Neil's Naim Preamp Tweaking Ramblings...

Introduction

A good while ago (when I was a poor student), I introduced myself to Naim amps via a second hand 42/110 combination. I couldn't locate a second hand Snaps, so I thought I would put together a homemade version to supply a single 24 v regulated power rail. Dead easy, and worked a treat. Then I thought I could turn the preamp into a pseudo-42.5 by cutting power tracks into 2 parts, and converting my power supply into a pseudo-Hicap by adding another regulated 24 v rail. That's when this tweaking stuff all began.

Since then I have had great fun playing around with all sorts of mods and tweaks to do with the preamps and their supplies, and also building power amps and the like. Some of it is still motivated by `saving-cash' but most of it is for fun, and the fact that I really like the results. Now, I realise that not everyone agrees with the idea of delving into a Naim piece of kit. In fact since I have put this web site together, I have got all sorts of emails. A lot of tweakers enthusiastic to have a go, and a load of people just interested to read the pages and `enjoy' them. There is a small contingent of people who seem to be personally affronted by the idea of taking a soldering iron to a piece of Naim kit, which I find a little strange, but their view is fine by me.

I imagine these tweaks to be primarily for people who have older Naim gear and are natural tweakers anyway. For those people who like that sort of thing, I would encourage them wholeheartedly to try the tweaks out. But I would add a little word of warning if you feel a little tentative about the whole thing. I would advise against anyone `hacking' their cherished Naim if they aren't totally happy about the idea, and I certainly would not want to get someone playing around with electricity when they aren't really up to it.

It's quite hard to describe the results of the tweaks. "Better" doesn't really tell you much. Different people will have different opinions I'm sure, although it is worth noting that I have haven't had any reports from people who have tried this stuff saying "it sounds worse". I would like to think this is because I really try not to loose sight of why I liked Naim gear in the first place, and so try to judge these tweaks in that context. I am aware that you can do things that can make the sound more `impressive' but that somehow diminish the musicalness of the amp..... the very reasons why I liked the amps in the first place. And so I have tried to be very careful in assessing whether I feel a particular tweak really works in this context. At the end of the day, I don't want a very different sound, I want a Naim sound, but `more of it'.

Am I a business?

And one last thing. I get the impression that some people have been given the idea that I'm some type of `business' doing this stuff. I am not. And although I am not adverse to supplying the odd regulator pcb etc, or even doing the very occasional mod if someone really wanted, (ie, to people who are really keen and can't do it themselves), I would not like to be seen as a some whacko with a Naim-tweaking business. In general, mods I have done have been for friends who have heard my system and said `I want some'.

So having said all that, lets get on with what you can get up to. Credit should also go to Jon Nicoll and Pete Wood as much of this stuff has been a joint effort.

Naim Preamp Mods

Naim preamps run off a 0-24v power rail. Older preamps (eg, 32 and 42) have a single 24v whereas newer preamps (32.5,42.5,62,72 etc) will run from 2 discrete 24v rails supplied by a Hicap. Basically by splitting the preamp into as many individual parts as possible, and supplying each one with its own 24v power rail, the performance improves greatly. The way Naim supply these rails, is to have a conceptually simple dc power supply (ie, mains coil, a bridge rectifier, smoothing caps) and then hang off this `raw dc' rail, 2 or more LM317 encapsulated adjustable regulators. Each regulator supplies a 24v `rail' from which to power your preamp.

Knowing this, you can see that it is relatively easy to hang lots of regs off a raw dc rail in order to get as many 24v rails as you like. This spiel describes some of the ways you might like to try this.

Multi-channelling a 42,42.5,32,32.5,62

These preamps are pretty cheap second hand, and work absolute wonders when modded. All these preamps are pretty understandable, in that there are basically 3 sections to them. Firstly the phono stage (these are on plug in boards supplying about 30dB gain for moving coil) nearest the phono sockets; secondly the buffer stage (this is a unity gain circuit nearest the selector switch); and the line output stages (these are sited on the left, towards the rear of the box and giving a bit more gain). There is also a little bit of circuitry at the back left of the preamp - this is a relay switch which ensures that the output signal doesn't go off to the power amp immediately after switch on (stops any nasty thumps etc).

