I made an electric violin. Here are some details for people who might want to build their own.
The body was mostly made by my father as a high school graduation present. It is made of one piece of black walnut up the middle, with separate pieces of walnut glued on for the “wings”. The lighter wood on the top and bottom is bird’s eye maple, in five slats per side, also glued. The only finish on it is Tung oil.
The pickups and electronics are my half of the project. The pickups are industrial pressure-sensitive piezo washers. They’re meant to be mounted under machinery to measure pressure and vibration, probably with a bolt through the middle. Instead, I have them mounted under the bridge to measure vibration.
As these were given to me third- or fourth-hand, I don’t know where to get them. Google search for “piezoelectric ring sensor” or “piezo ring transducer” should find something. Here is a company that sells small quantities. I haven’t tried them or anything, but people keep asking where to get them. If you try them and they work for you, leave a comment. You can also check eBay for odd parts like this.
I wouldn’t recommend the popular piezo buzzer type elements often used as “contact microphones”, since those are thin and designed to produce sound by bending. They aren’t designed to have things crushing them, and can be easily cracked.
To mount the pickups, two holes were drilled in the top of the violin, the same diameter as the piezos but about 3 times as deep. (Very professionally drawn MS Paint diagram.) A metal disc and a metal washer were made to the same dimensions as the pickup. The metal washer goes in first, with a wire soldered to it. The piezo sits on top of this, and then the metal disc sits on top of that, with the other contact wire traveling up through the middle of the washer and piezo and soldered to the middle of the disc. (Here is another quality diagram of the assembly.) This way a good, flush, mechanical connection is maintained to the top of the pickups, reliable electrical contact is made to both sides of the piezo, and the only thing visible from the outside is a nice sandblasted metal disc. (Here is a high-res closeup of the assembled bridge and pickups.) I was worried that the metal disc might not be the best transmitter of sound, but it has worked pretty well. Originally I was using tiny wires or metal film wedged under the bridge, but it was very prone to falling apart. Before that, I used a flex-sensitive piezo just jammed under the bridge, like the kind found in buzzers. It worked ok. Before that, I tried bolting the washers to the side of the bridge, sticking pieces of pressure-sensitive piezo in the “kidney” of the bridge, taping flex-sensitive to the side of the bridge, etc. This method seems to have worked the best. The best professional piezo pickups are actually several separate pickups mounted inside of the bridge itself, two or more for each string. I couldn’t afford to buy one, and didn’t know how to make one, but this works pretty well. [Years later: I can afford one now; maybe I’ll buy one someday and compare.]
In building it, I learned that the predominant motion of the bridge is actually a side-to-side rocking. Theoretically, the bass bar (a piece of wood that runs lengthwise on the inside of the top of an acoustic violin) sits directly under the right foot of the bridge and the sounding post sits under the left foot. The bass bar keeps one foot rigid while the bridge rocks back and forth, transmitting vibrations through the sounding post to the back of the violin. I’m sure it’s not really that simple, but the rocking back and forth is pretty accurate. When I first hooked up the piezos I was surprised to see that the waveform coming out of one was upside down from the other. As the pressure increases on one pickup, it is decreasing on the other. Since this would sound weird in headphones, and I wanted to be able to sum the signals, I just flipped one of the piezos over during construction, so that the polarity would be the same for both. Summing them while inverted would cancel out all of the signal except for the tiny bit that is different between channels.
The low frequencies are almost exactly the same from left to right, although the left pickup is louder for low strings and the right is louder for the high strings. The overtones, however, have some phase shift, which gives it a little stereo image. This can be seen on an oscilloscope with an X-Y trace mode. Playing a single clear note results in a mostly diagonal line, with little difference between the two channels. Little loops are visible because of the difference between the overtones. Playing a “harmonic” or plucking the string usually gives more interesting plots.
The waves have a definite polarity to them. When you up-bow there is a positive peak in the waveform, and vice versa. Absolute polarity of sound isn’t audible by itself, but it becomes audible after passing through any kind of asymmetrical distortion.
