Upgrading Naim Pre-amps
I have had a naim pre-amp of the 32/42 variety since the mid 1980's. Being discrete designs they are amenable to a bit of DIY upgrading. They are also very easy to get into, just screw off the feet and the innards slide out of the case. If you only put the feet back on finger tight then you will not even need a screwdriver to open it up next time. The easiest level of upgrade is a parts upgrade. I have tried this and have some suggestions. More complicated is a circuit upgrade. Finally I have tried power supply upgrades, which if done externally like I have, are relatively easy and leave the option of going back to the original.
The Naim pre-amps are constructed on a printed circuit board with a number of daughter boards plugged into it. As the daughter boards just plug into the main PCB they are easily removable making them easy to work on. Removing the main PCB would be a lot more work as it connects to the sockets but by disconnecting the knobs and other pechamical parts it is possible to get to the underside of the PCB without disconnecting anything electrically.
Suggestions for component changes are:
- Change the red Roderstein electrolytic capacitors for equivalent value Sanyo OSCON electrolytics.
- Replace all tantilum capacitors, they are usually small brown blobs with equivalent value Philips solid aluminium capacitors. As an additional tweek wrap Blue-Tak © round each one.
Change the transistors, primarly those in the phono stage daughter
boards. The daughter boards vary depending on the type of
cartridge you have, I have modified the moving coil variant. To
achieve low noise, five transistors are used in parallel. To be
able to put transistors in parallel like this you either have to
use closely matched transistors or use a resistor to enusre the
current is balanced between them. If you buy sufficient
transistors to get 2 sets that match closely you can either reduce
or replace the resistors. Use those that do not match in other parts of the circuit.
The first choice to be made is the transistor type to change to. Neil McBride has suggested changing to the 2SC2547E which is a faster type with lower noise, and it compliment the 2SA1085E. Avondale Audio use the 2SD786 and its compliment 2SB737. I chose to use the 2SC1775 based on the advice of Hartmut Quaschik whose advice I include here.
- put 220R resistors in the collectors (preamps) this cleans the high frequency range.
- use 2SA872 at 600uAmps for the voltage amplifier stage. I tested more than 20 different types of PNP BJTs at that place, and this is exactly the best one I found.
- on input stages, there are several choices:
- 2SC1775 - clean, but lacking some slam in the bass good at 50uAmps to 125uAmps.
- BD139 Philips - very much slam, but dry treble, resolution is not the best, good at 125uAmps, if you go more, noise becomes obvious.
- 2SC2546 - clean, but not airy, not much slam, sometimes fatigue.
- BF199 Philips, BF240 Signetics - slam in bass, good dynamics, a bit coarse, good at 25uAmps to 50uAmps.
- voltage follower: you have to try, sometimes SC1775, but at at most 2mAmps, it can't deliver 6mAmps or more without bass problems, sometimes BF199, which is very fast, try to use a current source, like a 2SK30Y - FET, Y is the current range from 1.5 to 3 mAmps.
don't modify circuit to cascode circuit, it spoils the music.
- resistors: use 1watt Beyschlag, as suggested from Audio Note, these are definitely the best below -.50UKP
Hartmut Quaschik from Munich in Germany.
- Some further parts upgrade advice was given by Neil McBride in June 1997.
In reply to Martin's query...
Geoff Mead, a member of the London DIY HiFi Circle has analysed the Naim pre-amp circuit and come up with some improvements. Many are easily incorporated as they only involve component substitution. I have gathered the information together and it can be found at the web site of the London DIY Hi-Fi Circle.
I have experimented with using DC-DC converters to power the pre-amp. Naim pre-amps run from a 24V supply and this is a common output voltage for DC-DC converters. By choosing a converter with the right input voltage it can be run from the pre-amp's existing supply.
I have written an article about my experiments with different converters. The initial improvements are a faster and more dynamic sound but extended listening has revealed roughness in the treble. This seems to be related to the amount of high frequency noise generated by the converter. There is still work to be done here before getting something I'm 100% happy with.
Last modified: Thu May 23 11:32:48 GMT Daylight Time 2002