Trouble with interference on electret mic BoB clone

I bought an electret mic breakout board (https://www.sparkfun.com/products/9964) and was able to get it to work quite well, so I decided to use the circuit in an open-source sensor module that I am working on. I received the PCBs from OSHPark yesterday and assembled the first prototype, but am getting some strange readings.

Without doing any sort of buffering or averaging, I’m just outputting the direct results of an analogRead to the serial port and monitoring it in the Arduino IDE. Without making any noise, I’m seeing a very regular oscillation occurring in the signal that is making it impossible to use. When I tap the electret I am able to make the signal top out, but the oscillating noise is still there.

I put together a Processing sketch that just graphs the analog data being received straight from my Arduino. Here is what I’m looking at:

I’ve never designed an audio circuit before, so I’m not sure what is happening. I don’t own an oscilloscope, nor do I know how to use one to troubleshoot a problem like this. Does anyone have some ideas about what is going on, and what I can do to solve the oscillation?

I’ve attached the Eagle files (schematic and PCB file) for my board, as well as an image of the PCB if that helps.

I noticed that when I measure two of the 10k resistors with a multimeter I see ~5.76k instead of 10k. However, I see the same thing when I measure the same resistors on the Sparkfun BoB. Odd.

can you upload a PDF of the schematic.

are you feeding raw-audio into the ADC, or has it been rectified first ?

-mark

The schematic is exactly the same, literally copied and pasted, as the Sparkfun breakout board (https://www.sparkfun.com/products/9964). Sparkfun forums do not allow PDF attachment, so an PNG is uploaded instead.

I’m assuming it is raw audio, as I don’t see any diodes anywhere in the design. The breakout board works perfect, my clone does not.

Might be a speed difference between the units. Can you toggle a pin after every ADC conversion, and check frequency of the pin changing state in each case ?

-Mark

What are you referring to with “units”? The op-amps? Or the overall circuit?

I don’t own a scope, so the best I could do is see how fast an LED blinks when I flip the pin. What should I be looking for when I try that?