There is an adjustable power supply kit sold on AliExpress for less than $7 postage included. Excellent design and quality parts. 5-40VDC depending on which transformer you use. Mine came with Nichicon Capacitors that measured 0 ESR. I am feeding it with a 115/230/28/28 transformer and it works very well. Getting +/- ~25VDC out of it.
Lusya DIY Kits LM317t LM337t Linear Regulated DC Power Supply Adjustable Filtering Board 5 40V DC F7 007|Amplifier| - AliExpress

Just a note about this kit. I received mine recently and it contains junk brand Chinese caps that don't have the proper [over-]voltage rating. I was hoping to get the same deal SamR got, but didn't. Lessons learned.

 

Duly noted. That is a shame... The PCB holes for the caps did come so that 3 different sizes could be installed. Still not a bad price even having to supply the 2 largest caps and a good design. I'd complain to AliX and get a refund.

 

Duly noted. That is a shame... The PCB holes for the caps did come so that 3 different sizes could be installed. Still not a bad price even having to supply the 2 largest caps and a good design.

Yeah, agreed.

I'd complain to AliX and get a refund.

I'm not sure. If this was a $50 item, I'd agree. But for $7 and knowing that it IS a Chinese product I'm probably asking for too much. Good caps aren't cheap. You got lucky, Sam.

 

When I buy something I expect it to be what it advertises as. Most of those kits they scrape the barrel to get the cheapest parts possible to put in them to boost profit. Typically it's the 20% ceramic caps being 50% or greater out of spec. I've become vigilant about testing parts for quality and when off-spec. or missing I have no qualms ringing their bell about it. I'd be more than happy to return it or a discount for the replacement part cost but invariably they tell you to keep it and issue a refund. I was pleasantly surprised that kit came complete and on spec. for all of the caps and resistors with the added advantage of apparently real Nichicon caps which tested 0 for ESR and on spec. That vendor has many kits and most are nowhere near as cheap as that one. I'd buy from them again but ring their bell about the caps. Just what voltage were they? I just checked mine and they are 35V so undersized, but I am not trying to get the rated Voltage out of the PSU and only supplying it with a 28V XFMR so it's good for me since I really never go over +/15V. Should have come with higher-rated caps since they advertise 40V output. Missed that one, sorry. Open a dispute and include a pic of the cap showing the rating and they will refund you. Then put in some higher rated caps which the PCB has room for.

 

I got 35V caps, same as you. And that was my complaint; they should be 50V. Good points, though. I think I will open a dispute. Thanks.

 

There is an adjustable power supply kit sold on AliExpress for less than $7 postage included. Excellent design and quality parts. 5-40VDC depending on which transformer you use. Mine came with Nichicon Capacitors that measured 0 ESR. I am feeding it with a 115/230/28/28 transformer and it works very well. Getting +/- ~25VDC out of it.
Lusya DIY Kits LM317t LM337t Linear Regulated DC Power Supply Adjustable Filtering Board 5 40V DC F7 007|Amplifier| - AliExpress
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Another note about this kit...It's been sitting in its bag and the only things I had looked at before were the underrated no-name caps. I had another look at the parts today and realized that the voltage regulators are USED...That is, they were pulled from equipment. So consider new electrolytic caps and voltage regulators as part of the true cost if you buy it.

 

Wow! Mine had all new parts W/ Nichicon Electrolytic capacitors (albeit 30V).

 

One possible upgrade to the mentioned Chinese power supply kit: Negative references can be added to the regulators to get down to 0V. And since the PCB has multiple holes for the main electrolytics it should be straightforward to add the necessary parts. See attached schematic. Add the two sections marked in green, add the two wires marked in blue (running to the opposing regulators), and remove the grounded regulator legs as marked.

Can someone tell me the purpose of D9 and D14? I pulled the image from the Web.

 

One difference between using one or two bridge rectifiers is that the single bridge acts as a full-wave, center-tapped rectifier, so there is only one diode-drop from the transformer, not two as with a bridge.

 

Here's the original schematic as supplied with the kit. Except that the kit came with 2000uF main regulator caps instead of 1000uF.