Need a very basic op-amp for electret mic

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yusraf

Joined Jul 15, 2019
1
Hello,
We're building a very basic sound detection circuit that listens to instrument sounds (repetitive loud beeps) and sleeps most of the time. The frequencies I want to detect are 700MHz, 1kHz, 1.5kHz, 2.2kHz, running a simple FFT algorithm on an ATMega328p. This is a low power application and I've managed to bring down the average current consumption to ~4mA (in 24hrs). So, the low power part is really important. The sampling rate on the ADC is 18kHz. Very basic stuff.
We've used electret mic breakout boards from Adafruit that had the MAX4466 chip which were very nice. Now we're looking for our own PCB, and we've had an excellent experience with JLCPCB and would like to continue using that service. Our initial design used the classic TI-OPA344 (sound detected perfectly fine so those noise/bw/slewrate specs work) but JLCPCB and LCSC have run out of stock and we can't continue using that. I've searched high and low for something that would be equivalent to the OPA344 if not on par with the MAX4466.
Can someone please suggest an alternative?
I'm not an audio person, and we're not looking for the best audio experience, just enough to detect the frequencies I mentioned.
I've tried looking at OPA333 but it has very low slew rate and very noisy above 1kHz. The AD8605 has a high supply current (1mA). The LMV321 has the same issue.
Is there any general-purpose/instrumentation/other op-amp that I can use as a suitable low power alternative?
Any other specs needed to narrow this choice down?
 

BobTPH

Joined Jun 5, 2013
3,498
I do not see why you would need anything special in an opamp for those frequencies, assuming the 700MHz is a typo. Can you explain further why such high performance is needed?

Bob
 

Ian0

Joined Aug 7, 2020
2,656
OPA333 slew rate is 0.16V/us - so on a 5V supply it will swing full scale at 10kHz, which is above your Nyquist freqency.
I also doubt that the noise above 1kHz is anything to do with your op-amp - is it on your power supply, or is it just ambient noise?
Don't forget that you need an anti-aliasing filter, otherwise any frequencies that your microphone pics up above 9kHz will make your FFT wrong.
I've attached a condensor mic preamp I've used many times, to which I have added a double pole (Q=0.5) filter at 3kHz, which should deal with the anti-aliasing requirements.
I’ve used it successfully with just about every cheap and nasty op-amp you could imagine.
It's a transimpedance amplifier, because the mic capsule's output is the drain of a JFET so effectively has a current output.
12k resistor biases the mic at about 2V (they usually run at 250uA). Adjust accordingly if yours doesn't.
Reduce the 1M resistor if you have too much gain (and increase the 47pF capacitor to keep the roll-off frequency the same)
If you do have a noisy supply, change the 12k for 10k + 2.2k in series and decouple the junction to ground with 100uF
 

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Audioguru again

Joined Oct 21, 2019
3,317
You do not say where in the world you cannot buy common and available OPA344 opamps. Digikey (in the USA) has thousands of them. Have you looked at Farnell who are world-wide?
 
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