Full disclosure: the lead designer at SpeakerFilters is also the "editor at large" of AudiophileMusings. Note the article below is entirely Clive's own words without any edits. Enjoy, and thank you Clive for your contribution.
NAIM SBL: FROM SNAXO ACTIVE TO LINN EXAKT by Clive Gidney
At the beginning of the year, having been running a pair of SBLs with a pair of 250s sort of bi amped (one for left channel only & one for right - just because I had 2 x 250s & hadn't got round to swapping them for a pair of 135s) I decided that it was time to do the proper thing & go active.
So SNAXO & Supercap added and this gave a significant improvement in detail, soundstage, PRaT & all the things that SBLs are famous for... But I was still left with the feeling that they are a bit forward & a bit bass light. In fact a non audiophile friend described them as "a bit shouty". And yes - I'd done all the set up things, as close to a solid brick wall as the skirting board would allow, re-sealed the bass loading box etc.
Then a couple of months ago I was at Brian & Trevor's in Manchester for a musical evening listening to a Linn/Kudos Exakt active system thinking this system sounds amazing (although I would have liked to hear it with Naim power amps) but the transparency that the Exakt technology brought, which performs the crossover in the digital domain, was breath-taking. In the interval I remarking to one of the other attendees how great Exakt was & that I wished it was available for SBLs! And he said "I write Exakt filters for speakers that Linn doesn't support" OMG!! That person was Neil Hallworth of speakerfilters.com
So we embarked on the Exakt SBLs project.
Step 1 was to physically measure the SBL cabinets, the position of the drive unit centres, the front baffle slope etc. These numbers were then plugged into Linn's system to enable Linn's Space Optimisation to be enabled. That is available now for free for anyone using a Linn DS or DSM with SBLs via Konfig!
Step 2 involved Neil calling at my house to take some electrical measurements of the drive units to measure their impedance.
Step 3 Neil then took these measurements together with some information on the crossover frequencies & slopes obtained from Naim to work some digital magic to create a first cut of the digital Exakt crossover filters. In fact he sent me three versions with different crossover slopes, 2nd order, 3rd order & 4th order. It was very quickly apparent that the 3rd order (which is what Naim used) was the best. The 2nd order sounded congested in the mid-range while the 4th order sounded as though there was a hole in it. He then sent me a 3rd order v2 without telling me what he'd done. When I tried it, it was clear that this was an improvement over v1 with better definition & spatial separation. The sound stage was more clearly defined & I was in heaven. The slightly bright & comparatively bass light presentation was gone without losing any of the strengths of musicality & PRaT that SBLs are renowned for. Neil assured me that there would be more to come in step 4 but I was so impressed that my SNAXO & Supercap were already up for sale!
Step 4 Neil called again to fine tune the filters. Firstly we agreed that in taming the brightness, they had lost a little of their sparkle. Now apparently, the sound waves emanating from all drive units have an interaction with the speaker cabinet front baffle and to compensate for this, within the Exakt crossover Neil had created a 0.8 dB treble filter. Now I'd been advised by John Jackson (ex Naim Senior Test & Service Engineer & currently Witch Hat Lead R&D Engineer) that the SBL cut away foam grill was designed to overcome the treble baffle interaction issue. So Neil removed the filter, updated the Exaktbox & sparkle was back. Hi hats shimmered, room acoustics were tangible, the space around instruments & performers improved.
Next we played with the actual crossover frequency. With v1 the bass/mid roll off & treble roll on were at the same point 2.4khz which we'd got from one Naim source. In v2 the bass/mid was rolled off at 2.3khz & the treble rolled on at 2.5khz (from a different Naim source) So we tried widening the gap by 50hz either way - not as good. Leaving the bass/mid where it was but bringing the treble back again - immediate improvement!
Adding baffle filters for the bass/mid drivers for both the length & width of the cabinet added further improvements and the subtlety of the changes that could clearly be heard was mind blowing; -0.5 dB - too much, - 0.25 dB, not enough so we settled on - 0.4 dB. Adding a 1.5 dB lift in the bass from 0-60 hz - too much, 1.2 dB - still too much, 1.0 dB - bingo! In all we went through 17 variations fine tuning the Exakt crossovers in just over two hours. Imagine trying to do that with resistors & capacitors in the analogue domain!
The end result? Absolutely stunning. If you thought going from passive to active with a SNAXO was a big step... from SNAXO to Exakt is like having seven league boots on! Really, it's that good.
Oh, the cost? Well I'm going to make a few assumptions. If you're already active with SBLs, then presumably you've got a good front end - LP12 around Akurate spec, a Naim NDX minimum, maybe NDS, or a Linn Akurate DS/DSM Katalyst, Naim 82/Supercap or more likely 52/Supercap, SNAXO / Supercap.
Hardware. You keep whatever power amps you're using - in my case a pair of olive Nap 250s which will retain your signature sound. Ideally you'll need a Linn Akurate System Hub which becomes your pre amp - £2700 new or about £1600 pre owned, & a Linn Akurate Exaktbox Katalyst which is where the majik happens (see what I did there) - £5000 new or I picked one up pre owned with 4 years warranty for £3200; And, you'll need a set of Exakt filters from speakerfilters.com which are just £295. These can be tested for a free 2 week trial period before purchase. Total cost about £5400 pre owned or ~£8000 going new
But you can recoup say £1500 on your NDX, or £2000 on your Akurate DS, £2000 on your 82 /Supercap & £1500 on your SNAXO /Supercap = £5/5500 so approx. cost neutral going pre owned
And if you've got a 52 or an NDS you might even have some cash left over
A huge shout out for Neil of speakerfilters.com for all the work designing & refining the filters.
Clive.
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