Monday, 25 January 2021

System Synergy and Expectation Bias

A couple of phrases you might hear or read from time to time when researching or in dicussion about music playing hardware.  What are they about, and why would I be writing about them right now?

Sometimes how to phrase an audio experience is very elusive.  Some readers might say that continues to be the case, even for published posts!  This particular case has been more elusive than usual - its been a year since I made my notes on this project. How to frame those notes have been through about 6 iterations and I'm not sure I'm there yet. But a lot of time and effort went into this, and its time to just put it out there, good or bad, as the products involved might be old hat before I'm content with the wording.  I wanted to not just list the reviews, but to try and put them into some kind of relationship - finally settling on the synergy and expectation bias frame to hang it all from. So here it is, some kind of a review from circa Christmas 2019:

A 6 week or so period of frenzied (at least by my standards) box swapping, mostly for research purposes, but also to inform some purchasing options, mainly not for me, but for a good friend of mine, who's system features here. And it's all given the opportunity for another few blog posts too.  Elsewhere on the blog you'll soon find some impressions of some of the products involved so I won't go into huge amounts of detail here, but will cover a "flavour" of the systems involved. Long form, no photos and no apologies for that.

Coming to Terms

What do these words mean, in an audio system context?  Well, let me have a go at describing how I would use them, so these aren't definitions as such, but more setting the context of this post.

System Synergy - when putting together a whole series of components into a system - CD player, turntable, amp, speakers, cables, the room - do they work together in the way you would like them to?  Or do they lead to some kind of imbalance in a system?  Do you hear a coherent musical message, or is there something that grates like a bloated bass when you wanted tight tunefulness or a laziness to the sound where you wanted pace and energy?  Well, when it all comes together to meet your hopes, that, to you, would be a system that displays good synergy - the components complement each other to give an overall positive result.  If it doesn't work, the synergy's not there, then there needs to be a further search to find what's causing the imbalance, to work out what needs changing or improving.  Note that there is no mention of cost in this description - it has nothing to do with cost, it has to do with balancing characteristics of components to get a good result.

Expectation Bias - we'd probably all like to think we're impartial and can evaluate what we're hearing without regard to what we think we're going to hear or prefer.  Are we going to prefer one brand over another because its been a favourite for years? Are we going to like the sound of the component that has the nicest finish or a preferred species of wood on the front? Will we expect the more expensive bit of cable to sound better than the cheaper one?  Of course we all have these biases - its part of what makes us human.  I think its possible to try and minimise them by being concious of the issue, but probably not to eliminate it.  Blind testing (where you don't know which component is playing) is often advocated, but it just introduces different problems (such as an unnatural focus on the listening process), so I don't consider it to be the answer.  Just accept that a level of bias is inevitable and that it influences your opinion, but try to minimise this as much as possible.  That's one of the reasons why this blog is just opinion - there's nothing scientific about most of it, and that's just the way I prefer to keep it - the disclaimers make this clear.

So How?

If we're susceptible to expectation bias, how do we know if a system really has good synergy or if we're just expecting it to have good synergy?  Well I don't think there's a way to be totally objective (objectivity is restricted by the measures used anyway, as there's no way of knowing if a set of measures is comprehensive or relevant enough, that's why science keeps evolving) so just embrace the subjective and try to deal with it.  Its also worth noting that listening to music is meant to be an emotional experience and therefore, to me, it seems that choosing a system on some level of emotional evaluation is entirely reasonable.

Rooms

Mustn't leave the room out of the equation either.  Lots of impact on the way a system sounds comes from the room and the interaction of the speakers with the room, so I've included that in these musings to try and cover a little of the less obvious.  Where possible (ie with the Linn front end) SPACE optimisation was set up for each system.  What is often referred to as "Tune Dem" or "Tune Method" was used to position speakers and adjust SPACE.

Expectations

I'll go straight into listing the systems that I listened to over those few weeks a year back and that have fed this post, along with my expectation bias - as without that the rest of the post will be a bit meaningless.  The premise of the post being that my expectation bias wasn't met and I'll describe where and in what way.  Perhaps this is part of trying to be aware of my own bias and trying to rationalise how I have, at least partially, probably, tried to put it to one side and listen to what's actually going on. Still a big "if" though.

