USB-C experiments – part 2…

As I wrote about a few weeks ago the USB-C power supply standard might open up a few new possibilities for DIY designs. Having recieved some PCBs now, it turns out I was right… 🙂

The first is very much an old idea – a rail-splitter or “virtual ground”. A rail-splitter does exactly what it says on the tin, it splits one supply rail into two equal rails with a “virtual” ground in the middle. Tangent described this idea in a fair amount of detail around 20 years ago and it was used successfully in many of the original Tangent/head-fi designs such as the original Pimeta. Most railsplitter designs have a fair few drawbacks so to me they have also been a bit of an edge-case that I didn’t see much need for.

However, I “reacquainted” myself with the railsplitter concept last year when Neal over at enjon.uk posted about building a “high-power” rail splitter for testing use. I started doing my own version of the layout more or less as soon as I saw Neal’s post because it seemed useful not just for testing, but as it often happens I couldn’t really “crack” making a good layout the first time and so I abandoned it. Picking up the layout again this time worked though, mostly because I started routing with the BUF634 in a DIP package to give me a good basic layout which I could then switch to the SO-8 outline that the new BUF634A comes in.

Having managed to crack the railsplitter layout I started looking at my “back catalogue” to see what else could be made to operate on a single 20V rail. The obvious choice was a Pass B1 because it basically does not require any changes except some optimised power filtering. I might still get to that at some point, but I realised the JFETs for the B1 are even harder to find than they were back in 2013 so a new design would be of limited use.

It then occurred to me that something else that has moved on since I first learned about railsplitters is opamp-design. The original constraint was the the TLE2426 doesn’t really like supplying more than around 20mA, but as most of the newer opamps has a much lower power consumption and so even driving headphones should be doable. I therefore decided to convert my “MiniPre”-design to single supply using the TLE2426 railsplitter. Originally I started looking at the TLE2426 in TO-92 format because it’s very compact, but the noise reduction graphs in the datasheet quickly convinced me that using the “full” 8-pin TLE2426 and the noise reduction capacitor would be a good idea when the source PSU is a potentially noisy switching unit.

My prototype amp uses the OPA1656 and I’m quite pleased with how this one turned out to be honest. I only had to extend the original board by around 6mm (0.25”) in length to fit the new PSU arrangement. While I’ve only briefly tested the amp it works very well, even on USB power. No noise, hum or outrageous turn-on/turn-off thumps while running off the decidedly non-spectacular PSU that comes with my HP work PC :).

This isn’t an “endgame” design by any means, but within the limits of the topology and space constraints I think it’s a very capable amplifier. USB-C supply clearly isn’t an “endgame” PSU solution to begin with and you might get a little bit better performance from these opamps at a higher supply voltage. Very heavy headphone loading is probably also a potential issue, but using this as the basis for a small monitor-controller or pre/headphone amp would be pretty obvious. If you can find a small case and shoehorn it in, I guess it would even work well for occasional office use without having to worry about mains supply and carrying a external PSU.

Project files for both of these designs coming within a week or so once I’ve had a chance to compose my thoughts on them 😀

5 Responses to USB-C experiments – part 2…

  1. Neal says:

    I thought this amp was already a great layout, but it’s really nice to see you keep revisiting and revising. I’ve been sat on a couple of OPA1656 for well over a year now, this might spark me into doing something with them.

    • theslowdiyer says:

      Thanks Neal – and thanks for the inspiration this time round. If you want a couple of my spare boards to get those OPA1656s playing some music then just let me know, because I definitely will not need all of them 🙂

  2. Neal says:

    Thanks, that’s very generous of you, I will take you up on that. It will be good to do a direct comparison.

  3. Pingback: Project files: Single-supply experiments… | theslowdiyer

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