Piezo Film and Cable for Audio Pickups
INSTRUCTIONS FOR WIRING AND USE
Piezo films and cable manufactured by Measurement Specialties, Incorporated (www. http://www.meas-spec.com)
For sale through Experimental Musical Instruments (www.windworld.com)
This information is provided by Experimental Musical Instruments. More detailed technical specs can be found at the MSI web site.
SDT1-028K with quarter-inch jack: If you purchased this item, no additional wiring is needed. You can use a standard lead cable with quarter-inch plugs (such as a typical electric guitar cable) to run from the pickup’s jack to the instrument input on an amplifier.
SDT1-028K without the jack: This pickup comes with a shielded cable already attached, but with no jack or plug on the end. If you purchased this item, you have a choice as to whether to solder a plug (male connector), or a jack (female connector) to the end of the cable. Alternatively, you can wire it to some other connection such as a volume control. In this pickup, the white wire and the silver shield wire (which you’ll find already soldered together) will serve as the ground conductor, and the red wire will serve for the hot conductor. Various hookup options are outlined below under the heading “And Connect It To … “
2.5″ piezo film or the 6″ piezo film: If you purchased either of these, you’ll find that they come with lead wires already attached. These lead wires, however, are not shielded, so they’ll be prone to pick up unwanted electromagnetic noise from the environment. You have a couple of choices:
a) If the idea of a bit of extra noise in the pickup signal doesn’t bother you, you can use the existing lead wires. Connect the red wire (labeled “Neg”) to the ground terminal of whatever you’re connecting it to, and the black wire to the hot terminal. See “And Connect It To…” below for more details.
b) For better signal-to-noise ratio, replace the existing wires with a shielded cable (available through Experimental Musical Instruments and other electronics supply outlets). Don’t solder the cable directly to the existing terminals on the pickup. Instead, cut off the unshielded wires from the piezo film, leaving a segment of each wire about an inch long still attached. Connect the red wire (labeled “Neg”) to the braided shield (the ground conductor) of the shielded cable. Connect the black wire to the inner wire (the hot conductor). With the shielded cable in place you can proceed with one of the options described below under “And Connect It To…”
For anyone who is not familiar with procedures for connecting to shielded cable, here’s more information:
Shielded cable, which is standard for carrying low-level audio signals, is cable containing two conducting wires: an insulated inner wire to carry the hot signal, and a ground conductor in the form of a braided wrap around the inner wire, which also serves as the shield. To prepare the cable for soldering, first get some shielded cable and cut a suitable length of it. Remove about an inch of the outer insulation layer at one end, exposing the braided shield beneath. Coax the exposed part of the braided shield off of the center conductor, pulling it to one side and twisting it together. This will expose a little less than an inch of the center conductor. Remove a half inch of the insulation from that.
To connect the shielded cable to the shortened wires from the pickup, remove the insulation to expose about a half inch of the raw wires from the pickup. Join the red wire from the pickup (labeled “Neg”) to the braided shield (the ground conductor) of the shielded cable by twisting them together, then soldering. In the same way, connect the black wire to the cable’s inner wire (the hot conductor). Afterwards, you may choose to cover the terminal areas with electrician’s tape or other insulation to prevent shorting.
If you wish to take shielding one step further, you can shield the terminals and any short lengths of unshielded wire as well. Here’s one easy way to do this: Wrap the terminals and unshielded section of wire in electrician’s tape to prevent shorting, leaving a small section of ground wire exposed. Then over-wrap the area with a layer of adhesive-backed copper foil (available at garden centers, hardware stores or electronics supply stores.) Find some way to solder a connection between the adhesive-free side of the foil and the ground wire.
1″ piezo film tabs: If you purchased these, you’ll find that they come with solder terminals but no wires attached. You will connect a suitable length of shielded cable to the terminals (shielded cable is available through Experimental Musical Instruments and other electronics supply outlets). IMPORTANT: The heat of soldering easily melts the surrounding plastic and compromises the contact between the terminals and the piezo film itself. To avoid this risk, use conductive epoxy (available at electronics stores) to attach the lead wires to the terminals. If you do choose to solder, be very careful to not to use excessive heat. Use a low-wattage soldering tool, avoid lingering too long with the soldering tool, and use a heat sink. [A heat sink is a more massive metal object in firm contact with the metal being soldered, which helps to draw away excessive heat during soldering. A hemostat or similar small, spring-mounted clamping device works nicely. Even an alligator clip, binder clip, or small spring clamp will help.]
To determine which terminal to connect to ground and which to hot, look at the piezo tab so that you can read the words “MEAS” and “meas-spec.com” as printed on the tab. In this position, the lower terminal is the one to connect to the shielded cable’s ground conductor (the braided surrounding wire). The upper terminal connects to the hot conductor (the center wire).
If you don’t already know how to prepare shielded cable for soldering, see the instructions on the previous page. After making the connections you may choose to cover the terminal areas with electrician’s tape or other insulation to prevent shorting.
