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Piezo Film Tab troubles

Discussion in 'Amps, Mics & Pickups [DB]' started by vertkurt, Aug 8, 2012.

  1. vertkurt


    Aug 8, 2012
    I am having troubles with my DIY pickup. I have it all soldered up and ready to go but it is not working.

    I have this cable or something similar to it http://www.markertek.com/Cables/Bul...Cable/Mogami-Wire-Cable-Corp/W2552-00-A.xhtml

    Right now I have it wired up to where the thick copper wire is soldered to one tip of the piezo and the red and white are stripped and twisted together and soldered to the other tip. Is this how they typically go together?

    On the jack end it is soldered the same way. I also have some copper shielding but am not sure how it wraps around it and every way that i have tried i get no response.

    I have found similar thread and one with a link to a website with detailed instruction, but the website no longer exists. http://www.talkbass.com/forum/f4/how-make-piezo-pickup-212443/

    Also, this pickup will be underneath the bridge of my homemade electric cello that looks similar to this one (Scroll down to see his pickup) http://www.thallenbeck.com/projects/ecello4/page8.php
  2. bssist


    Jun 23, 2007
    St. Louis, MO USA
    The way you have it wired is not optimal but you should at least get some type of noise out of it. Is there anything else you have done? Wrapped it in foil, etc?

    It might be better to solder the red wire to one tab, the white wire to the other tab. At the jack end solder the red wire to the center conductor, the white wire and the shield (the braided copper surrounding the red & white conductors) to the ring conductor. You will need to put an insulating layer around the piezo element & connections to prevent shorting them together. Then solder the pickup end of the shield (braided copper) to your copper foil that you place around the whole piezo element. That will help reduce hum and other unwanted noise.

    If you hear no buzz or any type of noise you most likely have the two conductors shorted together somewhere.

    Good luck!
  3. vertkurt


    Aug 8, 2012
    I have not wrapped it in foil yet because I'm not sure what I am doing.

    I will try soldering just the red and white ends to separate tips on the piezo.

    The jack I am using is a cliff jack like this one http://modulusamplification.com/14-CLIFF-UK-JACK-SOCKET-SWITCHING-MONO-METAL-NUT-P1930088.aspx So I'm not to sure what you mean by the center conductor.

    Thank you for the help! I'll be back once I solder the red and whites to separate ends.
  4. If you really have piezo film and not ceramic piezo disks the cable and connection is not the (main) problem.

    Your problem is the output impedance of the piezo film. You need a buffer with about 20 MOhm input impedance, which you need to make yourself or modify an existing preamp. Larger film area might have a smaller output impedance.
    Get the manual for the piezo film sensors and look out for data and connection examples.

    The cable needs to have a rather hard an not too thin inner isolation or you get noise by knocking on the cable. So make the cable from the piezo film to the buffer/preamp as short as possible. Good Microphone or instrument cable is generaly good enough but rather stiff. Thinner cable is sensitive to knocking. The best idea is to mount the J-FET directly near the piezo foil, like Shadow does on their SH-965NFX-B.

    If you do not understand what I'm talking about better stay away experimenting with piezo film and use piezo disks instead. You still might need a preamp, but "normal" ones with input impedance somewhere between 1 MOhm and 10 MOhm should work with them.
  5. vertkurt


    Aug 8, 2012
  6. vertkurt


    Aug 8, 2012
    Alright i have it soldered the way bssist instructed. At the piezo end the red and white are on separate tips and i have the shielding just twisted up and out of the way. The jack end i have the red soldered separately and the white and shielding are twisted up and soldered.

    I plug it into the base amp and when I flick the piezo end i hear it pop out of the amp. But when I put it underneath the cello bridge I get nothing coming out of the amp.

    As of right now every wire is exposed and I haven't insulated anything with electrical tape. Do I electrical tape every exposed wire AND the tips for the piezo? Or do I tape one side (either red or white) and leave one exposed THEN wrap copper foil around the entire piezo? What about the jack end?

