Mini Audio Amp/Preamp

This is a quick-and-dirty mini amplifier / preamp that lets you use a piezoelectric pickup to convert an acoustic guitar to an electric one. Because the pickup is simply a piezo disc, you could attach it to anything, really -- you could make an electric spork if you wanted! Here's what it sounds like on a guitar, when the gain is turned up to produce clipping distortion.

1 qty. 9V battery clip

1 qty. TL082 op-amp

1 qty. 5mm power LED (I used green, but any color from red to UV will work)

1 qty. 10kOhm resistor, 1/4W

1 qty. 2.2kOhm resistor, 1/4W

1 qty. 100kOhm resistor, 1/4W

2 qty. 2.2MOhm resistors, 1/4W

2 qty. 56kOhm resistors, 1/4W

1 qty. 20kOhm linear potentiometer

1 qty. knob for 20kOhm linear potentiometer

1 qty. 1kOhm linear potentiometer

2 qty. 1N400X diodes (I used 1N4004)

3 qty. TRS jacks (I used 1/8" (3.5mm) because that's what I had on hand, I know "real musicians" use 1/4")

2 qty. SPDT switches

1 qty. 27mm Piezoelectric disc

1 qty. 32 ohm speaker

1 qty. 4700uF 16V electrolytic capacitor (higher voltage ratings may be used if desired)

Solderless breadboard (for testing)

Perfboard

Tic-tac box (or project box of your choosing)

The piezoelectric disc produces a varying voltage with flexing. The amplitude of this voltage is comparatively high, but the piezoelectric disc has a very high equivalent series resistance (it's a high-impedance signal source) and its output current is tiny. To correct this, an amplifier is used. The TL082 op-amp actually contains two separate amplifiers on one chip, and because it uses a FET for input, its input impedance is very high -- for most practical purposes, it may be considered to be infinite. The amplifiers are also capable of having their outputs short-circuited indefinitely without damaging the TL082 due to its internal protection circuitry, which is a convenient feature.

Going from left to right on the schematic, we see the battery, then the power switch (drawn as a SPST switch, though in the parts list I specified two SPDT switches to minimize parts cost -- simply leave one pole unconnected to use it as a SPST switch, which is what I did when building this device). After the power switch is a 10kOhm resistor and LED which will illuminate when power is supplied. Next up, two 2.2MOhm resistors, which form a voltage divider, holding the noninverting input of one of the TL082's amplifiers to half of the battery voltage. This point is connected to one lead of the piezoelectric disc. The other lead of the piezoelectric disc is connected to ground. When sound waves flex the piezoelectric disc, it will produce a voltage which perturbs the center of the voltage divider, pushing its voltage up and down at the frequency of the sound wave.

This first amplifier of the TL082 has its inverting input connected directly to its output; this produces a configuration known as a buffer amplifer -- the output will be equal to the input. Why bother? Because the output from the TL082 has a much higher current capacity than the piezoelectric disc does, and a much lower output impedance. This prepares the signal for the next stage of amplification.

After leaving the first amplifier, the signal then goes to a 2.2kOhm resistor, and then a 20kOhm linear potentiometer, which is connected as a rheostat (its wiper is shorted to one end of its resistive track). These, combined with the 100kOhm resistor between the next TL082 amplifier's output and its inverting input, form an inverting amplifier. The output voltage will be equal to (input voltage)*(-Rf/Rin), where Rf, the feedback resistor, is 100kOhm and the input resistor is 2.2kOhm + the potentiometer setting. The gain of the circuit can thus be adjusted between -4.5 and -45.5. It doesn't matter that the amplifier is inverting because sound waves are symmetrical; flipping them upside down does not alter how they sound. This inverting amplifier requires a bipolar supply with both positive and negative voltage supplies. To get around this requirement, so we don't need to use two batteries, we tell it that 4.5V is "zero", that the positive battery terminal is +4.5V, and that the negative battery terminal is -4.5V. This is achieved with another voltage divider, the two 56kOhm resistors, whose center point is connected to the noninverting input of the second TL082 amplifier, setting its "ground" point.

At this point we have our amplifier circuit completed, with a buffered input and variable gain. The gain can be turned high enough to cause clipping -- cutting off the tops and bottoms of the input waveform -- which is the primary source of the "harsh/electric" heavy metal sound we all know and love. The only remaining compenent before the speaker is the coupling capacitor, whose purpose is to allow the AC portion of the waveform through while blocking the DC component; DC is bad for speakers and amp inputs!

As I was partway through assembling the core circuit on perfboard and fitting it into the case, the client, my stepchild, requested that it be possible to attach headphones to the output -- this also makes it possible to use the circuit as a preamp and to attach it to a larger external amplifier. While doing so makes the circuit more functional, it would also require a different case design to fit the components elegantly. Elegance? Not here! So I just made a ball o' wires with hot glue, as one does. To accommodate the modifications, a number of parts had to be added -- a switch to switch between the internal speaker and the external output, a potentiometer to adjust the output volume, two diodes in parallel pointed opposite ways to cap the output voltage amplitude at 0.7V, and finally the output jack. The pickup is connected through a standard AUX cable.

During testing, I found that this circuit had undesirable oscillation if the gain was turned to maximum and it was powered with a brand-new alkaline 9V battery. Using a carbon-zinc Leclanché (a.k.a. "heavy duty") battery avoided this problem. As this was a hasty afternoon project, I did not bother to tune the circuit to avoid this problem and simply supplied a carbon-zinc battery with it. Be advised.

After assembling the core schematic on a breadboard, try (gently!) dragging the edge of the piezoelectric disc along your work surface. With the 20kOhm potentiometer adjusted for high gain, you should hear it out of the speaker. Adjusting the 20kOhm potentiometer should adjust the volume. If the circuit passes these tests, it is in working order.

Having first tested the circuit on the breadboard, you can now solder the components together on perfboard in a more permanent fashion and put the resulting circuit into your project box of choice. For the pickup and amplifier input the shield of the TRS jacks should be attached to ground (the negative battery terminal on the amp, the black wire on the piezoelectric disc) and the signal input should be attached to the tip, or both the tip and ring. For the output jack, it's advisable to connect the shield to ground and the signal output to *both* the tip and ring. This makes it so that if you connect the preamp to a stereo with a normal AUX cable, the signal will appear in both the left and right channels. If you only connect the tip or the ring, you'll only get audio out of one channel.

You'll also need to attach the piezoelectric disc to your instrument. In my case, I simply taped the piezoelectric disc to the body of the guitar, under the bridge. Hot glue would probably result in higher sound quality, but I haven't gotten around to that yet. Enjoy your new electric instrument!