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. An attenuator is also present, allowing attenuation from 0dB to -10dB in 2dB steps. This allowed mixes to be produced according to Bob Katz's K System.

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 bridged out on the output module if not required and the attenuator module can be left out by linking the input bus to the output bus directly.


Front of Audio Monitor Selector













Monitor Selector Block Diagram













The balanced XLR inputs are switched in pairs using signal relays, each relay handling a pair from the balanced input connector.
The output from the two input switch cards are connected together to form a bus (since only one of the inputs will be active at any one time).

The bus was then fed into an attenuator module.

ISM-1 CIrcuit (pdf) ISM-1 BOM (pdf)
ISM-1 PCB (pdf)  


The attenuator uses a switched voltage divider, with identical settings for each channel. Resistor values are kept low to minimise Johnson noise.

Since two channels were required, the switch had to be 2 pole, and the most readily available 2 pole switch has 6 positions. This allowed attenuation values from 0dB to -10dB in 2dB steps.

Since two channels pass through the attenuator, I converted the signal to single ended for attenuation and then back to balanced to feed the output bus. This enabled one pole of the switch to be used for each channel.

The output of the attenuator drives the output bus, which is connected to the four output switch modules.

The attenuator assembly consists of two boards, one containing the rotary switch and resistors for signal attenuation, and one that comprises the balanced input and output buffers. Buffering is required to present the passive resistor attenuation network with a constant low impedance source and a high impedance load.

AAM-1A Circuit (pdf) AAM-1A BOM (pdf) AAM-1B Circuit (pdf) AAM-1B BOM (pdf) NE5532 Data Sheet (pdf)
AAM-1A PCB (pdf)   AAM-1B PCB (pdf)   SSM2142 Data Sheet (pdf)



The output switch module switches the pair of balanced output buses to the XLR output connectors using a pair of relays (as in the input switch module).

The only difference is the addition of trim controls for each of the outputs. The trim controls are used to trim each stereo monitor pair to the same SPL for the same input signal.

In order to take advantage of readily available dual gang pots, so that the trim value was the same for each channel, the output bus is converted to single-ended, passed through the trimmer control and then converted to a balanced signal to feed the output.

If the trim control is not required, this part of the module can easily be bridged out and the trim controls on monitors can be used instead.

OSM-1 Circuit (pdf) OSM-1 BOM (pdf)
OSM-1 PCB (pdf)  



Illuminated pushbuttons were used to select the required input and output and also a mute function.

A PIC16F628 microcontroller was used here to read the switches and switch the appropriate relays according to the selection, and light the appropriate lamps as required. The firmware HEX file and source code are available in the Downloads section.

A short switch-on delay to the power supply driving the relays is also present on this board, so that amplifier circuitry has stabilised before connecting the monitors.

Timing diagrams show the various delays for changeover and powerup.

An additional feature was added allowing the selection of several monitor pairs at once by muting the output and selecting the outputs required to be on and then unmuting the output. All of the selected pairs will have signal routed to them when unmuted. To deselect the outputs, select one output only when un-muted and all others will be disconnected.

ASCM-1 Circuit (pdf) ASCM-1 BOM (pdf)
ASCM-1 PCB (pdf) ASCM-1 Firmware

Timing diagrams:

Input Changeover Mute ON Multi Output Powerup
Output Changeover Mute OFF Multi to Single Output  


Power supply

An external power supply was built using the PSU-8 module to provide a clean +/-15V rail for the op-amp audio circuitry and a +12V rail for the relays, lamps and microcontroller circuitry.


Rear panel showing XLR in/out connections











Back Panel showing XLR Input and output switching modules.

Audio switch modules mounted on rear panel
















Input and output audio switching modules mounted on rear panel.

Modules on front panel













Attenuator module and front panel components

VU meter and Monitor selector together























Monitor Selector and VU Meter together