amplifierelectretmicrophonesensitivity
Is it possible to connect multiple electret microphones in order to increase sensitivity (gain) of the microphone(s)? If yes, how do I connect them?
Update:
Obviously one solution is to increase the number of amplifiers, but I am curious if multiple microphones can be serial connected – so this is more like a theoretical question (unless it is possible to do it – then it becomes a practical question) 🙂
Electrets don't need phantom power for the diaphragm, but the small capsules usually have a FET buffer inside which needs bias voltage. This can of course be derived from a phantom power source if necessary.
Here is a typical electret circuit:
The value of the resistor is usually between 2k and 10k (cap say 10uF or higher) The datasheet for your capsule will probably give recommended operating conditions.
Here's a good link on simple electret circuits.
Another page with some more advanced ideas.
It gives you:
As shown, you get DC feedback via R1+R2. But R1/C1 forms a low pass filter for signals coming from the collector and going to the base. Less negative feedback means less reduction in gain for high frerquencies. Cancel out all those double-negatives and you're left with a high-pass filter.
The transistor below corresponds to Q5 in your drawing.
simulate this circuit – Schematic created using CircuitLab
Best Answer
I am a technician at a boutique mic company. I like to build electret mics in my spare time.
You can increase sensitivity, gain, and signal to noise ratio by running multiple electrets in PARALLEL-not series. This works because:
1) each capsule has an onboard JFET. When combining noise from multiple FETs, some of it cancels. Two amps= -3dB, four amps= -6dB of noise reduction [see Small Signal Audio Design by Douglas Self, Routledge 2010].
2) Multiple elements will have lower output impedance, which can mean less noisy makeup gain down the line.
3) More diaphragm area means more sensitivity, which makes sense. It also means less self noise, for reasons that I don't really understand - but that's the principle behind the $3500 Audio Technica AT5047 (FOUR diaphragms!).
If you end up with a signal that is too hot and distorts the next stage, put a 10ohm resistor on the audio output of each capsule before wiring them in parallel. This averages rather than sums the outputs.