Systems

I won't go fully into every detail of the systems here, such as precise cables etc., but the main components that differentiate the systems.  I suppose its fair to say that my expectations are in comprison to my everyday system.  They're based on a mix of hearing 2 of these systems frequently and hearing the other components in other systems. It is fair to say that every single one of the components in these systems is highly respected, so there are no "duds" at all, and that's where the synergy element becomes so intriguing.

System 1: 

Melco N1ZH server, Linn Akurate DSM / Exaktbox Katalyst streamer, 3x Lejonklou Tundra Stereo 2.5 power amps, PMC Twenty.26 converted for active use, medium size pretty much rectangular room. This Exakt system uses filters for the PMCs available from SpeakerFilters.com, not the standard versions.
Expectation Bias 1: this is my everyday listen - its taken a long time to get to this system - it has a great deal of  detail but also a very musical flow to it.  Its not the last word in dynamics and slam but I can listen to it for hours (and do so, most evenings), following the tunes, enjoying the emotional impact of the music.  Its not perfect, no system ever is, but its what works very well for me.  What's my expectation?  Well, its more a reference point for comparison than an expectation really, but I would say that I expect it to be very difficult to beat in this evaluation with other systems having some benefits but not the overall balance of this system.

System 2: 

Melco N1ZH server, Linn Akurate DSM / Exaktbox streamer, 2x Lejonklou Tundra Stereo 2.5 power amps, Kudos Titan 505 on their dedicated stands and configured for active use, medium size pretty much rectangular room
Expectation Bias (comparative to System 1): more detail, a more dynamic presentation, a touch of brightness. Perhaps a little looseness in the bass.

System 3:  

Melco N1ZH server, Linn Akurate DSM / Exaktbox streamer, 2x Naim 250DR power amps, Kudos Titan 505 on their dedicated stands and configured for active use, medium size pretty much rectangular room
Expectation Bias (comparative to System 1): more detail, a more dynamic presentation, a touch of brightness, possibly assertiveness approaching aggression, more impact in the lower frequencies.

System 4:  

Melco N1ZH server, Linn Akurate DSM / Exaktbox streamer, 3x Naim 250DR power amps, PMC Twenty.26 converted for active use, medium size pretty much rectangular room
Expectation Bias (comparative to System 1): a more dynamic presentation, a touch of brightness, probably assertiveness approaching aggression, a loss of musical flow.

System 5:  

Naim NS01 server, Naim NDS streamer with 555DR PSU, Naim 252 pre with PSU, SNAXO crossover, 2x Naim 250DR power amps, Kudos Super 20 configured for active use, a medium sized room but L shaped, using the larger part of the L, which is smaller than the other listening room.
Expectation Bias (comparative to System 1): a system I'm already very familar with - a more dynamic presentation, too bright for my taste, assertiveness approaching aggression, a loss of musical flow somewhat compensated for by the enthusiasm.

System 6: 

Naim NS01 server, Naim NDS streamer with 555DR PSU, Naim 252 pre with PSU, SNAXO crossover, 2x Naim 250DR power amps, Kudos Titan 505 on their dedicated stands configured for active use, a medium sized room but L shaped, using the larger part of the L, which is smaller than the other listening room.
Expectation Bias (comparative to System 1): a more dynamic presentation, slight brightness, assertiveness approaching aggression, perhaps retaining the music flow, extra weight (because of the smaller room mainly), more information delivered.

So there we are, cards on the table. Something else worth mentioning - those who say "all competently designed solid state amps that measure within a gnats whisker of measurement-of-the-week all sound the same".  I have an expectation bias about that too - its just not true - I've heard far too many amplifier changes in systems to know that this is not true.  Unless I've only ever heard incompetently designed amps with bad measurements, which is also not true.

LISTENING WITH BIAS, OR NOT. MAYBE.

