Arcam A75, P75 Plus service manual A65/75/75P Plus Circuit Description

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A65/75/75P Plus 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.

Pre-amp

The direct signal path (line in, tone controls defeated) consists of a single op-amp gain stage, buffering the j-fet input multiplexers and Tape Monitor and AV switches. The selected input is AC coupled to the op-amp, the output is DC to the Pre-out, via a j-fet "pre-mute" switch. This switch is controlled by the uC in such a way that when ever the output relay changes state signal is removed from the power amplifier (A65/75).

The MM phono stage (by passable A75 only, SW100) is opamp based. The circuit is AC coupled to give RIAA/IEC low frequency response. The cartridge is loaded by R100,101,C119,120. The 1kHz gain is 56dB, and RIAA eq set by R110,112,111,113,C127,129,128,130.

Each line input has static protection afforded by the back to back diodes (e.g. D101,103), and RF filtering is set by R118,C134 etc. The selected input is buffered for Tape output by Z104 set as a follower. The output resistor is inside the feedback loop so to keep the output impedance low and short protection is catered for. C121,122 maintain stability into capacitive loads.

From the tape monitor switch (Z105) onwards the signal is AC coupled (C151,152). From there the signal is fed to the tone controls. These consist of two stages: Balance and buffer (Z160), and Bass/Treble Z161. The output of this stage is again AC coupled. Switch SW162 selects whether the tone adjusted signal or a direct signal reaches the gain section. In the A75 Z164 can be used to bypass the volume control when AV input is selected. This fixes the gain to allow an external AV processor to control the volume setting.

The gain stage (Z163) has a gain of 2.2, and the same low impedance output arrangement as the Tape output. The signal from this stage to the Pre- amp outputs and the Power amp input is controlled by a pre-mute (Z171). This removes thumps and clicks, and in the case of the Power amp relay changing state, removes signal so arcing will not take place.

Power Amplifier

The new topology is a high gain voltage feedback design, with a bipolar output stage. Its signal path is designed to be fully symmetrical, despite not having a symmetrical circuit (symmetrical rail to rail). This is achieved with a new dual differential input gain block, where the input differential sees a symmetrical active load (Q600,601,700,701) before driving the second voltage amplification stage (VAS) differential (Q603,604,703,704). This could be described as two Wilson mirrors with their bases clamped together. The second differential also has an active load of a more traditional mirror (Q610,611,710,711), and this gives a push pull signal to the input of the output stage, improving on the traditional single ended current source of the often called "blameless" topology.

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 output of the protection IC is sent to the micro controller (uC) to signal the output relay (RLY1) status. Also, for output muting purposes, the uC can switch the relay via Q100.

Input Selection

The uC controls the operation of the unit. IR commands are received from RX900 on a break-off PCB found on the front panel and fed via LK900/LK903 3-way ribbon. On power on/off the current state of the pre-amp switching stored in an EEPROM Z906. The input switching is 5V logic level and is direct from the uC to the DPDT analogue switches in the case of Tape Monitor (Z105), AV (Z164,A75 only), Pre-mute (Z171) and Mute Q100. The source selector is achieved by 8 way analogue multiplexors (Z102,103) and the control signals are decoded from the uC from MUX1,2,3 by

Z907 which also drives the appropriate front panel LED. The input selection is set by the remote control or the rotary encoder on the front panel display PCB. Tape monitor and AV mode(A75 only) is set by SW900.

Volume Control

The volume control is a motorized manual type, where the signals are routed through the 8-way ribbon LK189. The motor drive comes from Z905, which is a logic level controllable motor driver. It has its own unregulated PSU (D907,C920),and the speed of the volume knob is dictated by the REF pin. The circuit (Q900,R905,901,C901) allows the speed to vary from just above stall to full as the volume control button is depressed on the remote.

SIM Module

The connector PL161 (P75 only) allows the fitting of the ARCAM SIM module. In order to allow this to operate LK172 & LK173 must be removed. This will allow 12V trigger signals to operate the P75.

Headphones

The Speaker outputs (SK600,700) are connected to the headphone socket (SK162) via an 8-way ribbon LK170,171. The output is attenuated by a resistor network R161,162,236,237. The Relay drive is fed through the headphone socket's switch (HPSW1,2) so that when headphones are inserted the speakers are disconnected. Switch SW163 allows the separate switching of the second speaker terminals on SK600,700.

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Contents DiVA A65, A75 & P75 Plus Amplifiers Contents List A65/75/75P Plus Circuit Description A65, A75 & P75 Service Guide L942C1 USE exlude A65 Filter for BOMLeft Channel Power Amplifier L942C7 IR RX Support PAD SP1 Motor Driver Volume DrivePhono Stage & Input Selection Screen L942C2Right Channel Power Amplifier Pre-amp Out, Tone & Headphone L940C1 A75 Plus AMPL940C5 Left Channel Power AmplifierL940C7 L940C3 A75 Amplifier PlusLK1 LK6 L940C2L940C6 0VSIG L809CA L940C4Display PCB for A75 AMP Page Page A65/75 Display Board L8851.1 Arcam Thermistor Siemens B59008 Jumper 2 W Lead APL5 VOLMOT900 220NF Class X2 CAP 275VRMS CS1 Trans LF SS N BC546B Jumper 8-WAY Lead Alpha 8 AMP Multi Voltage Transformer Thermistor Siemens B59008 LK149 LK150 LK151 C606 C626 C706 C709 SW Push 4PCO LK168 A75 Heatsink Clamp LK151 LK152 LK153 WAY DIL PIN Header A65 Plus General Assembly HL4SA M4 Shake Proof RR PAN ‘E HL4SB A75 Plus General Assembly HJ4COOD Brass NUT Earth HL4SA M4 Shake Proof HL4SB P75 Plus General Assembly
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