Here are instructions for using an off-the-shelf polymer film piezo transducer to make an inexpensive pickup for either double bass or cello. Construction is very simple. The resulting pickup sounds excellent, rivaling commercially available pickups using similar under the bridge foot placement. If you can drill and tap a blind hole in a piece of wood and solder wires to a connector you have the skills needed to build this pickup.
There are a lot of commercial piezo pickups available for bass and cello. All work well, and each has distinctive advantages and disadvantages. The most common trade off associated with transducer design for these instruments is between tone and feedback resistance. In general, the closer the transducer element is to the strings, the greater the feedback resistance, and the closer the element is to the body the more the sound of the pickup will resemble that of the unamplified instrument. This pickup uses a polymer piezo transducer under the bass side foot of the bridge. Like other such pickups using this placement the tone is excellent, while the feedback resistance is fair.
Last updated: Saturday, August 15, 2015
The heart of the pickup is a polymer piezo film transducer made by Measurement Specialties. The product name for the transducer is SDT1-028K (catchy, huh) and the model number for the transducer is 1-1000288-0. You can order it directly from the manufacturer at the wholesale price but be forewarned that they have a high minimum order requirement which will not be met by an order of one piece. If you only want one, you can get this from Experimental Musical Instruments. The transducer comes with 18” of light shielded cable and is essentially ready to use. The instructions that follow tell how to make a simple tailpiece-mounted jack and how to install the completed pickup assembly on the instrument.
The first step is to build the jack holder. Start with a piece of ebony or rosewood 0.75” square by 2.25” long. Drill a blind 0.5” hole 2” deep, centered in one end of the wood. This is the hole for a threaded end pin jack. The hole does not go all the way through the piece. Next drill a 0.125” hole right through the piece so the wire can extend out the back. The 0.5” hole is then threaded using a 9/16 – 12 tap. The first photo shows the completed jack holder. Two of the edges of the holder have been rounded over for appearance. Sand and polish this piece as you see fit.
Photo 1 - The jack holder is drilled and tapped for a threaded guitar endpin jack.
The jack holder will be attached to the bottom of the tailpiece using Velcro. Cut a piece of 0.75” wide self-stick Velcro to fit the bottom flat surface of the jack holder. Peel off the backing and press in place.
Now the jack is wired. Trim all but 12” of the shielded cable from the SDT1-028K transducer. Run the wire through the small hole in the end of the jack holder. Strip about 0.5” from the jacket of the cable and brush back and twist the shielding. Then strip about 0.125” from the center conductor.
The jack is a Switchcraft threaded guitar endpin jack. It is available from either Stewart MacDonald or Luthier's Mercantile. You can also get the 9/16 - 12 tap from them if you don't have it, and they also sell a nice little key that can be used to screw in the jack.
Solder the center conductor to the tip lug of the jack, and solder the shielding to the sleeve lug. If you want to you can provide additional shielding for the tip connection of the jack by wrapping a small piece of copper foil with conductive adhesive around the base end of the jack. Such foil is available from the the lutherie suppliers mentioned above.
Photo 2 - The jack holder will be attached to the underside of the tailpiece using Velcro.
Thread the jack into the jack holder using a really wide screwdriver or the special jack installation key mentioned above. Using a machinist's scribe or tiny screwdriver, poke a tiny piece of foam weatherstripping into the hole in the jack holder that the wire comes through. This will keep the wire from rattling against the inside of that hole. The pickup is now complete.
Installing the pickup on the instrument is straight forward. Loosen the strings on the bass side of the instrument enough so you can lift the bass side foot of the bridge slightly. Slip the thin silver plastic part of the transducer between the underside of the foot and the top of the instrument. The black rubber cable connection part of the transducer should be positioned either on the in side of the foot (as shown) or on the side of the foot that faces the end of the fingerboard. In either case the silver plastic part should be completely covered by the bridge foot. You should be able to fold the black rubber cable connection part of the transducer up against the side of the bridge foot, too. Tighten the strings to secure the transducer in place.
Photo 3 - The transducer is wired to the jack.
The wire of the transducer on this pickup and on all piezo pickups is microphonic close to the transducer. It will pick up sound from anything it touches for about 12” from the transducer. So the wire must be routed such that it does not touch any part of the bridge, instrument top, or tailpiece for its entire length, and also so that it will not be accidentally touched by finger or bow. Route the wire through the bass side “eye” of the bridge using a big loop from the transducer to the eye. Now position the jack assembly on the underside of the tailpiece at a point where it is both clear of the string anchor holes and where the transducer wire will not touch anything. Stick the mating piece of Velcro on the underside of the tailpiece here. The easiest way to do this is to put the Velcro on the mating piece already stuck to the jack holder, peel back the backing paper, and stick the whole thing onto the underside of the tailpiece at the chosen location. Press it on hard so the adhesive sticks. You can then remove the jack holder and replace it as necessary.
Photo 4 - The completed pickup assembly.
The bass side “eye” of the bridge must be padded where the wire passes through it so the wire does not touch the bridge. Do this by wrapping a small piece of foam weather stripping (sans adhesive) around the wire where it passes through the eye, and push it into the eye. The foam will expand to fill the eye and keep the wire from touching the insides of it. The picture gives a much better idea of how this works.
The last step in the installation process is damping the black rubber cable connection part of the transducer. Since this thingie has considerable mass and is not well damped it will behave as an accelerometer if it is not strapped down. Place a cushion made of a small piece of foam weather stripping (again, sans adhesive) between the cable connection part of the transducer and the foot of the bridge. Strap it down to the foot using either a rubber band or a small flexible plastic cable tie. Installation is now complete.
Photo 5 - The pickup installed. Note that foam weatherstripping is used to isolate the wire from the "eye" of the bridge and the connection part of the transducer from the foot of the bridge. The jack can be seen (barely) on the underside of the tailpiece behind the bridge.
As with all piezo transducers, you'll need to connect this one to an amplifier through an impedance matching preamp. Very nice preamps made specifically for this purpose are made by Fishman Transducers as well as other manufacturers. If you hear any rattling or buzzing noises when you first fire the thing up, check to make sure the entire length of the wire running between the transducer and the jack is not touching any part of the instrument. Also make sure the cable connection part of the transducer is not touching the bridge or the top. Fixing these problems should eliminate any rattles. Play with the preamp controls to get the best settings for the preamp. As a starting point, add a little gain and roll off the bass a little bit.