Microphone Bias on Tympan Rev C

I recently was asked a question about microphone bias on the Tympan microphone input jack. Specifically, someone asked whether both inputs (ie, Left and Right) were supposed to have the bias voltage, as it appeared to this person that only the Left channel was given a bias voltage.

Well, the user is right…only the Left channel is given the bias voltage.

If you do want a bias voltage on both channels, and if you’re handy with a soldering iron, there is an easy modification to bring the bias to the right input. Simply solder a 2.2K resistor onto the empty pads of R16:


As with every modification, there is a trade-off. Below are the reasons for the two different configurations:

Default Configuration (R16 is empty):

  • Allows for one 2-wire microphone on this jack
  • Allows for one 3-wire microphone on this jack
  • Allows for stereo line-input on this jack.

Modified Configuration (R16 is 2.2K):

  • Allows for two 2-wire microphones on this jack
  • Allows for no 3-wire microphones on this jack
  • Allows for mono line-input on this jack.

Either configuration is totally valid. We chose the default configuration because it seemed to be the most flexible. You may prefer the modified configuration. If so, all you need is a soldering iron and 2.2K resistor!