NAIM SBL Loudspeakers: From SNAXO Active to Linn Exakt: Guest Contributor

Here's something a little different.  The first ever "guest contributor" post on AudiophileMusings.  Below is Clive Gidney's experience of working with SpeakerFilters on development of Linn Exakt filters for the Naim SBL loudspeakers.

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. 

Linn Exakt and PMC Twenty.26. World First Review Part Two - Converting The Speaker

Part Two Of A Three Part Review

Part One of the Review Is Here

EDITED 30 September 2016:  The official Linn conversion kit for the PMCs are now available from Linn dealers - there's now no need to convert DIY as per the described below - although the official kit still requires much of the same actions :)  Note that the PMC warranty will still be voided by the Linn kit.

After conversations with Linn about the timing of the conversion kit launch which is still not clear, back in April it was time to take the plunge.  So here's what is involved in the conversion from standard Twenty.26 to an Exakt compatible version. To be fair, this conversion could be done for any project to convert the PMCs to any form of active operation, it is not specific to Exakt.

FIRST AND MOST IMPORTANT: THIS CONVERSION WAS DIY AND IS NOT APPROVED BY LINN NOR BY PMC. THIS CONVERSION WILL INVALIDATE THE WARRANTY ON THE SPEAKERS. THIS IS NOT A COMPREHENSIVE GUIDE, IT IS NOT INTENDED AS A SET OF INSTRUCTIONS.  ANY FOLLOWING OF THESE STEPS IS ENTIRELY AT THE RISK OF THE PERSON MAKING THE CONVERSION. THERE MAY BE ERRORS OR STEPS MISSING. NO ACCURACY NOR COMPLETENESS CAN BE IMPLIED. YOU HAVE BEEN WARNED!

The basic principle here is to remove all electronics that sit between the rear terminals and the drive units in the speaker. Fortunately, PMC have provided the .26 with pairs of terminals for each individual driver unit within the speakers. This is intended for bi-amp or tri-amp operation of the speakers, still in passive mode.  Exakt needs a direct connection to each drive unit within the speaker, so the 3x pairs helps considerably in this conversion. The latest Twenty5.26 from PMC has reverted to a single pair of terminals and uses a metal panel, so would be considerably more work to convert. The plan was to make the conversion reversible, so making as few modifications as possible whilst still achieving the objective.

Dismantling and Removing The Passive Crossover

The back panel with the vertical terminal links already removed - this is how the speaker would be used in a tri-amp passive arrangement.  Note the textured paint finish on the back panel shows every bit of dust and finger mark, but these are easily removed with a lightly dampened cloth, as you will see in the finished article pictures at the end of the post.

The panel is held in place by multiple allen headed countersunk machine screws.  They're not particularly tight as they're threaded directly into the MDF of the main cabinet.

Once all the allen head screws are removed, the panel can be lifted out.  Here, a cloth has been used to support the panel on the speaker's stand without damaging the finish of the stand.  The panel is a fairly straightforward piece of machined MDF.  The crossover is very substantial and looks to be populated with good quality components. Along the top of the picture the cables go out to the individual drive units - these will need to be connected directly to the speaker's rear terminals to enable Exakt operation.  At this point they are kept connected until they can be correctly identified.
Here the damping foam for the Advanced Transmission Line can be seen.

Fortunately, the drive units for each cable are clearly identified and each cable has a clear + side marker on the cable itsself.  Before disconnecting the cables, the treble cable was given a single cable tie as a marker, the mid-range cable 2 cable ties and the bass cable 3 cable ties.  This makes them identifiable once they are removed from the passive crossover.  Note different gauges of cables used

The crossover circuit board is attached to the back panel by 4 crosshead corner screws and the mounting bolts for the 6 terminal posts.  Here an 8mm spanner is being used to loosen the terminal post nuts.  There are 6 nuts, washers and shakeproof washers to be removed.  Note these were only nipped up and not tight - presumably to avoid damage to the circuit board.

Once the 6 terminal nuts were removed, the corner crosshead screws were removed

The crossover board then just lifts off the terminals.  Here the plastic spacers that keep the crossover board supported away from the back panel can be seen.  This is all the dismantling completed - the posts and spacers all remain in place as this is where the cables that go to the individual drive units will need to be connected

Conversion

At this point, all the passive crossover components have been removed.  The speaker cables that go from the passive crossover to the drivers have female spade terminals, but they now need to be connected to the back of the cylindrical terminal posts.  It would be easy enough to just cut off the female spades and solder on some loop terminals, but that would make the conversion back to standard passive more complex and that was something that was to be kept as straightforward and as standard as possible.  This maintains the future resale value of the speakers as there is a much bigger market for standard passive speakers than there is for Linn Exakt compatible speakers.

