"David Price thinks Kerr Acoustic's new transmission line K320 floorstander is a really serious performer"

Hifi Choice Magazine

Product reviewed: K320

Reviewer: David Price

Published: February 2019

Taken from issue HFC 446:

There is no single correct way to design a box loudspeaker and yet so many people end up with reflex-ported cabinets. These generally offer the best bang for your buck, being straightforward to design, inexpensive to make and a relatively easy going drive for the partnering amplifier. Trouble is, reflex loading often results in a non-linear bass response; less well-designed examples impart a ‘one-note’ bass sound as the port kicks in, smearing transients and giving a rather cushioned sound. That’s why Jes Kerr – former professional drummer, music producer and now founder and designer of Kerr Acoustic – likes transmission line loading so much.

“With transmission lines it is easier to distinguish the detail and nuances in individual instruments in the lower octaves”, he says. “Take double bass and tympani playing together for example; with a reflex enclosure, these sounds are more likely to merge together, with the cabinet and port’s natural resonance being excited and colouring/smearing the presentation.” If properly done, transmission line designs give a faster, deeper bass for a given cabinet size and lower distortion, too. The reason that TLs aren’t as popular as reflex designs is they’re harder to get right, costlier to build and tend to be less efficient.

Transmission line designs have a complex, damped and usually folded duct inside, coming from the rear of the bass unit, that vents to the outside world. Jes says that he “learned by doing”, listening and experimenting with different enclosures. “I was fuelled by a love of music from the age of eight, but it wasn’t until my degree dissertation that I attempted a transmission line – a large three-way featuring a 12in woofer. I studied in great depth Martin J King and John Risch, which gave me a solid grounding in understanding transmission line technology and its implementation.”

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“You can peer right into the mix and get a great feel for the recorded acoustic”

The tidy rear panel is home to the binding posts and a tweeter level switch

His company was launched in June 2017, following 18 months of R&D. “I identified a gap in the market for a high-end transmission line speaker featuring premium materials and components, incorporating a planar tweeter. I felt that the benefits of TL loading could be taken further through superior cabinet materials, advanced drivers and a more fastidious approach to design methodology and construction.”

The K320 – £3,695 in satin finish, £3,995 in high gloss (pictured) – is the middle model of the company’s range; it’s essentially a floorstanding version of the standmount K300 Mk 2. Normally a floorstander sounds worse in some ways than its stand- dwelling sibling because you get to hear much more cabinet resonance, which results in time smear and boom. Here, the real wood cabinet is rigid and superbly damped for its price – being made in-house by hand using routers and jigs, with no CNC machining or automated processes involved. “We exclusively use a combination of 18mm and 24mm Baltic birch plywood, with 90° alternating cross-grain composition, infinitely varying natural wood grain structure, and damping resin between each 1mm plywood layer,” explains Jes before adding: “As a material, it is a great deal more costly than MDF and more difficult to machine.”

A modern 165mm Scanspeak 18W/4531G01 mid/bass driver with a wood-fibre cone is used, and the tweeter is a modified Fountek Neo X 2.0; it’s a 60mm true-ribbon design with a very low 0.027g diaphragm mass. These are laced together at 1.95kHz with a six-element, second order (12dB/octave) crossover with selected components, hand soldered using point-to-point wiring. Unique air-core inductors, hand wound in-house, are used, along with polypropylene film capacitors and carbon film resistors. Claimed frequency response is 24Hz to 45kHz, sensitivity 90dB and nominal impedance 6ohm. Around the back is a pair of 4mm pure copper binding posts and a treble level cut switch.

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Sound quality

This excellent compact floorstander far exceeds expectations for a product at this price – indeed it even has a touch of the exotic that its price rivals lack. Rush’s The Camera Eye is a torture track for any loudspeaker, with its three-dimensional soundstage, amazingly fast and propulsive bass guitar work and firecracker drums. The K320 loves it, showing just how impressive a well-done transmission line design can sound. The bottom end is so tight and taut that you can hear how Geddy Lee’s bass guitar sounds just as percussive as the drum kit. The bottom end is deep and strong, yet eerily well controlled.

Surprisingly, the stellar bass is not in my view the best facet of its character. At the other end of the frequency band, the ribbon tweeter delivers a delicately etched performance that’s full of filigree detail. Supertramp’s Bloody Well Right has some beautiful hi-hat cymbal work, with each strike tailing off into an inky blackness as the next arrives with its lightning- fast ‘clink’. The K320 delivers this perfectly, letting high frequencies shimmer with a lovely metallic sheen without ever becoming tonally harsh or texturally coarse. Instead, the music takes on a deliciously dextrous yet floaty feel that makes the song an absolute pleasure to play. In the presence region, things continue to sound great. It’s clear how the K320 reaps the benefit of that high-quality tweeter, showing a delicacy and vibrancy to the sound that’s often lost by clumsy fabric domes, and/or lesser mid/bass units. Things are pleasingly neutral, if not quite translucent. My aged copy of Wrapped Around Your Finger by The Police comes across in a most impressive light. You can peer right into the mix and get a great feel for the recorded acoustic. Things are marked out in space with great accuracy, and the stereo image is satisfying wide – if not quite as cavernous as a true high-end offering. For me, what absolutely defines this design is its combination of speed and smoothness. The super-light tweeter, allied to its ultra-tight bass and highly inert midband, make it great fun with fast, propulsive music. Kraftwerk’s Die Roboter sounds smooth and sophisticated yet hugely engaging. This speaker’s transient speed is excellent, and every type of music it touches sounds highly involving, vibrant and alive. The great thing is that everything ties in perfectly together; there’s no sense of the treble being ahead of the bass or of either driver interfering with the other to cloud the midband. Instead, it presents a cohesive single object in space and time, which is just what a great loudspeaker should do.

