More PVDF

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).

I don't know enough about electronics or piezoelectricity to diagnose the problem. I suppose it could be the material I used for electrodes (commercial films use silver ink), the dimensions of the film, the composition of the film (there are different kinds of PVDF), or something else altogether.

PVDF

Polyvinylidene Fluoride (PVDF) is a piezoelectric plastic. If you purchase a piezo pickup made of "film," it is made of PVDF. PVDF is said to generally have a broader, more balanced frequency response than ceramic piezo discs. If one or both of my readers remember, in my January post On Film I recorded a steel-stringed acoustic guitar with a commercially-produced piece of piezo film taped by the bridge. The frequency response was excellent but the signal was very weak, resulting in having to crank up my preamplifier and getting some hiss.

Hoping to resolve this problem, I purchased some PVDF film from McMaster-Carr in various thicknesses (.003", .005" and .01"), planning to make my own, louder, piezoelectric film pickups. Guidance on doing this is not readily available. There is a fairly good guide on using PVDF from Measurement Specialties, although it's not focused on musical instrument pickups. I also found some good information on making connections to piezo film from Trout Cove Lutheries.

Three things I have learned are: 1) you have to attach electrodes to the film, preferably each side, for only between these electrodes will mechanical stress be translated into voltage; 2) PVDF has some problems with hum; and 3) the dimensions of the film affect the low frequency cutoff (the lowest frequency for which the output of the circuit deviates less than 3 dB from the nominal value). So far I've focused on making the electrodes.

I cut a small piece of the .003" thick PVDF into a 1" by 4" strip, slightly larger than my commercial piezo film strip. For electrodes I stuck strips of conductive copper shielding tape on each side. I placed the stripped end of a wire under each and stapled them (unfortunately backwards) for a more secure connection. The result:


I taped this to a nylon-stringed guitar, ran it through my homemade mint-box preamp (see my first post) and recorded. The result was not stellar, but it did record audio and didn't sound awful, although not worth posting. I'm going to experiment with conductive glue painted on each side and possibly with silver ink from pens sold for circuit board repair (most commercial films have electrodes made from silver ink painted on surface). I'm thinking that different sizes and thicknesses of film will affect frequency response and signal strength .

Wooden Chips (wrapped in heat shrink, almost free)

I built two new pickups today, both using the same new design. I've become frustrated with dipping piezo discs in Plasti-Dip. I can't get the dip to dry evenly so that the surface of disc will be parallel with the layer of Plasti-Dip and thus flush with the soundboard of the guitar. I also worry about breathing in the allegedly carcinogenic fumes.

For my new design, I've mounted two discs on small squares of wood: one of balsa and one of basswood. I had intended to buy only balsa. But, at the hobby store, they sold large rectangles of basswood along with balsa for only $2.50, so I thought I would try it as well. I've learned since that basswood is from the linden tree (see above), used for electric guitar bodies, and supposedly has good acoustic qualities.

I soldered some lightweight, unbalanced Mogami lavalier microphone cable ( #W2368) on to two small unidentified piezo discs. I then glued each disc to a square of balsa and basswood, respectively. I tried to glue them so that the entire surface of the disc would contact the wood. These small piezo discs are made to buzz, rather than pick up sound, and are designed to resonate at a certain frequency, the resonance emphasized if a only specific point on the the surface of the disc is contacted. By having the entire surface contact the wood, the resonance is dampened and the frequency response broadened.


I then put each disc/wood combination into a tube of heat shrink and heated. I soldered 1/4 inch female jacks to the other end of each cable and wrapped the cables in shielding foil. I then placed each jack it heat shrink (clear the only kind I had that was large enough) and ended up with an respectable-looking pickup.


Both pickups sound good when taped behind the bridge on my nylon string guitar. The balsa pickup, maybe since balsa is lighter and not as dense, sounds and little hollow, emphasizing the midrange. The basswood pickup sounds great I think. Although it lacks a some bass (despite its name), the midrange and high-end sound very clear, i.e. it doesn't produce the slightly muffled "underwater" sound like other pickups I've built with piezo discs. It's also loud, but the wood seems to dampen some of the frequencies that otherwise peak to the point of distortion with my other disc pickups. I'm hoping that if I move the pickup nearer the sound hole, I'll get some more low-end. I think this basswood pickup is the best disc pickup yet. It doesn't have quite as broad a the frequency response as the film pickup, but its signal strength and clarity are excellent.

