by Fred Nachbaur, Dogstar Music ©1998, 2001


3D-2: A Simple Power Supply for Standalone Preamps

The power requirements for the standalone phono preamp are very modest; 12 volts at less than 0.5 amperes, and 430 volts at about 10 milliamperes. Figure 5A shows a simple supply using readily-available 12 volt, 1/2 ampere transformers (units capable of 1 ampere preferred for continuous operation). The 12 volt output from the first transformer is used to power the filaments, and is also fed to the second transformer connected as a step-up device to provide about 110 VAC. This supplies a voltage tripler circuit that provides the 400+ volts required for the preamplifier plate circuitry.

Note that, thanks to the self-levelling properties of our DC feedback circuitry, the actual value of the B+ voltage is non-critical; this is what allows us to use a voltage tripler, with notoriously poor voltage regulation, while maintaining a high degree of precision and independance from power supply fluctuations, tube characteristics, and component aging.

Fig. 5a: Power Supply for Standalone Preamp
Fig. 5a: Power Supply for Standalone Preamp

Filaments are powered from a DC supply to minimise (i.e. virtually eliminate) AC hum induced from the filaments. The four diodes D4-D7 form a full-wave bridge rectifier, and CRC filters provide a sufficiently clean DC voltage to the filaments to satisfy even "sticklers".

Phono Preamp, Power Supply section
Power supply section of the standalone phono preamp

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