The σ22 Regulated Power Supply

The σ22 Regulated Power Supply

Single-pass, series regulator design.
No IC (integrated circuits) are used. This allows complete design control over all operating points and parameters for superior performance.

The negative regulator is a complementary mirror image of the positive regulator, except for the voltage reference (see "Tracking rails" below).
Thematic resemblance to the topology of the β22 amplifier.

A constant-current source feeds a zener diode as a stable voltage reference. A low-pass filter (with a corner frequency of 1.6Hz) prevents zener noise from being introduced into the error amplifier. This is an effective yet lower-cost alternative to expensive voltage reference ICs. The low-pass filter also provides a soft-start characteristic.
The output noise (unloaded) is less than 12µV (measured using a Tangent LNMP (low-noise measurement preamplifier) and a Fluke 187 50000-count DMM in ACmV mode). This is several times lower than the noise of an IC regulator based PSU tested under identical conditions.
The error amplifier is a discrete implementation of an opamp with a high open-loop gain of 102.5dB. The voltage supply to the error amplifier is isolated with capacitance multipliers to boost its PSRR (power supply rejection ratio). This greatly improves the line regulation performance of the PSU.
A long-tailed pair differential amplifier with current mirror and constant current source forms the first stage of the error amplifier. The second stage is the voltage amplification stage (VAS), also with constant current source load. The 3rd stage is comprised of the power MOSFET output devices configured as a source follower.

The positive regulator's output voltage is based on the reference zener voltage and the gain of its error amplifier.
The negative regulator's voltage reference is the output voltage of the positive regulator. Its error amplifier has a gain of -1, so that its output voltage is the inverse of the positive regulator's output voltage. The negative regulator dynamically "tracks" the positive regulator -- any small voltage fluctuations on the positive rail also appear inverted on the negative rail, improving the CMRR (common mode rejection ratio) of the amplifier being powered.
The tracking behavior means that the voltage on both rails rise and fall equally. When used to supply a fully-complementary amplifier such as the β22, no "thump" noise is heard as the power is turned on or off.

Two paralleled high-current, highly reliable MOSFETs (rated at 17A each) serve as the "pass" transistor of each rail.
The high current rating provides a very high safety headroom against overcurrent damage.
The use of paralleled MOSFETs divides the heat dissipation, simplifying thermal management. Onboard heatsinks can be used which would allow the σ22 to supply up to 1A continuous (with much higher peak currents). More sustained currents are possible by using larger, offboard heatsinks.
The negative temperature coefficient of MOSFETs prevents damaging thermal-runaway conditions that may plague conventional BJT devices.

The high-current MOSFETs are not normally the limit of how much current the σ22 PSU could supply, as long as they are adequately heatsinked.
The maximum current limit is determined by the rating of the power transformer, the rectifier diodes (the specified MUR820 devices are rated at 8A), and the AC line fuse.
The AC line fuse rating should be selected to protect the power transformer from overcurrent damage.
There is otherwise no current-limiting circuit in the σ22, which allows it to supply peak currents of many amperes. High transient bursts of current are always available, which some amplifiers require to avoid clipping and distortion.

The all discrete topology allows the σ22 to be optimally tuned for the best combination of wide bandwidth and solid stability. Since the σ22's output impedance is much lower than even the best low-ESR large aluminum electrolytic capacitors, having wide bandwidth allows the σ22 to respond to fast changing current demands better than a large capacitor (or a bank of capacitors) ever would.
σ22's bandwidth extends beyond the audio band, and maintains supremely low output impedance in the µΩ range. (in fact, the hookup wire will dominate the output impedance).
As such, only a 1µF decoupling capacitor is used on each output rail onboard the σ22. The PSU can supply an amplifier with little additional capacitance for very fast response.
σ22 is also stable with a large capacitive load (tested to 10000µF), making it suitable for use in a wide variety of applications.

Configurable for rail voltages up to ±36V. The voltage is selected by using an appropriate reference zener diode, and choosing the value of a resistor. No further adjustment is needed.
Typical output voltages are ±5V, ±9V, ±12V, ±15V, ±24V, ±30V or ±36V. These are popular voltages specified for many headphone amplifiers, preamplifiers, and class-AB power amplifiers up to around 20Wrms power output into 8Ω.
Multiple onboard capacitor footprint options.
Four sets of output terminal blocks.
Can be used with dual-secondary or center-tapped secondary power transformers.
σ22 is the default power supply for the AMB β22 stereo amplifier.
Other popular applications include power supply upgrades for the Kevin Gilmore Dynalo, Dynahi and DynaFET headphone amplifiers, various stereo preamplifiers, etc.

See the Board & heatsinks section for more details.

Higher performance than IC regulators (such as LM317/LM337 or 78xx/79xx): Dramatically lower noise, lower output impedance, wider bandwidth, superior transient response, better line and load regulation.
Does not use expensive, hard-to-source parts.

Glass epoxy 6"x3.5" printed circuit board, double-layer with plated-through holes, silkscreen and solder mask.
Heavy duty 2oz. copper layers provide lower trace impedance.
The layout of all parts and traces have been carefully considered for maximum performance.

A ground plane on both sides of the circuit board, covering the entire board area (except under the heatsinks on the top side) provides a low impedance ground reference, shields against interference and allows optimized component arrangement on the board.