With the shielded cable in place you can proceed with one of the options described below under “And Connect It To…”
As an added option, if you wish to take shielding one step further, you can shield the terminals and any short sections of unshielded wire as well. Here’s one easy way to do this: Wrap the terminals and unshielded section of wire in electrician’s tape to prevent shorting, leaving a tiny section of ground wire exposed. Then over-wrap the area with a layer of adhesive-backed copper foil (available at garden centers, hardware stores or electronics supply stores). Find some way to solder a connection between the adhesive-free side of the foil and the ground wire. It is usually assumed that the remainder of the piezo film itself does not need shielding; however, a couple of customers have told us that shielding the entire pickup (covering in the lightweight copper foil connected to ground) does reduce potential noise levels further.
Piezo Cable: This coaxial cable is constructed much like a shielded cable. As with shielded cable, the stranded wire at the center serves as the hot conductor, and there’s a surrounding braided wire that serves as the shield and ground conductor. Between them is the layer of piezo material. Prepare the piezo cable for soldering just as you would shielded cable: at one end, remove about an inch of the outer insulation layer, exposing the braided shield beneath. Coax the exposed part of the braided shield off of the center conductor, pulling it to one side and twisting it into a single strand. This will expose a little less than an inch of the center conductor. From that, remove a half inch of the surrounding piezo layer. You now have the two exposed wires, the ground/shield wire and the hot wire, ready for soldering to a suitable length of shielded cable. Prepare one end of your shielded cable for soldering in the same manner as you prepared the piezo cable, then make the two connections: 1) solder the braided shielding wire of the piezo cable to the braided wire shielding wire of the shielded cable. 2) Solder the center wire of the piezo cable to the center wire of the shielded cable. Use electrician’s tape or other insulator in the area of the soldered connections to make sure the two exposed conducting wires don’t contact one another. With the shielded cable in place you can proceed with one of the options described below under “And Connect It To…”
For an excellent how-to for using the piezo cable to make an under-saddle pickup, see R.M. Mottola’s “Constructing an Under Saddle Transducer” at http://www.liutaiomottola.com/PrevPubs/Piezo/CoaxTransducer.htm. Special note for guitar bridge installations: the cable will fit nicely in the existing bridge slot of many guitars. On some guitars, however, the bridge slot is too thin. In that case you can either 1) widen the bridge slot, or 2) carefully strip the outer insulation layer from a section of the piezo cable to allow it to fit. Then spray the exposed surrounding ground wire with spray varnish or similar coating that will provide insulation. Install carefully to avoid fraying the exposed wire or wearing through of the new coating.
We’re now assuming that you’ve got a shielded cable of suitable length running from your piezo pickup, whatever sort of piezo pickup it may be. You have some choices as to where to go from here. You can:
a) Solder a jack (female connector) to the end of your cable. Typically you’ll use a standard mono quarter-inch jack (available form Experimental Musical Instruments and other electronics outlets), but other types can be used as well, depending on your purposes. Solder the braided shield/ground conductor from the pickup to the jack’s ground terminal, and the hot conductor to the jack’s hot terminal. The hot terminal on the jack is the one leading to the springy contact that touches the tip of the plug when the plug is plugged in. With this arrangement, you can use a standard guitar cable to run from the pickup to an amplifier input, signal processor, or other device.
b) Solder a plug (male connector) to the end of the pickup’s lead wire. Typically you’ll use a standard mono quarter-inch plug (available from Experimental Musical Instruments and other electronics outlets), but other types can be used as well, depending on your purposes. Solder the braided shield/ground conductor from the pickup to the plug’s ground terminal, and the hot conductor to the plug’s hot terminal. The hot terminal is the one leading to the tip of the plug. With this arrangement, you can plug pickup directly into an amplifier input, signal processor, or other device.
c) Run the hot and ground conductors from the pickup to volume and/or tone controls on your instrument, or wire it directly to an onboard mini-pre-amp. The possibilities in this case are too many to list here.
These pickups are extremely versatile and can be used many different ways. For full information, see our publication Getting a Bigger Sound, available through the Experimental Musical Instruments catalog on our web site at www.windworld.com. Following here are some rudimentary suggestions.
Piezo pickups produce a signal when they are attached to a vibrating surface. Our piezo films are sensitive enough that they can also pick up the air vibrations from many wind instruments. The films can be attached to the vibrating body by any of several means:
Double sided sticky tape (but avoid thick poster-mounting tapes, which may dampen the vibration)
Putties, such as the several brands of poster-mounting putties available
Glues. It’s usually best to use non-brittle, semi-permanent glues such as commonly available household contact cements, allowing removal with minimal damage to finishes.
Mechanical connections, such as embedding, springs, clamps or screws (but don’t break up the integrity of the pickup itself)
When it comes to deciding where to attach the pickup to the instrument, there are countless options. Experimentation is always in order. Here are some rudimentary suggestions.
