Filament Power Supply with 317 Voltage Regulator

The 317 voltage regulator incorporates all of the fundamental elements of a series regulator in one three-terminal package. We need to add only an external potential divider comprised of resistors R1 and R2 to produce an adjustable regulator. To obtain 6.3VDC values of R1 and R2 have to be equal to 511 Ohm and 110 Ohm respectively.



One end of the voltage reference is connected to the OUT terminal and the other is an input to the error amplifier. The third terminal is the ADJ terminal. The 317 therefore strives to maintain a voltage equal to the reference voltage (1.25V) between the OUT and ADJ terminals. In datasheet for 317, you will find that R2 is equal to 240 Ohm. The reason for this is that the 317 must pass 5 mA before it can regulate reliably. If the potential divider passes 5 mA, then 317 is able to regulate even if there is no external load. Datasheets usually show 317 with the ADJ pin bypassed by a 10 uF electrolytic capacitor which improves ripple rejection from 60 dB to 80 dB at 100 Hz.

Single-ended amplifiers, whether pre-amplifiers or power amplifiers, supplied with voltage from a power supply with a regulator add a specific musical characteristics to the sound of amplifier. This is because the amplifier draws a current proportional to the amplitude of audio signal, and the regulator strives to maintain a constant voltage in the face of this changing current. At high frequencies, the output shunt capacitor is a short circuit and maintains a low output impedance, but at low frequencies, it is the regulator that must do the work, and cope with the audio current waveform. the quality of the regulator is therefore inevitably audible. Nevertheless, regulator defects are still an order of magnitude below passive power supply defects.

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