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I've tried to add an indicator LED to a couple of electrets I have in parallel, plugged into a soundcard. At the LED I'm only reading 0.45V (I believe the bias is 5V). and obviously the LED isn't lighting up. Am I being a tad stupid? Do I need to add another power supply using a transistor (and some large resistors)? The LED is in parallel with the last mic. All the mics work fine (in fact I seem to have some added gain by having the LED in there).
Sorry to be dumb. Thanks in advance.
The microphone bias from your sound card is only intended to deliver a few milliamperes of current to an electret microphone. It isn't a power source like you might think.
The bias voltage is connected to the microphone wire through a (fairly) large value resistor.
Like this:
simulate this circuit – Schematic created using CircuitLab
Since the microphone draws a little current through that resistor, the voltage drops. You have multiple microphones, so they draw (together) more current so the voltage drops a lot more than usual.
If you had no microphones connected, but put an LED on the microphone input (paying attention to the polarity) then it would (probably) light up dimly.
It might not light up at all, though.
It takes a certain minimum voltage for an LED to "turn on." It varies by color. The lowest is the infrared (just a little over 1V) followed by red (1.2V.) Green and yellow take higher voltage, and blue takes the highest (around 3V.) If you have a white LED, then it is really a blue LED with some phosphor so it will take 3V as well.
That's all less than 5V, so you'd think it would work - but not all sound cards use 5V for the microphone bias.
So, you can't really add an LED in parallel to a microphone on a PC sound card. Even less so when you have multiple microphones connected.
The bias voltage is not usually 5V. For example, this typical USB Audio Codec offers 3V microphone bias – plenty for an electret microphone, even with a hefty series resistance.
So, as JRE says, it's unlikely you'll light up any LED.
Imagine that worked, though: That would be terrible!
A diode is a non-linear element; the current through the diode is an exponential function of the voltage across it. Now, your microphones are high-impedance voltage signal sources.
That leads to your diode in forward bias "swallowing" the signal a bit. But: that swallowing isn't like if you put a resistor in parallel to the microphones. Since the the diode is non-linear, you get intermodulation products due to having it. So, in the voltage signal the sound card sees, there's suddenly frequencies that weren't in the original signal.
Now, electret microphones aren't perfectly linear, either, but it would still be interesting to see what happens if you put a (lower band-gap) diode in parallel to your microphones and made them record a single tone without harmonics, and compared the spectrum of the recording with the original tone's spectrum (which would be a single line).
