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.
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.
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.
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.
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.