If however you want to build your own psu, then no problem. Here's how to do it.
First you need the mains coil. The secondary voltage must give less than 40v dc (about 27v rms) as that's the maximum the LM317 can take (and personally I would recommend keeping it below 35v as then you can use 35v tantalum caps on the input of the first reg stage). So buying a coil of max 24v rms is recommended. The VA rating is a contentious issue. Until recently I used a specially wound 600 VA coil (and before that a 700 VA one). Naim have a 700 VA or so in their SuperCap. The Hicap has something like a 500VA. However I have had success with considerably smaller coils too, and now use a couple of 300 VA audio grade coils made by Antrim Transformers (a good firm) in two separate supplies (one for the phono stage and one for the rest). So takes yer pick. In terms of the electrical load, you don't `need' a big coil. However, a meaty coil does seem to sound better. If you are in the UK, Maplin do a nice 330VA 22v rms affair and this coil is conservatively rated. On the other hand, you might like to look into getting one wound. One-offs are expensive, but if you order a few (perhaps you know one or two fellow Naim tweakers) the price drops drastically. I recently got 10 500VA excellent audio grade transformers wound for 35 quid each.
OK, wire up the coil to the bridge rectifier, and stick some large smoothing caps across it:
mains coil rectifier smoothing (eg) o-----o o----------o----o o-----------o----o----o------o mains ( ( V1 | | | | | | V2 ac ) ) rms | | | | | | dc ( ( | | | | | | ) ) 0v | | ------- | | | ( o------o | | | o+ | |+ |+ |+ ) o------)---o o--|o~ ~o|--o === === === ( ( V1 | | o- | | | | | ) ) rms | ------- | | | | ( ( | | | | | | ) ) | o------)------o | | | 0v ) ) 0v | | | | | | 0v o-----o o------o--------o o-----------o----o----o------o
I have shown the coil with 2 secondaries as that's what most off-the-shelf coils come with. I find connecting them together as shown, and having one large smoothing reservoir is best (this what the SuperCap does). The Hicap uses the 2 secondaries separately (ie, 2 lots of rectifiers and regs) but this means each side only has about 250VA with about 15000uF smoothing. I reckon having one bigger supply is better.
I have tried making the bridge rectifier out of discrete Schottky diodes, with pleasing results (ie, better sound), though I would recommend putting a capacitor across each diode (eg, a 1uF 100V polyester).
The smoothing caps are....whatever you like. Basically I would recommend that you don't skimp. I would have at least 20000uF in there. Personally I have always used 6x10000uF....which admittedly is way over the top, but then again Naim use 60000uF too in their SuperCap I think.
Recently I have taken the route of paralleling more smaller caps, and have used 15 of the 3300uF Panasonics Low ESR caps that you can get from RS. I think the results have been pretty good, although I am not altogether convinced and I think now would recommend (as a first attempt) to just use a few big caps (eg, 22 000 uF). Though the type of cap is again your choice. I have heard nice things said about the Elna Cerafines, and BHC Aerovox 30 series (in fact I have used these in power amps and they seem very nice). My gut feeling is that, if the ESR figures look quite low and the ripple currents quite big then it will be ok. I also have the feeling that if the cap is physically big for its capacitance, then it seems to sound pretty good (not sure why). Caps that offer a huge amount of capacitance in a small can are probably to be avoided.
o----o o----o o----o | R1| | R2| | | | - | - | | | | | | | | | | Voltage | - | - | | Out to | | | | | | Preamp o---o o---o o---o o---o----o------------o----o-------o | | | | | | | | from | | | | | | |5uF |10nF === === === === === === === === rectifier | | | | | | | | | | | | | | | | o--------o---o----o---o----o---o----o------------o----o-------o 0v Six x 10000uF caps Polypropylenes
where values for R1 and R2 could be R1=0R1 and R2=0R47, so by my reckoning, using freq=1/2piRC, giving -3dB points at 80Hz and 17Hz respectively (give or take 20% error at least!). Use 2.5 W resistors. There's very little power dissipated in the resistors when the psu is running, though at turn-on there is a huge current inrush. The actual wiring is as the diagram shows ie, the caps are connected by short lengths of wire running straight from terminal to terminal to terminal (ie, current goes into the first cap at one end, and out of the last cap at the other end!).
To cut to the kill, the big coil does add a bigger, more expansive sound with noticeably bigger and deeper bass. It isn't night and day... the 300 VA coils are still very good, but the big core does give a sense of scale and weight which I find very attractive.
Note that these coils are the Antrim Audio Grade ones... which use a low flux density core made of GOSS material (apparently). I say this as buying off-the-shelf coils may not give same results (comments anyone?). Getting this big coil wound costs around 130 pounds including delivery. Not cheap, but the improvement these coils make is well worth the extra cost I feel.
And that's about it.