OK, the first mod is a case of what can be removed from the preamp!

The buffer stage is a unity gain circuit which sits right before the balance and volume controls. I think it is there for historical reasons ie, some old tapedecks or tuners weren't happy driving an interconnect and a volume control, and so a buffer stage was put in. Nowadays, with better output stages of CD players and tuners etc, I feel this buffer is basically redundant. I find the phono stage is hefty enough to drive the volume pot no problems. It isn't doing anything as far as I can tell....except of course forcing the signal through 2 tantalum caps, 4 transistors and a load of resistors! So basically you can try cutting it out. If you use a source like a tuner, CD or tape there might more of an argument for a buffer stage, but even then I'm dubious (I have a tuner and tape deck which work happier without it) and would recommend trying it without. If you just take the tantalum input/output caps out and bypass the circuit with some wire (ie, reversible change) then you will soon see if it's better or worse. I suspect you'll find it's better. Note that the 32 series preamps (and 72 in fact) also have buffer boards sitting on the tape output path. If you don't use tape at all, you can just take them out (at least when you are not using them). If you do record to tape, then I would recommend keeping them and running them off separate regulated rails. Note that if you find you don't need your input buffer section on your 42, 42.5 or 62, you can put it to good use by wiring it in the tape output path so getting a free tape output buffer.

The second thing you might like to cut out, is the relay stage. While this doesn't do anything too nasty to the signal, except put an extra unsoldered contact in the signal path, I once did try bypassing it and tested having it powered up versus disconnected. I am convinced it slightly degrades whatever power rail it is sitting on (though nothing to get too hung up about). But basically if you really want to get minimalist, cut it out and feed the signal `out' straight from the line stage output decoupling caps to the output socket (note that the preamp must always be on before turning power amps on though).

OK, having thrown away the buffer stage, we simply have four `music' circuits left ie, phono left, phono right, line left and line right. If you've kept the buffer stage (perhaps as a tape output buffer) then obviously you need 6 rails. And these can be powered from separate regulators. So now we want to install the regs INSIDE the preamp. There is loads of room in the 42, 42.5 and 62 to do this (in the 32 and 32.5 it is more of a squeeze, though loosing buffer boards helps). Naim use the LM317 regs mounted in the Hicap. They run the 24v rail along a metre or so of cable, and into the preamp. If you look carefully at the line (and buffer for that matter) circuits, you will see that the power enters the circuit via a 27 ohm resistor. This is a little odd if you ask me. Any LM317 application note does bash home the importance of having the regs mounted as close as possible to the circuit they are supplying with as low impedance connection to the circuit as possible. Naim choose not to do this, and on investigation you can see why.... indeed just taking the resistor out while you have regs in another box (like the Hicap) does indeed degrade the sound. And I do acknowledge the resistor (together with the 47uF cap across the power rail) does reduce noise.

But it turns out that when you do make the effort of mounting the regs inside the preamp box, right next to the circuit, and whip out that 27 ohm resistor (so allowing a true low impedance power rail) then I find the preamp performance (particularly timing) suddenly blossoms. And this is why you should stick the reg circuits INSIDE the preamp box. If you decide to keep the regs in the psu box, then leave the 27 ohm resistor in.

