I'm frequently asked how to make clean sounding contact microphones without the hum that usually goes along with them. The process of building a contact microphone is simple, just solder a couple wires to a piezoelectric disc and you're done. With this tutorial, I'm going to get a bit more into specific details I've learned over the years.
An important thing to remember with any low power audio application is that somewhere the signal is going to be amplified and any electromagnetic hum that gets picked up by the signal wire in your cable is also going to be amplified along with it. To lessen the hum picked up by the signal wire, it is important to always encapsulate it in shielding connected to the negative or ground of your circuit.
The type of cable is important in this application. Most instrument and unpowered audio cable is stranded coaxial, meaning there is a stranded signal wire surrounded by a stranded negative shielding. For contact mics, a thinner flexible cable works best because it will flex when bent near the contact microphone rather than pry off the solder joint on the delicate ceramic piezoelectric element. I prefer to use braided shielding, which has less voids than the unbraided type, but can be more difficult to find. A typical application for thin braided coaxial cable is to connect a pcb to an rf antenna.
Step 1: Setup
Parts and Tools list:
10-25mm unsoldered brass disc piezoelectric transducer
~30 AWG braided shield stranded coaxial cable such as Belden 83265 or Alpha 9178
brass sheet (shim stock) .015 - .025" (.4-.5mm) (thick enough to not crumple, thin enough to not require a hammer to form) such as K&S
rosin core solder
variable temperature soldering iron
1-1.5cm dapping punch
small quick clamp
mixing sticks for epoxy (toothpicks or barbecue skewers)
paper for mixing epoxy
fine permanent marker
not shown in photo - scissors
Mount helping hands to the edge of your work table with a quick clamp.
Set soldering iron to 700F / 370C.
Step 2: Prepare Brass Shielding
Choose a piezoelectric transducer for your project. Small piezos are easier to fit into small areas but are not as loud or clear sounding as large ones. I've also found that small (10-12mm) piezos are very easy to damage by bending. The bond between the ceramic piezoelectric element and the brass disc on which it sits isn't very strong and with some inexpensive piezos it is possible to peel the ceramic off by accident without even being able to visually notice that the bond has broken.
After choosing a transducer, trace its outline on a brass sheet and cut it out using scissors. This disc will be used to shield the signal wire after the piezo is solder together.
Place the brass cutout in the largest depression in the doming block and form by hand to a shallow dome by crushing and stretching it to shape using a dapping punch.
Step 3: Strip and Tin Cable and Cut Wires to Length
Cut a length of cable for your project. Its always good to have extra and cut it down afterward.
Strip about 1.5cm from the end of the cable by selecting a gauge size on your wire stripper just slightly smaller than the diameter of the cable.
Using a pushpin or a thumbtack, separate the braided fibers of the cable shielding and twist into a bundle.
Coat the fibers in the shielding bundle with solder by briefly holding the soldering iron to it and then feeding the solder into the heated metal near the iron's tip. This is called tinning. This is done to both hold the strands together so they don't jumble and snap and also to make it easier to solder by preventing contamination and oxidation.
Strip the signal wire so that there is about 2 mm of insulation at its base and tin it.
With the piezo held in place with the helping hands, cut the signal wire of the cable so that it's tip is centered on the ceramic element and cut the negative shielding wire so that there is about 2-3mm of material exposed to solder to the brass backing disc.
Step 4: Solder Cable to Piezo
Position the cable over the piezoelectric transducer using the helping hands so that the tip of the signal wire is centered on the ceramic element and the shielding bundle is touching the surrounding brass disc. Try to keep the external insulation just off the edge of the disk. Keeping everything as flat as possible ensures that there won't be any pivot points where one could pry off the solder connection to the ceramic piezoelectric element.
Solder the shielding bundle to the brass disc by heating both the disc and the shielding together by holding the soldering iron against both surfaces and feeding the solder in-between. The solder should flow beneath and will solidify quick after the iron is pulled away. A strong solder joint should have a concave curve and the solder should appear shiny after it cools. This solder point is the strongest connection between the cable and the transducer.
