Project files: STEPS clone PSU

What is it?
The board for my “STEPS-clone” single-rail linear PSU as described here. This PSU is suitable for low-power streamers, DACs, headphone amps etc. that run on a single DC-voltage rail and require less than app. 15W maximum. This isn’t really a 100% clone of the original STEPS supply (see here), but I’ve drawn quite a bit of inspiration from the STEPS so I think the credit is well-deserved anyway 🙂

Note that the transformer primary connections are hardwired on the board, so there are separate 115V and a 230V versions of the board files.

How big are the boards?
The board measures 3.95” x 4.7” (app. 100 x 119 mm)

What is the status of the boards?
The published board files are for version 1.0 which is the version I have prototyped. There are a few minor changes I could do, but it’s mostly cosmetic and it might be a while before I get to it anyway so I have decided to publish this version.

Does it use any special/expensive/hard-to-find parts?
If you can order from Mouser, then nothing here is hard-to find. If you can’t, then the only thing that might be difficult to substitute is the Murata common-mode choke and that is optional anyway 🙂

Anything else I need to know?

  • The original idea was that the board should be able to slide into a eurocard-sized enclosure (that’s also the reason for the two extra mounting holes). However, in practice this isn’t possible as the primary pins of the transformer are way too close to the enclosure walls to make this safe. My recommended enclosure is the GX1xx-types from modushop, but there are many other options. If you have more devices, you can of course use larger enclosures to hold multiple PSUs.
  • The transformer secondaries are in parallel, so with the standard Talema range from 7VAC to 22VAC, it should be possible to make the STEPS with outputs from around 3-25VDC.
  • The 2-pin header near the output can be used to connect a volt meter to display the output voltage (or it can be used for something else – your choice! :D).
  • The solder pads on the board can be used either as test points or to tap the AC or unregulated DC-voltage from the board to another PSU board for an AUX-voltage of some sort (additional circuit, trigger voltage etc.). Remember to watch the total load on the transformer and the maximum heat dissipation in all regulators.
  • You can use my spreadsheet here to calculate the adjustment resistors for various output voltages. This will show you the upper/lower limit voltages if you use a trimpot for variable output, and also the power dissipation in the adjustment resistors which you need to be careful with at higher outputs.
  • The only really tricky bit of this circuit is (potentially) managing heat dissipation if your load draws a lot of power on a continuous basis. You’ll have to balance the heat dissipation in the regulator and the pi-filter resistors, while still keeping the voltage to the regulator high enough so that it doesn’t drop out – even if the mains voltage varies a bit. A little tip can be that if your load device isn’t sensitive to output voltage, then turning up the output by app. 0.5-1V will shift some heat away from the regulator. Be sure that you stay within the specs of whatever you are connecting to the PSU at all times of course!
  • As usual for these circuits, you can use both standard and LDO (low-drop regulators). The low-drop types are normally not “better”, but can be a bit less tolerant of circuitry and load conditions so it’s actually better to stick with standard LM317 unless you have a good reason to use an LDO.
  • The only time it really makes sense to use a 3A rated regulator (LM350 or Lx1085 types) would be if your PSU is 5-7V output with a 25VA transformer. If your output voltage is higher or the transformer is smaller, the 1.5A+ current limit of a standard LM317 (or Lx1086) should be just fine.

Downloads:
Download design files here

Related information:
1) Read the original STEPS page linked above. Even if the circuit isn’t completely the same, there is still lots of great info about the LM317 type regulators and how to get the most of them.
2) Read the manufacturers datasheet for the regulator that you are working with. Pay specific attention to recommendations around output capacitance and bypassing of the adjust pin as there are some differences between regulator models and manufacturers here.

Note: Always read the “intro post” for additional important information about my designs.

 

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Linear PSUs are better…

…aren’t they? 😀

No, I don’t really want to start up that discussion here because in my opinion it’s much more complex subject than most audiophiles believe. However, one thing that is obvious is that as more and more small audio components run on single DC rails from an external PSU (streamers, DACs, headphone amps etc.), a fairly large market for aftermarket “upgrade” PSUs has opened up. Some manufacturers (e.g. Auralic) even offer separate PSUs as upgrades themselves. Well, a linear PSU is normally a relatively simple thing so why not DIY it?

Since I now have a DAC, a preamp, a streamer and quite a few other things that run on single-rail DC this seems a worthwhile project and it’s actually been on the drawing board for a while. I did have a bit of trouble getting started on the circuit and layout though, and I didn’t manage to really break the deadlock until remembered a design called STEPS by headwize/head-fi user Tangent from (many) years ago. The design isn’t up anymore, but thankfully I managed to locate it on the wayback-machine.

It’s basically a standard LM317-based PSU, but with a few tweaks added to tease as much performance as is possible out of the LM317 regulator (or one of its many derivatives). My version isn’t a straightforward copy of the STEPS, but I owe a big thanks to the the STEPS all the same. Compared to a “normal” LM317-based circuit this one includes:

  • A simple mains filter on the primary side of the transformer.
  • A snubber circuit on the secondary side of the transformer.
  • Space for high-speed/soft recovery diodes and snubber caps.
  • Space for 2+2 18mm filter capacitors in C-R-C (pi-filter) configuration before the regulator.

Everything else looks like the “high-performance” circuit variation from the data sheet of any LM317-type regulator. The onboard transformer is a 25VA Talema PCB-mounted toroid type meaning the design should be good for most applications requiring less than app. 20W power. The 15VA type transformer will fit as well and allow for mounting in a 1U enclosure, but the constraints on heat sinking and capacitor height might then be an issue.

The pictures show the completed 12V prototype for my Arcam IRdac as well as a partially completed 16V board for an Auralic Aries Mini (a recent purchase) – I’m waiting for a transformer in the mail before I can finish that and test it 🙂