So a conversion from cylindrical terminal posts to male spades is required.

Parts required are 5mm gold plated loop terminals (its hard to find any of these. So here, 8mm are used ones and they're a little larger than ideal so stanless steel 5mm washers were also used to help with the fit), male gold plated 6mm spade terminals, some solder and tubes of insulation

On the left the 6mm male spade, on the right the 8mm loop.  Using pliers, the cable grip part of the male spade was gently squeezed together to the point at which it was a tight push fit into the cable tube on the loop terminal

Here the male spade has been push fitted into the loop terminal

Working through the 12 connectors - 6 for each speaker

Using a 45W soldering iron, heat was applied to the interface between the loop terminal and the spade terminal at this end of the loop terminal's cable tube.  Only once the flux would melt on the outside top of the loop terminal tube was the solder applied to the joint - this makes sure that all the surfaces are hot enough to receive the solder at the same time and eliminates the chances of dry joints.  Soldering is being carried out on a ceramic tile work surface

After cooling the male spade on each terminal was bent upwards to approx 45 degrees to assist with connecting to the speakers' internal cables.  Later, after fitting the loop terminal to the speaker terminal posts, the loop terminal was also bent upwards by about 20 degrees as other wise it would have been easy to trap the cables between the back panel and the speaker carcass

The terminals are then attached to each of the terminals on the inside of the speaker back panel using the stainless steel washers and the shake proof washers and nuts that originally held the crossover to the terminal posts

Then insulating soft plastic tubes were added over the male spades.  Probably not necessary.  Additional pencil markings show which way up the panel goes and which column of terminals is +ve

Here the panel is offered back up to the cabinet and the cables connected.  You can see the extra bend put into each of the loop terminals to move the cables away from the cabinet.  Tweeter to the top pair, mid to the middle pair and bass cables to the lowest pair, using the previously fitted cable ties to ensure correct connection order.  Note, the cables are only just long enough to reach the terminals.

Once the panel was refitted, a 1.5V AAA battery was used to ensure that the speaker cones all moved in the correct direction.  Put the positive of the battery onto the positive of the speaker and negative to negative and the speaker cone should move outwards,  Just a brief touch is enough to check but doesn't damage the drivers

The back panel was then masked up to create guides for adding a set of old Linn active system labels

Labels installed

So that was it, all done.  Checked, checked and checked again.  The connection sequence was as follows:

- Connect bass amp to bass terminals
- Play music quietly and ensure only bass info was coming out of bass drivers.
- Switch off amps
- Connect mid amp to mid terminals
- Play music quietly and ensure mid information is coming out of mid drivers
- Switch off amps
- Connect treble amp to treble terminals
- Play music quietly and ensure treble information is coming out of treble drivers

Enjoy the music.  Pack the passive crossovers carefully in bubble wrap and a strong box and store in cool dry place.

Part Three Of The Review Is Here 

EDITED 30 September 2016:  The official Linn conversion kit for the PMCs are now available from Linn dealers - there's now no need to convert DIY as per the instructions below - although the official kit still requires much of the same actions :)  Note that the PMC warranty will still be voided by the Linn kit.

"Portable" Project Part Three

Part One is here

Part Two is here

So we have a box, some vents, and amp and some crossovers.

Next up was cutting and fitting of the connections panel.  This covers a pair of RCA phono inputs, a combined mains socket / fuse / switch and a USB port for charging.  This was cut from a small black perspex panel, drilled to fit etc.

The full sheel of perspex with the appropriate holes drilled, but before cutting.  It has protective sheets on both sides

All the holes drilled, cut to size and slots cut to fit all the necessary bits

Soldering the input cables to the RCA phono sockets for the connection panel

The panel was then fitted with all its parts and then screwed into the main box.  No sealant necessary as the centre amp chamber is ventilated anyway.

This will be the outside, once installed

Connection panel all cabled up

Installed and cabled up - here both the inlet and outlet vent holes can be seen, and the 3 holes through to one of the speaker chambers

The white plug is actually the USB charger - the cable running over the top and then into the USB connection on the connector panel

Here the crossover, power amp, speaker cables, input cables, volume control, connection panel, mains sockets and USB charger are all in place, as is the rear panel warm air vent.  Output speaker cables not yet silicon sealed into place.  Note Linn black interconnects and bi-wiring of amp to speaker crossovers (the crossovers require bi-wiring as that's how the Mission 773e rear connections were arranged)

Part Four is here

"Portable" Project Part Two

Part One is here.

Using hole cutters on the drill, holes were cut for the drivers in the front panel, and some repair work was done to the wood as a result of my handiwork (PVA glue is a good thing).  Positioning the drivers was decided by putting the tweeters as far apart as possible, the need to squeeze 2 mid-bass drivers per side in the space available and the position of the hinges for the lid of the box.  Some suprisingly long and substantial nails were found in the panels by the hole cutters, which quickly went blunt, so won't be any use for future projects.  Ho-hum.

