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XR3801 Circuit Description and Service Notes Introduction These notes are intended to assist maintenance and service of the XR3801 amplifier. It is recommended that reference is made to the relevant schematic diagrams and system diagram whilst 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, although attention is drawn to the surrounding text which explains circuit operation and may qualify such measurements. Mechanical Construction The mechanical structure of the unit is made up of two identical side panels, a rear panel, and a 3mm steel structural front panel. The rear panel is fixed to the side panels by means of six M5 screws (three each side) which also secure the two rear rack mounting brackets. The front panel is fixed to the side panels by means of two side fixing brackets secured to the front by way of the handle screws and to the sides by three M5 screws each side. The main circuit board is fixed to the two heatsinks by the power device fixing screws and is spaced from the circuit board by way of insulating bushes. The circuit board/heatsink assembly is fixed to the rear and side panels by a total of eight M5 screws. The mains transformer is supported on the internal "transformer tray" which is fixed directly to the structural front panel. The extruded cosmetic front panel is fixed directly to the structural front panel by two M5 screws. Top and bottom covers complete the assembly each fixed in place by eight M4 screws. Access to all the major components may be gained by removal of the top and bottom covers, further disassembly is rarely required. Circuit description Input Stage The input stage is built around TL071 operational amplifier IC1A, configured as a unity-gain differential amplifier. Its correct operation is dependent upon both of its input terminals being correctly terminated and, therefore, any gain errors around this stage may be a result of a fault in the screened cable connection to the input PCB P1030. Preset potentiometer VR2A adjusts the gain of this stage and thereby provides adjustment to optimise the Common-Mode-Rejection of the input stage. This preset is factory set for optimum rejection and should not be re-adjusted unless it has been necessary to change any components around the input stage. The trimming 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 51 Ohm resistors. Observe the amplifier output and adjust VR2A for minimum output. Power fo