The system needs a way to listen to the sound around it and the natural way to do this is with an electret microphone. I will probably include a 3.5mm jack input to allow you to plug music straight in too.
I was looking online for some guidance on designing the circuit and I found a good reference design from Texas Instruments. I followed through the steps that they took in coming up with the design. This design is basically what I need so I am copying it almost exactly (hey, that’s what reference designs are for) except that I am replacing the TI op-amp for an NE5532 (my go-to op-amp), and the 75kΩ resistor with a 200kΩ potentiometer so that I can vary the gain.
I built the circuit up on a breadboard.
The red trace shows the audio signal after it has been amplified and it looks pretty clean. However, the power supply has a very repetitive ~45mV pulse on it every ~600µs and this is getting coupled through to the audio signal. You can see that every time there is a pulse on the power supply (blue), there is a little bit of noise on the audio signal (red).
It’s not clear right now how much of an issue this will be. As the pulse is occurring with a frequency of ~1.5kHz, I’m worried that the 1kHz or 2.5kHz filters will amplify the noise and that this will mess with the output. This can be fixed by adding a filter to the power supply or by getting a cleaner power supply. Rather than waste time trying to fix this now, I’m going to wait to see if this is actually an issue.