For testing the pickups, I made a little assembly with a block of wood to sit on top of the body, under the bridge. This way I didn’t drill big holes and then regret it. Your fingers start hurting pretty bad with all the tension from the very high bridge. The edge of the test base cracked when I was drilling it, so I made the actual holes a little closer to the center. The bridge feet don’t sit right in the center of them this way, but it still works fine.
The preamp circuit is an op-amp configuration called a charge amplifier. Piezo sensors can be modeled as a voltage source in series with a capacitor (as shown in the schematic), and charge amplifiers are the perfect complement for a capacitive source. (Similar circuits: 1, 2, 3) The other alternative for piezos is a very high input impedance resistive amplifier.
The high resistances create a lot of thermal noise, though, making the charge amp superior. [Edit: Where did I get that idea? Any thermal noise should be shunted by the capacitance of the pickup.]
J. Donald Tillman points out that an FET preamp’s non-linear response is actually better than an ultra-linear op-amp’s, since an op-amp will clip hard (important as the battery dies). An FET will distort sooner, but not as harshly, which is probably better for an audio preamp. Might be something for others to consider. I haven’t had any noticeable problems with clipping, and gave it significant headroom, but, then again, I play pretty distorted most of the time, anyway…
I measured the voltage output of the piezo by plugging it directly into an oscilloscope and playing hard. It was hard to read an absolute value, since it tends to make very large short duration spikes, but I measured about 10 V peak-to-peak at max. A typical signal was only about 4 VPP though. I decided to cut the voltage in half because a non-clipping signal is more important to me than a high signal-to-noise ratio, and it would probably be plenty acceptable anyway.
The gain for this charge amplifier is
- G = Cinput / Cfeedback
Notice that the terms are reversed compared to the typical resistive inverter equation. I measured the capacitance of the piezo with an LCR meter and got approximately 680 pF. So I am roughly halving the voltage (680 pF / 1500 pF = 0.453) to get the peaks well within the ~8 VPP rails of the op-amp (0.453 × 10 V = 4.5 VPP).
The feedback resistor determines the lower cutoff frequency, or the speed at which a DC voltage on the piezo would discharge back to 0 V, depending on your perspective. When I built the first preamp circuit (and there were several failed versions), a “constant” DC voltage would appear all the time, sliding the signal up past the power rails and clipping it off. Because the piezo is under the bridge, any change in the tension of the strings (fingering) or the pressure from the bow translates into a huge pressure difference for the piezo, which would then output a DC voltage. Since there is some inherent resistance in the piezo element itself, this charge slowly dissipates, but it can be very slow. In resistive amplifier designs, a large resistor needs to be connected across the leads of the piezo to dissipate this charge, but it has to be a large value so as not to dissipate the signal, too. In the charge amplifier, the feedback resistor performs the same function. The lower −3 dB point is
- fc = 1 / (2 π Rf Cf)
So mine has a low cutoff frequency of about 100 Hz. The lowest note on a typical violin is G3 (depending on your numbering system) = 196 Hz. So I have almost an entire octave below the lowest note I would typically play left over for down-tuning and flat frequency response. One problem with this is that the piezos pickup a lot of low-frequency handling noise from the body of the instrument, but it’s not that bad. Higher-order filters could be helpful to keep frequency response flat for frequencies of interest while cutting out handling noise.
The non-polar electrolytic capacitor is for blocking any DC that might be present, even though there shouldn’t be any, since the output is referenced to the bias point. It isn’t hurting anything, though. Regular electrolytics are not ideal, since there is no constant voltage to keep them polarized.