Given the price and its enormous ability, it is almost churlish to criticise – but no speaker is perfect, including this one. In absolute terms the K320 doesn’t quite have the sense of effortlessness that you get from larger three-way high-end designs. All this shows us is that the Kerr is – like everything else – not immune to the laws of physics. Truly free-flowing loudspeakers need wide baffles, big drivers and deep boxes with far more air inside to really breathe.

Conclusion

As one of the most impressive upmarket floorstanders I’ve heard in a long time, the K320 is a great advert for transmission line loudspeaker designs. Just remember to partner it with a muscular solid-state amplifier and decent source to enable it to deliver its very best.

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"The K300s deliver a wide and spacious sound with very good resolution"

TNT Audio

Product reviewed: K300 Mk2

Reviewer: Mike Cox

Published: March 2019

In my experience the loudspeaker is the single most important choice when putting together a satisfying music reproduction system. The loudspeaker is very room dependent and also depends on the position in the room, get this wrong and it does not matter how good the rest of the system is you will not be able to extract the best from your system.

Not only is the loudspeaker important from the sound quality perspective it is also extremely important from the cosmetic point of view, they need to match the decor of the room. The Kerr Acoustic K300 speakers we have here for review are a relatively small size transmission line stand mount speaker and are available in any of the RAL colours. Matching these speakers to your colour scheme should not be a problem in most situations.

The drivers are a Fountek ribbon tweeter with a Scanspeak for the bass/mid duties. The bass uses a shortened and loaded transmission line. The crossover is second order 12dB/Octave and the sensitivity is 89dB/W which is above average for a speaker of this type and size. A good 50W per channel solid state amplifier should be adequate. During my review I used a 500W Emotiva amplifier to start with moving on to a Leak Stereo 50 (25W/Channel) and finishing with an IQAudio DigiAmp+ (35W/Channel) and at no point did I feel any of the amplifiers were not powerful enough.

The K300s were supplied with stands for the review as I had nothing suitable. The stands are spiked and supported K300s as well as looking good. I positioned the K300s as usual in my room firing directly at the seating position and placed about a third of the way in from the side walls. They were also about 1.5m from the rear wall, I cannot place them much closer to the rear wall because of the dormer window and roofline that you can see in the first picture. The K300s worked well as placed but as Kerr Acoustic are only a few miles from my house I invited Jes over to take a listen to ensure his speakers were operating at their best. Jes changed the speaker position and tow-in so they were about 75cm from the side walls and firing almost straight down the room. This position change really helped, the ribbon tweeters had been sounding a bit forceful and the tow-in change really helped to balance the sound. Placing the K300s slightly wider apart opened up the soundstage giving greater scale and depth.

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The Listening Experience

Now the speakers had been optimally positioned in my room and after a few hours of burn-in the serious listening began. Recently I have been listening to "The Jazz Groove" internet radio station (78kbs), the sound quality of his stream is remarkably good even with the low data rate. The K300s deliver a wide and spacious sound with very good resolution from the low data rate stream. The bass is deep and controlled, in the past I have heard transmission line speakers that were over the top and bloated. In my room the K300s were perfectly balance with good resolution and timing.

Turning up the data rate some what via Diana Krall and the album "Turn up the Quiet", the K300s easily reveal the improved quality. Diana's voice is presented beautifully centre stage, with a natural and realistic breathiness. The Fountek ribbon tweeter is something special delivering fabulous detail without being aggressive or harsh, something I have heard with other speakers using ribbon tweeters. Moving on to Gregory Porter and the title track from his hit album "Liquid Spirit" the bass extension from the transmission line let the double bass really shine, it was like a spotlight had been turned on the bass player and they had equal billing alongside Gregory.

Next up on the play list are The Tallis Scholars and the Allegri Miserere, a fabulous recording with an expansive soundstage that some speakers just seem to gloss over. The K300s managed to deliver a very respectable rendition, getting very close to speakers costing much more. This recording has huge depth to the soundstage which is where many speakers fail and the K300s only start to get a bit vague when the voices are a long way back of the venue. This sounds like I am being very critical, actually this is a complement as this is difficult recording to deliver well.

In order to explore the capabilities of the K300s I turned to vinyl and Elton John's second album "Elton John" from 1970. Considering this album is nearly 50 years old the recording quality is amazing and the K300s let this quality shine through in spite of the rather noisy background from my copy of the album. Compared to modern recordings this album sounds like it was produced simply so you hear all the musicians in their place in the studio and Elton's voice with wonderful tonality and detail. This simplicity worked well with the K300s and they delivered a very realistic performance with good timing, the transmission line did not get in the way of delivering the tune.

