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Preamps and Microphones…

A few weeks ago I did an experiment that stored an 8 bit PCM file in the flash memory of an Arduino, and then used PWM to flash a laser at a high rate so I could transmit that audio over a long distance to a solar cell based receiver. A few days ago, I assembled a nifty little gadget called the bliplace, which interfaced a small electret microphone to an AVR ATtiny processor. For the last couple of days, I considered merging the two: the idea was to build a small interface circuit to condition the signal from an electret microphone to feed into the A/D converter of an Arduino. Then, it would be simple enough to use the Arduino to modulate the PWM output.

I'm a bit confused about the bliplace, it doesn't seem to employ any kind of preamp to increase the output. My understanding is that the output from these kind of microphones is typically quite low, so a preamp would seem to be necessary to boost the overall voltage swing to something that the Arduino can measure with some precision.

My idea was to use a simple op amp circuit to boost the voltage from the microphone, and center it around 2.5v. It's not hard. You create a resistor divider to halve a 5v supply, and put it into the non-inverting input of the op amp. You then feed the input signal through a blocking cap (I used 0.1uF) and a 1K resistor into the inverting input. Then, feedback from the output of the amp is fed through a 100K resistor back into the inverting input. Here's the schematic in LTSpice:

I put it together today on a breadboard, and it seems to work. I couldn't dig up a suitable microphone (I know I have some somewhere), but I'll get to it in the next day or two and get it all soldered up so I can hook it to my Arduino. Since the LM324s are a quad operational amplifier, I could even use the spare op amps to do some additional filtering. I'm still working on that part.

Through some strange coincidence, a couple of other people I know have been playing with amplifiers too. Whisker over at tymkrs has been working on a preamp for a microphone, and MakerDino's weekly hack at HackAWeek was to build a single transistor preamp. It's a simple little class A amplifier, and he did a great video:

I played around with the circuit in LTSpice a bit, and it seems to work pretty well. It provides about 36db of gain, and seems reasonably linear. You can get distortion of the input signal is too high, but other than that, it looks pretty good. The gain is enough to convert a 10mV signal to a 600mA signal. Current draw is about 700 microamps with a 3v supply. Not bad! I could use similar circuit for electret preamp. I'll play with both circuits some more. Thanks Dino!

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