I have the two pickups output separately, in a “stereo” approach. Since my amp and effects pedal (and most everything with an instrument input) uses mono, I needed a way to mix the two together easily. Originally I had an elaborate circuit planned to output stereo to a TRS plug and automatically convert the stereo output to mono when a TS plug was inserted, with only one op-amp. It used capacitive summing amplifiers and involved summing the left channel and two copies of the right channel, one of which was the inverted R channel output. L + R − R = L. Then, when a TS plug was inserted, it would ground the inverted R, leaving L + R at the tip (+6 dB). So it would output L + R in mono mode and −L, −R in stereo mode. However, I then realized that I couldn’t use that circuit and also have a gain or volume control (though this may be useful to someone else). Eventually I decided to just use a “backwards Y” connection to mix the inputs.
Connecting two outputs together is almost always a very bad thing, but you can actually do it in certain situations (as I learned). The problem with connecting two outputs together is that they tend to have very low source impedances. When one output is tied to another, it ends up putting a lot of power into that low impedance, which tends to burn them both up. The 5 kΩ resistors on the outputs of the amp prevent the two op-amps from “seeing” each other; instead they see a large load impedance in parallel with a 5 kΩ resistor to ground. So now I have two cables; one is a normal TRS cable for stereo output, and the other is a homemade TRS to TS cable which connects the tip and sleeve together for mono output.
The capacitors from the power supply to ground prevent it from clipping early and doing other weird things. From what I understand, the changing impedance of the output stage of the op-amp causes signal frequency changes in the supply voltage, which is smoothed out by these caps before they can have a feedback effect on the inputs. Before I had them, it was clipping strangely at about half the voltage it should clip at, depending on the signal.
Both the chin rest and shoulder rest are standard models meant for a regular violin, which has a “lip” on the edges for them to grab onto. Mine just has 90° angles, so both tend to slip off. I might try to attach something permanently. I also might try to make a shoulder rest that extends farther down the chest, to give some leverage to counteract the violin’s weight.
It’s very hard to hear yourself while playing with noisy guitars, drums, and the like. It turns out that hearing yourself is much more important for the violin than a fretted guitar, though, as that’s how we stay in tune. We get as close as possible to the right note with muscle memory and then shift into tune with the ear as the note forms. When I can’t hear myself, I’m very out of tune. So I am probably going to build some type of belt-pack in-ear monitor. Another option might be a little speaker on the violin itself, near the left ear. That would use up a lot more power than the preamp alone, though.
If I made another one, there are a few things I would change.
- Weight: It is way too heavy. I would not have made it as thick or put the “wings” on it if I realized how much it would weigh. It’s not heavy for a piece of wood, but it is heavy compared to a mostly hollow acoustic violin. I can hold it up with my chin fine, but it gets tiring, and then I tend to use my arm to hold it up, too. It’s not really something I can fix, though the expanded shoulder rest idea might remove some of the torque.
- Tuners: Machine heads are far superior to pegs. Especially pegs in homemade holes, which do not turn smoothly and either end up too loose or too tight in different positions. If I could go back in time, I would have done it like most modern electric violins do, with the strings fixed to the end of the violin, and machine heads at the base for tuning; no separate fine tuners, no scroll, and less weight at the end, where it matters.
- Update: I bought a set of Knilling “perfection” planetary gear pegs. They’re expensive (especially compared to the guitar machine heads I wish I had used), but seem to work, and I have removed the fine tuners from the tailpiece. An alternative is Pegheds, which are even more expensive. I haven’t used them for very long, so they may have wear issues that I don’t know about. They don’t feel as impeccably manufactured as they are priced; the resistance to turning changes slightly as you turn the peg in a full rotation, which wouldn’t happen if all the parts were perfectly symmetrical. There’s also an obvious manufacturing seam, which I sanded off.
- Strings: I would have given it 5 or 6 strings. Sometimes I think I should have taken up cello or viola instead; I always want to play lower notes than it is capable of. Sometimes I play with the strings tuned down a step or two, but you can only take that so far. Octave effects work, and can sound really nice, but only for some applications. I have read about specially made “baritone” violin strings, which are very thick and sound an octave lower than normal…
- Power: The battery lasts a long time, especially with a jack switch to disconnect it when not plugged in, but phantom power might be a better idea. The in-ear monitor and phantom could be in the same box.