Guitar Contact Microphone Preamp

In a prior Instructable, we showed how to construct a contact microphone from a piezo element that could be pick up sounds from any vibrating object and play them through a guitar amplifier. A well-known issue with piezo contact microphones is that they have poor bass response when connected directly to a typical amplifier. This is especially noticeable if the contact mic is used as a pickup for an acoustic instruments such as a guitar, which results in a "thin" sound. In this Instructable, we show how to build a very simple preamplifier for a contact mic that restores a fuller, richer tone with more bass. The design has only a handful of electronic components and fits neatly inside a candy tin (such as Altoids or Sucrets).

The following sound clip demonstrates the preamp on an acoustic guitar first strumming without and then with the preamp, followed by fingerpicking without and then with the preamp.

1 10k resistor

1 100k resistor

2 22M resistor (or 10M will work)

1 0.1uF capacitor

2 10uF electrolytic capacitor (4.7uF is also fine)

1 2N5457 JFET (J113 and some others will also work)

2 3-position 1/4" phone jack (such as Switchcraft 112BX or equivalent)

1 9V battery

1 9V battery clip

Protoboard for testing circuit before soldering

Breadboard for soldering circuit together. I use this Proto Breadboard from Adafruit that has the holes wired in strips exactly like a protoboard which is very easy for transferring your prototype to a finished product and simple to use in general. It is very well made and you can cut it to size with tin snips.

Tin candy box (such as Altoids or Sucrets tin, full size, not mini)

Poster mounting putty and (gaffer) tape for attaching contact mic to guitar

Where Did the Bass Go?

The schematic above is the complete design of the preamp circuit. To understand how it works, we'll first take a look at the problem we're trying to solve, and then build up a solution in steps.

A piezo element operates like a voltage source in series with a capacitor--when the piezo crystals vibrate, they produce a voltage. This voltage may be high, but the current is very small. Because of the series capacitance (Cpiezo), virtually no DC current flows through the piezo. The input of an amplifier acts like a resistor to ground (Ramp), known as its input impedance. Together, Cpiezo and Ramp form a high-pass filter that cuts out low frequencies. For typical values of Cpiezo and Ramp, the cutoff frequency is well into the audible playing range of a guitar. In order to reduce the cutoff frequency so that more bass tones get through, we need to increase the input impedance that the piezo is driving. This is where a preamp comes in. The role of the piezo contact mic preamp isn't really to amplify the signal (the output voltage of the piezo is plenty high), rather the goal is to provide a buffer with a very high input impedance of 10M ohms or so, which is more than 10 times or so greater than the input impedance of a typical guitar amp, so that more of the bass gets through. It also provides more current to drive signals on the cable to the amp, which can also improve the highs.

Amplifiers use transistors (or tubes) to amplify or buffer a signal. There are three types of transistors to consider, bipolar (BJT), MOSFET, and JFET. Bipolar transistors don't have a high enough input impedance. MOSFETs have an extremely high input impedance, but tend to be fragile (static electricity can destroy them) and also can be noisy in audio applications. JFETs have a very high input impedance and are less fragile and noisy than MOSFET, so that's what we'll use. The vast majority of contact mic preamp designs found on the web also use JFETs.

A Simpler Example: The Basic JFET Source-Follower Amplifier

As a starting point to understanding the complete preamp, let's consider a simpler version shown below:

A JFET has 3 terminals called the gate (g), drain (d), and source (s). In general, increasing the voltage Vin on the gate of the JFET causes an increase in current flowing through the device from the drain to the source. Having nowhere else to go, the current flows through the source resistor R1, which in turn produces a voltage across R1 (remember Ohm's Law, V = IR), causing the output voltage Vout to also increase. One of the unusual things about a JFET is that if the input voltage is 0, meaning that the input is grounded, there is still a current that will flow from drain to source, and you actually need to apply a negative voltage to the gate to turn the device off. The plot below shows the relationship between the input voltage Vin and the output voltage Vout, given a supply voltage Vdd = 9V and a source resistor of 10k ohms.

Note that when Vin = 0V, Vout = 1V, indicating that current is flowing through R1. Vout goes to 0V when Vin is around -1.25V, which is the point where the JFET turns off. In the range between Vin = -1.25V and Vin = 7.5V, Vout increases linearly with Vin. In fact, for every increase of 1V of Vin, Vout also increases by 1V. The ratio of the change in Vout over the change in Vin (the slope of the line) is the gain of the amplifier, which in this case is just equal to 1.

