You can fabricate your own reference supply with a determined effort. What's important is that your supply have a few features that are not so common in low-cost supplies.

I'd suggest you look at the old app-notes and databooks available at the National Semiconductor website, perhaps the Fairchild site, too.

Features desirable in the "classic" triple-output supply are a narrowly adjustable Vcc (normally 5-volts) supply at about 3 amperes with current limiting, and a dual (bipolar) adjustable output supply with a range from 0 to 20 volts to either side of GND, with current limiting on either side and with optional tracking. This latter feature allows you to use a single control to adjust the voltage on both supplies, whether they're set for the same voltage or not. The bipolar supply often is limited to about 1 ampere. All three outputs typically have foldback current limiting settable for each output, and, of course, indicators for voltage and current on each. An indicator for the current-limit setting is quite useful, too.

If you decide to build one yourself, you should acquire a good transformer to use as a basis, so you don't have to replace it later. Its a very good project, but ... keep in mind that it is intended to be a useful tool, which you'll use again and again, and taking shortcuts in its construction will annoy you for the rest of the time you use it. Design the user interface carefully, based on what you want, and not on how long a piece of wire you have in your hand. Make the enclosure's front-panel convenient, including the controls and the output connections. If you want outputs in the rear, well, that's up to you, but front-panel connections are very, Very useful.

I would advise you to take as much time as you need to arrive at a comfortable and convenient front-panel layout. Don't be afraid to look at existing instruments in www-published catalogs (Agilent, Fluke, Tektronix, etc.), and try to determine why the features are arranged as they are. The people who design these commercial supplies do what they do for a reason. It's best to know what that is before, rather than after, you've built your front-panel.

RE