Lower the temperature of the soldering iron to 650F (350C). The solderable metallic skin of the ceramic element of the transducer can burn away at temperatures higher than this, and may still do so if the soldering iron is held to it more than a fraction of a second. Luckily this skin readily accepts the solder if you can get the signal wire positioned just right.
Adjust the position of the signal wire so that it is touching the ceramic element. Sometimes its possible to bend the entire cable back away from the ceramic element using the soldered shielding as a fulcrum. Then adjust the angle of the signal wire towards the element using tweezers, the wire strippers or even the wooden mixing sticks. Then with the signal wire bent toward the ceramic element, bend the entire cable back, flat with the plane of the piezo. This is how I usually combat the springiness of the signal wire.
Be careful when bending the cable. As I explained previously, it can be easy to separate the ceramic piece from the backing disk. This component is also easy to shatter, and both situations will often result in a dead microphone.
Now, with the signal wire touching the ceramic element and the soldering iron set to a lower temperature, position your solder on the signal wire and quickly tap the soldering iron onto it to melt the solder and connect the signal wire to the ceramic element. The smaller the joint the better. The connection between the metallic skin of the ceramic component and the ceramic itself can become compromised at the solder joint (the skin can bubble outward slightly), so the less solder, the more intact electrical connection. Be careful with this solder connection, it can be easily torn by bending the cable the wrong direction before everything is glued. If it gets torn off and you need to resolder the connection, the output volume from the microphone will be lower. Usually, replacing the piezo disc is a better option than trying to fix anything.
Step 5: Epoxy Solder Points
I've tried a number of types of adhesive for holding everything together in contact microphones, silicone-based and polyurethane-based adhesives, but I still find that epoxies work the best for securing the cables. Five minute epoxy works the best for these, but keep in mind, it can take anywhere from 5-20 minutes for a five minute epoxy to cure (dry).
I use masking tape to hold the piezo to the table while securing the solder connections with epoxy. Masking tape (painter's tape) is just sticky enough to hold the disc flat, but not too sticky so that I don't accidentally flex the brass disc when removing it. More often than not, when I would do this part of the process using double sided mounting tape or vhb, I would ruin the mic by shattering the ceramic when peeling it off the table after the glue dried.
First, fold a short piece of masking tape in half lengthwise so that half of the adhesive can stick to your work table and the other half is facing upward. Hold the tape in place and creased with two more pieces of tape on each end. Place the soldered piezo disc on the tape and secure the cable to the table with another piece of masking tape. If you are working on more than one contact mic, line them up on the tape and apply the epoxy to all of them in one step.
Mix the 2-part epoxy with a wooden stick and using that stick cover the piezos solder points, extending the surface area about 2mm beyond the solder. I don't cover the entire surface area of the disc because the added mass deadens the sound, killing some of the high frequency response. Keep the stick and the paper that it was mixed on to determine when the epoxy has hardened.
Step 6: Attach Brass Shielding
The final step is attaching the domed shield created earlier.
Clip the piezo disc to the helping hands (try not to crush the ceramic with the alligator clips).
Cut a notch in the brass shielding with the scissors to the shape of the shielding wire's solder joint so that the brass shield sits flat along the edge of the brass disc. Be careful not to accidentally touch the ceramic component with the domed brass shield.
Place the brass shielding on the piezo disc and hold in place using the alligator clips of the helping hands. Sometimes it slips a bit and using a bit of masking tape can help with the first solder connection. I like to hold the shield to the piezo at three equidistant points. Two points can work, but this can act like a hinge and can also buzz at high frequencies. Heat the edge to the piezo and the shield with the iron and carefully feed solder in-between. After the solder appears fill the gap, let it cool and adjust the shielding (it almost always shifts a bit on this first connection) then solder at the final two points.
And there you have it. Sometimes I dip this in a rubberized coating like plasti-dip, but typically I'll just epoxy this assembly bare to what I want amplified. After this is completed solder on a jack or plug and you are on your way.