If you are thinking of soldering a piezo disc directly to a guitar cable, stop and get some help. This is the help. It is a high fidelity piezo disc preamp on a circuit board. Put it between the disc and cable and you will get much better sound.
The core amplifier part of the circuit is the same as is in
Metal Marshmallow Pro. It has super-low noise even at high gain. It has a proper high-impedance input buffer so that it has good bass response and you won't get that awful sound associated with pizeo discs soldered directly to guitar cables.
Included
- Marshmallow DIY Piezo Preamp
- 7-Pin Header
Note that this is only the board and headers, if you want other accessories with it, like a piezo disc and a gain pot, check out the
Marshmallow DIY Kit.
Pin Description
- p+ Connection for the positive lead of a piezo disc. By convention the inner crystaline part of the disc is considered as positive.
- p- Connection for the negative lead of a piezo disc. By convention the outer brass part of the disc is considered as negative.
- 3-5.5V Supply power to the board here. Acccepts voltage in the range of 3 to 5.5 V. Absolute maximum is 6V and there is no overvoltage protection, so be careful.
- GND Supply ground to the circuit here.
- DC OUT DC-coupled audio output. In the absence of input this will output half of the supply voltage, and in response to input it will range ± half of the supply voltage. Use this to connect the output of the preamp to a microcontroller or other digital circuitry.
- AC OUT AC coupled audio output. In the absence of input this will output 0V, and in response to input it will range ± half of the supply voltage. Use this to connect the output of the preamp to audio equipment like a recording interface.
- GAIN Connect a resistor between the two GAIN pins to set the gain. Without a resistor, the gain will be unreasonably high and the audio will be very distorted. The gain is given by G = 1 + R/1000, where R is the resistance in Ohms of the resistor. Useful resistor vaules range from 0 Ohms to about 50k Ohms.
Application Notes
Piezo
- The piezo disc should have a capacitance of 1nF or greater. The preamp was intended to be used with brass piezo discs which usually have on the order of 10s of nF of capacitance. It can also be used with piezo films, which often have single-digit nF of capacitance, so long as this condition is met.
- The reason for the above is that the disc interacts with the preamp to form a high-pass filter whose cutoff frequency is F = 100 / (2πC), where C is the capacitance in nF. The cutoff should normally be below the audio range (around 20 Hz).
- It is intended that the piezo disc should have short leads and should be close to the board. It may also be on the far end of an instrument cable or similar, so long as the combined capacitance ofthe cable and disc are greater than 1 nF.
Gain
- If a fixed gain is desired, a chip resistor may be soldered across the solder pads adjacent to the GAIN pins. If a fixed gain of 1 is desired, these pads may be bridged with solder. No additional resistor will then be needed between the GAIN pins.
- If an adjustable gain is desired, it is recommended to use a potentiomenter with a logarithmic taper.
- The gain is given by G = 1 + R/1000
- Alternatively R = (G - 1) * 1000, where G is the voltage gain and R the resistance in Ohms.
- Useful gains range from roughly 1x to 50x (0dB to 34dB), corresponding to 0 to 50k Ohms. See the table below for more details
| R |
G |
G dB |
| 0 |
1 x |
0 dB |
| 1k |
2 x |
6 dB |
| 2.2k |
3.2 x |
10 dB |
| 4.7k |
5.7 x |
15 dB |
| 10k |
11 x |
21 dB |
| 15k |
16 x |
24 dB |
| 33k |
34 x |
31 dB |
| 50k |
51 x |
34 dB |
Shielding
To reduce hum, the board and piezo disc should be enclosed completely in metal, which should be grounded. The metal could be a metal box or aluminum foil. If using foil, isolate the disc by covering it in a layer of tape before covering it in foil. The 2 large holes on the sides of the board are ground and are meant to facilitate connecting the board ground to a metal box.
Audio Cable
The 2 adjacent holes labelled AC OUT and GND are intended for connecting a guitar cable to the board for audio output. Connect the inner conductor of the cable to AC OUT, and the outer shield of the cable to GND. The other end of the sable may then be plugged into a mixer or audio interface.
Headroom
Line level is approximately ±1.5V. The output of the board will swing ± half of the supply voltage, which for example would be ±1.65V if it is powered with 3.3V. If the gain is 1x (R = 0 Ohm) and the input is greater than ±1.65V, then the output will be clipped by the preamp. On the other hand, if the board is powered with 5.0V, the output will swing ±2.5V. In this case, at 1x gain the input could be as high as ±2.5V without the preamp clipping. Because this voltage is greater than line level, it might still be clipped by the audio interface or mixer. However, the input of the mixer could then be padded down by a few dB to bring the whole signal back into range so it does not clip. Therefore, higher voltage is a little better for louder sounds.
Tec Specs
| Size |
0.8 x 0.535 |
in |
| Size |
23.32 x 13.335 |
mm |
| Current |
1.5 |
mA |
This board is not Phantom-Powered. There is a
Phantom Powered version here.
