Piezo preamp fitting to my equipment?

Thread Starter

benam

Joined Apr 19, 2018
7
Hello,

I'm interested in building a preamp for a piezo contact mic like http://www.richardmudhar.com/blog/piezo-contact-microphone-hi-z-amplifier-low-noise-version/#comment-4719.
I already tried buillding something like this with a mpf102, and it worked (a bit), but it only worked on a guitar amplifier, has no overvoltage safety, and I had the fear of breaking my equipment when plugging it in. Now I want to go with the circuit described in the link. But I don't know if it will actually work with my piezo and my power-amp/recording tool (sorry, I'm a noob in calculating electronical compatibilitys). For example, I use a piezo of type 717770, which has a impedance of 800 ohms, but I thought that piezos usually have a very high output impedancy? Also, I don't know if the circuit is compatible with 'normal' line-ins, for example my ax-390 amplifier or a common notebook mic input (in my case, a t400). Can you maybe help figuring out if I can use this circuit with my equipment? It would be cool to be able to plug it into the most common inputs. Thanks!

Please, I really need an answer. It would already be cool if you can point me to some technics to calculate if the circuit is compatible (regarding the impedance, voltage and current levels).

Do I have to edit my question or why are there no replys?
 
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bertus

Joined Apr 5, 2008
21,595
Hello,

Looking at the schematic, the gain is about 11 times.
Can you post some datasheet of the used piezo?

Bertus
 

Thread Starter

benam

Joined Apr 19, 2018
7
Yes, I want a contact mircophone, but as far as I know piezos can actually act as both (and I already measured with an oscilloscope that it can pickup sounds).

How did you figure out the gain? And are there some ways to calculate the output impedance, the output voltage and so on, basically how the circuit will work?
 
Last edited by a moderator:

bertus

Joined Apr 5, 2008
21,595
Hello,

I have used such piezo element as pickup for a vibration measurement.
I had a soft piece of metal strip with the piezo glued to it.
One end of the strip was fixed to the stand, the other end was free and had a little weight in it.
When I bumped the floor, I could see a reaction of the piezo.

Bertus
 

bertus

Joined Apr 5, 2008
21,595
Hello,

The gain of the circuit is about (R4+R5)/R4:

Benam_piezoopa.png

The output impedance will be about the value of R6 = 470 Ohms.

Bertus
 

Thread Starter

benam

Joined Apr 19, 2018
7
Thanks really much! So when I want to increase the output impedancy, I can just increase the R6 resistor without side-effects? (Because, I think, a little higher impedancy wouldn't hurt)
 

Thread Starter

benam

Joined Apr 19, 2018
7
And is it a voltage gain or a current gain? Because I would like to know which voltages I can expect in normal use.
 
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