The S-reg Voltage Regulator
A shunt-source voltage regulator for low current audio circuits.
Introduction
There are two kinds of linear voltage regulators, one, called a series regulator, adjusts the current flowing from the power supply to match the demands of the load. The other, the shunt regulator, keeps the current from the power supply constant, matching the demands of the load by bleeding off the unwanted current.
There are advantages to 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 regardless 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 developed for the CrystalFET phono stage, a demanding circuit that required a regulator with low noise and extremely high ripple rejection. It was used on the last Phonoclone revision (Phonoclone 4) and is now the standard regulator in the Emerald phono stage. A general-purpose, 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 2x12 V secondaries, such as (the VSPS or Phonoclone power supplies, for example.)
The total program current for the evaluation version is set to 60 mA, so care must be taken to ensure the load current does not exceed about 50 mA or the shunt function won't work properly. No heatsinks are required.
The evaluation board is double-sided board and measures 5x8 cm.
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