I wanted to build a guitar input circuit for an arduino due ADC.
The circuit is intended to be connected to an arduino due analog pin. These ADC input pins require a voltages between 0 and 3.3V when the guitar pickup output is typically 100-500mV AC centered on 0V.
Step 1: Design
I found a nice simple circuit simulator for android called EveryCircuit. I used that app to design the amp.
The guitar signal is modelled by a signal generator on the top left corner of the diagram. There's also a switch there that can be used to supply a square wave to the simulated circuit. This where a guitar input jack socket will be in the build.
Underneath the signal generator, a simple voltage divider circuit lowers the voltage from the 5 volt arduino due power supply pin to 2 volts. This gives the output signal from the op amp a 2 volt dc offset.
The first op amp is working as an inverted amplifier. Gain is controlled via the 10K potentiometer. The op amp I used was a OPA2132. This has a max output voltage (if run at 5V) of about 4V. This is why the DC offset described above is 2V not 2.5V.
The gain can be reduced by connecting the switch that is near the op amp output. You might want to do that of you have another signal boosting pedal before this circuit.
The rest of the circuit lowers the output to 3.3V and uses a second op amp buffer that maintains the voltage level if it connected to something with a low input impedance. Its the output of this second op amp that I intend to connect to the due analog pin.
Step 2: Build on Breadboard
I built the circuit on some breadboard. Here are some pictures. I think its now safe to connect to the arduino due analog pin. I didn't bother to put in the gain boost switch.