The reg circuits themselves are pretty simple. There are 2 similar circuits I use (one being better than the other). For the first one you need a LM317, 2 resistors to set the output voltage and a little cap hanging on the input and output to make everything stable. You can improve things slightly by adding a third cap too, so the basic circuit is :

    
    o------------------------o-----------------o
                   -----     |                Vin 
    this part --> |  o  |    |                  
    is Vout too    -----     |
                  | 317 | o--o         Adjustable Voltage Regulator
                  |  T  | |  |         LM317T in package 3\TO-220
                   -----  |  |
                   | | |  |  |
         o---------o | o--o  |
         |           o-------)------o----------o
         |           |       |      |       Vout
         |        R1 -       |      |           
         |          | |      |      |
         |           -       |      |                  
         |           |       |      |
         o----o----- o       |      |
         |    |+             |+     |+    
      R2 -   ===            ===    ===                            
        | |   |10uF          |1uF   |1uF                    
         -    |              |      |
         |    |              |      |         0v
    o----o----o--------------o------o----------o

Actually, the 10uF helps filter ripple better if its slightly larger; 47uF for example (note this cap is actually optional but I would recommend it... it reduces hiss). And the 1uF can also be bigger if you want... the output cap perhaps being the obvious one to enlarge (ie, you could happily put a couple of hundred uF in there.... though I'm not convinced you actually get any improvement). Voltage is set by resistors. Vout is approximately given by

Vout = 1.25( 1 + (R2/R1) ) .

If you are a stickler for getting the exact voltage you want, you can use a trimmer pot as R2. R1 wants to be somewhere around 120 ohms. Some examples of the approximate voltages obtained for some combinations of R1 and R2 is given here (I find they are normally correct to within +/- 0.5 volts or better):

  Out/V   R1     R2              
  20.0    120    1800     
  21.7    110    1800     
  22.4    130    2200     
  24.0    110    2000     
  24.2    120    2200     
  26.3    110    2200     
  28.5    110    2400     
  31.3    100    2400     

Note I have chosen relatively low values of R1. This ensures that at least 10mA or so is drawn from the regulator. Using larger values, say greater than 2k (and the appropriate value of R2 obviously) does theoretically make the LM317 work slightly `better'. I don't think it's particularly significant, but you might like to do it that way. Just ensure that at least 10mA is drawn from the reg (which it will be as soon as it is connected to a circuit.... the circuits eat of order 50mA).

People have commented that most application notes show a little diode sitting across R1 which is to protect the circuitry when the circuit is switched off and caps are discharging. I realise this, but ages ago I tried the circuit without the diode and tried my hardest to destroy the LM317, and I couldn't. Since then I dropped the diode just to makes things a touch simpler and more compact, and I've never had any problems since. But feel free to leave it in if you like.

Ok, now having said all that, I would recommend actually running 2 reg circuits back to back ie, output of one supplies the input of the other. You might have one `pre-regulator' supplying all the other reg circuits, or for maximum quality, each reg will have its very own pre-reg circuit. The improvement in sound when you add a second stage of regulation is really very impressive. I'm a total convert. I would recommend using the circuit I described above as the pre-regulator (saves on cost slightly and I get the impression the quality of the pre-reg is somewhat less important than the final reg stage), and for the final reg stage powering the circuit use the circuit below.

This second regulator stage is slightly different and incorporates a voltage reference, which improves the output stability, line regulation, thermal regulation, drift and noise by a factor of about 7, and in use it's a lot better audibly than the first one:


             -------------- 
            |              |
            |    LM317T    |
            |              |
            |adj   out   in|
             -------------- 
             |      |      |
             |      |      |
         o---o      o      o--------o--o
         |          |               |  Vin
         |   o------o------o--o     |
         |   |      |      |  Vout  |
         |   -      -      |        |
         |  | |    | |     |        |
         |   - R5   - R3   |        |
         |   |      |      |        |
         o---o--|<--o      |        |
             |      |      |        |
            +|      -      |+       |+
        10uF===    | |    ===1uF   ===1uF
             |      - R4   |        |
             |      |      |        |
             o------o------o--------o--o 0v

where the voltage reference ( --|<-- ) is given by 2 legs of the 6.9 V LM329CZ:


                       ------ 
                   /          / 
                  -----------  |
                 |           | |
                 |  LM329CZ  | |
                 |           | |
                 |      -|<- | |
                 |     |    || 
                  -----------  
                  |    |    |
     Cut this     |    |    |
     leg off ---> |    |    |
                  |    |    |

I note that Farnell stock 3 versions of this voltage reference, the LM329BZ, LM329CZ and LM329DZ which are 3 grades of temperature stability (and hence vary in price). The LM329CZ seems a reasonable choice between stability and cost.