This configuration of an amplifier, where there is a resistor connected to the source of the JFET and the output voltage "follows" the input voltage, is called a source-follower. The value of the source resistor affects the gain of the amplifier slightly. If the value of this resistor is high enough, the gain will be very close to 1 and if the resistor is smaller, than the gain will be somewhat less than 1. I've seen designs on the web where the size of this resistor ranges from 1k ohm to 220k ohm. If the value of the resistor is too high, it will heat up (slightly) as current passes through it, which can result in noise. 10k ohm is a good middle-of-the-road value. You may also see some preamp designs on the web where there is also a resistor connected to the drain of the JFET. This style of amplifier is called a common-source and generally has a gain greater than one--that is, the change in output voltage is greater than the change in input voltage--true amplification. This can be useful for an electric guitar with coil pickups, but since the voltage output of a piezo contact is already high enough, this isn't really necessary in our case.

When Vin is greater than around 7.5V, Vout starts leveling off at a little below 9V. This is because the JFET is turned on as far as it can go, and any further increase in Vin won't produce any additional current through the device. More technically, the JFET stops behaving like a current source controlled by the gate voltage and starts behaving more like a resistor. Note that the transition into this region of behavior is much more gradual than the behavior at the other end, where the JFET shuts off more abruptly when Vin drops below -1.25V.

Avoiding Distortion: The Purpose of the Gate Bias Resistors R1 and R2

What does this leveling off at extreme low and high input voltages mean in terms of the sound quality of the amplifier? A piezo contact microphone on a guitar produces a signal that varies plus and minus above and below 0V, that is, to either side of the red dot in the graph above. As the input to the amp changes, the output changes smoothly with it. If the input dipped below -1.25V, however, the output would be "clipped" at 0V. This causes distortion in the sound, which is sometimes desirable, but generally not for a preamp for an acoustic guitar. If somehow the input signal got above 7.5V, it would also get distorted, but this would happen more gradually. This type of gradual distortion is one of the prized properties of tube amps for electric guitars, which many players feel has a nice "warm" sound.

How likely is it that the voltage produced by a piezo contact mic will dip below -1.25V? Depending on how the contact mic is attached to the guitar, it can happen when you play hard, but for the most part from our experiments, the signal from the mic stayed mostly in the range of plus or minus 0.5V. There is a very simple fix, however, that can give the input signal more headroom by shifting the middle operating point to a higher voltage, as illustrated in the plot below.

This is the purpose of R1 and R2 in the schematic at the top of the section. If we only had R1, the operating point at the input of the amplifier would be 0V. But if we add R2, which has the same resistance as R1, the two resistors act to both pull the input up towards Vdd = 9V and down towards ground with equal strength, resulting in an operating point bias voltage equal to Vdd/2, or 4.5V. In practice, we've found that leaving R2 out is generally okay and we don't get much distortion. Most of the contact mic preamp designs that I've seen online just leave the bias voltage at ground and omit R2, but moving the bias point closer to the middle of the "safe" range just adds one more resistor. The values for R1 and R2 should be at least 10M ohms--remember that the whole point of the preamp is to provide a high input impedance and you wouldn't want to compromise this with a small resistor! Using resistors over 22M ohm doesn't really add any value.

Removing DC Bias from Input and Output: C1, C2, and R4

As we just saw, increasing the average voltage of the input signal where it hits the gate of the JFET can be a good thing so that the JFET can operate in a distortion-free "safe zone." Shifting the average voltage like this is called adding a DC bias to the signal. It's bad form, however, to pass along a signal with a non-zero DC bias to another component, such as a stomp box or amplifier downstream. Coupling capacitors in series with the input and output of a circuit remove any DC bias from an incoming and outgoing signal. That's the purpose of the input coupling capacitor C1 and output coupling capacitor C2. Since a piezo has a series capacitance at its output, C1 probably isn't necessary in this case, but most preamp designs include it and it doesn't hurt. If the JFET were biased to operate around 0V by leaving out R2, C2 may also be optional. However, I once had a case where a volume pedal produced noise in an audio component that was before it in the effects chain and putting a 10uF coupling capacitor between them solve the problem, so it's best to leave C2 in there. R2 also helps insure that the output is biased around ground.

Storing a Little Extra Energy for Voltage Spikes: C3

Lastly the 10uF capacitor C3 between Vdd and Gnd serves to store a bit of extra energy from the power supply (9V battery) if it's needed to adjust to fast changing signals that may cause noise. You may never notice this problem if you leave C3 out, but it is considered good circuit design practice to have a largish capacitor across the power supply.

So much for the little electronics lesson, now we're ready to build!

