I've tested the frequency response of two hydrophones for use with an AudioMothDev. The Aquarian H2a uses plug-in power and doesn't require any modifications to be used with the AudioMothDev board. However, I found that its flat frequency response only extends to 3 kHz. Signals levels are down 20 dB by 10 kHz and were undetectable at 30 kHz. The HighTech HTI-96-min requires the addition of a resistor (10-12 kOhm) from the audio input to the ground. It also requires external power (3 v is enough from one version, but I could not use the external voltage provided by the board without adding noise). The HTI-96-min has a flat frequency response to 30 kHz and a useable frequency response up to 140 kHz.

I'd like to use an Audiomoth Dev with an external hydrophone. Most hydrophones with a single-sided pre-amp require at least 3 v power in a 3-wire system (ground, power & signal). However, the 3.5 mm external input jack on this board is designed for a 2-wire plug-in-power pre-amp. I'm testing the Aquarian h2a hydrophone which uses plug-in-power, but it is not as good at low signal levels as the HTI-92-WB hydrophone, which requires 3-wire power. Can anyone tell me if it is possible to modify the existing 3.5mm socket to provide phantom power to a 3-wire system? If not, this would be a good option for future versions of this device.