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I'm needing to build an electronic stethoscope.

I'm thinking to use this piezo film because I already used a piezo disc (ceramic piezo element I suppose) 25mm large (no idea which producer, eBay purchased) plugged into a preamp of an usb audio interface (Behringer UC2020).

Very good response obtained if layed on hard surfaces, almost nothing on body even at max gain. I think preamp does not give so much gain but anyway I suppose the problem lies in the mechanical characteristics of this piezo disc: it is a metallic plate which to be bent needs a considerable amount of mechanical energy; tiny chest movements (the mechanical energy is soft) are not enough to excite the piezo material.

Now I'm wanting to go forward to a piezo film, totally another mechanical properties, but the high output voltage may became problematic (tech specs says 90V). What would you suggest?

How can I make a circuit to make the output safe? It will be plugged with a XLR connector directly in the preamp of the audio interface.

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    \$\begingroup\$ You don't need to be an expert, but you do have to provide better information if you want anyone to bother helping you. We have no idea what exact issue you had with the ceramic part (or even what part you used), no idea what piezo film part you are using (beyond I guess it being a Meas-spec part). It doesn't help that your question is poorly put together/formatted, and not easy to follow. \$\endgroup\$ – I. Wolfe Feb 24 '16 at 22:52
  • \$\begingroup\$ ok, thanks for commenting. I'll add some infos. Keep teaching me!! \$\endgroup\$ – lazzaro Feb 25 '16 at 10:41
  • \$\begingroup\$ @lazzaro please do some testing, or at least collect and present your thoughts about real anticipated voltage/current outputs from the film in your use, input impedance, sensitivity and max 'safe' levels for your preamp, etc. Then do a little research on different existing audio preamp/transformer topologies to decide which direction you think will work well. Then modify this question to give use that information so we can "have something to go on" to help you here. \$\endgroup\$ – Robherc KV5ROB Feb 25 '16 at 19:10
  • \$\begingroup\$ Thank you for not scoring me down. I'll make some tests and when I'll get something I'll be back. \$\endgroup\$ – lazzaro Feb 26 '16 at 6:17
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Try using a voltage divider to take it down to about 3V for the amp. This is a nice calculator : http://www.raltron.com/cust/tools/voltage_divider.asp

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