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.
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Phantom power for a hydrophone or microphone
Phantom power for a hydrophone or microphone
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@jaybarlow33 Dear Jay, Hope all is well. I recently built a recording unit with the H2A and an Audiomoth 1.2.0 to record fish sounds. I was able to record some fish sounds and would like feedback on the unit I built in terms of sound quality. I can share pictures and audio files with you if that's okay with you. We can also communicate through email or phone. Thank you, Carlos
Thank you Edwin, I will I will give that a try.
Cheers
I would like to attach a JrF (Jes Riley French) 'D' Type Hydrophone (cos I have one !) to one of my AudioMoth recorders via the additional 3.5mm jack socket. I understand from JrF that the hydrophones do not need PIP etc., so theoretically should work. The information here seems to suggests that a resistor might need to be added for the system to work ?
Footnote: The Aquarian H2a hydrophone (which uses plug-in-power - the bias voltage) still works with this 12kOhm resistor. Apparently, a 2.8v bias voltage is sufficient to power it. Also, note that the full-scale voltage given above (0.02 Vrms with low gain) applies to the external input jack on the AudioMoth Dev 1.0.0. Other versions may have different full-scale values.
Again, thanks for the explanation. The maximum full-scale input to the AudioMoth (i.e., with the lowest gain setting) is only about 0.02 Vrms, so reducing the bias current to 2.8 V should provide plenty of headroom for the maximum AC signal.
It is nice to hear that you can see some sound on the microphone input. The resistor you placed helps lowering the voltage on the analof input circuit. In the Audiomoth it seems that there are two resistors in series creating the bias voltage for an electret element. If there is no load this voltage is 3.3V on the input of the analog switch IC. I drew an image showing the pass band of the switch which is between 0V and the supply voltgage. If the bias voltage still is too high you can get into trouble. If your microphone, hydrophone or other audio source does not lower that bias voltage you will probably not hear a lot. Without the added resistor over the microphone input you can only hear the bottom halfs of the sine wave of the stronger signals. Adding a 10 or 12k resistor will lower the voltage to about 2.8V or so, which is low enough for most small audio signals. Ideally one would like to see this around half the supply voltage (1.65V) but that might take a resistor of about 2700 Ohms as load to reach that voltage which might give additional load to the microphone and consume a little more energy.
Thanks Edwin. I found what appears to be a solution there. I added a 12 kOhm resistor between the input and the ground to short out the bias voltage. Signals from HTI hydrophones look good (in air). This does not appear to have created an RC filter; at least I'm not seeing any attenuation of the lower frequencies. I'll add a picture after I'm done testing.
There are some ideas in this external microphone thread. https://www.openacousticdevices.info/support/device-support/external-ultrasonic-microphone