AXT Systems

MSU-3 Stereo Monitor Selector
MSU-3 Unit
This audio monitor selector unit was designed and constructed to allow a single balanced 2-channel input to be routed to one of 6 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 ASCM-4 control module will also allow the selection of two stereo balanced inputs (using ISM-2 modules) if required. 

The balanced XLR inputs are first unbalanced to maintain maximum CMRR from the incoming signal cable using BIM-3 modules.
The output from the BIM-3 modules are fed to the master attenuator (a VOLCM-4 module), which allows continuous adjustment of the stereo signal.

The output from the master attenuator is fed as a bus to the inputs of each of the trim control modules (also VOLCM-4 modules). The output from each trim control module is then fed to an OSM-2 output switch module which is responsible for balancing the signal and connecting it 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-4 control module, which allows for two selectable inputs (not used in this project). 

An external PSU consisting of a PSU-8 triple-rail module was used in this case since space was limited in the enclosure.
MSU-3 Unit Block Diagram
MSU-3 Unit
MSU-3 Modules before cabling
MSU-3 Unit
MSU-3 Modules with cabling
MSU-3 Unit Front
MSU-3 Front view
MSU-3 Unit Back
MSU-3 Back view
MSU-3 Unit in sleeve
MSU-3 in timber sleeve