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I've been suggested this circuit as a good low-noise high gain piezoelectric transducer preamp (used to pick up really faint sounds in water).

enter image description here

It is nice because it includes a high pass filter (yet no AGC or a compressor, which would be useful for long recordings with equipment left in the field and later retrieved).

How would you add either AGC or a compressor?

It uses two 12V batteries. Could I use just one yet be able to give both + and - voltages?

Gain is set with R2 and R3. Could I replace both of them with a potentiometer to have variable gain?

The high pass frequency is set with R3 and C1. Could I replace the R3 with a potentiometer and add a switch to disable the filter when it isn't needed?

Since I would probably be building just one, I'd like to pick up the best IC for this use so that I can't later think about "I'd have implemented that other IC".

Both AD743 and AD797 are suggested in the diagram. AD743 lists a 2.9 nV/√Hz at 10 kHz while AD797 lists 0.9 nV/√Hz at 1 kHz. Since I'd be using this in the 3-45 kHz range, does this mean that the AD743 has less signal distortion at upper frequencies (hence the reference to 10 kHz instead of 1 kHz), or are they equivalent?

Thank you

Allison

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  • \$\begingroup\$ Are you able to use a solution with multiple op amps? I built a low noise pre-amp with AGC a few years ago with four op amps. The first stage was a low noise op amp with a small constant gain, the second was the AGC stage with two op amps, and the third was a bandpass filter (you want to reduce the bandwidth of your pre-amp to the frequency range of your signal in order to reduce noise at frequencies outside that range). \$\endgroup\$
    – Null
    Commented Feb 10, 2017 at 16:58
  • \$\begingroup\$ DO you care about calibrated AGC gain being recorded? There are many AGC or compander designs and use the lowest noise /root Hz up to 100kHz and use 2 or 3. stage LPF with AGC. Such that bandstop is defined at 1/2 sampling rate to avoid aliasing . The specs in your design are missing \$\endgroup\$ Commented Sep 25, 2018 at 12:41

3 Answers 3

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For low noise designs, it's best to keep your input amplifier with few or no adjustments. For noise analysis, you can model the hydrophone as a simple capacitor at low frequencies (at least 5 times lower than the resonant frequency). Just measure the capacitance on a capacitance meter. Trim pots in low noise circuits are generally avoided since the wiper can cause issues. LTspice is handy for doing noise analysis since the opamps you are interested in are in the simulator's library.

Op amps:
The AD743 is a FET input opamp. This is desirable for hydrophone amplifiers since the input noise current is in the fA range. Noise-wise, this is one of the better FET input opamps.

The AD797 is a bipolar input, which is why it can achieve such low input voltage noise, that has an input noise current in the pA range. Depending on the impedance of your hydrophone and the lowest frequency you need, pA noise current is usually bad. If you really need low noise, you can use a JFET follower before the opamp. JFET followers inherently have low voltage noise, but you'll have a gain of apx -1dB.

Input Protection:
Hydrophone amplifiers need input protection. Ceramic hydrophones can build up high DC voltage due to thermal changes. When you connect the hydrophone to your amplifier, you could have a couple hundred volts hit the input of the opamp. Also, if the hydrophone hits something solid, it can produce fairly high voltages. Using a pair of reverse biased diodes, perhaps 1V, will solve this issue and still maintain low input capacitance. A pair of back biased diodes will have an impedance around 3Mohms, so be sure this meets your low frequency requirements.

High pass capacitor:
Use a proper capacitor dielectric. X7R, and other Type II dielectrics, are like little microphones. They will pick up mechanical noise. If you're on a boat, this can be problematic. Use plastic or NPO (COG) dielectric.
Choose your high-pass cutoff frequency wisely. If the cut freq is too low you will get acceleration noise (low freq noise caused by the movement of the hydrophone) which can blank the input.

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It uses two 12V batteries. Could I use just one yet be able to give both + and - voltages?

The AD743 will work down to supplies as low as +/- 5 volts so try using a mid-rail generator like this: -

enter image description here

All the ground points in your circuit go to the mid-rail point. Choose an op-amp that is low noise.

Gain is set with R2 and R3. Could I replace both of them with a potentiometer to have variable gain?

Replace R1 and R3 with just a single potentiometer - the junction of R1 and R3 is where the wiper goes.

The high pass frequency is set with R3 and C1. Could I replace the R3 with a potentiometer and add a switch to disable the filter when it isn't needed?

I would just choose a bigger value for C1; after all you don't need it to work down to 0.000001 Hz do you!

Both AD743 and AD797 are suggested in the diagram.

Read the data sheets. Swings and roundabouts. Look at the noise graphs in the data sheets. It has nothing to do with distortion.

How would you add either AGC or a compressor?

I would start by justifying that some non-linear process was justified so, over to you.

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Swap C1 with R3 and you have the gain controlling resistor at "ground". Because of C1, the voltage R3 goes to can be any quiet DC voltage.
R3 to the positive supply would be fine. Replace R3 with a voltage controlled resistor AKA a N-JFET and you have a voltage controlled gain.
Some AGC action can be done with a simple circuit driving the gate of the JFET
Look up how to make a "voltage doubler"
The AC into the doubler is the output signal
The The doubler makes a negative voltage but uses +12V as its ground.
A capacitor on the output of the doubler is charged when there is a signal on the op-amps output. A resistor across it discharges it when there in no signal.

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