So here we go, here are the descriptions of systems 2 to 5 and a conclusion, that's not what I thought it was going to be:

System 2: the biggest suprise for me really.  The Titans give a far more detailed presentation than the PMCs, its suprising how much more they reveal in the recording. The extra brightness is certainly there, as per expectation, but is this just brightness or is it more detail too?  I'm going with the latter as there's more expression too in the vocals.  In fact the greatest strength of the Titans in this and the other systems is their vocal expression - its quite outstanding by any standards of the many systems I've heard.  Another element that was as expected is that, compared to the Titans driven by the Naim amps, there is a softness in the bass with the Tundras.  And here I think we see the first of the synergy issues - perhaps the Tundras and the 505s don't sit all that comfortably together as there was another element here that I wasn't expecting as the Tundras have never sounded like this in any other system I've heard - a sense of relentlessness.  Not shouty aggression, just a bit too much going on and losing some of that lovely flow that the Tundras present in partnership with speakers such as the PMCs and Graham Audio LS5/9. That, juxtapositioned with the lack of pace in the bass lines, meant that this wasn't the best of the combinations compared here.  There was a fair bit of experimentation with toe-in with the Titans in this room as there's definitely not the width or precision of soundstage compared to the PMCs. Don't get me wrong, we're talking about a decent sounding system, but in absolute terms, it lacked the right synergy.

Expectation bias 0: Systems 1

System 3: its long been a thing, a Linn front end with Naim amplification into a pair of speakers of choice.  Traditionally, of course, that was an LP12 and a passive Naim pre-power combination. Here we bring the tradition up to date with Linn streamer/pre and Naim power. Yep, more impact in the lower frequencies, no doubt about that - nearly reaching as low frequencies as the PMCs too - impressive. Mind you, the 505 standmounts on their dedicated and structurally massive stands are actually much more intrusive in the room than the much more constrained PMCs (which I always use the the grilles on). Definitely more detail from that (very) expensive tweeter - is it too bright? Its touch and go and track dependent, sometimes just tipping over the edge. But here's the thing - music, because listening to the music on this combination was very engaging - but its different to the regular system 1. There's an immediacy without shoutiness and that detail mentioned earlier but there's not the same musical flow that a Tundra delivers (in partnership with speakers they work happily with!). Whilst the Titans have very impressive bass power and immediacy, they still didn't quite fill the room like the PMCs do. Vocals are excellent again and there's plenty of engagement into the meaning of the music. This is good, but is very much along the lines of the expected.

Expectation bias 1: Systems 1 

System 4: before I go any further, just consider this: moving from Tundra to Naim power amps is a 50% price hike in amplification prices. And they do have a very different mix of abilities. I can also say that the Naim NAP250DR is a heavy beast.  Not too easy to lift onto a shelf and keep the shelf edges in pristine condition.  Those massive toroidal transformers are very heavy indeed. And noisy, good grief they're noisy. Not through the speakers, no issues there at all. But my goodness do they hum. I know my incoming mains typically carries about 2.5% distortion (the PS Audio regenerator displays this information) - I'm not sure if that's high, typical or low, but its certainly enough to get the NAPs a buzzin'. Just to be clear - neither of the power amps in this sequence of tests were plugged into the PS Audio, only the low current source electronics are fed from the regenerator. So there is no doubt that the 250 boxes have a tighter grip on the PMCs. There is more slam, there is more immediacy. There is a little more enthusiasm in the upper frequencies but it never becomes wearing in this system. All of this as expected.  What I didn't anticipate at all was the way everything hung together as a piece of music in very much the same manner of the Tundras into these speakers. Even that very taughtly controlled bass didn't disrupt the ability to follow the tune. Very good indeed. Except for gaps between tracks when the boxes from Salisbury irritated me significantly. Expection or surprise - calling this one a score draw.

Expectation bias 2: Systems 2

System 5: nearly all change.  Change of front end, speakers and room. As I'm very familiar with this system as it as grown and developed over the last 5 years with the Super 20s, then my "expectation bias" notes above are really all you need to know - they're more about experience than expectation. So let's call this one a no score draw!

