JBL LSR28P Technical & Service Manual

Also see for LSR28P: Owner's manual

18POWER SUPPLYInitially, raw alternating current enters the LSR28P from the IEC connector on the signal inputprocessing 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 thetransformer connected. The voltage that has been selected is then connected to the toroidaltransformer 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 rectifierD13. It is filtered by capacitors C16 and C17. . . the end result is a stable power supply ofnominally positive and negative 36 volts. Mainly, this voltage is used for the rails of the poweramplifiers, which provide a full voltage swing from peak to peak of 78 volts. The rectifiedvoltage is also passed through a resistive ladder network which steps down the magnitude furtherand is regulated at positive and negative 15 volts to drive the housekeeping/fault detectioncircuitry. Voltage doubling circuitry supplies the positive and negative 70 volts necessary tosupply the drivers and predrivers. By utilizing this bootstrap configuration, the adjacent stagesare prevented from scavenging the voltage from the rails during demanding informational ormusical passages.The main power transformer that is employed in this product has its core in the form of atoroid. This physical shape provides better magnetic flux permeability between the primary andsecondary windings, and, thus, has a better instantaneous power delivering ability thanconventional bundled, stamped-steel. cores. A lower hysteresis loss also is beneficial since lessheat is generated. It has long been known that the toroid allows better flux lockup between theprimary and secondary windings and, therefore, a better instantaneous voltage delivery thantraditional transformers. The combination of using this type of transformer and the bootstrapcircuitry almost guarantee that sufficient voltage will be available on demand for full bassresponse. . . without the “bottoming out’ of the low frequency driver at crucial moments in themusic.