Connect your sound source to the 1/4-inch "INPUT" jack on the rear of MIC100 unit. Consider a guitar as the source for the purpose of example. Plug the headphones into the 1/4 inch "OUTPUT" jack on the MIC100. While the MIC100 is a monophonic unit, the 1/4 inch output will send signal to both sides of the headphones.
Rotate the "OUTPUT" control fully counterclockwise and set the "GAIN" control at the twelve o'clock position. Make sure your guitar's volume is turned up, then slowly turn the "OUTPUT" control clockwise until you can hear sound at a comfortable level in the headphones.
Adjust the "GAIN" and "OUTPUT" controls to taste. Because the MIC100 is based around a vacuum tube, high "GAIN" settings will produce a gentle overdrive tone, particularly suited for guitar. As you add gain, use the "OUTPUT" control to reduce the headphone level. The "20 dB PAD" and "LIMITER" buttons will also help keep the level manageable.