Expectation bias 2: Systems 2

System 6: Would we expect (or have a bias!) that the 70% more expensive 505 with their stands would have a family feel when contrasted with the Super 20 floorstanders? Would there be a family resemblance from the North East parentage? Yes. And No. The Titans have that same family immediacy but the way they feel more "grown up" was far greater than my expectations.  Especially in the treble - where the Super 20 appears to major on detail, I find it gets too much after a while. What seems to be the case with the 505 is that the detail is real and it knows that it doesn't have to try too hard. I was a little worried about the size of the bass that comes with the upgrade to Titan (yes, the standmount has far more bass output than the floorstanders, in part due to their isobaric loading) in this smaller room, but it worked very well indeed - and there's an important reminder that system synergy includes the room.  Here, of course, there is no SPACE optimisation to make the best of the room so its all down to component choice and speaker positioning. What was just a touch underwhelming from these speakers in the bigger room is just right here. I don't get the same sense of ease of musical delivery compared to the Linn front end with Naim amps and these speakers, but I can see how their immediacy could appeal. No stridency, no experience of fatigue.

Expectation bias 2: Systems 3

OUTCOME

So you can now decide if any of this tells you anything about expectation bias or system synergy (which needs to include the room).  I certainly was surprised that the weight of the 505 could work in the smaller room, that the Tundras and Titans just didn't get on together at all and that I'd enjoy the Naim power amps in my regular system anywhere near as much as I did.

I wish the Naims amps didn't have 2 faults: that hum I can't just dismiss nor excuse, and their price. So I shall continue to be extremely happy with the Tundras as they work very well indeed with the PMCs.  As for my friend with the largely Naim system - he bought the 505s and has them working even better now the speaker cables are Super Lumina.

Wednesday, 15 July 2020

LINN Exaktbox-i: Photo Comparison Between Majik (Pre-Katalyst) and Akurate Katalyst Versions

Linn's carefully hidden Exakt separates system consists of the following key components:

An "Exakt ready" streamer / pre-amp (such as Klimax DSM / Akurate System Hub / Majik DSM amongst others)
A proprietary Exaktlink cable which uses standard Ethernet cables
An Exaktbox which is essentially a digital active crossover and digital signal processor, amongst other things (such as Akurate Exaktbox10 / Klimax Exaktbox)
Power amps, which can be from Linn or any other manufacturer.

However, there is an option to combine the last 2 elements - the Exaktbox and the power amps.  This is called an Exaktbox-i and combines 8 channels of DAC / digital active crossover and 8 channels of power amplifiers in Linn's normal just-a-bit-narrower-than-standard box.  There's really only one of these in the range, but it has evolved in the 4 or 5 years it has been available:

1 - Intitially it was a Majik Exaktbox-i containing Majik level DACs and 100W power amps, all in the Majik level casing
2 - Then it was re-badged the Akurate Exaktbox-i but still contained Majik level DACs and 100W power amps, but now in the more substantial Akurate casing.  This version was very short lived, soon replaced by:
3 - Akurate Exaktbox-i with Akurate level Katalyst architecture DACs but sill with Majik level 100W power amps in Akurate level casing

As is often Linn practice, those who bought the earlier products don't get left behind when there's an upgrade.  So it is possible to have a Majik Exaktbox-i upgraded to Akurate Katalyst Exaktbox-i internals but retaining the Majik casing.  The first Akurate Exaktbox-i can also be upgraded but I expect most of those were done pretty much automatically as there was a deal where the cost to upgrade was only the difference in price between Katalyst and non-Katalyst versions.  It would be very odd to find one of these that hadn't been upgraded, given the small price to do so.

I've been using an original "pre-loved" Majik Exaktbox-i for a while for Exakt filter design activities.  For some reason, now seemed to be the right time to take that up to Katalyst specification.  So that has allowed me to make comparisons between the circuitry of the non-Katalyst and Katalyst versions.

A similar comparison of the Akurate Exaktbox10 (April 2018) is here.

