AE-231: Week-4REGULATION: Making the output voltage independent of load variations.
Our power supply begins with an external 24volt wall-wart type power transformer, chosen because it can be located far away from our microphone preamplifier, which is sensitive to the radiated magnetic energy that is typical of power transformers. (Toroidal power transformers can be rotated to minimize / null the radiation, but they are more expensive...)
FIGURE-1a: Our voltage-doubler derives two voltages from the wall-wart's single transformer winding. Keep in mind that the 24-volt specification is always RMS. To predict the AC-to-DC conversion, dividing 24 by .707 will yield the voltage produced by each diode, upper and lower, approximately plus AND minus 33.9 volts DC. The zener diodes will clamp at 24 volts after which, "-24vdc" will be come our Zero Volt Common "reference" connection so that the supply will produce 24 volts for the preamp and 48 volts for phantom power.
Figure-1b: One of three alternate methods for voltage regulation. The zener diodes are now used for reference only (by the "pass" transistors) so that load changes do not affect the voltage drop across the resistor. The base-emitter junction loses .6 volts.
Figure-1c: "Fixed" 78xx and 79xx voltage regulators are substituted for the zener diodes. These TO-220 case devices are an affordable and efficient way to get the job done.
Figure-1d: The fixed regulators from Figure-2b are replaced with LM 317 and LM338 adjustable regulators.