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disable plugin power

Is there a way to disable the plugin power (PIP) for the external mic, eg. through the software or by removing the 1k1 resistor from the PIP RC circuit? Where can I find the resistor on the USB-Audiomoth? I would like to connect a hydrophone that is self powered and might get damaged from the PIP DC.

5 comments

5 Comments

Gerwin Meijer

Apr 17

Thank you Alex, I will give it a try and will report back on the forum. I was afraid of introducing a filter in the audio line, but, when I understand the circuit correctly, this configuration would result in a high pass filter of 5-10 Hz, which is no problem for me.

Alex Rogers

Apr 17

Replying to

Yep. The issue with un-powerd hydrophones is that they are typically just a bare piezo tranducer that has a very small capacitance (around 10nF) so when combined with the bias resistor of a few kilo-ohms you end up with a high-pass filter with a corner frequency of about 20kHz. You lose all the low frequency components. With your power-hydrophone and this configuration you end up with a high-pass filter again but the corner frequency is very low - around 20Hz - so you should get good sound quality.

Gerwin Meijer

Apr 16

Hi Alex,

I would be using Audiomoth 1.2.0 as USB mic device or Audiomoth USB microphone, probably the latter, so without the battery holder. Do you mean the hydrophone needs to have a DC offset voltage with PIP active or with the resistor removed? Is there no capacitor in the audio input line to the preamp to remove any DC offset and PIP DC? My guess would be the first 20 dB preamp does not need any DC....? My hydrophone has a built in preamp 20dB powered by a 9V battery, the signal is balanced XLR, but I will use only one pin (unbalanced) to connect to the Audiomoth USB mic connector. Actually, I hope it will not have any DC offset.

Alex Rogers

Apr 16

Replying to

The input of the AudioMoth assumes you will be connecting an electret microphone and the AudioMoth input needs to end up with a DC offset of between 1 and 2V. If your hydrophone is self-powered, you probably don't need to modify the AudioMoth in this case. Just add a resistor and capacitor to the output of the hydrophone. This will ensure that the AudioMoth receives the correct bias voltage and will prevent the hydrophone seeing the DC offset.

The input to the AudioMoth pre-amp will effectively then look like this.

And you should have good frequency response.

Edited

Alex Rogers

Apr 15

Hi Gerwin, You would have to remove the resistor from the PCB. Are you using AudioMoth 1.2.0 or AudioMoth Dev 1.0.0? Do know the DC offset voltage of the hydrophone output? The output signal will need to sit between 1-2V for the hydrophone to work correctly with the AudioMoth input. Alex