The subject of amplification is surely complex and highly personal when it comes to the guitar. However, when choosing a method of amplifying an acoustic instrument, it is often desirable to pursue a sound that closely reflects the original, non-amplified version. For example, a recording engineer will often place microphones around the guitar to best reflect this. The live performer rarely has as much control as what is offered in the studio, and must use alternatives. Most commonly with instruments having nylon strings, the piezo microphone is used, often under the soundboard of a guitar with the signal sent to a preamplifier for the first step of gain staging.
Since there were many electrical considerations with choosing a transducer and matching its impedance with the preamp, I decided to follow the design of Alex Rice’s piezo preamplifier. It had very good reviews and I understood the basic theory of why it works so well. The piezo transducer is unbalanced and it connects to a balanced preamplifier that is wired as a differential amplifier. This allows the ability for vibrations represented by positive and negative half cycles to get louder, while signals that are common (like hum) get rejected and are sent through a separate ground via the XLR cable.
I wanted the ability for the guitar to remain fairly portable, so I terminated the piezo directly to a 3.5 mm socket, with the wires being fed to the outside through a pre-drilled hole I created from another project. I realize there is some connector loss in signal power by doing this but figured it was worth it. I also use an additional unbalanced cable from the guitar to the preamp, so that is again less than ideal, but…an outstanding sound was achieved that I am extremely pleased with.
I highly recommend using this circuit for amplifying acoustic instruments. The only drawback I can see is I need to connect a balanced XLR connection to a mixing desk with phantom power (48 V). I planned on using this in my setup anyway, so it worked out ok, but others may want to follow instructions in the design for powering the circuit using 9V batteries. Since I know everything works, I plan to fix the electronics to a small chassis box that I will drill holes to mount the jacks – plug in and just play.
I would exercise some caution with your levels though, a feedback loop could be easily created with the amount of signal gain achieved and the piezo is definitely not immune.