Some samples with me noodling around on a nylon string guitar:

Balsa Pickup

Basswood Pickup

My Beautiful Balloon

Bart Hopkin writes in his excellent book, Getting a Bigger Sound, that "A piezo attached to a balloon will give you a result almost like a rather poor microphone." Specifically, he notes that a balloon piezo will be weak "in the high frequency response." I was intrigued by this idea and so built a balloon piezo using one of the piezo disc pickups I had built.

The setup was simple. I taped the pickup to an inflated balloon and hung it by a string from a mike boom in front my guitar amp. As a preamp for the piezo I used an Art Tube MP Studio preamp I recently purchased (for $29). I also miked the amp using an Audio-Technica Pro 37 for a comparison recording.

The sound from the balloon is surprisingly good, but as Hopkin wrote, some high end is missing. Although there are certainly better ways to mike a guitar amp, I suspect this would make an interesting microphone for vocals. Imagine a group of singers standing around the balloon.

Listen below to my shambling (con rubato) reproduction of Jimmy Webb's Up, Up and Away. I include both recordings for comparison.

Up, Up and Away - Balloon Piezo

Up, Up and Away - Pro 37

Mogami Cable

My friend Jeff and I have a running joke about guitarist Eric Johnson who said that he wanted to write a book on cables, such a notion seeming to epitomize the indulgence of obsessive musicians and engineers more interested in eliminating every last bit of noise than in making compelling music. I haven't yet become an Eric Johnson fan, but am now more convinced about the value of good audio cables.

So far, I've built all of my pickups by chopping guitar cables in half (so I don't have to resolder the plugs) or by using scrap, balanced, 1/4 " microphone cable. These cables work fine but they're heavy and tend to get twisted. They pull on the piezo element and the plug or jack, which are taped on, resulting in noise from extra vibration or the pickup becoming detached altogether.

Again following the lead of David Fittell's site, as I did when I built a piezo film pickup, I purchased some lightweight Mogami cable. I bought three types from Redco Audio:

• W2368 - miniature unbalanced microphone cable, only 2 mm in diameter;
• W2697 - miniature balanced microphone cable, only 2.5 mm in diameter; and
• W2319 - guitar cable built for high impedance things, only 5 mm in diameter.

I've only built two pickups with this cable, using about 2" lengths of the two microphone cables with a $0.40 Murata piezo disc at one end and an 1/4 phono jack at the other. I didn't do a good job with the Plasti-Dip or the soldering and so have left these as rough-looking prototypes, i.e. I haven't added any foil or heat shrink, but I haven't heard any hum yet. I also haven't heard any difference between the balanced and unbalanced cable.


These pickups are excellent. I modeled them after the designs of the commercial pickups I discussed in my last post. Each unit is so small I can tape it in its entirety almost anywhere I want on the guitar. There is little stress on the pickup and no resulting vibration. Also, in the pickups made with guitar cables, the cable itself, attached directly to the piezo element, is very sensitive and will make noise when touched or rubbed. In these new pickups, the cable between the pickup jack and the amp or preamp is not so sensitive and doesn't make any noise when touched. I haven't recorded anything yet with these but will soon. And, the cost for each, given the cheap Redco prices, is about $1. Subarashi!

Cost-Benefit

Along with the almost indescribable pleasure derived from making piezo pickups come significant savings. Below I’ve compared the prices of three commercial piezo pickups to the cost of making your own. I chose three pickups very similar to my piezo disc pickups, i.e. piezo discs soldered to a cable with a jack or plug at the other end. The price for the Hot Spot is from Experimental Musical Instruments; for the next two, from Amazon. None of the prices include shipping.

K&K Hot Spot	$32
Barcus-Berry Piezo Transducer Interior Mount	$54.95
Fishman National Resophonic/Resonator Guitar– Passive	$109.95
Average Cost	$65.63

Now consider the approximate costs of making your own:

Piezo disc	$0.50
Shielded Cable	$3.00
Jack or Plug	$1.50
Solder, foil, etc	$1.00
Total	$6.00

The cost for the homemade pickup is per unit, assuming you make two to four, and thus decreases the more you make. I haven't included the cost of tools you might need (soldering iron, wire clippers, etc.). But, if you didn't have them, their costs would likely be comparable to the shipping you would pay for any of the commercial pickups.

I don't own any of the commercial pickups and maybe they sound great. But, I would be surprised if they sound 5X to 18X as good as the homemade pickups. It's likely these manufacturers purchase their piezo materials from the same source I do or you would. In regard to discs specifically, I haven't come across any "really good" expensive discs. They all seem to be intended for cheap buzzers.

Another benefit I came across recently is that because piezo-electric materials don't pick up sound, multi-tracking is headphone-free. For any successive track, I can play along with my monitors. Also, I can record while my 5-year olds run around the house screaming. These benefits would accrue however, even if you paid $110 for piezo disc with a wire soldered to it.