Much has been written about the musical capability of the upgrade from Linn's previous generation DAC to the Katalyst DAC, and I can't think of anyone who has anything but praise.  An example is here regarding the upgrade in Linn's onboard active Akubarik speakers.  So I'll not write about that here as it is well trodden path. With the upgrade being applied to the Exaktbox-i back at the Linn factory, there's no ability to directly compare the 2 products back to back.

To the photos.  Note that every single audio board (DACs are combined with the power amps) and the Exakt processing board are changed in the upgrade - the photos with the green circuit boards are the Majik non-Kat version, the black circuit boards are the upgraded Kat version.  The PSU and the casework is untouched, save for the addition of a Katalyst sticker on the underside.



Externally the box is unchanged


BEFORE AND AFTER UPGRADE COMPARISON PHOTOS:

The most obvious difference from an overall perspective is the change from green to black and gold boards

Each side of the enclosure has 4 channels of power amps (the chips bonded to the heatsink are the obvious components) and 2x stereo DACs.  The DACs above are the cross-shaped cluster of an oblong chip surrounded by 8 smaller chips

Here you can see the difference in the DAC architecture - now we have the 2x stereo DACs which are the 2x flat square chips - the board layout is very different












The "flyover" board down one side of the enclosure is the Exakt processing board.  The above original layout has 3x FPGA processors - the large flat square chips. The one on the left is the "master" processor with the other 2 processing 4 channels each
On the new board there is one more powerful FPGA chip in the centre doing the work formerly done by 3x separate chips




Board labelling for channel 5 to 8 DAC / power amp board has been relocated


Relocated labelling for the Exakt board





The increased complexity and density of the components surrounding the DACs is clear in this comparison.  The Katalyst DAC has multiple different regulated power supplies which probably explains this increase in components





SOME KATALYST ONLY PHOTOS:


Xlinx FPGA that does all the Exakt processing





DACs and power amps for channels 1 to 4 live in the shadow of the Exakt board




Monday, 29 June 2020

Another Project Underway

Actually, its been underway for about 2 years, but there's more time at home right now so its pregressing a little quicker.
No more details for the moment but here's a sneak peak at 2 of the components ready to go into it.

Tuesday, 26 May 2020

Building a Nindex Speaker - Part 3 Crossover Design, Measurements and Listening

PART 1 OF 3 HERE
PART 2 OF 3 HERE

As explained back in Part 1, the speaker is intended only to be operated in active mode with a Linn Exakt system. So a bit of an explanation of Exakt first.

In a traditional hifi system, an amplifier feeds a full range signal to the back of the speaker cabinet, then a passive electrical system inside the cabinet (the crossover) splits the signal into the correct frequency ranges for each of the drive units to handle.  This crossover is made of high power components which sap power from the signal, have wide tolerances and introduce their own problems such as phase errors.
In Linn Exakt, the full range signal from a digital source is fed to a component called an Exaktbox. In here the signal is split into the frequency range appropriate to each driver in the digital domain - this means it is done far more accurately than can be achieved in the speaker's own passive analogue crossover. The Exakt system allocates a DAC (digital to analogue converter) to each speaker driver and then the signal is passed to a power amplifer for each driver.  This is then connected directly to the appopriate driver in the speaker cabinet without any further components getting in the way, so the amp has better control over the movement of the driver.
So Exakt has very precise control over what gets to each driver and it introduces no phase errors of its own.  In addition to this, the system corrects for any phase errors within each driver and any frequency response "features" of the driver by applying an opposite signal to the error. Finally it also introduces time delays to drivers that are closer to the listener, in order to better time align the sound getting to the listener's ears. Some information here plus if you Google Linn Exakt you'll get lots more information and opinion.
Personally, I call it Linn Lessinexakt, but I don't suppose that would go down well in the marketing department...

Digital Design

As a consequency of the Linn Exakt approach, the Exaktbox system can be programmed to address pretty much any loudspeaker available, as long as there is someone who can measure the speakers and then carry out the design work to be loaded to the Exaktbox.  This is the service I offer under the SpeakerFilters brand.