To finish off I switched to my Leak Stereo 50 power amplifier, Revox A77 reel to real tape deck and a sampler tape from Analogy Records. The recording quality is in a different league to anything digital, it is like vinyl without the background noise and is a great test for any system whatever the price. With this new front end to the system the K300s stepped up and delivered the goods, the high frequencies were smooth and detailed in a way you just cannot get with a digital source.

Analogue tape is still a relatively new source for me and every time it just amazes me. While the K300s are comparatively inexpensive, they do not mask the music delivering a thoroughly enjoyable experience. I cannot easily put into words how good analogue tape is, music seems to break free from the replay system and transport you to the recording venue and the K300s deliver a fabulous peformance. The analogue recording has a better dynamic range, fabulous detail, with excellent rhythm and timing, the K300s letting this all through. I would love to hear the larger and more expensive models from Kerr Acoustic with the analogue tape front end, it should stunning.

Conclusions

Over the years of my journey with music reproduction I have preferred speakers of a smaller scale. My listening rooms have never been large and a speaker that dominates the room visually has usually got in the way of the music, probably because there is not enough room to let the speaker breath. Because on my preference for smaller speakers bass performance has always been a compromise, a good example was when my regular speaker was the classic LS3 5a. The Kerr Acoustic is a different beast to the venerable LS3/5a with a more modern presentation, more dynamics, yet retaining a lot of the neutrality of the LS3 5a. Where the K300s really excel is bandwidth, the bass is no longer such a compromise, and the ribbon tweeter is so good with lots of detail without being harsh.

To get the best from the Kerr Acoustic K300s you do need to spend time getting them positioned correctly as I discovered. To my ears, in my room, a valve amplifier delivered the best results with the ribbon tweeter sounding sweet and smooth with fabulous detail. Using a solid state amplifier is still fine, just don't compromise on quality as this could compromise the K300s.


Interview with Kerr Acoustic designer and owner Jes Kerr


What is your background, in terms of speaker design?

In the beginning, it was very much “learning through doing”. I've had a keen fascination for loudspeakers from a very early age, fuelled by a love of music. Being surrounded by live music since birth, I became fascinated with the concept of reproducing it as accurately as convincingly as possible. I built my first (mono) speaker together with my father around age 8. From then on, it was a case of pleading with my parents each weekend to go to various local car-boot sales to collect as many “junk” speakers as possible, taking them home, dismantling them and making various “Frankenstein” speakers out of the components.

The advent of the internet made it far easier to access information, which helped to underpin and substantiate the knowledge I had gained through experience. Through my early/mid teens, I began experimenting with different types of bass enclosures, ranging from reflex, infinite baffle, and various band-pass designs - learning all the time through trial and error. I used these to reinforce the mediocre bass response from my low-budget system. It wasn't until my late teens that I began collecting more serious speaker systems (from the likes of Infinity, Cerwin Vega, Rogers, Mordant-short, Sony APMs etc). In many cases, I found I was able to improve the performance of these speakers through subtle crossover modification, reinforcing the internal cabling and cabinet bracing, modifying the internal absorptive material, etc.

It wasn't until my degree dissertation that I attempted a transmission line - a large 3-way design featuring a 12” woofer - essentially an early predecessor to the K100. I studied, in great depth, the writing of Martin J. King and John Risch, which gave me a solid grounding in understanding TL technology and its implementation.

When did you start Kerr Acoustics, and why?

Kerr Acoustic was launched officially in June 2017, following 18 months of product R&D and market research. I started the company because I identified a gap in the market for a high-end transmission-line speaker featuring premium quality materials and components, incorporating a cutting-edge planar tweeter. While there are undoubtably some very effective transmission-line speakers on the market, I felt that the benefits of TL loading could be taken a step further through the implementation of superior cabinet materials, advanced driver technologies, and a more fastidious approach to design methodology and construction. Early prototypes proved that the concept worked, and following positive feedback and encouragement from various industry professionals, I believed that there was commercial potential for my designs. Now, another 18 months in, we're selling them through a number of dealers, and as far afield as Hong Kong.

Where do the cabinets come from, and what materials are used in them?

Our cabinets are produced entirely in-house by a 2-man team, crafted by hand using routers and jigs - without the use of any CNC machining or automated processes. We exclusively use a combination of 18mm and 24mm Baltic Birch plywood, for it's numerous beneficial structural and acoustical properties (namely, 90 degree alternating cross-grain composition, infinitely varying natural wood grain structure, and damping resin between each 1mm plywood layer), all of which greatly helps to reduce resonance and mechanical energy transfer. As a material, it is a great deal more costly than MDF and significantly more difficult to machine. In particular, the resin layers between each “ply” make it particularly tough on our saw blades, which we have to have sharpened regularly to maintain clean cuts! However, I believe the structural and sonic benefits fully justify this extra cost and work - producing cabinets which are extremely non-resonant and acoustically inert.

Where do the drivers come from, and what are the exact make and model?

The K300 and K320 feature a Scanspeak 18W/4531G01 mid-bass driver, with a sliced wood-fibre cone and oversized motor assembly. The tweeter is a Fountek Neo X 2.0, which we perform some slight modifications to before installing into our cabinets. In our larger K100 model, the woofers and midrange drivers are from Volt:. In the K300 the woofer is from Scanspeak:. The tweeter is from: Fountek: 

How complex is the crossover, and how many elements to it?