Tequila!

I haven't made any new pickups this week and so thought I would entertain both of my devoted readers with a delightful recording of Tequila!

I recorded everything with a piezo pickup except for the shouting, which was provided via microphone by my three in-house vocalists. Everything went straight into the Firebox. The details:

• Ukulele -- I doubled tracked the ukulele, both times using the small piezo disc taped in front of the bridge. One of the ukuleles has a bit of VST chorus I forgot to take off before I mixed down to two tracks.
  

• Telecaster -- I taped the large piezo disc, which has a very tinny sound, between the pickups. I then applied the free VST Electri-Q equalizer, using a setting named "Electri-Q Guitar."

• Electric Kazoo -- a very noisy instrument, made by taping the small piezo disc to a kazoo. I applied the now outdated Amplitube LE VST amp simulator to beef up the sound and then a VST limiter to get rid of some of the noise.

• "Cymbal" -- this is a pot lid, hung from a mike boom by a string and played with a wooden chopstick. I taped the piezo film pickup to the bottom of the lid.
  

I added a little bit of reverb (using the free Vosengo OldSkoolVerb) to the final mix.

Tequila!

It Takes Two (or Maybe Three)

I've built a new pickup that sounds very good. I wired two small piezo discs in parallel and laminated them.


I soldered on a short piece of shielded cable with a 1/4 phono plug and then wrapped the square of laminate in copper shielding foil, letting the conductive adhesive contact the ground.


Connecting the piezo discs in parallel halves the output impedance, which might account for the good frequency response. It also could be that the two discs pick up a more representative sample of the soundboard vibrations, but they're not very far apart. Having two discs wired in parallel cuts the signal too, so I needed to add some more gain from the preamp.

Below is a sample recorded with the pickup taped behind the bridge of my nylon string guitar. The selection is my attempt at Beck's version of Skip James' Jesus is a Mighty Good Leader. The sound has an odd muffled quality, but overall is good. I may try three discs next.

He's a Mighty Good Leader

Pink Noise

Adam Kumpf, a really smart guy at MIT, built a piezo pickup much like mine, although he installed his permanently in a guitar. On his page, he posted frequency response curves for his pickup versus a commercial piezo and a Fender Stratocaster. I liked this idea and given the varying results with my three pickups, decided to do something similar.

I'm guessing that the vibration of a guitar soundboard, especially when contacting human bodies of varying density, is complex. So, to replicate such an environment when testing each pickup, I conducted the following highly-controlled experiment. I taped each element to a thin wooden board using painters' tape and hung the board by string from a wooden ruler in front of my right monitor. The ruler was held down by the plastic lawn dwarf who resides on top of the monitor. I then played pink noise though the monitor and simultaneously recorded through each pickup.


The results show some of what I've heard so far with the three pickups. First, here is the frequency response curve of my pink noise sample (when adjusted for a 3 db slope, it's flat):

Here is the curve for the small piezo disc pickup:

For the large piezo disc pickup:

And, for the piezo film pickup:

All of the pickups have jagged responses, even when averaged. This isn't entirely bad, since you can use different microphones and pickups for various purposes, to emphasize or deemphasize different frequencies. But, these curves don't compare well with an average microphone curve.

Nonetheless, I think the small disc sounds pretty good. Its curve, though jagged, has some high peaks throughout the range. The big disc sounds loud and tinny. I'm not sure if that's evident in its curve, especially when compared to the small disc. The film pickup sounds the best but produces very little signal (evident in the amplitude of its curve; I had to blast the pink noise just to get that). Otherwise, I'm not sure what these curves show, except that frequency response in these pickups is uneven.

By the way, if you compare these curves with Adam's, you'll see that his are much smoother, even though his pickup is almost identical to my disc pickups. He told me that he didn't use pink noise but instead "recorded audio data over a series of chords and notes that spanned the entire range of the guitar and then did post-processing in audio software." This is, in a way, a more accurate way to test these pickups since an acoustic instrument would never produce pink noise but instead a smaller range of frequencies. I may try it, but all this has made me tired.

Size Matters

In the case of my piezo disc pickups, it's bad.

I made a new pickup using a larger disc (about 1 5/16" diameter). I constructed it in the same way as I had the first pickup -- by soldering it to a short piece of guitar cable and then dipping the disc and about an inch of the cable into Plasti-dip.

The large disc gives a strong signal, but distorts very easily, regardless of where I tape it on the soundboard. I don't know whether this is entirely a function of the disc's size or if there is something different about its composition compared to the smaller disc I first used. The sound quality is not great.

