Preamp for underwater piezo transducer. - Page 1
Forum section: electronics / projects

author: Tone Deaf
author_rank: Newbie
timestamp: June 30, 2022, 02:38:10 pm
content: Hi Troops.I am looking to build a piezo transducer to listen to and record sub aquatic sounds from inshore areas around the coast as well as rivers and lakes. My question is with regard to preamp and cable for the prototype transducer ... So It seems logical to have a pre amplifier in the transducer (which will be enclosed in a plastic housing, potted and coated with a flexible membrane). My thinking is that the stronger signal will be less prone to being corrupted by hiss and interference picked up by the lead. I have a couple of tiny Hi Z 9v preamps that I bought for a guitar project and was thinking that... and here we go... I could build one into the transducer head and supply it with power from a screened microphone lead. So what do I need cable wise? Can I use a three or four core screened cable with two cores supplying power to the preamp (9v from a pp3 battery), the third core supplying the signal from the preamp to the onshore amplifier and the screen supplying the negative signal from the preamp to the onshore amplifier. Is it best to have a metal case for the onshore amplifier and if so should the screen be grounded to the case as well as the amplifier ground.I have another question regarding the Piezos. I plan to use five in fact have already prototyped one which works okay. I connected the Piezos in parallel in this first prototype. Is this best or would it be better in series?One last thing. What are your thoughts on decoupling in a system like this?I look forward to hearing from anyone with views on, experience with and advice for my project.Many thanks in advance.Kind regards.Tony.

author: richnormand
author_rank: Unknown
timestamp: July 02, 2022, 01:26:33 pm
content: This might be helpful as it touches several design issues:https://www.instructables.com/Lets-Build-Some-World-Class-Hydrophones/

author: chris_leyson
author_rank: Super Contributor
timestamp: July 02, 2022, 02:49:08 pm
content: For best performance the preamplifier input impedance has to match the piezo transducer impedance for maximum power transfer. A high Z input impedance amplifier will generate more voltage noise gthan your source. Series or parallel ? Parallel connection works well for ultrasound transducers so should also be applicable to audio. Also you should make the preamplifier output balanced to eliminate common mode noise pickup from supply lines in the same cable. That's my 2 cents worth from an RF design perspective.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 02, 2022, 08:37:09 pm
content: The piezos are mainly capacitive. So the impedance depends a lot on the frequency and the frequency range can be quite large. The frequency range of interest also determines if it makes sense to use the piezos in series, in parallel or just a single pizo. With higher frequencies the 5 signals would not be in phase and one would be a rather complicated directional dependence.5 seprate channels with 5 amplifiers would allow to later decide later and in the extremes do some beam forming / directional tuning or localization.Chances are a FET based amplfiier would be the best choice. This could be similar to a electret microphone with just a JFET and high ohms (e.g. 10 MOhms range) resistor and only a 2 wire link - so no extra wire for the supply.