I've built a circuit using an ISD4003 ChipCorder, which is an IC for recording and playback of audio (in a telephone answering machine, for example). I've written code to send the SPI commands to record and playback audio. The problem is that whenever I command the ChipCorder into record mode, I get loud low frequency noise recorded over top of my voice. The noise only exists while the ChipCorder is in record mode. The noise is less than 200 Hz with a dominant frequency of 40 Hz. (It sounds like a cross between a buzzing bee and flatulence.)
This is an excerpt of my circuit schematic. That's an electret mic going through a high pass filter and a preamp before the AUDIO_IN signal goes into the ChipCorder.
I know the noise is generated by the ChipCorder because the noise only occurs while the ChipCorder is recording. I believe the noise is going over the power lines and being picked up by both the mic and preamp. If I power the mic and preamp with a separate power supply then the recording is clean with no noise. But that was just a test, not a solution.
I changed the corner frequency of the high pass filter to 300 Hz to try to filter the noise but that wasn't enough. I even tried adding another first order high pass filter after the preamp (not shown on schematic) and that still wasn't near enough. I also tried adding more bypass capacitors (100 uF and 220 uF) in various places but that didn't help.
What else can I do to prevent the low frequency noise generated by the ChipCorder from affecting the mic and preamp?
Also, I'm testing this circuit on a solderless breadboard. Is it possible that moving to a PCB with a proper layout, ground plane, etc. will have a dramatic affect on this noise issue? I'd rather fix it on the breadboard before moving on to a PCB.
Best Answer
For starters, you haven't decoupled the microphone power at all. You should split R1 into two resistors (e.g., 4700Ω each), and put a fairly sizable ceramic capacitor (0.1 to 10.0 µF; 1.0 µF gives you a cutoff frequency of 34 Hz) from the middle junction to ground. This is the most direct path by which power supply noise can get into the audio input chain.
The next way would be if the opamp has poor PSRR, in which case, you should decouple its supply with a dedicated resistor and capacitor. The problem here is that you don't have a lot of headroom to work with, which will limit the size of the resistor you can use (voltage drop based on the average current that the opamp requires), which in turn will require a relatively large capacitor in order to get a low cutoff frequency.