There are several stages involved in creating the filters:
  • Physical measurement of the speakers which are then uploaded to Linn's database
  • Acoustic measurement of the passive speaker prior to modification to get a view on its existing characteristics (not part of this design work as there's no existing passive design to measure)
  • Electrical measurement of the speaker drive units (in and out of the cabinet if a ported design)
  • Translation of the electrical measurements into the parameters expected by the Linn Exakt Design system
  • Creation of initial crossover points and slopes, along with approximate attenuation of individual drivers - usually based on the passive crossover design
  • Acoustic measurement and listening with lots of experimentation to get the right mix of measured results and the best listening experience - this is the most time consuming with lots of iterations
So these were the next steps to the project.

Electrical Measurements

These are based on pretty straightforward impedance sweeps - showing the impedance in relation to the input frequency range from 20Hz to 20kHz - the generally accepted range of human hearing.

An example of the bass driver in the Nindex project is shown below:


The upper (red) line shows the phase response of the driver, the lower (blue) line shows the impedance curve which is fairly typical of a driver in a sealed cabinet - the peak is the driver's resonance frequency when mounted in the cabinet. By analysing various elements of this trace, the "thiel-small" parameters for the driver can be derived and fed into the Exakt design tools.

Looking at the response curves of the TM1-A array, it was a fairly straightforward choice of where to start with for the crossover points:

Mid Range Dome Frequency response - shows that the crossover points need to be above 850Hz at the lower end and below about 7500Hz at the top end (when the off-axis response starts to fall away)

The tweeter has a very wide response range and looks like it could handle from 1500Hz or so upwards - so it would be a good design to use with a larger mid-bass in a two way system.

I started with nominal crossover points of 1500Hz and 7000Hz, both with Butterworth 4th order crossovers - for those of you concerned about such steep slopes, remember that, unlike a passive crossover, the Exakt digital crossover doesn't introduce phase distortions based on the steepness of the slope. So the slope can be adjusted to suit integration requirements and the subjective listening experience.

The first acoustic measurement to take is just a quick sweep to check if the relative volumes of the drivers are somewhere near close.  Using the bass driver with attenuation of 0dB I usually put in -3dB for a mid-range and -4dB as the starting point.  Then work from there, usually by ear first, then refining with acoustic measurement. Its possible to hear changes of as little as 0.1dB which most people find very surprising until they hear it for themselves.

Then starts the refining of the crossover points - but only one at a time of course - listening for where the instruments sound more believeable but most importantly where the music itself is more engaging plus best for the most stable central imaging but widest overall spread of image - its not scientific, other than staying within the sensible ranges for each driver.  A crossover point is a nominal value - once this seems about right, more listening is done to various combinations of each driver roll-off point and therefore which mix is right and how steep the slopes might be.  For the bass to mid crossover, its easy enough to hear changes of around 25Hz in the values.  For mid to tweeter, I usually work in chunks of 100Hz.

Design work was completed using a Linn Majik DSM/3 network streamer / pre-amp and a Linn Majik Exaktbox-i which combines 8 channels of Exakt processing, 8 DACs and 8 power amplifiers - a remarkable bit of kit all in one box. The Exaktbox is the pre-Katalyst DAC version (at time of writing and design work - the upgrade is planned for the future).

Linn Majik DSM/3

Linn Majik Exaktbox-i


Here's the data that I ended up with for the Nindex:

Bass Driver
  • Roll-on = 10Hz
  • Slope Type = Butterworth
  • Slope Gradient = 4th Order
  • Roll-off = 1100Hz
  • Slope Type = Butterworth
  • Slope Gradient = 3rd Order
  • Attenuation = 0dB
  • Time Alignment = 0
Mid Driver
  • Roll-on =  1600Hz
  • Slope Type = Butterworth
  • Slope Gradient = 3rd Order
  • Attenuation = 4.0dB
  • Time Alignment = -119 microseconds
  • Roll-off = 4800Hz
  • Slope Type = Butterworth
  • Slope Gradient = 3rd Order
Treble Driver
  • Roll-on =  6700Hz
  • Slope Type = Butterworth
  • Slope Gradient = 3rd Order
  • Attenuation = 9.1dB
  • Time Alignment = -87 microseconds
  • Roll-off = 28000Hz
  • Slope Gradient = 4th Order
In the simplest terms, the resulting graphic looks like this below. Note that this is the electrical output required to achieve the crossover points and a flat response from each drive - so the curves aren't straightforward because Exakt is allowing for the characteristics of the drivers themselves.  Plus, in this image there are actually some filters hidden from view but having an effect - for example there is a small lift below 60Hz to add a little bit of weight to the bass.