The output voltage is approximately given by

Vout = (R4 * a) + 8.125

where

a = (8.125 / R3) + (1.25 / R5)

Below are examples of the sort of values we might want. Take the decimal place with a pinch of salt as these are only guides (probably good to +/- 1 V). And I haven't tested all of them either. The ones I use most is the 19.6 V one.

  Vout  R3    R4    R5  
  19.6  1200  1500  1200
  20.1  1500  2000  2200
  20.6  1500  2000  1500
  21.0  1500  2000  1200
  21.5  1500  2000  1000
  22.0  1000  1500  1100
  22.5  1500  2200  1100
  23.1  1200  2000  1800
  23.5  1100  1800  1100
  24.0  1000  1800  1800
  24.5  1100  2000  1600
  25.0  1500  2700  1500
  26.1  1500  2700  1000
  27.5  1000  2200  1800
  28.3  1200  2700  1800
  29.2  1200  2700  1200
  31.9  1000  2700  1800
  32.3  1000  2700  1500

You can of course use this 2nd circuit for both reg and pre-reg (I don't bother generally as the first one is a bit cheaper). If you only end up with one reg circuit, then use the second one with the voltage reference in it as it really is better.

Note that the 10uF cap (could be bigger) is again optional but I'd recommend leaving it in again.

When you run 2 reg circuits back to back, the output cap of the 1st one becomes the input cap of the second (ie, just use one cap). This cap be the 1uF or bigger. Someone recommended to me that in this case it would be beneficial to have this cap quite large (say 470uF) though I haven't tried this myself.

I have one report from someone who reckoned the second circuit produced more hiss than the first one (he ended up just using 2 of the 1st circuit back to back). Now, I haven't been able to reproduce this at all... the second circuit has given me (and others) nothing but success, but I mention it as in the `investigation' it did throw up the question of `are all regs the same'? And of course they are not. Some fixed voltage types are decidedly noisy and hissy. I would recommend sticking to the LM317 unless you have a good reason to use something else (ie, the spec sheet blows the LM317 away..... in this case please let me know!). I believe the LM317AT is a slightly better spec version of the LM317T so that is probably worth using.

STOP PRESS

As a drop in replacement for the LM317, the LT1086CT sounds considerably better. It gives a firmer bass and just sounds more musical all round, and is not too expensive. I would recommend using it in all applications instead of the LM317. News flash over....

The voltages you run the circuit from is actually not too important. Although the preamps are nominally 24v beasts, they are happy running on anything from 18v to 25v (Hicaps give almost 25v). This means you have a fair bit of room to play with when picking your voltage levels for the respective reg stages. For example, if you already have a Hicap (ie, call it 24v) you could use that as your power supply; and remember it already has 2 regulated rails in it so a couple of pre-regulators are taken care of for free. Then just modify your preamp, setting the regs inside it to about 20v. Make sure there is at least 3v (4v to be sure) between the pre-reg and main reg voltages as the LM317 needs about 3v to work properly. I usually set them to 19.6 V with values of R3, R4 and R5 shown above.

And that's about it. Believe me, it works!

OK, some layouts....

In the past I've always left it up to individuals to sort out the pcbs, perhaps by using matrix board or doing there own layouts via etching boards. But I thought I'd put some layouts on here just to help people out. The layouts I tend to use are pretty rough and ready, although do have the advantage of being compact.