I recorded with the large pickup taped behind the bridge of my nylon string guitar. I plugged it directly into the Firebox. I then recorded the same bit with the smaller disc pickup, also taped behind the bridge and plugged directly into the Firebox. The difference in frequency response is significant, with the larger pickup sounding like it's playing through a portable AM radio. This isn't necessarily always bad though as I can used this pickup when I want a lot of midrange.

Large piezo disc pickup

Small piezo disc pickup

On Film

Today I built a pickup using piezo film. I purchased the film in two lengths from Edmund Scientific. I'm reluctant to recommend them as a source because, even the though their prices were low and the delivery was prompt, their website was not entirely honest about shipping costs or availability of items.

Of the two sizes of film they sent me, I used the larger which is 16mm X 73mm (almost 3 inches long). I thought this would fit nicely behind or in front of a bridge. Also, in his fantastic book on pickups, Getting a Bigger Sound, Bart Hopkin writes that the larger films give a stronger signal and a more representative sample of the sound board's movement. I may use the smaller films for a ukulele pickup.

To build this pickup, I followed the guidance from David Fittell's site. I followed most of his instructions, including using shielded cable and a female phone jack. I did not glue cork to the pickup or put heat shrink on the jack.


Using the new film pickup and later the chip pickup I'd already built, I recorded the theme from The Munsters, played on an Epiphone with very old strings. I recorded directly into my recording interface, a Presonus Firebox. This time I taped the pickups behind the bridge using painters' masking tape. The difference between the new film pickup and the disc pickup is significant. The disc gives a much stronger signal. But, the film pickup seems to have a more realistic frequency response.

You'll hear some hiss in the sample with the film pickup. I'm not sure if it's a hum resulting from not yet effectively contacting the ground of the jack to the foil or if it's because of a gain problem. The signal from the film pickup was so low I had the preamp on the Firebox turned up all the way.

The Munsters' Theme - piezo film pickup

The Munsters' Theme - piezo disc pickup

Hello

I've started making my own piezo pickups for guitars and ukuleles. I don't want to install them permanently in any instruments and so am experimenting with different means to attach them, scotch tape being my current favorite.

I've built only one so far: a 3/4 inch diameter piezo disc soldered to a short length of guitar cable. I bought the disc from Electronic Goldmine in a three-for-a-dollar package. I bought other discs as well, but these were the only ones with pre-soldered leads. Soldering wires to these discs is incredibly difficult. I pulled off the leads and used the solder already attached to the disc.

After I soldered together the the cable and the disc, I dipped the disc and an inch of the cable into black Plasti-Dip, a weird, and according to the State of California, carcinogenic substance. The idea for dipping the pickup in Plasti-Dip from Nicolas Collins' book Handmade Electronic Music. According to Collins, the Plasti-Dip strengthens the pickup connections, reduces hum, and reduces some of the high frequency resonance of the disc. All these things turned out to be true.
Because piezo pickups have high impedance outputs, they require preamplifiers for better sound. I've used some on hand and built one so far. Here are some sample recordings of the above plasti-dipped pickup and some preamplifiers. The samples are a Mel Bay scale exercise played on a nylon string guitar. I taped the piezo right near the bridge under the high "E."

The first sample was recorded with my recording interface, the Presonus Firebox. It contains its own preamplifiers so I could plug the pickup directly into it. Notably, all of the other samples must go through the Firebox. (Click on the pictures to play the clips.)




The second sample I recorded with the Digitech Genesis 1, an effects unit and amp emulator now discontinued. I bought it new about 6 years ago for $70. All of the settings were set half-way and all of the effects (compressors, noise gates, etc.) turned off. I set the amplifier to "Brit Combo," which supposedly emulates a Vox AC30 -- an odd setting for a nylon string guitar, but it sounded natural.


The third sample I recorded with the "Mint Box Piezo Buffer," a preamplifier I built. I was under the impression when I started building the first pickup that I would need some sort of special preamp, i.e. one that could handle very high impedances. This turned out not to be the case, but building this was lots of fun. The design is by an engineer named Scott Helmke. Scott is a very nice guy and answered many of my dumb questions while I was building this, such as why the tantalum capacitor started smoking the first time I plugged in the battery. The mint box sounds great through an amplifier, much better than a piezo alone. I am disappointed that the recording sounds not so good: too much midrange. But, I believe the Mint Box was designed for live settings.


Finally, I recorded the same exercise using an Audio-Technica Pro 37 microphone as a reference.


I'm happy with the sound of this pickup and think that, for accurate reproduction, the Firebox alone sounds best. I have some more discs and some piezo film I'll write about next time.