Neil's Naim Preamp Power Supply Page...

Getting that power...

OK, you need something to power the preamp. If you have a Hicap or Snaps then you could use that. If you want more than the nominal 24v it gives (eg, if you want to use it as the pre-regulator stage but still insist on running your modded preamp circuits from 24v), then open it up and you will see the LM317 reg (in a TO3 can) in there with R1 and R2 (note they actually use 2 resistors in parallel for R2)..... so you can re-set the voltage by changing R1 and R2 to whatever you like. Obviously there's a limit - check what the raw dc level is across the caps (modern Hicaps usually give about 38v) and leave at least 5v between that and the reg voltage.

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.

A recent experiment...

A while back I tried adding a couple of resistors into the psu to add as a low pass filter and try and filter some crap off the mains. This was at a time when I had a Lingo powering my LP12 and had found that it was definitely degrading the sound of my system somehow (it seemed to be dumping crap on the mains which my preamp didn't like). So I added the resistors in the following way (and this was on this web page, more or less as my recommendation as the way to try things...)


          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!).

But....

With the resistors in there, the sound definitely cleaned up and overall I found them to be a success. However, recently I got rid of the Lingo, and moved to more revealing speakers, and because of this am going through lots of tweaks and reassessing whether I still like them. I find that without the resistors, I find the music flows along better. The resistors seemed to sit on the music somehow. And so I would now say to people.... give them to try as an experiment although I have now banished them from my system. Feedback on this would be welcome.

Bypassing...

Note on the above diagram, the extra little caps. I reckon having some very good grade small caps after the main smoothing caps does audibly improve things. The 5 uF polypropylene makes the biggest difference. The little 10 nF is perhaps overkill, and note that it can make the sound a little brighter. My choices for these caps are Answar or Kimber. I have tried so called 'audio grade' ones from RS Components (don't waste your money) and also more normal loudspeaker crossover grade (again don't waste your money) and feel if you are going to do it at all, splash out of quality.

Another word about coil size...

Recently I have made a direct comparison of 2 approaches with coil sizes. I have 2 identical power supplies at the moment... one for the phono stage and one for the line stages. They both have 300 VA Antrim Audio Grade coils (well worth the money). Well, I got another Antrim coil wound. The secondary windings are actually rated at the 300 VA or so level, however the core is a 1000 VA! By dropping this into one of my supplies I then had the means to directly compare these 2 high quality psus, with the only difference between them being the coil core rating.

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.