Fitting the crossovers, amp, cables etc. into the space available wasn't too bad, but sequencing what to when was pretty key.  I also used 5mm p-clips to keep stuff tidy.  Wherever possible, connections were soldered and covered by heatshrink can cables p-clipped to the inside of the box.  Bi-wiring was used from the back of the amp to the crossover boards, as that was just as easy as it would've been to re-bridge the connections on the crossover itsself.

Crossovers mounted on the insider of the front panel - the hole lower down is for the ALPS pot volume control

Crossovers are on small plastic "stand offs".  The multi-coloured cables pass from this central amplifier space, through the 18mm dividers into the speaker cabinet spaces.  The gold coloured cables will go to the amp

Wiring up the volume pot was straightforward, using a wiring diagram found using Google.  It uses Linn black analogue cables from the input panel and then onwards to the amp.

Part Three is here

FRANKENKEILIDHS - Zombie Speakers Rise Up: PART 4

PART 1 here
PART 2 here
PART 3 here

I only have reasonably basic tools available, so no fancy routing of baffles or anything like that.

Let's start at the back of the cabinets.  Normally, Keilidhs have 6 terminals on the back panel to facilitate connecting up to 3 pairs of cables from 3 amps.  They're an integral part of the crossover circuit board and, once the external collars are screwed onto the posts, hold the crossover into the cabinet:

Original Keilidh Crossover (from www.costrruirehifi.net)

So, no crossover also means no binding posts to connect speaker cables to.

I also decided that I wanted to make conversion of Frankenkeilidhs from passive to active and back again as easily as possible, as I have a vague idea that I'd like to play with some low cost DSP crossovers at some point.  So the approach is just to fit a set of terminals into the existing holes in the back panel of the Keilidhs and connect them directly to each of the speaker drivers, thus avoiding the need to house a crossover inside the cabinet, and facilitate active conversion without opening the boxes up again.

ebay again and some nice looking binding posts were sourced at £1.95 a pair.  They claim to be gold plated and look the part.  I guess we'll know in a year or two's time how true that is.  Anyway, they're long, substantial and provide the option of bare wire or 4mm banana plug connections.

One binding post stripped down to its parts, ready to fit

For speaker cables, I'm using some high purity 42 strand copper cable from Maplin - its left over from a 50m drum that was bought when putting the garden speakers in place.  Do I cost it in or not?

Tining up the speaker cables ready for soldering to the binding posts

The sequence for building up the posts, cables and cabinet need to go in the right order to avoid re-work by doing silly things like soldering up and having no nuts to tighten the posts in place. Fortunately, and unusually for me, I got the sequence right on all 12 posts.

At this level of quality, it probably doesn't matter at all, but I did as many of the little things as right as I could, as you might as well, if you're going to put the time in and it doesn't make a material difference to the costs.  So each of the 3 internal cables and across both pairs are identical lengths. Just in case it makes the tiniest of differences.

This is from inside the cabinet.  Thread the nuts onto the cable, then thread the cable through to the outside of the cabinet.  Note there are no plastic washers on this side as they're only need for identification or to isolate the post from the cabinet.  MDF is insulator enough.

Threaded through to the outside

Fit the plastic red or black washer first (for easier identification of + and - terminals), then the clear washer

I soldered the binding posts after threading the washers onto the cable (picture above), but before installing the binding posts into the cabinet.  This allows soldering of wires to posts outside the cabinet, making the job much easier than doing that job inside the cabinet

Then push the binding posts into place and tighten the nuts from the inside of the cabinet

Here all 6 posts are completed on the inside of the cabinet

And from the outside - looking very professional, even if I do say so myself!

FRANKENKEILIDHS - Zombie Speakers Rise Up: PART 3

PART 1 here
PART 2 here

The starting point, a set of cabinets in very good condition, but kinda missing tweeters, mid-bass drivers, wadding, crossovers, terminals and any sense of self esteem:

Empty cabs - no drivers, crossover, wadding, terminals nor bases

Inside, Linn fitted these "Kustone" blocks which are said to help with absorbing internal reflections

The Linn logo is moulded into these, but a bit tricky to see here

Its a loosely packed set of granules, held together with adhesive.  Water would probably pass through this material

Here you can see some of the internal cross-bracing and the thickness of the MDF in use.  These are suprisingly sturdy and well constructed boxes

Further internal bracing - the flat board with small holes is horizontal and the larger hoop is in the vertical

So I started with tweeters - and already there is some doubt over how to calculate the budget for this build, as I attempt to keep track of the spend.  I have decided the following (possibly dubious) rules:

- I only include the cost of what goes into the final version
- Parts tried along the way can be sold on or used in other projects, so are not included in the total
- Where I've used parts which are components of some other product or project, I've allocated a nominal value to them, based on a vague recollection of what I paid for the complete product
- Labour is not included, otherwise I'd be looking to buy a brand new pair of M140s or something...

