Upgrading the Naim SBL passive crossover - part 1
This and some later posts are intended to record the upgrades that I have made to the SBL passive crossover, and this may be of use to others. As a starting point I obtained a second set of crossovers bought from a pfm member so that I could upgrade it incrementally.
The standard crossover looks like this:
http://i575.photobucket.com/albums/ss196/isnchys/sblcrossover.jpg
The main problem areas are the blue capacitors which are bipolar (non-polarised) electrolytics and the ferrite cored coils. Both these component types have the reputation of introducing distortion in the audio signal. Also in the case of the electolytics there are also the problems of high ESR, short service life and stability.
The orange capacitors are polyester (better) and in two cases are in parallel with electrolytic capacitors, probably to reduce the ESR.
The target was to replace the electrolytics that carry the audio in the HF section, and the ferrite cored coils that carry the audio signal in the LF section.
The remaining capacitors do not need to be upgraded but need to be replaced due to their age. However it is better to replace them with low cost polypropylene capacitors to avoid the service life and stability problems of the electrolytics.
Here is the SBL crossover circuit diagram, including changed component values:
http://i575.photobucket.com/albums/ss196/isnchys/sblcrossovercircuit.jpg
Note that pictures are rotated by 180 degrees with respect to the circuit diagram i.e. HF section at the bottom of the pictures.
After upgrading the audio carrying capacitors in the HF section the crossover looked like this:
http://i575.photobucket.com/albums/ss196/isnchys/sblcrossover1stupdate.jpg
The 1.5uF and 20uf capacitors either side of the coil are in parallel which allowed their positions to be swapped to find space for the large 20uF capacitor.
At his point due to the lower ESR of the new capacitors the HF output was too high, but more detailed.
Changing the L-Pad resistors
In the crossover circuit the L-Pad resistors reduce the output to the HF driver so that the HF and LF drivers are correctly matched. As R1 is increased, R2 is decreased so that the impedance seen by the amplifier stays at about 6ohm.
Recommended values are:
R1 ohm............ 1.8---2.0---2.2
R2 ohm............ 15----12----10
Attenuation dB.. 3----3.5----4
I am currently using 2.2 and 10, but I find that the SBLs are a sometimes little bright with the standard crossover. I suspect that 2.0 and 12 corresponds to the standard crossover. There is obviously the opportunity to tune the HF output to suit the listener and the room acoustics.
Part 2 will be posted in a few days.
TTFN,
John
. One thing is for sure, the components Naim used were of only average quality or they would not have fitted into the small crossover case (vacuum formed ABS).
