I designed and built the GMPA- 4 (Guitar Mic
Pre-Amp, version 4) to allow anyone to connect to an
amplifier or house PA system, and adjust their
microphone or guitar pick-up for the proper tonal sound
and volume that they desire. The circuit is version 4
because of tweaks I made to its versatility, keeping the
cost low, and giving good battery life.
I tested the GMPA- 4 with my Ibanez acoustic guitar
with built-in electret mic and my Les Paul electric guitar
with dual hum-bucking pick-ups. It worked flawlessly.
The pick-ups put out several hundred millivolts and act
like a miniature generator when the steel string vibrates
across the Alnico pole magnet under each string.
The GMPA- 4 input impedance is roughly 100K ohms
which does not load down the guitar output. I also tried
using a Nady SP- 4 microphone with a high impedance
matching transformer. It worked perfectly, as well. If you
use a low impedance microphone, you need to use a
balanced low impedance to unbalanced high impedance
It would be possible to add the impedance matching
transformer and XLR balanced line jack inside the unit,
but this was not done in order to save cost and space. It
would also be possible to add additional channels by
adding additional quad op-amps and the necessary
At most venues I play, they have microphones
already set up, so I only need the one channel for my
guitar. I kind of like having the one unit per instrument
so each person can select their own levels and tonal
About the Circuit
The schematic diagram (Figure1) shows the MCP6024
quad op-amp which is the heart of the unit. U1C is the
input buffer and has two gain levels that are selected by
the HI-LO switch. When the switch is open, the gain is
about five. When the switch is closed, the gain is about
20. The output of U1C feeds the + 12 dB side of the treble
and bass controls; R7 and R11, respectively. The signal
November 2013 41
■ FIGURE 1. Schematic diagram.
■ FIGURE 2. PCB layout.