An apparatus for a guitar comprising a tremolo anchor, an upper post portion, a lower post portion, and a compressible material. The monitor can be moved toward and away from the base and can be angularly adjusted with respect to the base of the computer whereby the monitor has a plurality of degrees of freedom and adjustment with respect to the base of the computer. The collimating lens system has a focal point on the main axis thereof on which focal point its light source is located. A part of the signals outputted from the power amplifier are fed back to an adaptive controller via a coupler.