad into it.
I’m assuming the microphone needs it’s own power source or a preamp. I need advice on my next step in making this work?
Electret microphones normally have an inbuilt FET preamplifier. You need to bias it from a quiet power supply of some volts in series with a resistor of a few kohms. Obviously capacitively couple to the following amplifier. Experiment and see what happens. If you start with 10 V and 10 k, you're inlikely to damage anything.
Why not just modify the one you have? Most of what you need is already there in your headphone amplifier.
The CMoy is a (very simple) amplifier built from a readily available schematic.
The amplifier itself looks like this:
The power section looks like this:
It is intended to drive a headphone from a line level source.
You are connecting two electret microphones to it.
Electret microphones require a DC bias voltage to operate.
They will also likely require some amplification.
The existing amplifier can easily be modified to provide both of those things.
Bias for the microphones:
Connect a resistor from switched 9V to each of the inputs (the point marked IN from the amplifier section and the point marked 2 in the power section.)
You will need two resistors (one for each channel.)
I would start with a value of 1.2k ohms.
The value isn't terribly critical. Anything from 1k to 10k ought to work. I wouldn't go below 1k - too much current might damage the amplifier in the microphone. Above 10k and you might start picking up more noise. Between those values you might see some difference in frequency response - I couldn't begin to guess which way, though.
Try that, and maybe crank the volume of the amplifier up to full.
If that works, you are done
If you can hear a bit from the microphones, but it is not loud enough, then you can boost the amplification.
Amplification:
R4 and R3 set the amount of amplification (the gain.) The gain is R4/R3. The values given in the schematic make a gain of 10.
If you change R4 from 10k to 100k, you will raise the gain by a factor of 10. That makes the gain 100.
Remember that you have to do this twice as well - once for each channel.
That ought to more than make up for the weaker signal from the microphones
I have ignored R5 from the schematic - it probably isn't there.
I have also assumed that the microphones are wired to the stereo plug the same way a line level signal would be. It is possible it is wired differently, but I don't see why anyone would
Most electret mics draw 0.5mA and need a bias resistor of 10k from a 8.5V filtered source. If the resistor value is less than 10k then the level from the mic is reduced. If the supply voltage is less then loud sounds cause distortion unless the resistor value is reduced.
I use 1k in series with the 10k and a 47uF to ground where the resistors join to make the filter.
The Cmoy amp has a gain of only 11. A mic needs a gain of about 200.