There are two approaches really. Perhaps you already have a psu giving a regulated 24v (perhaps the rail from a 140 power amp, or a Snaps or a Hicap) and so you may just want to fit extra regs in the preamp (supplying 20v or so to the circuits). In which case you can use this layout (100mm x 160mm board giving 18 separate circuits):

Single regulators pcb tracks, in: Postscript, PDF

And the components are put on the following places:

Single regulators what goes where, in: Postscript, PDF

Or perhaps you have built your own raw DC psu (giving at least 28V dc) and you want to have a dual stage reg board sitting in your preamp (this is the approach I follow), then you can use this layout (100mm x 160mm board giving 12 separate circuits):

Dual stage regulators pcb tracks, in: Postscript, PDF

And the components are put on the following places:

Dual stage regulators what goes where, in: Postscript, PDF

The phono board mod

The Naim moving coil phono board has its own little zener diode based regulator circuit on it. I assume it is there so as to give some basic extra regulation when the preamps are used with a poor supply (ie, run from the small power amps). However, if you have your nice new LM317 reg circuit installed in the preamp, it is better to cut this zener network out, and use the LM317 network instead. The improvement in sound is big. All you need to do is find the components that make the zener network (ie, a transistor, 3 resistors and the zener diode; see below). They are the first connections from the phono board 24v input pin. Take them off the board, and connect the power input pin to the power rail.....


                          e        c
           21.5v o-------- o       o--o--------o 24v in
                      ZTX   \     /   |
  Get rid of all      384    \   /    |
  this and connect           -----    |
  your regulated              b|      |
  rail from the                -      -                
  LM317 to the            68R | |    | | 2K2           
  node marked 21.5v            -      -                
                               |      |                
                               o------o                
                               |      |                 
                               -      |                
                          2K2 | |     |                
                               -      |                
                               |      |+               
                               o     === 47uF (keep this but connect        
                               |      |        up to power rail above)
                         Zener ^      |             
                         Diode -      |                
                               |      |                
                               o------o-----------o 0v

The 47uF electrolytic capacitor is one of the the ones in the red/purple plastic cans. Keep this, but connect its positive terminal up to the power rail (ie, when you take out the 2K2 resistor that is connected to it, you can drop in a link instead). You can see that it is relatively simple to get the zener stuff off and use your new 24V rail. Note the actual circuit rail is at around 21.5v after the drop across the zener network. You might like to run your LM317 reg circuit at this, although the circuit is happy running at 24v too. Anyway, this mod to the phono boards is highly recommended --- it's pretty easy to do and does reap big rewards. Although ONLY do it if you have your regulator close to the circuit ie, inside the preamp. If you have moving magnet board, you'll find you don't have this zener network; just the 27 ohm resistor like the line stage sections... which, again, can be taken out if you have the regulators close by.

Miscellaneous mods

Whether you have done the 6 channel stuff or not, there are a few things you can do which will reap rewards. There are a few things which I would want to mention here:

1) There are between 20 and 30 electrolytic or tantalum caps in the preamp. These can be replaced by the new OS-CON type capacitor. Believe me these things are the best thing to happen in cap technology since sliced bread. Replacing all of the 10uF tants that sit on the input and output of each circuit (ie, ac decoupling) reaps big rewards. The 47uF ones are worth doing too. For UK people it's worth noting Farnell do 25v 10uF (664-583) and 47uF (664-601) `audio grade' ones. Recently I have tried using 47uF caps instead of 10uF for the input decoupling and liked the results (smoother, more bass) and so you might want to try that... although to date I have only tried them in one place (line level input stage after the volume knob). A cheaper option for the caps that sit across the power rails (the 47uF ones) are Panasonic Low Impedance caps. These are obtained from RS. I like the 82uF 50v Super Low Impedance Miniatures (394-945), and the 47uF 50v Low Impedance Radials (844-080). I note that 20V 100uF OSCONs are now available and they might be nice sitting across the rail (assuming you are running the rail at below 20V).

