Tuesday, 13 September 2016

Exaktbox Comparison. Something to Consider When Choosing Which Exaktbox To Use: Stereo or Dual-Mono?



How exact is Exakt?  Well, you’d think, from the branding, that it would be as exact as Linn could make it.  A recent online “conversation” revealed that all is not exactly as it might seem.  And its all down to processing power.

A quick review of Exakt – it’s Linn specific set of processes in the digital world that changes the signal being fed to a digital to analogue converter (DAC) specific to each driver in the speakers to allow the sound coming out of that driver to correct for room nodes (like boominess), for manufacturing tolerances (in the higher range of Linn speakers), time for the signal to travel to the listening position, to make sure that driver only gets the part of the frequency range particular to that driver, to correct for phase alignment issues between drive units at the crossover point and to correct for phase errors within the drive unit itself.  All very much in real time (it takes about 45ms).  Clever stuff.  But that takes a fair amount of processing power, something that happens inside a Linn Exaktbox which takes a feed from the Linn digital streamer / pre-amp, does all that processing and feeds the signal to multiple DACs – one per speaker driver – which then feeds to the power amp per driver and then onto the speaker driver.

So all of the above, applied to a pair of speakers that have 3 drivers per speaker (known as 3-way), requires 6 channels of processing and DACs.  As this is a particularly good “use case” to illustrate the differences between stereo filters and dual-mono filters, that’s what I’ll step through in this post.

Linn make an Exaktbox that has 6 channels (in fact, there are 2 of these, one at Akurate level, the other at Klimax level).  So, that it makes it very easy to choose an Exaktbox to suit a pair of 3-way speakers right?  3 drivers x 2 speakers = 6 channels.  So Exaktbox 6 (Akurate or Klimax level) will do the job.  Well yes.  And no.  Why not?  Well, we need to take a look at what’s in each of those boxes.  Exakt processing is done in a processor called an FPGA, and the number of FPGAs you get per Exaktbox varies according to which version in use.  Take a look at the diagram below.  On the left, Exaktbox 6 (Akurate or Klimax, for this discussion they’re identical on the Exakt processing side), and on the right the Akurate Exaktbox 10 (actually called Akurate Exaktbox without the 10, but lets not get into branding choices here!).  Clearly, there are 6 DACs in the Exaktbox 6 and 10 DACs in the Exaktbox 10.  But, more crucially for this post, the Exaktbox 6 has 1 Exakt processor, the Exaktbox 10 has 2 Exakt processors.  Note that these diagrams are simplified for the purposes of illustrating this comparison and test (for those who are really interested, in Exaktbox 6 there is a "Master" processor and 1x "Slave" processor, in the Exaktbox 10 there is a "Master" processor and 2x "Slave" processors).




Now, the Exaktbox 10 has to be capable of running 2 speakers of up to 5 drivers per speaker (5-way).  So, for the sake of argument, lets assume that a 5-way speaker will use 100% of each Exakt processor in the Exaktbox 10.  This might not be true, but it keeps the discussion relatively simple – believe it or not! So, with 2x mono Exakt filters – one mono filter per speaker – both Exakt processors will be fully busy:


 

But we’re talking in this post about 3-way speakers, which probably need less processing power than a 5-way speaker filter.  Again, using an assumption that is purely that, I’m going to say that a 3-way speaker will consume 3/5s of the processing power of a 5-way speaker.  So then we have the 2 Exakt processors in an Exaktbox processor, running a pair of mono 3-way filters might look a bit less busy than the above example.

















Now, if we take those left and right Exakt filters out of the Exaktbox 10 and put them into the Exaktbox 6’s single Exakt processor, what happens?  Well, it gets a bit over full.  I know these are all approximations and assumptions, but its just to illustrate what’s going on here.  The principle applies, regardless of the precise percentages of capability.  So we then have an Exaktbox 6 single processor that’s too full, you can see here that stacking up the 2x mono filters into the single processor doesn’t fit:



So what to Linn do to make it possible for an Exaktbox 6 to support 2x 3-way full range speakers?  Well, they don’t run the 2x mono filters like the Exaktbox 10, they run a cut-down stereo filter.  In order to achieve this, the stereo filter does less complex (and therefore less accurate) processing so it will squeeze into the space available in a single processor:

 

And what does that mean for sound?  Well, it’s less accurate, as you might expect.  Hence, Exakt is sometimes less exact than it might be.

Experiment

Its taken a while for this experiment to happen in Musings’ listening room.  Why?  Well, it took a while for Linn to get around to releasing the “dual mono” Exakt filters for the PMCs amongst others such as Linn’s own Keilidhs.  So if you were running an Exaktbox 6 or an Exaktbox 10 for PMCs back in May 2016 (like me), then you’d not hear much of a difference between them as they were both running the compromised stereo Exakt filters.  In fact, the Exaktbox 6 sounded slightly better due to the greater space and better circuit layout inside the box.  But this was a small difference and has nothing to do with the Exakt filters in use.


Let’s be clear here, even the compromised stereo Exakt filters, with my PMC Twenty.26 speakers, Exakt brought a step change in performance to the system.  See my review here of my first impressions of these speakers with Exakt.  Those impressions were written with stereo Exakt filters in an Exaktbox 10, per the diagram below:



But a few weeks back, Linn finally released the dual-mono Exakt filters for the PMCs.  So the system was set up with stereo filter in the Exaktbox 6 and dual-mono filters in the Exaktbox 10.  As below:


Everything else in the system was kept the same – same Akurate Exakt DSM source, same Chord C-Stream cable for the Exaktlink, same 6x Linn silver interconnects and same 6x channels of Akurate power amplification and speaker cables.  The same SPACE Optmisation settings were applied too, leaving only the stereo vs dual-mono Exakt filters being the only difference between the 2 set ups.  Yes, there’s the better layout of the circuitry in the Exaktbox 6 to allow for, but earlier testing has shown that as being fairly minor (worth having, but not worth trading equipment for, if you already have an Exaktbox 10, IMHO).

