Use Audiomoth with an external hydrophone

As I have described in an earlier post, I am working on an underwater setup for the Audiomoth, based on an earlier porpoise detector housing. It has a piezo-ceramic hydrophone with a frequency response suitable for recording dolphin and porpoise sonar clicks. I followed advice from Jay Barlow at NOAH and put a 12 kOhm resistor between signal and ground and connected the external jack detector to ground. I use an Aquarian Scientific PA-4 preamp, and it works fine in the high frequency range at lower gain levels but loses 11 dB from 30kHz to 150kHz. Of course this can be compensated for in the post-process, but it affects detection of weak clicks due to lower S/N ratio. I read in the Forum that you were working on a new external preamp for the Audiomoth – is there now a suitable one for my purpose? If so, what are the dimensions – the space in the housing is limited (diameter 67mm and length 80mm; most of this space is taken by the AM and an extra 3xAA battery casing).

Another possible issue is a suspicious “wrapping down” in the spectrogram. I recorded a pinger, with main frequency output in 50-85kHz range, but with some components between 100-120kHz, at a sampling rate of 250kS/s. The top spectrogram below is this AM recording, and the one below the same pinger (there is some variation in the pings) recorded with a Reason hydrophone using a NI USB-6351 A/D converter sampling at 333kS/s, and a custom-made software on my laptop.

As you can see, there are down-sweeping tones in the AM recording which are not there in the laptop recording. Is this an aliasing effect? As far as I have understood there is no anti-aliasing filter in the AM, right? I tried to remove this by applying a lowpass filter, with cutoff at 150kHz, but this did not work, possibly because it is not inserted before digitalization.

Any suggestions on how to deal with this?

Thanks for any advice!

mats