XLS 3U SERIES
CIRCUIT DESCRIPTION AND SERVICE NOTES
INTRODUCTION
These notes are intended to assist maintenance and service of the XLS 3-rack-space family of amplifiers. Please
refer to the relevant schematic diagrams and system diagram while reading this document.
The component references of the two amplifier channel electronics are appended "A" and "B" respectively. Shared
circuitry (such as that of the protection system) has no suffix. This document will refer to channel A references
only. Operation of channel B is identical except where explicitly noted. Voltage values in bold mentioned in the
text are test voltages which may be used for diagnostic purposes. Please read the surrounding text which
explains circuit operation and may qualify such measurements.
MECHANICAL CONSTRUCTION
The mechanical structure of the amplifier comprises three main components: chassis, front panel and cover.
The one-piece chassis incorporates rear panel, side panels, bottom cover and transformer mount.
The main PCB is fixed to the chassis using 4 M4x16 screws with insulating washers through the main
heatsinks and 4 M4x8 screws for the edges of the board, one of which connects the chassis electrically to 0V on
the PCB.
There are two heatsinks secured to the board with 3x No6 / 5/16" pozi pan 'B' point self tapping screws and 3x
M3.5 shakeproof washers each. Output devices are bare mounted (with zinc oxide heatsink paste) to the heat-
sinks with 2x No6 / 5/16" pozi pan 'B' point self tapping screws each. CAUTION: The heatsinks are LIVE at up to
+95VDC on one AND -95VDC on the other.
The plugable input module and output module are secured using 4x No4 / 8 plastite screws and 4x M3 / 8 screws
respectively.
The mechanical integrity of the amplifier is realised when the substantial frontpanel is fitted using 4x M4 /10
Flange Pozi Black screws at the sides and 2x M4 nuts behind the front. The cover is secured using 6x #8 / 3/8"
Flange Pozi Black self tapping screws.
All internal wiring is plugable to allow quick removal of main PCB for repairs and servicing.
CIRCUIT DESCRIPTION
Input Stage
The input stage is built around one half of a TL072 dual operational amplifier, IC1A on the input board,
configured as a unity-gain differential amplifier. Its correct operation depends on both of its input terminals being
correctly terminated and, therefore, any gain errors around this stage may be a result of a faulty cable termination
within the mating XLR connectors. There is no provision for trimming Common-Mode Rejection (CMR) but
checking this can easily eliminate the input stage in the search for a fault.