The 6-element 2nd order (12dB/ octave) crossover is a simple affair of very high quality and carefully selected components, which are individually measured, pair-matched, and hand-soldered using point-to-point wiring. We exclusively use unique air-core inductors (hand-wound in-house), polypropylene film capacitors, and carbon film resistors. Despite the simplicity of the design, the crossovers took by far the most time to optimise and fine-tune. It was not only essential to ensure they performed well sonically, but they also had to perform well electrically, presenting an even impedance load to the amplifier. A great deal of care and attention to detail was employed to fully optimise these circuits, with emphasis placed on them being sonically transparent and a low-loss as possible.

Why choose a TL? What the benefits, and the downsides of this approach?

Once I had experienced the true benefits of a well-implemented transmission line, it became very difficult to consider using other methodologies for cabinet construction.

Some of the benefits include:

More extension and a much more gradual roll-off in the low-frequency end of the spectrum, compared to ported / sealed cabinet designs. Bass response is also perceived as “faster” and better controlled than alternative cabinet designs, with bass notes starting and stopping with more precision and accuracy. Significant reduction in distortion, owing to the rear waveguide channeling the unwanted rearward energy away from the cone. In the majority of sealed & ported designs, the unwanted rearward energy often resonates and reflects inside the hollow cavity behind the driver, which can introduce distortion and non-linear behaviour, making it very hard for the cone to function optimally.

This is particularly the case in 2-way designs, where a single bi-polar driver has the responsibility of covering a very wide bandwidth of information. Incorporating a carefully damped rear waveguide means that the cone isn't fighting against reflections inside the cabinet, and by the time the drivers' rear energy reaches the terminus of the transmission line, all content above around 80Hz has been absorbed and internally dissipated by the dampening materials. The result is a significant reduction in distortion, and a much more transparent, cleaner, and more natural sounding midrange. Additional internal cabinet bracing - a very welcome benefit from the internal waveguide. This in turn helps to reduce unwanted cabinet resonance and colouration.

Some downsides:

Typically, TL loading requires a physically larger cabinet than sealed / reflex designs, to function optimally. In the mainstream, people's ears are typically calibrated to the high-Q bass output from reflex enclosures. Hence on first listen, our TL's can sometimes be perceived as sounding somewhat bass-light in rooms with significant axial room modes that don't have some degree of acoustic treatment.

Our speakers don't “flood” the room with a wash of exaggerated 40-80Hz content, as a lot of reflex designs do, and hence are somewhat more sensitive to boundary conditions than more conventional designs. However, in a correctly optimised room, the advantages in bass performance are obvious. In non-optimal spaces, bass output can be somewhat optimised by experimenting with rear boundary proximity.

Why not bass reflex loading like everyone else?

While reflex loading can give a fairly instantly-satisfying “cushioned” bass response, it is rarely accurate and often contains a lot of non-linearity. A reflex port performs very effectively over a narrow band, but falls down outside of this band, often making it very hard to achieve low bass without significant internal cabinet volume.

In my experience, reflex enclosures tend to sound “slower” and more “ploddy” in the bass. Moreover, they impart a stronger “one-note” sonic signature, making it harder to distinguish individual low frequency characteristics in the source material, owing to cabinet and port-induced colouration. Transmission lines make it easier to identify and distinguish the detail and nuances in individual instruments in the lower octaves - take double bass and tympani playing together for example - with a reflex enclosure, these sounds are more likely to merge together, with the cabinet and port's natural resonance being excited and colouring / smearing the presentation. Of course, this is just a general rule, and based on personal experience.

What sort of sound were you aiming for?

Being originally designed for professional studio application, the primary design objective was to produce a fast, accurate, and neutral loudspeaker. Great emphasis was placed on optimising the response in the time domain, which I believe to be of equal (and in some situations, greater) importance than amplitude / frequency response. This is one of the main reasons I opted for an incredibly light tweeter diaphragm (0.027g, compared to ~0.5g for a conventional dome and ~0.17g for an AMT-style ribbon tweeter).

This incredibly low-moving mass enables unparalleled response in the time domain, with an remarkable ability to respond to transient information effectively, particularly during dense program material where the tweeter is having to reproduce a wide band of information simultaneously. Our speakers do not flatter or compliment the source material, and resultantly, people are often surprised by how drastically different their recordings sound from one another, owing to the speakers importing very little of their own sonic signature.

Do you know of any other TLs that use ribbon tweeters?

I believe that IPL offer DIY speaker TL kits, that include ribbon tweeters. I don't know of any other company that combines these design features together.

What is your favourite speaker, not designed by you?

A tough question, but I do have a soft spot for my trusty Infinity RS 9 Kappas. A hideously complex and woefully impractical loudspeaker, but one which offers remarkable performance when fully optimised and driven appropriately. The infinite-baffle 7-driver 5-and-a-half-way design presents a particularly evil impedance load to the amplifier (dipping as low a 0.3 ohms in places!), and practically requires an arc-welder to drive them effectively.

