SERVICE - NEWS BULLETIN No 94BE003
These Power Supplies are build into products such as Studio Line Amplifier, RIAA pre-amplifier. The Power Supply are build up around the two circuits LM317T and LM337T. They have a variable output voltage, one positive and one negative. Max. Current are 1A if adequate cooled. You can vary the output voltage between � 5V and � 30V by changing the external components. By moving the earth reference in the controlling circuit between the positive (LM317T) and the negative (LM337T) You can balance the output voltages to be symmetrical around 0V (earth). To do that we used the potentiometer called BAL. It is 100Ohm in size.
We show only one half of the circuit to do calculations.
To find the output voltage (Uout) we use the following formula:
Be aware that R2 in the real construction are made up of a fixed resistor, R, and one half of the value of the potentiometer, BAL, Rp. That results in: R2 = R + Rp. C1 is to remove RF- noise and is normally on 0.1 uF, C2 reduce voltage variations on the reference and is normally 10uF and C3 smoothen the output voltage and are normally on 100uF.