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Table of Contents
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.
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Summary of Contents for Crown XLS 3U Series
- Page 1 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.
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Page 2: Power Amplifier
CMR checkout procedure is as follows : Inject a common-mode test signal at 1kHz and +4dBu to the channel under test. The common-mode test signal of the Audio Precision test system is suitable, otherwise connect the signal to both pins 2 & 3 via 56 Ohm or 47 Ohm resistors. - Page 3 amplifier. Emitter resistors R10A and R11A de-sensitise the performance of the input stage to parameter varia- tions of the two input transistors. The quiescent current for the input stage is delivered by current source Q3A. Diodes D1A and D2A provide a reference voltage of approximately 1.2V, which is applied to the base of Q3A. Approximately half of this (0.6V) will then appear across R13A (220R), which then sets the current, sourced from Q3A collector at approximately 2.7mA.
- Page 4 bias. The bias voltage across the Vbe multiplier should range between 2.4V (heatsink warm) and 2.5V (heatsink cold). Bias voltages outside this range indicate a fault with the Vbe multiplier and/or a fault in the sec- ond long-tailed pair (Q4A - Q5A, R19A, R20A, R23A, R24A). For example, too small a bias voltage could be caused by R19A being high, R24A being high, R21A being low, Q8A being faulty etc.
- Page 5 current flowing through R58A comes from ZD7A. To turn the LEDs on, the amplifier is required to produce an output voltage approximately 5V above the reference, at which point ZD7A is no longer in breakdown and the current flowing through R58A comes from the output stage via D13A, LED1A and LED2A.
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Page 6: Output Connections
OUTPUT CONNECTIONS The output of the amplifier is connected to Zobel Network R12A/C8A. This network presents a defined load impedance to the output stage at high frequencies to ensure stability. Either of R12A or C8A being faulty will result in the amplifier oscillating at high frequency, which may also be evidenced by mains "hum"... -
Page 7: Power Supply
protection system. R1 is connected via D11, D12 to the +/- 18V AC secondaries of the mains transformer. The union of D11 and D12 will have a negative going full-wave rectified version of the secondary voltage. This is averaged by C11 to a negative DC voltage, reverse biasing D2 and, therefore, has no effect on the protection sys- tem. - Page 8 • Over-bias of the output stage: If the lightbulb is glowing brightly then a reading of more than + 2V across the Vbe multiplier (Q8A, R22A, R21A) indicates excessive current draw in the output stage. XLS PROCEDURE TEST SPEC Load switch set to 4 W position unless stated otherwise. Test carried out on all versions unless stated otherwise.
- Page 9 XLS 202 I/P +0dBu @ 10kHz, 3kHz and 1kHz. O/P <0.2, 0.07, 0.07 % THD respectively. 6. Noise test. NOISEA.TST. O/P <-90dBr ref. 31.5dBu. XLS 602TX O/P @ 50Hz and 100Hz <-68dBu. XLS 402TX O/P @ 50Hz and 100Hz <-68dBu. 7.
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