I'm trying to attach an instrument cable to the DT4-028K/L w/rivets (Piezo film) and make something exactly like this Large PVDF Film Tab contact microphone. My only issue is I get large amounts of hum when I touch the film. I am using copper tape w/ conductive adhesive. Is this creating a problem? I have covered both the rivets with electrical tape before the copper tape application as well.

How do I properly replicate 2 and attach this PVDF to an instrument cable. I am currently using this unbalanced cable. I will use a separate Piezo geared pre amp.

I'm sorry but my knowledge is real thin here so please present your answers as if you were talking to a child.

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    \$\begingroup\$ I would suggest giving more details of your exact construction — photos would be good but also describe exactly what you've wired and how you are using the copper tape (and insulation where applicable). This will help you get a good answer if someone knows the relevant electronic theory but isn't familiar with this particular construction of contact mic. \$\endgroup\$ – Kevin Reid Mar 14 '19 at 16:19

Since the cable and amplifier are unbalanced, a tiny current from stray electric field is induced on the high impedance side only and not the ground side thus a difference potential is amplified.

It appears as if the example you show has EMI-type copper foil with adhesive substrate wrapped around the sensor for electrical insulation. The vibrations still couple thru it but the copper shields the electric field around the sensor and bypasses any current induced by your hands touching it.

You need a drain wire soldered to the copper with strain relief like hard vinyl tape or kapton tape.



simulate this circuit – Schematic created using CircuitLab

It is critical that any sharp edges on the sensor does not puncture the adhesive and electrically short itself to the foil for ground guarding.

Then use shielded twisted pair wire with the copper connected to the shield only at the sensor end and chassis ground at the amplifier end or signal ground for best results.

Grounding the copper at both ends may work but not as well. enter image description here

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  • \$\begingroup\$ Thank you @Sunnyskyguy EE75. I think i'm following you. So if I had a cable like this (redco.com/Mogami-W2697.html) And connected the inner wires to the Piezo Films leads, covered the thing in copper tape, then soldered the cables copper shielding to the copper tape as the ground? Would that work? Would I then have to use an XLR connector on the end of the wire, or could I just connect both inner wires to the tip of the 1/4" ? Thanks again. \$\endgroup\$ – user26575 Mar 14 '19 at 17:21
  • \$\begingroup\$ @user26575, no, you would not solder both wires to the tip. If you are connecting it to an unbalanced amplifier, you would ground one end (along with the shield) and connect the other end to the tip. If you are able to use a differential input, that would be better, in which case you'd use a TRS 1/4" connector, and connect one to tip and the other to ring, and the shield to the sleeve. \$\endgroup\$ – Nate S. Mar 14 '19 at 18:06
  • \$\begingroup\$ Thanks @Nate Strickland. I will have to hold off on the differential input, (as its out of my understanding at the moment). A point of confusion is that the product I am trying to recreate ( link above as Large PVDF) seems to just use a single 1/4". I would hope that for 100$ it doesn't have the noise/hum that I encountered. Here is an image of the PVDF. contactmicrophones.com/products-lftt.html \$\endgroup\$ – user26575 Mar 14 '19 at 18:50
  • \$\begingroup\$ You can do it single-ended, and it will probably work okay in a low-noise environment so long as you keep the cables short. Just don't touch the film. And I would not be at all surprised if the commercial version has the same issue. \$\endgroup\$ – Nate S. Mar 14 '19 at 18:54
  • \$\begingroup\$ @user26575 don't count on the $100 part being shielded. If you can't get a Diff. Input then connect the shielded cable and copper foil ONLY to the mic plug ground ring and NOT to sensor gnd. It is still unbalanced but with that and shielded cable, it may be adequate. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 14 '19 at 18:59

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