The S-Reg Voltage Regulator

A shunt-source voltage regulator for low current audio circuits.


Introduction

There are two kinds of linear voltage regulator, one, called a series regulator, adjusts the current flowing from the power supply to the load. The other, the shunt regulator, keeps the current from the power supply constant but balances the demands of the load by diverting a variable fraction of the supply current to ground.

There are advantages for either type. Series regulators are good at ripple rejection, while shunt regulators are good at reacting to dynamic loads.

The S-Reg is a simple shunt regulator with an active current source. I call it a shunt-source regulator.


How it Works

Each supply rail has four transistors. The first two make up the current source. As the name implies, it is programmed to generate a constant current output regarless of the input voltage, and as such provides good ripple rejection as well as built-in overcurrent protection.

The second two transistors form the shunt, though it may be more logical to think of this block as a voltage amplifier with a reference diode drop as the input and the output connected to the load. The shunt will try to keep the output at a fixed voltage, while the source is providing a fixed current. The shunt must therefore absorb as much current as needed to keep the voltage on the load constant.


Circuit Boards

The S-Reg circuit was originally developped for the CrystalFET phono stage, a demanding circuit which required a regulator with low noise and extremely high ripple rejection. For general purpose use, a standalone evaluation circuit is presented below that will generate±12 V rails from ±18 V DC inputs. It is intended to be paired with a transformer with 12 VAC secondaries. (The VSPS or Phonoclone power supplies, for example.)

The series current is 60 mA, so care must be taken to ensure the load current does not exceed about 50 mA or the shunt function cannot work properly. No heatsinks are required.

The evaluation board is double-sided board and measures 5x8 cm.

RJM Audio S-reg

Download Schematic (BOM)

Boards are available, see the products page page for details.


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