Schematic diagramThe following is the schematic diagram of the σ22 PSU. See the Parts list section for details about parts selection.Click the schematic below to see a larger version with all parts annotated. 
Here is the detailed schematic diagram in PDF format (53KB). Circuit descriptionAC input from the power transformer secondary gets full-wave rectified by discrete fast-recovery diodes D1-D4 and smoothed by capacitor C5. Each diode is bypassed with a snubber capacitor C1-C4 to reduce noise. The resultant positive and negative DC rails then feeds their respective voltage regulator sections.The positive regulator consists of a discretely-implemented equivalent of a high current output opamp acting as the error amplifier. Q3 and Q4 form a classic differential long-tailed pair (LTP), loaded by a current mirror (Q7, Q8) to increase the open-loop gain. This is desirable in a PSU application because the increased global feedback would help keep the output impedance low and enhance overall stability. The current through the LTP is governed by current source CR2. Q11 is the VAS stage, loaded by a current source CR3, and compensated by capacitor C14. Two 17A power MOSFETs Q13 and Q14 are connected in parallel to serve as the output "pass" transistors. The paralleling of two devices divides the heat dissipation, and to ensure that they would not fail under severe overcurrent conditions. This eliminates the need for current-limiting circuitry. Only a suitably-sized AC mains fuse is needed for protection against damage to the power transformer and other components. The reasonable maximum current capacity of this power supply is then basically limited only by the transformer, rectifiers, and the amount of heatsinking of the MOSFETs (and, of course, the AC mains fuse). In the positive regulator, the error amplifier operates as a noninverting amplifier. The input of this amp is fed by a DC reference voltage provided by zener diode D5. The zener current is sourced from the current source CR1. An RC filter formed by R3 and C13 (with a corner frequency of 1.6Hz) effectively removes any zener noise. The error amplifier then multiplies the reference voltage to the desired output voltage. The output voltage is determined by D5's zener voltage and the gain setting of the error amplifier (and can be varied by changing the value of R10). The voltage reference and error amplifier (with the exception of the output MOSFETs) gets their supply voltage through a capacitance multiplier formed by Q1, R1 and C9. This, along with an additional capacitor C11 and the use of current sources, boosts the PSRR (power supply rejection ratio) dramatically to improve line regulation. The negative regulator has the same topology as the positive regulator, except that all transistors are complements of the positive version, and it operates as an inverting amplifier with unity gain. The voltage reference of the negative regulator comes from the output of the positive regulator, thus making the output here an inverted version of the positive output. Whatever appears at the positive output is mirrored at the negative output, except it's out of phase. This is how the negative regulator "tracks" the positive. Four sets of DC output terminal blocks are provided so that a single σ22 could supply a balanced β22 amplifier (four amp boards). Two ground terminals are provided as the amplifier's output jack ground return point if the amp is to be configured as a 2-channel passive amp. There are pads for an LED "power on" indicator light. The LED provides a small nominal load on the regulators (from the positive rail to the negative rail) even when there is nothing connected to the output. It will also help slowly discharge the rail capacitors after the power is turned off. Main: σ22 Main | Prev: Tech highlights | Next: Board & heatsinks |
