18
POWER SUPPLY
Initially, raw alternating current enters the LSR28P from the IEC connector on the signal input
processing board and is directly connected to the power switch S3 through the main fuse F1. .
From there, it travels to the voltage selector switch that determines which primary on the
transformer connected. The voltage that has been selected is then connected to the toroidal
transformer and stepped down to a more useful voltage. This voltage then enters the main PCB,
where it is rectified or transformed to positive and negative Direct Current by bridge rectifier
D13. It is filtered by capacitors C16 and C17. . . the end result is a stable power supply of
nominally positive and negative 36 volts. Mainly, this voltage is used for the rails of the power
amplifiers, which provide a full voltage swing from peak to peak of 78 volts. The rectified
voltage is also passed through a resistive ladder network which steps down the magnitude further
and is regulated at positive and negative 15 volts to drive the housekeeping/fault detection
circuitry. Voltage doubling circuitry supplies the positive and negative 70 volts necessary to
supply the drivers and predrivers. By utilizing this bootstrap configuration, the adjacent stages
are prevented from scavenging the voltage from the rails during demanding informational or
musical passages.
The main power transformer that is employed in this product has its core in the form of a
toroid. This physical shape provides better magnetic flux permeability between the primary and
secondary windings, and, thus, has a better instantaneous power delivering ability than
conventional bundled, stamped-steel. cores. A lower hysteresis loss also is beneficial since less
heat is generated. It has long been known that the toroid allows better flux lockup between the
primary and secondary windings and, therefore, a better instantaneous voltage delivery than
traditional transformers. The combination of using this type of transformer and the bootstrap
circuitry almost guarantee that sufficient voltage will be available on demand for full bass
response. . . without the “bottoming out’ of the low frequency driver at crucial moments in the
music.
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