This post is part 2 of the article ‘High Quality Guitar Pickups On A Shoe-string’, and discusses the fitting of a peizo pickup under the soundboard of an acoustic guitar, together with a suitable pre-amplifier and tone-control.
The Peizo Capsule
The pickup is made from a dismantled peizo ‘buzzer’, which is widely available and very cheap. My local stockist ESR supply a suitable unit, part no. 030-026 at £0.63 (+VAT)
|Care is needed in dismantling the buzzer, as the capsule is quite brittle, and any undue force may crack it. To remove the cover, I used one jaw of a small round-nosed pliers inserted a short way into the cable entry hole and gently lifted the cover which popped out quite easily. The capsule was then gently prised from the container without damage, using a small-bladed (jewellers) screw-driver.|
Siting The Pickup
Ascertain where, on the underside of the soundboard on your guitar you are going to attach the pickup. Directly under the bridge, or close as possible to it is a good choice, though internal bracings may mandate that you have to compromise. It goes without saying that the sound obtained from the pickup will vary quite widely, depending on site. Two positions I have used with satisfaction are just forward of the bridge, to the north (with the guitar in playing position), and also in the north shoulder of the guitar. Wherever you place it, there will probably still be a dominant resonance – hence the provision of either a notch-filter or graphic equalizer in some commercial acoustic guitar amplifiers. I have found a simple tone-control usually suffices, simply to balance the output across the range of frequencies.
A short length of small diameter screened audio cable, sufficiently long enough to connect the capsule to a socket sited where the strap-peg was, should be soldered to the capsule in place of the existing unscreened wires. This should be temporarily connected to a jack plug and the capsule tested with an amplifier to ensure that it works. Note that a peizo capsule will not match properly into the input socket of an electric guitar amplifier, as it’s very high impedance needs proper matching, and also an amplifier with ’straight’ performance characteristics, such as a PA amp should be used, if a dedicated acoustic guitar amp is not available.
Peizo Pickup Pre-amp & Tone Control Schematic
I present below a suitable buffer and tone control setup for use with the aforementioned PA amp.
A few words about the diagram above. The op-amps used have JFET inputs, with a consequently very high input resistance, which suits the very high impedance of the peizo capsule very well. R8 sets the input resistance to 10Mohms. The two 100nF decoupling capacitors should be sited as close as possible to the op-amp supply pins V+ and V-. I tend to use dual-supplies of 15-0-15 volts with my op-amp projects, so if you wish to run the pre-amp on a 9v battery, (connected between V+ and V-) then the 2 X 10K resistors, together with the 2 X 100uF capacitors should be included to provide a ‘GND’ point that is one-half of the supply voltage. A higher supply voltage could be used to a maximum of 30volts – make sure any electrolytic capacitors are rated accordingly. The final 2.2uF output capacitor should be of the reversible type when used with a dual-voltage supply, but can be of the ordinary polarised type if a single supply rail is used.
Parts List for the Peizo Pickup Pre-amp & Tone Control
All resistors 1/4 watt
R8 : 10M ohms
R9 : 25K lin pot
R10 : 100K ohms
R11,R12 : 470 ohms
R13,R14 : 10K ohms
C3,C7,C8 : 100nF ceramic 63v
C4 : 10nF polyester 63v
C5 : 2u2 reversible (non-polarised) electrolytic 63v
C10,C11 : 100u electrolytic 35v
Op-Amp : Texas instruments TL072 8-pin DIL package
1 X 8-pin DIL Socket
J2 : 2-pin 0.1inch pin-header
X2 : 2-pin 0.1inch pin-header
X3 : 3-pin 0.1inch pin-header
Suggested Printed Circuit Board Design
Fitting The Pickup Jack Socket
If you are only fitting a peizo pickup to your guitar, then a simple 1/4 inch line Jack socket soldered to the short length of audio cable, can be mounted in a hole drilled in place of the strap-peg. On my guitar I also fitted a magnetic pickup, and so I fitted a 4-pin mini-DIN socket – see part 1 of this article: High Quality Guitar Pickups On A Shoe-string