However, they have been my reference loudspeakers for the last 8 years due to their ability to reproduce program material with a remarkable sense of scale and detail. They combine a number of desirable design characteristics - including sealed cabinets, very light injection-moulded-graphite 12” woofer cones, well-designed midrange domes, and planar tweeters. They also function as a di-pole above around 800Hz, and image fantastically - not just in terms of breadth and depth, but also height - something that I think a lot of speakers miss.

“The K320s punch way above their 165mm bass-mid driver and 25kg weight. Listeners will not find themselves suffering from stereo starvation or bandwidth deprivation"

TNT Audio

Product reviewed: K320

Reviewer: Mark Wheeler

Published: January 2019

Arriving at the TNT-Audio secret mountaintop lair, Jes Kerr, and his cabinet-maker father Steve, unloaded the gloss white finish K320 and carried them carefully into the dedicated listening room. The white automotive finish is reminiscent of pre-CBS Fender guitars and Steve is a life-long guitar player, which might have unconsciously influenced this decision. The range of custom colours available screams off towards infinity, the first time this reviewer has encountered such a range of finishes included in the price. Typically customers of other brands have to spend the retail price of the Kerr K320 as an extra cost option for custom finishes.

The Kerr K320 is in the middle of the Kerr range of three models. It features the same drivers as the Kerr 300 but with greater bass extension. This is achieved by a much larger cabinet and different tuning. The K320 and K300 both use the well-known Scanspeak 18W chassis driver (in this application 18W/4531G01) with the radial cut & stuck pulp/paper diaphragm designed to control resonances while minimising mass. Your Old Scribe used this driver in its low colouration heavy polypropylene version some years ago and compared that with lighter cones versions for some of the driver acceleration experiments. The motor system of this driver is therefore familiar to your Old Scribe as designed to prioritise low distortion (providing it is used within its excursion limits) and to reproduce fine detail even during dense complicated passages, so it will be interesting to discover if that applies in this application.

Designer Jes Kerr says of this bass-mid driver choice, "The K300's [and K320's] driver is actually a bit of a classic among speaker geeks. It's manufactured in Denmark by Scanspeak, who are long-established high-performance driver specialists, held in particularly high regard by speaker designers across the world. The particular driver used in the K300 has been in Scanspeak's range for a decade or two and is relatively conventional in terms of its technology (the cut and re-glued paper diaphragm is its most notable feature), but it's a seriously high-performance driver nonetheless. I've personal experience of the 18W over the years and have always appreciated its notably uncoloured and natural character, combined with its low levels of distortion and substantial magnet system and voice coil."

The smaller Kerr K300 utilise a short transmission line (as in the Bailey approach to minimise overhang and dissipate cone rear energy. The larger Kerr Acoustic K320 uses a hybrid of transmission line and tuned third wave pipe. The familiar Tuned Quarter Wave Pipe (TQWP, often mis-described as a transmission line) has some of the power handling advantages of reflex loading (controlling cone excursion near the system resonant frequency) but fewer group delay disadvantages than reflex. This results in lighter but faster sounding bass and more placement flexibility in the room. The tuned third wave pipe trades some of that excursion limiting advantage in exchange for even lower Q bass tuning knee and deeper (quasi second order slope over 1 octave) extension. There is a balance of compromises involved in system Q and box stuffing, most significant in bass loading by pipe.

The smaller model K300 systems were designed for control room near field monitoring while the K320 are more suitable for domestic duties, making more effective use of the floor area. This extends the frequency response (in room) to 24Hz – 45kHz from the 33Hz – 45kHz of the K300 for an extra 600 quid. There is a 1dB gain in sensitivity from the larger model, but given that 1dB is the smallest human reliably detectable change in level, this is unimportant. While the K300 is intended to monitor the content of a production, the £3,395 K320 exists to extract that content for pleasure.

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Ribbons

Jes Kerr says "The aluminium ribbon diaphragms used in our monitors are just 15 microns thick, and have a total moving mass of 0.027g. Conventional dome tweeters are around 0.5g. From this you can gather that our ribbons are approximately eighteen times lighter than most domes! The result? Flawless reproduction of high-frequency content, and unmatched transient response..." 

The Old Scribe would agree with Jes that it's difficult to go back from great ribbon tweeters. Mark Levinson & SME's founder ARA used Decca ribbons to supplement stacked Quads. They added the Stanley Kelly-designed Decca London Ribbon horn tweeters between the two Quads on each side, for duties above 7 kHz. Your old scribe enjoyed similar horn loaded Decca London Ribbon tweeters for many years from 1.8kHz up in a steep slope active system and can attest to their advantages, if only they can be integrated successfully with the relatively sluggish top end of a moving coil driver. However, managing that transition and ensuring adequate power handling (ribbons can drift in the magnet gap, permanently lose shape or burn out) are the reason they are so seldom used in commercial designs.

The choice of cabinet materials also matches the prejudices of your Old Scribe, following experiments he reported for Speaker Builder magazine, described in these pages. Jes Kerr's decision to utilise 18 and 24mm Baltic Birch plywood to contain the pressure in the transmission-line fits the requirement for structural integrity and vibration control. Birch ply also has a sonic signature with the least psychologically intrusive colouration of commonly employed loudspeaker panel materials. The detachable rear panels are well secured, even if this is a compromise compared with all panels glued.

