A compact and inexpensive hydrophone with an internal ultra-low-noise preamp

A simple and compact JFET preamp has been built with a gain of 25 dB, a quiescent current of 2 to 3 mA at 12V, and an equivalent input noise voltage of 1.7 nV/Hz¹2/ at 1 kHz. This extremely low noise specification was measured with a dummy source impedance that mimics the intended piezoceramic source impedance. Separately, a compact and inexpensive passive hydrophone has been built and measured to have a sensitivity of roughly -190 dB re 1V/uPa and a maximum dimension of 28 mm prior to encapsulation. We expect to finish the construction and measurement of an integrated hydrophone and internal preamp soon. This hydrophone should exhibit a noise performance far below Knudsen sea-state zero when completed. The component cost of the preamplified hydrophone is roughly $25 in small quantities, plus the cost of two simple machined aluminum parts, encapsulation, and cabling.