When coming to these recommendations, I tried a lot of different caps, and found that a lot of caps might look good on paper (and might even clean the sound up a bit) but could detract from the overall musicalness. For example, the good old red plastic-canned Roderstein 47uF that sit in various places took some beating in terms of musicalness and `Naimness'. So if you are trying any random component replacement, I would recommend keeping an open mind and be prepared to swallow your pride and admit `na not as good'. Oh, and when playing with caps etc, note what working dc voltage they are rated for (this especially applies to the stuff about the power supplies)!

2) If you do not use the balance control....then get rid of it. On the old preamps such as the 42/32 it is really quite a nasty double pot affair, and you tend to find it is never really balanced at the centre point anyway. If you bypass it, you will hear an improvement.

3) I definitely like this one. Naim use ZTX384/214 transistors for just about everything. These are pretty old, pretty bog-standard things. I tried dropping in some replacements ie, modern ultra-low noise, fast trannies. The ones I use are from Maplin (at around 40p each): the 2SC2547E (order code QY11) to replace the ZTX384, and the 2SA1085E (order code QY12) to replace the ZTX214. Try replacing all the trannies on your phono boards (which is where the biggest return in sound is had). I was really gobsmacked at the improvement. The only pain, is that these particular trannies have a slightly different pin-out to the ZTX ones ie, the ZTX ones go c-b-e whereas these go b-c-e. So you have to twist 2 of the legs around each other (without touching!) to fit them in. It isn't too much hassle actually.

4) Talking about caps again... basically any electrolytics in the preamps (including the 10uF input/output caps) can benefit from being `bypassed' with a decent polypropylene cap. This just means soldering a little polypropylene cap across the electrolytic. My favourites are the little green Wima polypropylenes (say 10nF ones). For the power supply caps, something bigger is in order such as a 4.7uF polypropylene. I have tried Kimber ones (nice but expensive) but now use Ansar ones. I find almost anything is better than nothing though. This bypassing stuff tends to smooth the treble and make things sound cleaner and more open. They can give the impression of making the sound a touch `brighter'.

Note that if you are using a Naxo active crossover, then all these preamp mods apply equally to the Naxo. There is one thing you have to note though. If replacing the transistors you would be well advised to put a small (say 47 pF ceramic or polystyrene) cap across the base-collector of the PNP transistors (there are 2 PNP ones in the circuit). This is because there are 2 NPN/PNP pairs which can be a bit unstable when you use nice `fast' transistors (or resistors for that matter). The cap just stops any chance of oscillation. It is probably worth doing this with all the NPN/PNP pairs in the preamp in fact.

One other word about a Naxo. Each circuit can have its power rail cut in 2 parts ie, sort of before the filter network, and after. And then you can run each rail from a separate reg (so a Naxo 2-4 would end up needing 8 regulators). This is the sort of thing Naim do with their Snaxo, and I have now tried it myself and it works very nicely indeed.

And that's about it.

I have to say that with all these things done, the performance of say, a humble 62 preamp, becomes something quite stonking. For me (perhaps not least because I'm a drummer) I think one of the most important areas of improvement is the subtlety of timing (I reckon that goes a long way to making the musical performance a heck of a lot more realistic), but the improved resolution and detail is extremely impressive too. As I said at the top, I would recommend anyone who is interested in these sort of tweaks to have a go. But basically I think you have to enjoy tweaking, such that the finished customised item is fun to make, and something to be proud of. That way you won't get too frustrated the first time you connect something up the wrong way, or can't find a dry solder joint.

Oh, and this may be obvious, but please take a BIG note of polarity. Electrolytic caps connected the wrong way explode (if you connected the 60000uF or whatever in the psu the wrong way, they would probably take your head off!). So be careful, and make sure you know something about electricity before playing with mains voltages. Needless to say I take no responsibility for damaged goods or damaged health!