Here I’d like to share my version of a piezo-pickup for a double bass (and most other instruments). Those pickups can be quite expensive if you buy them at a music store but actually there is not much to them. The same transducers build into professional pickups can be bought at your local electronics store for less than a buck.
The big advantage of piezo-pickups compared to microphones and magnetic pickups is their sensitivity and linear frequency-response. This is especially important for double-bass pickups in the low 20 to 600 Hz regime which is notoriously difficult to capture well. Also they don’t require magnetic strings which is nice for all you friends of nylon and gut strings.
The tricky part about a piezo-pickup is designing a good amplifier to go with it. If you use a piezo-pickup with an ordinary instrument amplifier you will often hear a shallow metallic tone, lots of fretboard noise and very little low frequencies. This is because piezos have a very high impedance. If you connect an amplifier with a lower input impedance to your pickup, you will get selective reflection of some frequencies (mostly the low ones) and that doesn’t sound well. So be sure to only use amps with a high input impedance (>1MΩ, better >10MΩ) or put an impedance matching (more precisely: impedance bridging) circuit in between. How to build one yourself will be described in a later post.
All you need to build the piezo-pickup is:
- a passive piezo-disc (I used a FT-36T), if you have an instrument in a low frequency range pick a large one, generally try to avoid a disc that has its resonance frequency in the range of your instrument, because this frequency will otherwise dominate all the others
- shielded, flexible, single channel (copper braid works best), coaxial cable
- a 6,3 mm audio jack socket (or 3.5 mm if you prefer to keep it small)
- tinfoil
- duct tape
- double sided tape
- wax/blu tack/glue
- a soldering iron and solder
And this is how you build it:
Solder the center wire of the coaxial cable to the white center area of the piezo-disc. Solder the shielding of the coaxial cable to the yellow (or sometimes silver) colored part of the disc. Make sure there is no short-circuit and maybe put some glue on the solder for stability. On the other end of the cable solder the shielding of the coaxial cable to the outer contact of the audio socket (GND) and the center wire to the inner contact (signal).
To minimize noise by induced external fields I added some shielding to the piezo-disc. This was done by covering the white part of the disc completely with double-sided tape and adding a layer of tinfoil on top. The tinfoil should have electrical contact to the yellow part of the disc but none to the white part. Because I didn’t like the color of the tinfoil I added another layer of black duct-tape on top of that.
On the other side of the piezo-disc (the just yellow or silver side) I stuck a thin film of blu tack. This is the side you stick on your instrument. Alternatively wax should work fine too. For the best sound use hard glue (i.e. epoxy) but only if you have no intention of ever removing it again. Try to use a thin layer so that no sound waves get muffled when they travel from the corpus through the blu tack/wax/glue to the piezo-disc. It is useful to know that piezo-discs primarily pick up sound waves through a bending deformation of the disc and much less through linear pressure on the two sides. Thus it is important that the whole area of the disc is connected to the corpus of your instrument. Hence sticking the pickup onto your instrument using a peg probably won’t sound very good. It also bears the danger of resonant buzz between corpus, peg and pickup. The same buzzing occurs when the cable touches parts of the bass and is allowed to swing freely. Thus avoid letting it touch the bass or tightly attach it to the tailpiece.
I’ve tested variants of this pickup with several piezo-discs connected in series or parallel. What I found was that they are inferior in sound and amplitude compared to a single large piezo-disc. I guess this is because of interference effects when combining the slightly phase-shifted signals of the individual discs that pick up the signal at different positions on the corpus or bridge.
Positioning of the pickup:
Now you should consider where to stick you brand new pickup. For the double bass I experimented a little, asked around and tried to come up with some explanations why the sound is so different at different positions.
On stringed instruments the bridge transmits the vibrations of the strings onto the corpus. The left foot of the bridge (see image on the right) transmits primarily low frequencies since the thicker (lower frequency) strings press on it. The corpus of stringed instruments is usually made of a soft-wood front and a hard-wood back. This is because soft wood resonates better in lower frequencies while hard wood resonates better in the higher frequency range. Hence below the left foot of the bridge there is a piece of wood glued to the front inner side of the corpus. It helps to spread the lower frequencies from the left foot of the bridge across the entire front of the corpus. So If you are interested in capturing primarily low frequencies stick it on position 2. Similarly below the right foot of the bridge transmitting higher frequencies from the thinner strings there is a piece of wood (the sound post) connecting the front to the back of the corpus. It transmits the higher frequencies from the left foot of the bridge to the back of the corpus. If you are interested in the higher frequency range stick the pickup on pos. 4 then. However the bridge also transmits sounds from the movement of your fingers on the strings and slapping noises very well. If you are a Rockabilly bass player and you want to capture slapping sounds stick it on pos. 5 or directly on the inner side of the fretboard. If not, I don’t recommend placing the pickup too close to the bridge. Finger and fretboard noises are primarily of higher frequencies and thus don’t transmit well onto the bass-corpus. If you want a base-heavy, very clear sound try pos. 1 close to the f-hole. There resonance is strong and you are pretty far from the bridge. My favorite position is no. 3. This is the one people in recording studios prefer because it gives a good balance between low and high frequencies, no exaggerated resonances in some specific ranges like you find close to the f-holes and a diminished but not inaudible slapping response.
Of course you can always stick one pickup on each position that interests you, amplify each signal individually and mix them together as you please.
