Problem connecting Electret breakout board to mono amp breakout board

I’m wanting to connect the output from SparkFun’s Electret Mic breakout board (https://www.sparkfun.com/products/12758) and feed it into their Mono Audio Amp breakout board (https://www.sparkfun.com/products/11044).

Possibly naively, I though that the output from the mic breakout board could be fed directly to the input on the amplifier board. (IN- on the amp board joined to common ground, IN+ joined to AUD on the Electret Mic breakout board)

However, when I do this the speaker output from the amp repeatedly steps through from 0v to the supply voltage about three to 4 times a second.

When I scope the output from the electret board without the amp attached I see what looks like a correct signal (if I whistle by the mic I see a sound wave oscillating centered at half the supply voltage. However, when I connect this output to the amp, the scope shows a completely different input signal, namely the voltage stepping from 0v up to the input voltage in a series of rapid steps before repeating from 0v.

Knowing that the output from the mic board is biased at half the input voltage, I have also tried connection IN- on the amp board to a voltage divider providing, as a reference, half the input voltage. I still received the same results.

I believe that both breakout boards are (individually) working correctly and that I probably have to do something to match the output from the electret board to the input of the amp board.

I would be really grateful if anyone could provide me with some pointers or clues.

Thanks!

Can you please provide detailed pictures of your setup?