The electrical outputs to achieve the crossover points

I then, by habit, add some "protective" filters to each driver - they're not shown above, but I'll put in a low pass filter for the bass driver than allows frequencies between 0Hz and 3000Hz. For the tweeter, for example, I'll add a high pass filter from 3000Hz upwards.  For example for the bass:

Example "safety" filter for the bass driver
Baffle step is a phenonmenon that means when a certain frequency and above reaches the edge of the baffle, it diffracts and becomes amplified for the listener - this is often cited as the reason that studio monitors are built into walls, to avoid any edges. This is obviously not possible for a speaker designed in a box so its best to put in step filters at the frequencies calculated on the baffle width and height.  So those calculations are done and appropriate filters added - with attenuation levels that are guessed to start with (they're often of a similar value between designs with the size of the attenuation rising as the frequency rises) and are typically in the range 0.2 to 2.5dB.  I usually start the step curve a little before the intended frequency and keep the slopes at a nice and gentle 2nd order. Here's the step for the mid driver which kicks in about 15Hz before the calculated point and is attenuated by 1.5dB:

Mid Range Baffle Step Filter
At this point, for confirmation of the rough acoustic measurements and subjective work, I'll take an acoustic 20 to 20000Hz sweep to make sure there are no glaring errors - preferring to work this way rather than relying entirely on measurements.

So now we have a pretty flat response through the design process, a choice of crossover points that give the best subjective sound and imaging.  Of course, in the background of all this the Exakt system has applied the calculations to each driver to deliver minimum phase errors, but all of that is invisible to the designer.

Now its time to do some detailed acoustic measurements and iron out any signficant frequency response anomylies in the drivers themselves.

I use REW software for acoustic measurements, paired with a calibrated UMIK-1 microphone connected to the laptop by USB. The laptop is dedicated to only the audio work, runs Windows 7 and pretty much nothing else. No fancy audio programmes nor drivers or anything else that will potentially add noise or unwanted processing effects.  For design work I always measure the speaker on-axis vertically with the tweeter as that is usually the most directional driver.  Horizontally I will measure on-axis and 30 degrees off axis.

My intention with this design was to get the 30 degrees off-axis frequency response pretty close to flat.  This will mean that on-axis the speaker will sound a little "bright" in that the treble output will be higher than the rest of the audio range.  This is deliberate to allow the speaker to be pretty much optimal with just a very small amount of toe-in.  Then for those who like a little more brightness or for older listeners whose hearing is starting to tail-off at the higher frequencies, a little more toe-in of the speaker will bring that change in balance.

Measurements are taken at 300mm to allow the mic to pick up the full frequency range but to minimise the effects of room reflections.  Not having an anechoic chamber to work with means that measurements below about 110Hz need to be completely ignored as they are prone to standing waves in the room.  Any tweaking below 110Hz is done purely by ear.


The camera angle makes this look more, but this is measuring 30 degrees off-axis in the horizontal

The software provides a sweep from 20 to 20,000Hz and measures the output from the speaker, corrected for the calibration of the microphone. I ignore all readings below 110Hz.

This then gives a picture of any interesting "features" of the frequency response of the drivers themselves, and, if there are any, where the drivers are perhaps overlapping too much, or leaving a trough due to not enough overlap.  Usually some quick adjustments to crossover points or possibly slopes get this sorted quickly.  The "features" of the drivers can be adjusted for with the Exakt digital crossover designs.

