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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 Feb 10 '17 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\$ – Tony Stewart Sunnyskyguy EE75 Sep 25 '18 at 12:41
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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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