PassLabs Aleph1 free download

File name PassLabs-Aleph1 pwramp.pdf

Aleph 1 Service Manual - Rev. 1.0, 1.1 PRODUCT DESCRIPTION The ALEPH 1 is a high performance Mosfet single-ended Class A audio power amplifier, intended for maximum performance in reproduction of music. It is a simple design, having only three gain stages: input differential pair, cascaded voltage gain stage, and output followers. All three gain stages are biased by current sources from the negative supply. The output stage will operate as a single ended system at lower power levels and will operate as a push-pull system at levels above the bias point of the constant current source. There are only marginal differences between the Rev 1.0 and 1.1. SIMPLIFIED SCHEMATIC To best understand the operation of the amplifier, refer to the simplified schematic Figure 1. The front end of the amplifier accepts a balanced or unbalanced input signal at two N channel Mosfets operating as a differential pair. They are provided with bias by a current source from the negative rail which operates at a constant 10 milliamps. The output of the differential pair drives a P channel Mosfet which provides voltage and current gain. At the output of this second stage you will see the full voltage swing of the amplifier. This second gain stage is provided with a single-ended Class A current bias from another current source from the negative supply which provides a constant 40 milliamps current. Between the current source and the drain of the P channel device is a constant voltage source which is used to provide voltage bias to the output Mosfet transistors. The amplifier has N channel output transistors operated as source followers, so that they provide only current gain. High current single ended Class A bias is provided by a current source from the negative supply. This current source provides approximately 4 amps of constant current, which can be modulated by the P channel level shifting transistor to produce greater amounts of negative current on demand past the bias point. The control circuitry of the current source is further set up to behave as if a negative 16 ohm resistor were placed in parallel with it, extending the bias range of the output stage. COMPLETE SCHEMATIC For purposes of clarity and simplicity, the complete schematic of the ALEPH 1 is broken up into the following sections: Power supply, Output Stage, and Front end. Figure 2 shows the power supply schematic. An IEC standard AC line connector connects to the primary of a toroidal power transformer through a fast blow fuse, a power switch, and power inrush suppression thermistor. Fig 2 shows the transformer wired for 120 VAC, and the transformer can be adapted to 240 VAC by connecting the two primary windings in series. The unregulated secondary system consists of a bridge rectifier, and four 25,000 uF capacitors. The secondary DC voltage is approximately plus and minus 67 volts. Note the power thermistor which connects the earth ground to the chassis and circuit ground. Fig 3 shows the output stage blo