Speck Electronics Xtramix - Manual (Page 48)

23. Power Supply Connector
If required, the VU meters may be calibrated to indicate -10dbV.
An AC voltmeter and signal generator will be required to perform this
adjustment.
This 4 pin connector will accommodate the connector from the power
supply cable. The connectors are keyed so they will only fit in one
direction. For installation instructions refer to the Installation Section in
Chapter 3 of this manual.
43 Chapter 4 Operation Section