AXT Systems

MSU-1 Stereo Monitor Selector
MSU-1 Unit
This audio monitor selector unit was designed and constructed to allow two balanced 2-channel inputs (eg DAW and CD player) to be selected and routed to one of 4 balanced 2-channel outputs (monitor speakers).

In addition, trim controls are present on each output, allowing each monitor pair to be trimmed to the same SPL.

A master attenuator is also present, allowing attenuation from +2dB to -INFdB as a continuous control. This allows calibration of mix SPL level to various level standards.

The resulting layout allowed everything to be squeezed into a 2RU high, half rack width box, which is the same size as my VU meter box.

Being modular, various sections can be removed if not required. The trim controls can be left out if not required and the attenuator module can also be left out if not required, leaving a simple switch.
The balanced XLR inputs are first unbalanced to maintain maximum CMRR from the incoming signal cable and then switched using a signal relay by the ISM-2 module.

The output from the two ISM-2 input switches are connected together to form a bus and fed to the master attenuator, which allows continuous adjustment of the currently selected stereo signal.

The output from the master attenuator is fed as a bus to the inputs of each of the trim control modules. The output from each trim control module is then fed to an OSM-2 output switch module which is responsible for connecting the signal to the approrpiate speaker set.

A MUTE button is also present which immediately disconnects all outputs from all speaker sets.

The control of inputs and outputs, buttons and LEDs is performed using the ASCM-3 control module.

An external PSU consisting of a PSU-8 triple-rail module was used in this case since space was limited in the enclosure.
MSU-1 Unit Block Diagram