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Pass Laboratories Aleph 5 Service Manual Rev 0 9/20/96 Aleph 5 Service Manual. The Aleph 5 is a stereo 60 watt audio power amplifier which operates in single-ended class A mode. The Aleph 5 has only two gain stages which are biased by current sources. Because of the inherent simplicity of the circuit, it is easy to understand and repair. There are no adjustments. Figure 1 shows the simplified schematic of the amplifier. Two p channel Mosfets form an input differential pair biased by a current source operating at about 20 ma. The drain of the input Mosfet is attached to the gate of an n channel power Mosfet which forms the active output stage. It is biased by a current source at slightly greater than 2 amps. The voltage rails of the supply are at 34 volts, and each channel draws approximately 150 watts. Figure 2 shows the actual schematic for the amplifier. F1 is a slow blow fuse, set at 4 amps for 100-120 volt operation, and 2 amps for 220-240 volt operation. S1 is the power switch, which has two sets of 25 amp contacts wired in parallel. T1 is a thermostatic switch rated at 75 degrees Centigrade. It is mounted to the rear heat sink. TH1 is a power thermistor used to connect the circuit and chassis ground to the AC outlet ground. It will normally operate at 5 ohms, suppressing ground loops in the system, but will drop to a low impedance if significant current is passed through it. TH2 is a power thermistor (Keystone CL-60) which is used to suppress inrush current. The power transformer drives a high current bridge and 120,000 uF of capacitance to form a plus and minus 34 volt supply. Referring to the channel itself, Q1 and Q2 form the differential input pair of Mosfets. They are biased by the current source formed by Q3. Q3 is biased to about 20 ma by the reference Zener diode Z5 which sets about 5 volts across R11. Z1-4 protect the input from static spikes. R2-8 form the input networks and feedback loops. The output of the differential input pair drives the gates of output devices Q18-29. These parallel devices are matched gate to source voltages as close as .01 volts, however variations as high as .1 volts will not impede operation. The voltage across R114 shows the drive voltage for the output stage, and it should have 4- 5 volts across it. The output of the amplifier is taken from the drains of Q18-20. Q18-20 are provided a bias current by the current source circuitry of Q6-8, also a matched set of n channel Mosfets. Q6-8 are set at 2 amps DC by the network consisting of Q5 and the components surrounding it. Q5 is biased by R17 and R18 in series. A capacitor C9 is used to reduce supply noise. R20 serves to sense the current running through Q6, and feeds that to the base of Q5, forming a loop that holds the output current at 2 amps. The pn junction drop of Q5 forms the reference voltage for the system. R19 is a