Passive Crossovers

Regular readers (and anyone spending any time within earshot of the Old Scribe) will know how repugnant he finds passive crossovers. Kerr have gone to great lengths to minimise the damage wrought by their passive crossovers, knowing that the domestic market mostly demands passive crossovers. In particular Kerr specify low resistance inductors. These are the lowest available with air cores; it is possible to reduce DC resistance further by employing iron-dust (ferrite) or laminated iron cores, but these bring their own distortions, especially at sustained high levels. Tuned quarter wave pipes, correctly executed, can have flattish impedance curves in the bass, which means that the frequency response, Q and bass phase performance is less affected by series inductor heating effects than a reflex alignment.

Kerr have sensibly eschewed the seduction of bi-wire terminals. Treating the crossover and drivers as a system, rather than a high-pass system and an entirely separate low-pass system, has many advantages that far outweigh the alternative bi-wire-able option. The primary advantage of the latter is the, somewhat nebulous, advantage of adding the loop resistance of 2 runs of loudspeaker cable between the bass section and the treble section in a parallel crossover, which is primarily of significance in high global feedback low output Z amplifiers. In such an application, bi-wiring could be argued to reduced transient intermodulation distortion but there are stronger arguments for replacing such an amplifier than trying to mitigate it with particular loudspeaker crossover topologies. The choice of solid copper input terminals is much more expensive than the brass and theoretically superior.

Economy of delivery, exemplified by the included exclusive finishes, is accomplished by a direct sales model. There are no importers, no middlemen, no retail network and therefore no clumsy acoustic emphasis tricks to impress potential customers in the demo room. The economics of wholesale & retail distribution mean that these would have to be double the price in that market. What you do not get, as a consequence of this sales model, is immediate delivery of an in stock item. No Kerr Acoustic loudspeaker is going to be an impulse buy because they are individually built to order to the customer's spec.

The package of the Kerr K320 thus comprises:

  • LF Driver: 6.5 Wood-fibre cone with extended linear suspension system and patented Symmetric drive motor design

  • HF Driver; 2 True Ribbon tweeter with 0.027g diaphragm mass

  • In Room Response 24Hz – 45kHz

  • Sensitivity 90dB (2.83V/1m)

  • Impedance 6 Ohms (nominal)

  • Crossover 1.95kHz - 2nd order (12dB/ Octave)

  • Construction 18 / 24mm Baltic Birch plywood transmission-line

  • Connections One pair of 4mm pure copper binding posts

  • Dimensions height 1020mm X width 195mm X depth 395mm

  • Weight 25 kg (per cabinet)

Set up & tuning the Kerrs in room

Kerr recommended firing straight down the room. This is based on pro practice an minimising local reflections in near-field situations. It is true that such an arrangement maximises soundstage size and accuracy, but at the expense of minimising listening area width. If the nearer loudspeaker is also closer to being on axis, the Haas effect is doubly counteracted by extra high frequency energy from the nearer tweeter. If a broad listening area (stereo seats?) is demanded, the ribbon tweeters dictate a compromise in soundstage width and accuracy.

The soundstage initially rarely extended much beyond the loudspeakers, which is usually likely to be a placement issue. The LEDR Test indicated where there might be sources of early reflections, which were quickly remedied. However, proximity to a rear wall is indicated by the Kerr K320 bass alignment, to provide sufficient low frequency loading, which is more acceptable in most domestic situations, but this does compromise freedom from early reflections.

Trying various positions and degrees of toe is useful with any ribbon tweetered transducers. This room usually favours toe in, pointing (very) slightly in front of the central listener's nose. This typically creates a more solid impression, with more soundstage depth. However, this much toe-in also stretches central images at high frequencies unnaturally wide with the Kerr K320. Many typical soft-dome tweeters with passive crossovers exhibit an effect of the highest frequencies jammed tight in the tweeters, regardless of soundstage scale at lower frequencies. The ribbon tweeters of the Kerr K320 differ with more of an artificial stretching of the central high frequency components of the soundstage, in relation to the remainder of the virtual stage. Second order (12dB/octave) crossovers are more prone to this sensitivity than third order (18dB/octave) crossovers, especially when loudspeakers fire straight down the room, but care and effort do result in a consistent image scale with the Kerr K320.

Moving the Kerr loudspeakers closer to the rear boundary, but firing directly down the room changes the soundstage and balance again. Unexpectedly, instead of reinforcing lower bass, this does not drive the room quite so well, the bass now losing slam but the soundstage moving further back than the change in distance from listener. Your Old Scribe continued to experiment long after Jes and Steve left, eventually settling on a position 450mm from the rear boundary (which acts as a diffuser, being slatted) firing straight down the room, closer spaced than usual, with the big Hammer Dynamics derived loudspeakers adjacent to the Kerrs' outer sides set only 300mm back from the Kerr baffles. This arrangement, similar to the 'wings' arrangement of some PA systems to project bass better, created the best balance of bass extension and slam combined with the best compromise between soundstage depth and width.