This is done by "modelling" the humps and dips in the response curve, then Exakt compensates electrically by applying the inverse to the signal delivered to the speaker.  So a 3dB hump at 1000Hz in the speaker will have an exact opposite - 3dB output applied at 1000kHz. It is possible to add bell curves, high or low pass and shelves. The height and width of a bell curve is adjustable.  It is possible to model each left and right speaker individually, or just go with a blend of the average of both.  In this project I worked on individual drivers - in commercial models it makes more sense to go with an average as there is little sense in trying to second guess the features of individual products out in the market.

So here is the resulting modelling for the left speaker with all the humps and dips modelled in for Exakt to correct for. It has to be said, apart from the need to add a lift from about 16000Hz upwards, the tweeter was the best behaved of the drivers.  Please be aware that I don't spend days and days dialling out every single ripple in the response curve. It would be rather unproductive - I focus on the main features and leave it at that.

The top line is the modelling of the "features" in the frequency response of the drive units.  The bottom 3 lines are the phase characteristics that Exakt has calcuated for correction.  Note, at this point the features include the effect of the cabinet, not just the drive units, so some of these could be due to the cabinet resonances mentioned in the build section (Part 2).

Top is the modelling of the frequency response of the drive units, including their "features".  The bottom is the Exakt modelling of the phase characteristics for correction

RESULT

So what is the ouput of all that work?

I've used 1/12th smoothing of the acoustic measurements to give a realistic idea of the reponse curve.  Below are 2 graphs to show why I'm mentioning this - if I was a speaker manufacturer, I would be tempted to publish the the 1/3th smoothing to make it look miraculously good!

Remember to ignore everything below 110Hz as there is too much room interaction.

Example with 1/3 Smoothing. Contrast With 1/12th Below. See how the above pretty much eliminates the room iteraction!  And those who really like their hifi to be "objectivist" rather than "subjectivist" sometimes forget that the numbers can be manipulated.  This is exactly the same data, just presented differently!

Exactly the Same Measurement But with 1/12th Smoothing - I use 1/12th in the results below
Single speaker result below - remember it is designed to be listened to off-axis.  Here you can see the more pronounced upper frequencies that would result in pointing the speakers directly at the listener compared to the intended only slight toe-in. Rotating the speakers with more toe-in allow a slightly brighter presentation if required.

Green line is on-axis response measurement. Red line is 30 degrees horizontal off-axis response measurement
 A word about phase correction with Exakt - one of its key selling points. It must be said that it is a phase error minimisation solution, it doesn't deliver a completely flat phase response.  However, I will provide a not to be named phase response example from a very well known domestic speaker maufacturer for comparison.  This is from their £16,000 speaker.  As usual, please ignore the measurements below 110Hz.  I do not post this comparison here to try and claim my project gets even close to being within sniffing distance of having a hint of the performance of the comparison speaker, but just to illustrate the effect of having no phase errors introduced by the crossover and the correction of phase errors within the drivers.

Nindex phase response, 30 degrees horizontal off-axis:
Phase Response of the Nindex Project Speaker

The dotted line in the above graph is the phase response of the £16k speaker
SOUND

So how does it sound?  Well, I'm not at all disappointed by the result. In fact, its rather enjoyable.  I don't think its the most dynamic speaker ever, but it does have a nice sense of flow - its definitely of the more laid back variety.  If you like most JBLs, you'll not like these.  Bass is tuneful but not "kicking", doesn't go massively deep but its pretty good for a box of this size.  There's no real noticable box artefacts, the treble is sweet and very detailed, the mids and vocals a touch polite but well articulated. Maybe it will appeal to those who prefer the BBC type of design?
Perhaps, at the next but one Wigwam Show (plans are already in place for the next one) you'll get a chance to come along for a listen.
Now perhaps its time to hook them up to the Akurate Katalyst / Lejonklou power amps in the main system and see how they get on.  Perhaps there'll be a post script later.

Finally, what's in a name. Well, not much thinking really. The base cabinet is an Index, but they're no longer Index, so they're Nindex. It works with the NINka drivers too. Kind of.

A few parting photos.  Thanks for reading!





PART 1 OF 3 HERE
PART 2 OF 3 HERE