Guitars and other plucked instruments with soundboards, including kalimbas: Attach the pickup to the soundboard on, in, under or near the bridge. The nearer the bridge, the stronger the signal and better the feedback resistance, but the sound may not be as warm as when it is attached a little way away from the bridge. For instruments with enclosed sound chambers, if you can get to the inside of the instrument, you can significantly improve the sound by attaching to the inside of the soundboard where the pickup can respond to internal air resonance. For inside-the-soundboard mounting, you’ll need to find a flat surface, unobstructed by soundboard struts, large enough to accommodate the pickup. If no such space is available for the pickup size you’re working with, you might consider switching to a smaller size. For instruments with bridges that are not glued in place but are held in place by the pressure of the strings, try placing the pickup under the flat bottom or the feet of bridge (yes, the films are durable enough for this sort of treatment). Piezo cable is usually often mounted in the bridge slot, under the saddle.
Bowed Strings: for bowed instruments with tall bridges, attaching on the bridge just under the strings often works well. Soundboard attachments like those described above for plucked strings also are effective, as are placements under one or both feet of the bridge.
Drums: Try affixing the pickup on the drumhead, fairly near the rim. If possible, attach on the inside of the head both for protection and for air resonance response.
Wind instruments: For instruments with bells, attach on the inside of the bell. In this application, a softer mounting is better. One convenient soft mounting is Velcro.
Free bar instruments such as marimbas and vibes: Attach a separate piezo to each bar, on the underside. Typically, attaching near one of the nodes (where the bar is supported) provides the most balanced signal. Attaching at the center of the bar will produce a stronger, peakier signal — so strong, in some cases, that it will overload the pre-amp.
Piezo contact pickups produce an ultra-high-impedance signal that doesn’t travel well over long cable distances. With increasing cable lengths, there tend to be losses in both signal strength and fidelity. In addition, amplifier inputs generally are not optimized for such extra-high-impedance signals. For this reason, the use of a separate pre-amplifier, located as near as possible to the pickup itself, is sometimes recommended. The pre-amp increases the signal strength and also converts the signal to lower impedance, making it less subject to loss over long distances and more suitable for typical amplifier inputs. However: piezo pickups can be and often are used successfully without a pre-amp. Here are some notes and suggestions concerning the “pre-amp or no?” question.
No Pre-amp: This is the way to go if you want an inexpensive, hassle-free, easy-to-install system. You’ll do fine without the pre-amp if you keep the cable lengths (the distance from the pick-up to your amplifier input) to a minimum – four or six feet should be OK; twelve may still be OK if you’re not a perfectionist. It also helps if you’ve got a strong signal to begin with – different instruments and different mounting configurations produce different signal strengths. While most amplifier inputs are not optimized for the ultra-high impedance of a piezo pickup, typical instrument inputs will produce acceptable results. Some of the new breed of acoustic guitar amplifiers (as distinct from traditional electric guitar amplifiers) provide one input that is optimized for piezo pickups.
Yes Pre-amp: This is the way to go if you are concerned about avoiding any loss of sound quality and signal strength, particularly when you need to use longer cable lengths. It costs more (you have to buy the pre-amp). The installation may be very simple, or may be quite a bit more involved, depending on the configuration you use. Tiny battery-powered pre-amps are now available, and these can be set-up with the pre-amp either on the instrument itself, or clipped to the player’s belt, or in some other convenient arrangement. Depending on the model of pre-amp you use, the pre-amp may provide a convenient way to give your instrument local volume and tone controls. Experimental Musical Instruments sells two suitable mini-pre-amps: the Power Pack, produced by K&K Sound, and the Power Jack, from Fishman. Another simple and fairly affordable option — not ideal, but still effective — is to use a guitarist’s stomp box, such as a little graphic equalizer, between the pickup and the amplifier input. Make the cable run from the pickup to the stomp box as short as possible.
What Sort of Power Amplifier is Best?
Electric guitar amplifiers will work with the signal from a contact pickup, but they are optimized for a type of sound preferred by electric guitarists, and are not ideal for accurately reproducing an acoustic instrument sound coming from a contact pickup. Look instead for a system designed for a flat frequency response. Such a system might be identified as a PA (public address system), powered speaker, keyboard amplifier, or acoustic guitar amplifier. You’ll probably need something with a quarter-inch, high-impedance input available, which is standard for musical instrument amplifiers. You can also run the signal through a mixer, if the mixer provides such inputs. For running through a mixer which does not provide such inputs, you can send the signal from the pickup to a device called a direct box or a DI, which converts the signal to a form suitable for the mixer’s microphone inputs.
For complete information on pickups and their use, look to our book Getting a Bigger Sound: Pickups and Microphones for your Musical Instrument, by Bart Hopkin. It’s available from Experimental Musical Instruments through any of the contact addresses below.
Experimental Musical Instruments
(415) 663-9691 (phone/fax)
PO Box 421, Point Reyes Station CA 94956 USA
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