I've spent the last few weeks attempting to build better piezo film pickups since the limited success with the prototype discussed in my last post. Unfortunately, the two I've built so far make noise mostly.Using the template above, I cut out two 5" by 1 1/2" rectangles of .003" thick PVDF film. Then, on the hatched out area on each side I put electrodes. For one of the pickups I painted each side with Amazing Wire Glue as the electrode. According to the label, Amazing Wire Glue is non-toxic adhesive and carbon. On alternate sides of the film, I also painted into either the (+) or (-) rectangle to provide a place for a contact to the cable. For the pickup made with Amazing Wire Glue, I stapled on the cable ends and then glued them, following the advice of Trout Cove Lutherie.
For the other pickup, I taped on copper tape with conductive adhesive as the electrode. On this pickup I soldered the cable ends directly to the tape on each side of the film. Here is what both pickups looked like before I had them laminated:
Then, my lovely wife laminated each using some self-sealing lamination. I soldered the ends of the cables to some 1/4" phono jacks which I covered in shielding foil and heat-shrink and then I was done.
I recorded with both and both work, but produce a lot more hum than signal. The foil pickup seemed to produce a little more signal (and hum). The hum isn't apparent until pressure is applied to the film (e.g by taping it to a guitar), but then it's overwhelming. I tried the following to no avail:
- I checked that to make sure that the positive and negative sides were not shorted (they weren't);
- I covered each temporarily with some EMI shield, which made the hum worse;
- I reversed the polarity;
- I tried a commercial film through the same preamp (and heard no hum).
2 comments:
Hey,
Just saw your post, it was interesting :-) If you want to play with piezo pickups for a vibrating instrument, I don't recommend PVDF... too 'mushy' for the amount of signal that you pick up. If you bond real PZT (lead zirconium titaniate) to the body you will get much more power out. The cheap solution is bond greeting-card piezo transducers (e.g. http://www.goldmine-elec-products.com/prodinfo.asp?number=G14378) to the body, e.g. using a thin as possible layer of epoxy (vacuum bag 24-hr epoxy. or use 5-min and hold it there very tightly til it sets). The botique solution is to use *large* pre-packaged wafers such as MIDE QPxx (http://www.mide.com/products/qp/qp_piezo_sensor.php - disclaimer, I work here!)...bonding process is the same, but there exist flexible, conformable versions (ask very nicely and you may be able to score cheap ones rejected for minor defects, but still very functional). No matter which PZT you use, expect a reasonable voltage (>1VAC) at very high impedance, similar to PVDF with higher power output. You might want to use some very high-impedance opamp to buffer the signal before it hits your big amplifier, as the output impedance of ANY piezo sensor (PVDF or otherwise) is on the order of a Mega-ohm.
The MIDE parts typically come with pre-attached leads you can clip directly onto. You can connect to greeting-card piezos with silver epoxy or carbon glue (Amazing Wire Glue), but direct solder connection is best.
Thanks Tim. I tried to make my own PVDF transducer because the commercial films I had purchased had such a good frequency response. Unfortunately, for whatever reason, even though I increased the output with my homemade PVDF, the output was just a lot of hum.
I've used several types of commercial piezo discs in various configurations with equally various results (see previous posts). I haven't tried cutting my own PZT wafers yet. I may.
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