I’m Valentin, I’m working in a research chair in Montreal specialized in in ear technologies.
I’m writing you after a problem we “solved” with the Tympan rev D hardware.
Our goal was to use our earpieces (using Analog Microphone Electret Condenser) connected to the Tympan Platform, especially the input 1 on the headers.
The first thing we did was to verify the sensitivity of our mic powered by the Tympan power supply (1.8V). We placed our mic in a Sound Calibrator Type 4231 at 94dB and 114dB, we measured the true RMS value and we compared our results to the microphone documentation. The results were correct.
Then, we decided to compare the signal produced by the microphone when connected to the IN1 and the same signal produced by a function generator connected to the IN1. We measured the voltage with a National Instrument card at the input pin and we noticed a big difference between those signals. The mic signal was clearly heavily distorted at 114dB.
We had the idea that something was pumping too much current for the microphone. So we decided to remove the capacitors C37 and C39 in order to only have our mic connected to the codec. We did the same measurements as before and we noticed the majority of the distortion was gone.
Our conclusion was the resistor R32 and R33 (1K) directly connected to the GND creates an impedance that is too low for our mic.
Did someone run into the same issue?
The data in the plots shows the 2 mic signals (recorded by the Tympan on an SD card) when in the calibrator at 114 dB before and after removing capacitors C37 and C39.