Before soldering up any circuit, it is good to test it on a protoboard to make sure it works. This is also the time to tryout different options, such as different JFETs, or seeing if biasing the circuit around an operating point of 0V or 4.5V by eliminating or including R! makes a difference. Once you've got the circuit configuration that you want and it works, you can solder it to a breadboard. There are many different breadboard styles to choose from, but I especially like the ones where the holes are connected by strips of metal on the back side in the exact same manner as a protoboard, with the holes in the middle of the board in vertical strips and the holes along the top and bottom edges in horizontal strips for power and ground. The biggest advantage of these is that you don't have to keep flipping the board over to see what gets connected to what as you are soldering, and that you can just transfer your circuit layout from the protoboard. A number of companies make this kind of breadboard, but the ones from Adafruit are especially good. I order the full-sized ones and then cut them to size as needed with tin snips. To fit the circuit board into the tin case along with the battery and phone jacks, the board shouldn't be more than 10 holes wide.

The picture above shows the detailed circuit layout on the soldered version of the Adafruit Proto Breadboard. Each of the vertical strips are labeled with the circuit node names from the schematic and the bottom horizontal strips are Vdd and Gnd. Note that two vertical strips are dedicated to the JFET gate node (g), tied together by a small jumper wire. Also note that the leftmost vertical strip is tied to Gnd and the rightmost to Vdd, to provide more holes for connecting the battery clip and phone jacks. When connecting the electrolytic capacitors C2 and C3, pay attention to which side is (+) and (-). The longer lead is the (+) terminal.

Finally, in a very subtle detail, if you look at the datasheet for a 2N5457 or J113 JFET, you'll see that I reversed the connections to the source and drain terminals. For these 2 JFETs and most others, the device is symmetric and the source and drain terminals are interchangeable, and this made for slightly simpler wiring.

Soldering Tip: When soldering to the breadboard, work your way from the shortest to tallest components. Insert the resistors and jumper wires into the board first, hold them in place with masking tape on the top of the board, then flip the board over and solder them on the back side and trim the leads flush with the board. Next do the ceramic capacitor and the JFET the same way, and then the electrolytic capacitors last.

Connecting the 1/4" phone jacks and battery clip is straightforward, with one small twist. Even though the male plugs on a guitar cable have two contacts, tip (T) which carries the signal and shield (s) which is ground, we are using female jacks with three terminals, tip (T), ring (R), and shield (S). The twist is that we'll use the ring terminal of the output jack to create a switch so that the battery is disconnected when nothing is plugged into the output.

Because the fit inside the mint tin is tight, you may need to clip off the tips of the jack terminals if they are too long before you solder to them as shown above.

Use a couple of inches of stranded hookup wire to make the connections. Stranded is more flexible than solid, which makes it easier to wrangle all the parts into place in the tin.

The top figure shows the wiring to the jacks and battery clip. The tip terminals of the two jacks go to the In and Out connections on the board. Both shield terminals go to ground. The (+) terminal of the battery clip goes to Vdd on the board. Now here's the twist: the (-) terminal of the battery clip goes to the ring terminal of the output jack. If no plug is in the jack, then the battery is disconnected, but if a plug is inserted, since the plug doesn't actually have a ring contact, the ring terminal and shield terminal end up getting shorted together, connecting the (-) terminal of the battery to ground. This is a common trick for extending battery life in guitar effects.

Drill 3/8" holes for the jacks in one end of the box. Make sure that you center the holes so that there is room for the washers and nuts when the top is closed. Remove burrs from the holes with a small file or Dremel grinding bit.

Line the bottom of the inside of the box with an insulating material, so that the circuit board won't short out when you place it in there. I use thin (2mm) craft foam that has an adhesive backing.

Mount the jacks in the holes and fit the circuit board and battery in as shown. Secure the battery so it doesn't bounce around--I used a little bit of soft packing foam that holds it in place when the box top is closed.

Attach the contact mic to the guitar top using a bit of poster mounting putty. You can further secure both the mic and cable using gaffer tape, which holds firm but peels off without leaving residue. The sound will vary depending on where you place the mic. Generally just below the bridge on the high strings side works well. Run a guitar cable from the contact mic to the preamp input jack and another from the preamp output to your amp input. Turn on the amp and rock on!

• Tutorial on how JFETs work

Some additional contact mic preamp designs

• A simple design that also uses the jack power switch trick
• Another mint box preamp. This design uses a voltage divider to reduce the gain if the mic is too hot
• A comprehensive set of pages on the design of a common-source preamp
• A design similar to this one that uses a pair of resistors to set the gate bias to Vdd/2