Piezo contact Microphone and Impedance matching

[epilot]

Hello guys...

I hope you all are doing very well...

Been away for years...

I own a Professional Preamp and a recorder with a Microphone input impedance of 1.7K ohms...
I want to connect a Piezo disk to it to record something around 20Hz to 20kHz sounds.
So I found myself having the problem of mismatching between the Peizo and the recorder...

Just decided to build this Piezo Contact PreAmp to connect my 25mm Piezo disk to it (Capacitance of my piezo disk is 19nF)...

I came to the problem of needing the fets of teh circuit to be matched by hand...
So I decided to move to another circuit containing OpAmps...

I found this one, by R.Mudhar...

It is not a phantom powered circuit (Cause my recorder also supports phantom power, so I wished I could find a phantom Powered OpAmp version of Piezo amplifiers, but I couldn't find anything useful in the net...

So I decided to make that circuit by R.Mudhar anyway and see what result I can get out of it...

I Know we have some great, professional guys in this forum who are OpAmp experts...

So I have a few question for you please...

I think the impedance of my piezo is something around 1M ohms or maybe more...

1. Would you please let me know what is the input impedance of R.Mudhar circuit then?
I am afraid the input impedance of his circuit is not high enough yet for my Piezo, right?

2. The output of this circuit will go to the microphone line of my recorder which as mentioned before, it has a Microphone input impedance of 1.7 K ohms.

Please, can you let me know the output impedance of R.Mudhar circuit then too? I think I need the output impedance of the guy's circuit to be at least 5 or 10 times less than of the input impedance of my recorder. Right?

3. Don't I need any cap between the Piezo and the non inverting pin of the OpAmp (Like this one circuit)?

4. You experts, do you see that circuit to be a neat one to be used to match a peizo with a microphone line of an OpAmp please?

5. Do you see the circuit to be OK to be used for a spectrum of around 20Hz to 20kHZ (audible sound)?

6. I want the noise of the circuit to be as little as possible... Any idea please?

Thank you so much for any comment.

 

[Nigel Goodwin]

My main issue is the opamp, it's an excellent low noise opamp, but isn't particularly high input impedance.

Have you tried a simple single FET as a source follower?, one FET (such as a 2N3819), two resistors (2.2M and 1K) - give it a go and see how the noise level is, I've only used them for ceramic PU cartridges where they perform superbly. You could also add a capacitor across the supply, and perhaos a resistor feeding it - I always just used them powered by aPP3.

 

[alec_t]

Would you please let me know what is the input impedance of R.Mudhar circuit then?

The 5534 datasheet says the input resistance could be as low as 30k.

 

[rjenkinsgb]

I'd use the near-universal "Schoeps" circuit used with many condenser microphones; that has extremely high input impedance.

With a piezo transducer you just do not need the bias generator board, required for a condenser mic capsule.

eg. As with this example, omitting most of the bottom section that produces the bias; keep R10, D3 and its shunt cap, to stabilise the FET stage supply:

Other examples:

https://i2.wp.com/jonpattonmusic.files.wordpress.com/2017/04/schoeps-style-transformerless-circuit.png

https://www.audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic_files/BM800.jpg