Although it takes about 10 minutes to swap all the interconnects over between the 2 boxes (and power up each drive unit one at a time, starting with bass and working up the frequency range to ensure all the connections are correct), its pretty easy to hear the benefits that dual-mono filters bring.  As is often the case, its not that easy to describe, especially given how good the stereo Exakt filters are. But lets say there’s just a lot more of everything without adding any clutter at all.  What you think is very stable imaging, becomes even more stable, wider and deeper too.  What you think is lots of space between and around instruments is a good bit better defined.  Bass appears to go a bit deeper, but room nodes are not excited any more than they were.  Some sounds evolve with even greater texture through the lifetime of a note.  There’s another improvement in the flow and effortlessness of listening to the music and following the tune – without thinking about it.  Are there any downsides?  Not that I’ve noted, its all good.  At least its all good if you don’t think too hard about the financial implications, or the idea that, with even greater processing power in future iterations of Exaktboxes, that there might be yet more to come.  But let’s not go there just now!

So what is the conclusion?  Well, Exakt isn’t always as Exakt as it could be.  And if you’ve got 3-way full range speakers like me, the Exaktbox 6 isn’t necessarily the “lower cost” route it might’ve been, as it involves a level of compromise.  But its also worth saying that the benefit brought by dual mono filters is worth having – if I knew this when starting out with Exakt, I would’ve saved up for the Exaktbox 10 over the Exaktbox 6.  It just so happened that the ex-dem Exaktbox available to me was the 10 channel version and it was part of the purchasing choice for a different reason – the option to go to the 5-way Akubariks, had I wanted to do that at some point in the future.  What does that mean for choosing an Exaktbox for other systems?  Well here’s the run down, as it stands at the time of writing, for a stereo pair of speakers:

  • 1x Majik Exaktbox-i – 8 channels – 2 Exakt processors – capable of running stereo or dual-mono filters up to 4-way
  • 1x Akurate Exaktbox 6 – 6 channels – 1 Exakt processor – capable of running stereo filters up to 3-way (and for some small 3-way speakers, that might be as much as is needed as the stereo filter is as good as the processing is going to get)
  • 2x Akurate Exaktbox 6 – 12 channels – 2 Exakt processors – capable of running dual-mono filters up to 6-way (actually, this might be up to 5-way as that’s the maximum number of channels in currently released Exakt filters, but Linn would have to confirm)
  • 1x Akurate Exaktbox 10 – 10 channels – 2 Exakt processors – capable of running stereo or dual-mono filters up to 5-way
  • 1x Klimax Exaktbox – 6 channels – 1 Exakt processor – capable of running stereo filters up to 3-way
  • 2x Klimax Exaktbox - 12 channels – 2 Exakt processors – capable of running dual-mono filters up to 6-way (actually, this might be up to 5-way as that’s the maximum number of channels in currently released Exakt filters, but Linn would have to confirm)

Some observations / thoughts:

  • Linn should update the Klimax Exaktbox to further differentiate it from the Akurate Exaktbox 6 by adding a second Exakt processor.  The idea of a 6 channel Exaktbox that could run a pair of 3-way speakers in dual-mono is very attractive indeed and would be something to aspire to.  Currently, the cost of 2x Klimax Exaktboxes to run dual-mono 3-way speakers is just financially so far out of reach.  Or maybe produce another model called the Klimax Exaktbox 2 with 2x Exakt processors, priced slightly above the single processor version.
  • The Exakt part of the Konfig interface doesn’t tell you which filters your Exaktbox is running.  Exaktbox-i and Exaktbox 10 will run dual mono filters by default, where they are available.  They will run stereo filters if dual mono are not available.  Multiple Exaktbox 6s will run dual-mono if the user / dealer install the mono filters into each one.  Either way, Linn really needs to make it obvious in Konfig precisely which filters are in use.
  • Should I stick with dual-mono filters in the Exaktbox 10, or take the plunge and go with 2x Exaktbox 6s to get that last chunk of performance?
  • Does a Klimax Exaktbox running stereo filters sound better or worse than 2x Akurate Exaktbox 6s running dual mono filters?  Maybe there’s someone out there with deeper pockets than me (or a dealer with the boxes to hand) who would like to experiment?
  • Thanks to MaH at the Linn forum and some pointers to comments made by Linn's lead speaker designer, its clear that not all 3-way speakers will benefit from dual-mono operation over and above stereo filters.  For smaller speakers that don't need such a wide range of low frequencies to be corrected, then the stereo filter is all that's needed and Linn won't be producing dual-mono for those (an example might be the Majik 109).
  • Does this apply to 2-way speakers?  Or does the lower processing requirement of 2-way speakers mean the stereo 2-way filters are just as comprehensive as dual-mono filters?  The bullet point above suggests that small 2-way speakers will probably have only stereo filters, but maybe the larger ones, such as Kudos Super 20 will still benefit from dual-mono filters due to their greater bass extension.

Conclusion

I like the effects of dual-mono filters in the Exaktbox 10 and will be sticking with them for the time being.  I was very happy with the system before the move to dual-mono, its just a very substantial layer of icing on my system cake!  I’m off to consume another slice…

FYI, for those systems using speakers with built-in Exakt processing and power amps, none of the above applies – this is because each speaker has its own Exakt processor on board and therefore, by definition, are running a mono filter each.