I am using the below circuit to amplify the signal from an electret condenser microphone for analysis by a microcontroller to detect short bursts of sound. The circuit runs off a ~4.5V supply from 3 AA batteries, so options for opamps are limited. I am using an NJM4580D. Datasheets for microphone and opamp linked below:
Circuit includes a potentiometer to adjust gain on second stage. The circuit is working fine at low gains with louder sound bursts, but I am having an issue with the noise at the higher gains required to detect fainter sound bursts.
The noise appears to be approx 580Hz. The room is quiet prior to sound bursts. The circuit is built on protoboard, and size is somewhat limited. Photo below:
So, given the limitations on power supply and construction, is there anything I could do to reduce the amplitude of the noise while maintaining the signal strength at higher gains?
Appreciate any feedback.
Brian appears to be correct. I removed the microphone circuit board and powered directly from 4.5V batteries. At max gain, the noise was acceptable:
The rest of the circuit board that this device is connected to is fairly straightforward. I don't have a presentable schematic at the moment but it is constructed in a manner similar to the microphone circuitand made up of the following components:
The microcontroller is an ATMEGA328P https://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061B.pdf
MAX7219 driven 8-digit segmented display https://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf
Pair of SN74HC165N shift registers for keypad input (momentary pushbutton array) https://www.ti.com/lit/ds/symlink/sn74hc165.pdf?ts=1607932478460
SN74HC125N buffer on SPI MISO line https://www.ti.com/lit/ds/scls104e/scls104e.pdf?ts=1608013220070
100nF decoupling is provided at each IC power pin and I have a couple of 100uF electrolytic capacitors on the power rails.
Any thoughts as to what might be causing instability/interference?
As suggested by Andy, I tried a different op-amp, the MCP6022, which I was able to drop into the existing circuit and the difference in output range is remarkable:
In addition to this, I took Brian's advice and added a 470uF capacitor to the power rails at on the mic board, which also helped significantly:
Finally, I added a 10ohm resistor in series with the power supply to the mic board and it also produced a noticeable reduction:
Original op-amp with the additional capacitor and resistor, for reference: