Arcam A65 & 75 free download

File name Arcam-A65_75 amp.pdf

Service Manual A65/75 Issue 1.0 DiVA A65, A75 & P75 Amplifiers ARCAM Contents List ! Contents list ! Circuit description ! Service guide ! Circuit diagrams ! Component overlays ! Circuit board parts list ! General assembly parts list A65/75/75P Circuit Description PSU In order get the output devices to turn fully on and achieve the greatest efficiency, an auxiliary rail is formed from a charge pump driven by the main AC outputs from the transformer. C304,305 & D301,302 charge on alternate cycles. In the other cycle the charge on C304,305 is pumped into C310,311 respectively. This creates a voltage much greater than the standard rails, but with low regulation and high ripple. This is then regulated to 38V via emitter followers Q300,302,303,305 from a zener stabilized voltage (D304,305) The Pre-amp ±15V supply comes from linear regulators Z300,301. The transformer is a dual voltage unit, the required voltage being selected by which fuse holder is fitted with the appropriate fuse. If an alternative to 115V or 230V is required then a different transformer is fitted. The secondary is centre tapped for the main rails, but a separate winding is added for the uC supply. The star ground point SP300 is not a component on the PCB and is on the schematic only for layout purposes. The input pair (Q606,607,618,619,706,707,718,719) are in a Darlington configuration to lower input current offset which in turn will lower DC offset at the output, as the impedances seen on the input nodes are necessarily high for the wide bandwidth required. The output stage is a simple bipolar darlington (Q614,615,714,715). However, the devices used are Darlington devices with integral diodes. This means the bias temperature compensation is part of the output device enabling smaller number of devices, no extra devices on the heatsink (vital for this mechanical layout) and more consistent temperature compensation of bias. Also the resistors are also integral, saving a large amount of PCB real estate and hand insertion cost, while also able to be used as bias set test points and protection sensing. Due to the decreased efficiency of this type of output stage over the previous bootstrapped Mosfet output, a separate set of rails is required for the VAS stage in order to saturate the output transistors. The operation of this is outlined in the Power supply section. However, these rails are individually regulated for each channel to decrease power supply crosstalk. The protection of the output stage is taken care of by a proprietary IC (Z101). This takes care of the switch on delay (R150,C149), DC offset (R102,103,C147), Thermal trip (TH100), and safe operating area. The safe operating area is monitored by a network which compensates for the voltage across the output devices, current through them, and duration of power (R627,606,625,605,C617 & symmetrical associated components) and this is sent to the protection IC via (Q605,612,613,616,617,712,713,716,717). The outp