Super Symmetric Power Supply

Super Symmetric Power Supply (SSPS) was conceived to be a major upgrade compared to typical power supplies. Experiences of our audiophile community related to the effects of AC mains quality led to a design brief emphasizing extreme filtering and isolation between SSPS's AC source and the resulting DC output. We believe an overemphasis on low impedance through power systems have left the way open for mains conducted and rectification noises into many audio circuits. Thus in addition to the typical capacitor filtering after rectification SSPS features two common mode choke (CMC) filter inductors in a CLC configuration between the positive and ground rails. Great care was taken with the circuit board layout to maintain symmetry between the rails. This helps keep the noise as a common mode signal the CMC can work on. A secondary goal in the design of SSPS is the ability to configure the supplies to meet a variety of outputs including both regulated and unregulated configurations. Thanks to a bit of serendipity the transformer and capacitor types selected for high audio performance include high voltage devices. Thus SSPS can be built to support a very wide array of applications from regulated 5 volts to high voltage FET and tube circuits.

SSPS circuit boards.

Top SSPS board is built as a +-18v supply. Bottom board is single +5v supply.

 

SSPS Circuit Description

Figures 1 and 2 show the schematic diagram, circuit board, and parts placement for one SSPS board. In a typical X-DAC 3.0 upgrade one SSPS board is used as a pre-regulator to supply regulated plus and minus 18 volts for the DAC chip and analog stage on board regulators while a second SSPS built as a uni-polar regulated plus 5 volt supply feeds the digital circuits. Follow the schematic and layout as we walk through the circuit from raw AC in to pure quiet DC out. The fuse and AC power switch are external and feed all SSPS boards used. AC enters on terminals P1, P2, P3, and P4 allowing the transformer primary to be connected in parallel or series to suit local mains voltage. The board is laid out to accommodate the entire range of TALEMA® encapsulated toroidal transformer line or SIGNAL® split bobbin IF line. Thus a variety of voltage and current requirements can be met by proper sizing for the desired voltage and current requirements. Between the transformer primary and secondary connections a guard band is used to further isolate the output from noise. The guard band ends in the corner mount hole so it can be grounded through a metal standoff to chassis ground. The dual secondary is connected to two full 4 diode bridges with isolated grounds for the positive and negative rails. The isolated grounds allow a balanced star ground system to be more fully realized, isolate the two polarities, and allow the use of positive regulators for both plus and minus rails or even building SSPS to provide two isolated outputs of the same polarity. The secondary can be paralleled when SSPS is being built as a uni-polar supply. The diode bridges on the PCB are laid out to accommodate our preferred Schottky radial lead diodes or HEX-FRED high speed soft recovery types in 2 lead TO-220 packages. Noisy 4-in-1 diode pucks need not apply! The area around the transformer secondary, diode bridges and first filter capacitors uses all the copper area possible on the PCB. This is because as the diodes rectify the transformer output high current high speed dV/dT bursts exist as the diodes conduct at the peak of each AC cycle. Designing this area for as low an impedance as possible keeps the energy going where we want it, into the filters, and not into the air as RFI noise. The first round of filtering comes from caps C1 and C7. With the worst noise spikes taken care of a switch to a RC scheme using circuit board inductance (labeled Zx on schematic) and more caps follows. Z1, Z2, Z5, Z6, C2, and C8 make up the second filter stage. The idea behind the serpentine 'Zx' PCB traces is at the desired DC frequency their impedance is essentially nil while at the troublesome radio frequencies they present an impedance right where the capacitors tend to loose effectiveness. Third filter stage is the common mode chokes (CMC). These CMC inductors are placed between the positive and negative rails. Our care in keeping these rails as identical as possible is to keep the noise components balanced allowing their capture in the CMC chokes. The forth stage of pre-regulation filtering is the CRC filter comprising Z3, Z4, Z7, Z8, C3, C4, C9, and C10.

The unregulated DC forms the input of the regulator section. The regulator design is based on 3-terminal integrated regulators. Only the positive devices are used as they have slightly better performance and are much easier to source, often as new unused surplus. The standard option uses the tried and true Linear Technology LT1086CT for up to 1.5A output. Standard '317' TO-220 positive regulator pinouts are used so a variety of second source devices are applicable. These are VR1 and VR2 on the schematic. Diodes D9-D14 are protection for the regulators against reverse voltage spikes from the big caps surrounding them. Voltage dividers R1/2 and R3/4 set output voltages. Resistors R2A, R2B, R4A, and R4B allow optional trimming of the output voltage. Caps C6 and C12 de-couple adjust pin nodes while C5 and C11 keep the output stable while lowering the regulator's output impedance. Be careful if replacing C5 and C11 with low impedance caps like plastic film types not to leave the regulator unstable and oscillating.

Figure 3 shows an enlarged close-up of the regulator section of SSPS. The right side of the figure shows both the parts placement and PCB tracks in an 'X-ray' through the board from the top view. The left side shows the identical second regulator section without the PCB tracks for clarity. A lot of time and thought went into this layout so lets review its features. As this is an area where lowest impedance is desirable the regulator's ground, input, and output traces are as wide as possible. Placing the output connection node between VR1/2 regulator and their output capacitors C5/11 keeps trace distances as short and direct as possible. The regulator is placed on the edge of the circuit board. This allows air flow and flexibility in heatsink selection. The PCB is laid out for extruded aluminum board mount heat sinks such as Avidâ Power Cooler series. Notches machined in the board edge either side of VR1/2 are for heatsink mounting screws or solder pins. The end of board location also allows the option of using the case wall as a heatsink, keeping in mind the regulator tab is at V+ and would need to be insulated. Between the regulator and output capacitor are the connection points for power supply output and ground. Four holes are provided spaced and drilled for Molex â connectors or soldered wires. These are labeled "+Vcc" for regulator positive voltage output and "-Vcc" for the regulator ground to recognize either could be voltage out or ground depending on if this section is acting as a positive or negative supply. As low impedance is less of a concern with the protection diodes and voltage set resistors they are relegated to the circuit edge. Note also the CxA surface mount 1206 size bypass capacitors. While shown on the upper parts side of the board they are actually mounted on the lower solder side. A final high frequency noise filter can be mounted in the form of C5B and C11B surface mount capacitors right beside the Vcc outputs. When configured as an unregulated supply regulator components are deleted and by connecting V-in to V-out output caps C5 and C11 can be paralleled with C4 and C10 to add that much more filtering and energy storage.

SSPS project status, pricing, and availability

SSPS has been prototyped, the prototype corrected, and production circuit boards built and tested. Low voltage (+5v and +-18v) versions have been fully tested. Testing of high voltage versions is next in the development plan.

Production SSPS printed circuit boards are thick 0.093 inch FR4 fiberglass single sided with 2 oz. copper tracks. Boards are professionally fabricated and include parts placement silkscreen, solder mask, and CNC drilled holes.

SSPS circuit boards are available for immediate shipment at $32 each. The 7 page manual includes a full parts list with all parts available from major suppliers such as Digi-Key, Signal, and often your local electronic surplus store. If you desire a full parts kit contact ACG with your requirements and we will quote a price.

Figure 1 SSPS Schematic

(hint: Use the 'view image' function on schematic to view full screen version.)

Figure 2 SSPS Copper Tracks & Parts Placement

 

 

Figure 3 Detail of SSPS Regulator Section