PSU-25 - Super-Quiet Power Supply for 25W Tube Power Amp


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PSU-25 supports a 25W tube power amp and preamp and is optimally matched to London Power's PA66-25 and PA66-10 Push-pull Octal Power Amp kits. 

Tube power amplifiers require various voltages for the plate circuit, bias and heaters. PSU-25 provides an integrated power supply solution that is compact while providing extremely clean support for tube audio circuitry. These supplies are so good we use them in our own London Power amplifiers!

The PCB accommodates wiring for world-wide mains compatibility. The board itself supports the following:

  • dual mains fuses
  • Japan-compatible 100-120-240V mains
  • plate supply: 320Vdc or 460Vdc at 210mA
  • active hum filtering for high-voltage plate supply
  • bias supply for fixed-biasing
  • 6V/12V London Power heater system at 5A
  • supports four octal tubes plus 12x 12A_7 preamp tubes
  • supports ftwo octal tubes plus 22x 12A_7 preamp tubes
  • supports 33x 12A_7 preamp tubes without octals
  • DC stand-off for heater supply (linkable option)
  • regulated ±12Vdc-175mA auxiliary supply
  • optimized ground tie to chassis

The paralleled main filter cap positions provide ample ripple filtering and long life. The plate voltage can be set to a high or low value: use the 460V wiring to match PA66-25, and the 320V wiring to match 2x PA66-10. These low-noise PSUs can be used to support large numbers of preamp tubes, such as for a mixer or multi-channel active cross-over.

The London Power 6V/12V heater configuration allows the actual heater wiring to be small-gauge wire, typically #20 or #22. Each 6V heater has its own wiring connection, and there are a couple of 12V connections to support various preamps, effects loops, etc. A DC-stand-off is provided for a 30db reduction of heater noise.

The bias supply is a typical cascaded RC filter following a full bridge. Proper bias supplies are built using separate windings, as all London Power power transformers (PTs) have.

The auxiliary +/-12Vdc supply provides active regulation of both polarities, supporting opamps and other solid-state devices, and switching circuitry.

PSU-25 is optimized for use with London Power's PTs, which have dual primaries, plate and bias windings that use a full bridge rectifier (not a half-bridge), CT-ed auxiliary and the 6V/12V heater winding.

Doubled output wiring blocks are provided to facilitate connection of multiple power amps, preamps and auxiliary circuits. All wiring should be #22 twisted as appropriate. Transformer leads should be #18 or smaller.

Extra fuse blocks are recommended, as it is highly beneficial to protect every secondary and also to protect each output stage. Use only slow-blow fuses around the power transformer, and fast-blow types for output transformers.

PSU-25's PCB measures 6" x 3.5" x2"H (155 x 89 x 52mm)

PSU-25's power transformer size: 4.6"D x 2.3"H (125 x 63mm)

Kit requires assembly. If you would like this kit pre-assembled, add PSU-BLT to have the PCB pre-assembled,. The transformer fly-leads will NOT be attached to the boards. The assembled boards are bulkier than the loose parts, so shipping is slightly higher.

Add GND Ground-Link-Isolator to avoid ground loops with other mains-powered equipment.

About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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