On the simple basis of 3 feet good 4 feet bad (with apologies to George Orwell) because the fourth foot will always have less pressure and might even ‘chatter', the supplied 4 spikes (and footers for easily marked timber floors) were eventually substituted by 3. This was achieved with Yamamoto PB18 loudspeaker spike receiver bases under the front two spikes, and a Michell Tenderfoot aluminium cone placed point down at the rear centre of the base. The Yamamoto PB18 loudspeaker spike receiver bases have become the default loudspeaker spike interface here, usually more applicable than Michell Tendercups or RDC cups. The truly excellent Polycrystal Point Bases are no longer available and are also more applicable under turntable spikes. This 3 point support is clearly superior to the standard 4 spike configuration, acknowledged by Jes Kerr when he heard the effect of this change and now being considered for future production.

During their visits, Jes Kerr and Steve Kerr were introduced to the delights of the all valve (tube) recording of Lightnin Hopkins' Goin Away before they went away, leaving the white Kerr Acoustic K320's behind for a rigorous workout on the end of three different types of power amplifier.

The result of further careful set up is a noticeable increase in resolution at the expense of ‘weight'. The remastered Pink Floyd Relics (always remembered as a bargain bin disc in 70s Record shops) achieves a soundstage extending beyond the right loudspeaker in this configuration, when heard from the precisely central position.

Listening could begin in earnest.

Sound quality

Immediately apparent is the lack of tweeter resonances The absence of tweeter resonances was immediately apparent Coming straight after the B&C DE-35-8, which was in use primarily because it's dispersion pattern exactly matches the Hammer Dynamics 305mm bass-mid driver, at the 7kHz crossover point.

Notable too is an absence of bass resonances, either in terms alignment, loading, or drive unit. Phil Lesh's bass always had the character of the woodwork, wiring and player intact through a 12 side Grateful Dead marathon. Dave Clarke's debut album Rise 1 has some really deep bass and usually demands power or high efficiency to shift some air. The really deep bass was apparent from the first track. Notably there were no resonances at these lower frequencies. There is no high Q hump to trick listeners into imagining more extension. There is surprisingly deep extension from these compact floor standers. There were no chest thumping transients when fed by an 8 Wpc SET300B, because ye canna change the laws of physics captain.

The Kerr choice of line tuning & damping has the advantage of limiting driver excursion near to fundamental resonance (as in an optimum reflex alignment) but with less phase shift, or group delay than ported enclosures. Indeed, the accurate rhythm portrayed down to the lowest octave implies phase alignment akin to a medium Q sealed box like a Bessel alignment. Transmission lines have been accused of poor rhythm or even sluggishness [plebs here?] but the Kerr K320 do PRaT as well as any decent sized sealed box. That they achieved this even when driven by 8 Watts of puny single ended triode (SET 300B) implies a well controlled impedance characteristic, as well as coherent phase performance.

"How can a loudspeaker affect rhythm or Pace?" demand Plebs, stage left, "Surely only a source component can affect the subjective pace and rhythm of music"

Low frequency phase response (especially group delay defined by the bass alignment in room) distinctly and predictably change the time domain relationship between the frequencies comprising bass notes and thus affect our perception of their rhythmic interrelatedness. This can result in sluggish or incoherent sounding bass even though the source signal is at the correct speed and can even be heard in live PA systems where the source is obviously correct. The Kerr K320 succeeds in its balance of trade-offs between bass extension, power handling and low frequency phase response, even at higher levels. The larger cabinet will help at higher levels, the internal air load remaining linear within the maximum SPL limits of the design.

Cabinet resonances are also strikingly absent and cabinet rigidity is apparent by the clean tight bass and midrange quality. From deep male voices, which never become over-chesty, to high, clear female voices, which never became shrieky, the driver cone never draws attention to itself. Good high frequency phase response is demonstrated by the stable soundstage positioning.

In the tradition of the British school of studio monitors, the Kerr Acoustic K320 are extremely even handed. The Kerr's are properly neutral without being neutered. There are designers who pursue low colouration and neutrality at the expense of all else, but Kerr have not arrived at low colouration by this route. The Kerr avenue has pursued following the loudspeaker's abilities rather than mitigating a loudspeaker's disabilities. Kerr have used a system approach, hence this is not a product driven by a passion for a particular driver configuration, nor is it driven by a slavish adherence to a particular bass alignment or crossover alignment. From this listener's perspective, it seems that time was spent finding out what worked well together for the smaller near field monitors, then thinking about how more could be wrung from the concept if size compromises were slackened. It may be coincidence that the crossover slopes are in-phase second order and the bass response tends towards second order (in initial roll-off where it matters) and the tweeters extend way beyond human hearing range, but it comes together in a coherent system.

Returning from Womad, where Leftfield played their last ever complete live performance of Leftism, inevitably the newly remastered triple vinyl pressing of Leftism found its way onto the Audio Files Spoke modified Linn Sondek. A review sample of the Burson Bang (review soon) 40Wpc solid state power amplifier replaced the modified Assemblage SET300B. Driving the Kerr K320, the Burson Bang displayed remarkably valve like characteristics, despite coming so soon after the epitome of vacuum tube amplification. The soundstage of Leftism, in this new edition, fills the room, extending as far as the walls on either side of the listener, as well as beyond and behind the plane defined by the loudspeakers themselves. This is not merely attributable to lack of early reflections and good set up, but to the excellent phase performance of these loudspeakers (crossover as well as tweeter) at high frequencies. The extended HF response will bring benefits of more linear phase shift as well as the inherent acoustic positioning advantages of extended top end to beyond human audibility. The dog liked it too.

