Input for an electric guitar
I'm afraid this is not really a DSP question, even though it must be answered in order to get to some DSP later on ...

A guitar input is a high impedance ("Hi Z") input with an input impedance of typically \$500\, \text{k}\Omega\$ to \$1\,\text{M}\Omega\$. This is different from a microphone input or a line-input, both of which have significantly lower input impedances and they're also designed for  different voltage levels. If you have an impedance converter with an adjustable output voltage you should be OK.