    Again thank you for helping me. Atleast now I can hear a noise come out of the amp when tap the piezo :D.
  7. bssist


    Jun 23, 2007
    St. Louis, MO USA
    Ok, you hear noise when you tap the element. You are getting closer. Unfortunately, that means you need to look at the info about impedance matching. You'll have to check the specs on the piezo strips you bought. Just because someone else made it work doesn't mean you have the same element specs or amp input specs. Based on your original post I would not expect you to understand all the impedance, resistance, reactance, blah, blah, blah. That is ok, there is nothing wrong with not knowing. You'll just need to be willing to do some research and learn. I don't have time to try to explain it all (honestly, I learn it, use it, forget it, then go re-learn it the next time I want to do a project). You'll have to start searching the web for info.

    Here's what I can explain in the time I have. It looks like you have a "switching" jack. If you plug your cable into it the tip of the cable touches the "center" conductor. You need to solder your red wire to the lug that is connected to that arm. Then you solder your shield to the tab connected to the outside part of your jack, the one that is physically mounted through the mounting hole. You will solder the white wire to the lug connected to the arm that touches against the longer shaft of the plug just behind the black insulator on the plug just below the tip.

    If that does not make sense try looking for pictures online. Try looking up "TRS" connections and/or "stereo" connections.

    I hope this information is helpful to you but again, you need a lot more information about what you have and what you want to accomplish. You're not really likely to just stumble onto success the first time out.

    Good luck.
  8. vertkurt


    Aug 8, 2012
  9. The piezo foils are extremly heat sensitive.
    You might have dameged the foil if you soldered the cable without a heat sink. I did this myself once, I think, even using a heat sink then.

    The mint buffer is not bad, just try to use a 20 MOhms input resistor or two 10 MOhms in series.

    Which size is your foil? The smallest ones have a really high output impedance. I would not recommend them for a bass.

    Since I already experimented a lot with the foils of this manufacturer and know what difficulties arise, I recommend a lot of reading and understanding before you continue with your experiments. ANd you should have at least some basic knowledge of electronics.

    Get the manual and study the examples and technical data of the foils.
  10. vertkurt


    Aug 8, 2012
    The Piezo tab is 1" long by 1/2". I did my best to shield the foil with a pair of linemans pliers to keep the heat from melting the plastic. I think it turned out alright because nothing seems to be damaged on it. I will look into some different types of preamps and see if I cant make that mint buffer.
  11. vertkurt


    Aug 8, 2012
    Alright everything is good to go on the piezo. Everything is soldered up and it works.

    Unfortunately, I think I understand what you're talking about. I have a 50w bass amp and I have to crank it to the highest volume to get the sound to come out, and when it does it is hardly audible.

    So I will look into the prebuffers some more and see what I can learn about them and try my hand at building that mint box one after i learn a little more about it.

    Thanks for all the help everyone!
  12. The preamp/buffer generally doesn't make the signal louder (sometimes even a little bit softer), but the output could be loaded higher before the signal breaks down (starting in the bass frequencies). They are also more sensitive to bending than to pressure, so putting them under the bridge foot gives a rather low signal. But it also depends how much pressure is on the sensitive part of the foil and how much it differs by bridge movement. Putting a thin sheet of a hard material on the sensitive part only might help a bit to concentrate the pressure where it should go. (But this makes it even more sensitive to the mounting position.)

    I'm afraid yours is the smallest size (like mine). There are larger foils from this company, even one, more expensive, with a good size for a bass with a mounted cable. You can download literature about these foils from the manufacturers site, technical manual and product guide, I think.
    The small ones are good for first experiments, but for a bass you might want a foil that fits under the whole bridge feet. Response and good position for the foil also highly depends on theshape of the bridge foot (does it make contact with the outer parts first or is the pressure concentrated on the middle of the foot...).

    The electrical data for the larger foils show a lower output impedance (higher capacitance) for them, which gets a better bass response for 10 MOhms input impedance.
    I even tried to model all this stuff and decided I need the larger foils (but didn't order them yet).

    One side of the piezo foil is not isolated! If you just wrap copper foil with a conductive sticking layer around, it is highly probable you will short the two terminals.
    If you remove the copper foil, the sticking layer will remove the conductive layer of the piezo foil, so better put a layer of scotch tape around the foil before apply the copper foil.
    Then isolate one of the connectors (after you soldered the inner hot wire to it) before you apply the copper foil. BEFORE YOU DO THAT: check which connector goes to the unisolated layer and use the other one for the hot wire.

    Remember you might get a microphonic cable if you choose a thinner one to avoid stiffness. If you need to choose a thin one: keep it as short as possible. And put your buffer/preamp as close to the pickup as possible.

    Good luck.

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