A few basic design principles stayed the same. A reflex system with a paper coned woofer with a massive magnet, Two-way construction with AMT HF unit, again based on a Mundorf core. A cabinet with everything needed to avoid unwanted radiation from the box and a highly optimised crossover with high-quality parts.
We wanted an enclosure large enough to deliver realistic low frequencies and scale but was small enough to be relatively discreet. It had to be low enough to sit below windows and work well close to walls.
Knitting together an eight-inch mid-bass unit with an AMT HF unit is more straightforward than a 10.25-inch mid-bass, but a seamless transition is not easy.
The basic crossover topology is 4th order acoustic Linkwitz-Riley but with an all-pass delay for the HF unit and some simple impedance compensation at the low end to make KIM easier to drive.
All the inductors in the crossover are zero distortion air-core designs with polypropylene film capacitors also chosen from Mundorf. Resistors are a combination of Mundorf low inductance and Bifilar types.
The result is a loudspeaker system with an average impedance of eight ohms, a minimum impedance of 5.9 Ohms and 86db sensitivity for 1 watt. Note here that 1 watt at this impedance represents a lower current draw than with most loudspeakers.
The trend since the 80s has been for slim loudspeakers, which has necessitated the use of smaller mid/bass drive units. The classic two-way loudspeaker went from being an eight-inch with a 19mm tweeter to a six and a half to a five or even four-inch mid/bass, and at the same time, tweeters generally moved to 25 or 29mm.
Materials technology and better adhesives have allowed massive changes to power handling, maximum excursion, and reliability, encouraging smaller drive-unit systems.
With KIM, we wanted to design a speaker we would use ourselves, forcing us in the direction of an 8inch mid-bass driver.
Similarly, as we said with the 10.25inch driver unit of Borg, a great 8inch will not beat a great 10.25 incher, but it can deliver a full range performance with only a small compromise on maximum SPL.
Unlike our previous bass and mid-bass drivers, we needed a sizeable half-roll surround to allow the excursion due to the smaller pulp paper cone. The softer rubber half-roll surround also terminates the cone correctly to eliminate colouration caused by energy returned to the cone from the cone surround.
From our learnings from Borg development, the mid/bass has an oversized magnet. This ensures a Q that is too low and which alone would reduce the low-frequency performance. Combined with the higher than convention resistance of the primary inductor feeding the driver, the Q is corrected, enabling the system to deliver clean, accurate in-room bass to 32Hz—a clever solution to using an air-core inductor for the lowest distortion.
AMT HF Unit
The tweeter is an Air Motion Transformer (AMT) operating according to the principles established by its inventor Oskar Heil. Developed by Mundorf together with FinkTeam and manufactured specifically for FinkTeam by Mundorf, the AMT has a strong, 25μm-thick pleated Kapton diaphragm with 50μm aluminium strips. This material has excellent internal damping, resulting in particularly low distortion. We developed a unique etching process to produce it, and the diaphragm configuration optimised through many tests.
The choice of 110mm AMT delivers a wide dispersion horizontally – giving good stereo across a wide listening area. The directivity vertically is deliberately controlled to reduce floor and ceiling reflections that may harm the imaging and reduce the apparent image height.
Reflex ports or vented enclosures have many amateur loudspeaker design experts confused. Over the years, they have heard many loudspeakers with poorly designed ports. Poor system design produces slow lumpy, and ill-timed bass, and many have decided this is relevant to all ported loudspeakers. It is not.
Porting a loudspeaker correctly gives significantly more low-frequency power handling, more bass in the important octaves, and with two-way loudspeakers can bring an added realism to the midrange.
The slot port integrated into KIM’s rear panel is tuned to 37Hz. An additional resonator in anti-phase called CleanPort removed the pipe resonance of the main port.
Terminal Panel & Controls
The two heavy terminals are nickel-plated solid copper designed for low resistance and high connectivity.
There are two additional controls: an HF control which allows a subtle change in the overall HF level to suit different rooms.
The other control offers a choice of damping to suit the owner’s choice of amplifier. The three positions possibly suiting a tube amplifier a solid-state amp with a lowish damping factor and a solid-state amp with a high damping factor.