Here were my choices of tweeters.  Why Mission?  Well, I have some lying around in the workshop, there's loads of them about and they sell at very reasonable prices.  The left hand flat faceplate driver looks very similar to other Linn drivers from earlier designs such as Isobariks and early Kabers.  But they also look very similar to a million other soft dome tweeters out there and probably originate from a limited number of manufacturing plants, regardless of the branding on the product.  Of course, there will be many subtle variations in design, materials and sonic outcome.  This particular pair are from a pair of Mission 737R speakers from the 1980s.  Sourced from a pair I had that suffered a little too much from aging cabinet glue and the mid-bass drivers having brittle rubber surrounds.  So I paid about £45 for those originally, breaking them after about 3 years of ownership.  Let's call it £15 the pair.

At the time I was looking into this, a pair of much more recent Mission 782 tweeters came up on ebay and I picked those up for £18.  As it happens, later testing revealled suprisingly little difference in the performance of these drivers - the 737R driver sounding a little more enthusiastic, the 782 smoother and more cultured, but these were very slight differences.

At £15 and £18 a pair, both options look like they're going to be within the budget.

Tweeter options.  737R (1980s) and 782 (90s - 2000s). Front

Rear views with the 737R at the top

Here they are in the original homes:

Mission 737R

Mission 782

Trial mounting these in the cabinets immediately gave the need for the use of a file (to provide clearance within the cabinet for terminals in the case of the 737R, some head scratching about the way the face plate is designed on the 782.  Much use of blu-tack too.  Mocking up stuff is a bit crude, rough and ready, but hopefully gives an approximation of the way stuff will work when finished from a sonic and physical perspective.  I had a pair of old Linn tweeters in the workshop too, but there faceplate is huge and the magnets won't go through the hole in the Keilidh baffle.  I don't have any wood routing equipment so major changes to the holes in the baffle were of the radar.  This is my first real attempt at doing stuff with speakers, so I wanted to keep it as straightforward as possible.

Mid-bass drivers:  the main challenge.  Where to get something that will fit, and at the right price.  I'm sure Monacor's drivers from the likes of Wilmslow are very good, but they're out of budget for this build.  Pretty much nothing else fits in the size and power bracket.  The size though, is common place in the car audio arena.  But then they always seem to come coupled with a matching tweeter (and a not so sophisticated single capacitor crossover, in the main).  So an ebay automated search was instigated and a couple of weeks went by.

The up popped something quite interesting from Monacor's CarPower branded speakers - a 165mm woofer, boxed, new old stock, no tweeters and 2 pairs available, model CRB-165PS.  At a buy it now price of £25.95 a pair, worth a punt.  Only 4 ohms per driver though - which is the standard in the car hifi world, but something which could present amplifiers with a load challenge.  The cones on these are polypropylene, which might fit well with the tweeters, with the 737R also using that material in its mid-bass driver.  The look like a metal cone, with their blingy sliver finish.  With their 4ohms and 35W nominal power handling, they were always going to be a bit of a gamble.  Manufacturer's details here.

The back of the Mancor CarPower mid-bass driver.  The basket is very thin steel (in line with the price of the drivers) and I added some foam pads in an attempt to damp them a little.  Pictures of the fronts come later.

More on mid-bass drivers later!

The final component is the crossover.  Another rake around in the workshop and I dug out some old Mission 773e terminal / crossover units.  The Missions, like the Keilidhs, are a 2 way design with 3 drivers - again the mid-bass drivers doing identical work and arranged vertically above and below the tweeter.  They look like this:

Mission 773e - similar to the Keilidh in concept, but newer, cheaper and a somewhat less well built cabinet.  Decent sounding for their price though.  Might be a good donator of a crossover.  Or not.

So what are these crossovers worth, when considering the budget?  They're from a pair of 773 that I bought from ebay which were described as "immaculate".  When they arrived, they'd clearly been stood in a puddle at some time and the side panels were separating from the base, with the wood de-laiminating in the way that MDF does. The cabs were good only for the wood recycling skip down at the local tip.  Fortunately a full refund was paid immediately and the speakers weren't required to be returned.  So I have 4 drive units, 2 tweeters and a pair of crossovers spare to help out with any problems I have with the pair we have in the dining room.  I'm therefore going to count the crossover costs as £0.  If you're not convinced by this logic, no worries, it all comes back to bite me later...

So the major components are selected, a few bits and pieces such as terminal posts have been sourced and build can commence.