The Burson Bang has quite soft bass quality for solid state amplification, which the Kerr's make the most of. The combination extends very low in room, to the extent that no musical programme material contained low enough frequencies to be found lacking, if anything going a little deeper than the much larger Hammer Dynamics 305mm (12") nearly full range TQWP/TL system usually in place. After 6 sides of Leftism, the only place to reach for is Leftfield's Rhythm and Stealth over 10 sides of 250mm (10") vinyl. This enables wide groove spacing and a high level cut, with great dynamic range. Finally, the Graham Nalty built 100Wpc monster was brought in, its 1.2kVA power supply offering much more heft. This brought the Kerr K320s to life in terms of bass, midrange and above all, dynamics. The only downside is the effect on the treble. Despite operating in class-A for most of its headroom, the top end of a push-pull bipolar transistor output stage is never going to be as sweet as Western Electric 300b's, especially through as revealing a transducer as the Kerr badged ribbon tweeters.

The extra welly offered by the big muscle amp is just what the Kerr K320 needed. The sublime mid-range and treble delivered by the SET300B (NOS valves equipped) was sacrificed for the oomph required to achieve the bass extension and dynamics of which these loudspeakers are capable. The typical two-way with upper midrange crossover downside of the same driver moving maximum low frequency air quantities also trying to deliver vocal subtlety. At high levels there can be a degree of midrange suck-out, but this is because these can go very loud for such a modest bass-mid driver size. The small signal frequency response, on axis, has a dip around the crossover point (typical for a 2-way in phase second order crossover) which becomes exaggerated as the bass-mid driver voice coil gets hotter at higher levels. The remarkably even bass remains resonance free under duress.

The Kerr K320 acquitted themselves well at high listening levels in a large room. At low levels the Kerr K320 also achieved good detail retrieval. The sound stage is good and solid, the only compromise being the narrower seating options dictated by loudspeakers firing straight down the room.

Conclusions

That your old scribe spent hours just enjoying music, whether lively funked up offerings after coming home from a George Clinton Parliament-Funkadelic gig, or emotional roller coaster opera, or 12 sides of Grateful Dead into the small hours of a Sunday morning, might imply that the Kerr Acoustic K320 offer all that a listener needs. They punch way above their 165mm bass-mid driver and 25kg weight. Listeners will not find themselves suffering from stereo starvation or bandwidth deprivation.

The Audio Show at Woodland Grange - Leamington Spa 2018

We were delighted to have the opportunity to exhibit our loudspeakers at The Audio Show in Leamington Spa this year, which were featured in three different rooms. The K300s were being used in the Longdog Audio Room, fed by an outstanding selection of components designed and crafted by Nick Goreham. Our Reference K100s were in the MCRU room, being powered by a superb system assembled by David Brook. Finally, in The Oak Room we were demonstrating our K320 floor-standers, which certainly seemed to capture the attention of our visitors. It was a marvellous weekend, and we are very grateful to the organisers and venue staff for arranging such a fantastic show.

Below is just some of the public feedback we received following the show:

“To come out with such a brilliant product as the K320 almost straight off the bat is the stuff of Hi-Fi legend.”

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“It's so refreshing to see a young chap like Jes Kerr coming in to the UK audio scene with credible and desirable products that don't cost the earth.”

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“I've heard quite a few loudspeakers costing quite a lot more than the K320s sounding quite a lot worse. These must be a serious contender for the loudspeaker bargain of 2018.”

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“I liked the diminutive floor stander K320 from Kerr Acoustic. Under £3k a lovely little ribbon tweeter and 6.5 inch bass mid in a transmission line managing to fill a room three times the size of most people's lounge” 

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“Biggest pluses for me were the Kerr Acoustic speakers, particularly the K320 floorstanders, which sounded ace on the end of some nice vintage but not mega expensive gear. Also liked the Kerr standmounts in Long Dog Audios room” 

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“MCRU were playing their music through a very nice sounding pair of speakers, pretty new to the market, that I could happily spend more time with” 

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“I was also impressed with the Kerr Acoustic speakers being shown in a few rooms. The K320 seeming to hit a sweet spot of sounding excellent without having to mortgage the youngest child to afford them.”

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 “Agree with previous posters that the Kerrs (all 3 sizes in different MCRUS rooms) were nice.” 

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“The things that stood out for me were the Kerr Acoustic speakers that were in three rooms I think, particularly the K320 which were on the end of relatively modest vintage gear. Long Dog Audio's room sounded nice, again with Kerr Acoustic speakers” 

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“Some superb sounds especially from Kerr acoustic”

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“Best sounds for me were in Nicks room, Longdog Audio. Also the Kerr K100’s in the MCRU room were very good indeed”

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“Oak room had Kerr acoustic speakers, they were also in the mcru room. Great speakers and relatively affordable. They also offer colour matching at no extra cost. Great sounding speakers.”

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“Kerr superb as stated above.”

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“Was helping out Nick in the Longdog room. We usually use Graham 5/9’s, but this time used Kerr Acoustics. Very pleased with them, definitely better than the Graham’s, which makes them a bit of a bargain IMHO.”

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“I also loved the floor-standing Kerr K320s and was surprised at how beautiful they sounded in that large room.The longer I listened the better they sounded. Highly addictive sound.”