PassLabs 0s free download

File name PassLabs-0s pwramp.pdf

Aleph 0s Service Manual Version 1.0 - 1.3 PRODUCT DESCRIPTION The Aleph 0s is a high performance Mosfet single-ended Class A stereo audio power amplifier, intended for maximum performance in reproduction of music. It is a simple design, having only three gain stages: input differential pair, cascoded voltage gain stage, and output followers. All three gain stages are biased by constant current sources from the negative supply. The output stage will operate as a single ended class A system at lower power levels and will operate as a push-pull class A system at levels above the bias point of the constant current source. 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 8 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 30 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 complementary N and P channel output transistors operated as source followers, so that they provide only current gain. High current single ended Class A bias is provided by yet another constant current source from the negative supply. This current source provides greater than 1 amp of constant current per channel COMPLETE SCHEMATIC For purposes of clarity and simplicity, the complete schematic of the Aleph 0s is broken up into the following sections: Power supply, Front end, and Output Stage. Figure 2 shows the power supply schematic. An IEC standard AC line connector connects to the primary of a toroidal power transformer through an inrush suppression thermistor, fast blow fuse, a power switch, and a thermostat. 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. 100 volt operation requires a special transformer. The secondary system consists of a bridge rectifier and four 31,000 uF capacitors. The secondary DC voltage is approximately plus and minus 40 volts. The front end circuitry of the amplifier is decoupled from the main supply by RC filters. Figure 3 shows one half of the output stage. Both halves run exactly in parallel. A B C D V+ 1 1 R205 R207 .33 221 IRF244 2 Z201 9.1V 47.5K +D R201 Q201 2 OUT R208 .33 Q202 G D Z202 9.1V Q203 R203 221 R202 221 IRF9240 IRF244 S -D 3 3 R204 221 Q204 4.7 C201 R206 MPSA42