History of the phonograph: KLH Model Two Analog Computer Speaker – Strong bass in a small enclosure

The KLH Model Two Analog Computer Speaker amazes with its bass performance.

Text: Kari Nevalainen
Photos: AVPlus delivery

The story of this speaker can be started with, for example, the Tivoli Audio Model On model, that all-wood tabletop radio that in the turn of the millennium launched the most visible retro fad in recent history and attracted a lot of imitators in its wake.

The American businessman Tom DeVesto once had a whiff of the kind of table radio that would have been the Markenson Model One, and he wouldn’t have been able to design it alone. So he brought in his former business partner, the legendary instrument designer Henry Kloss, who didn’t have to design the radio from scratch.

In 1957, Kloss, together with Malcolm Low and Josef Hofmann, had founded the company KLH (Kloss-Low-Hoffmann), for which he designed the KLH Model Twenty, a compact table radio with excellent sound quality (1965), an external and technical direct predecessor to the Tivoli Model One.

Kloss, who studied physics at MIT, was a true designer’s dream. He is remembered for, among other things, the highly selective and fairytale-like FM radio KLH Model Eight, the world’s first full-range electrostatic speaker KLH Model Nine, the world’s first portable transistor record player KLH Model Eleven, and the world’s first consumer-market-ready stereo speaker with Dolby noise reduction technology KLH Model Forty. The Advent VideoBeam from 1972 was, in turn, the world’s first affordable projector.

The bass computer necessary for operation of the KLH Model Two Analog Computer Speaker is a kind of active frequency corrector or limiter that controls the signal fed to the speakers. Its internal components are not particularly exotic by modern standards.

Enclosed enclosure

With over 30,000 loudspeaker units produced annually, KLH was one of the world's largest loudspeaker manufacturers during its golden age. Before KLH, in 1954, Henry Kloss and Edgar Villchur founded at least one equally legendary loudspeaker company, Acoustic Research. AR became known for the pioneering acoustic suspension loudspeakers developed by Villchur, whose impact on later loudspeaker manufacturing was enormous.

Simply put, an acoustic suspension speaker refers to a closed bass box. Like a spring, the air cushion behind the speaker element helps to dampen the movement of the bass sound field. Thanks to the air damping, the stiffness of the bass field and its suspension can be reduced, resulting in a greater frequency response and more bass output. The principle allows for compact speakers that reproduce almost the entire frequency range and at a relatively low cost. Sensitive acoustic suspension speakers require more amplification power than, for example, bass reflex speakers, but the quality of the bass output and linearity are good.

After leaving AR, Kloss began manufacturing licensed acoustic-suspension speakers under the KLH brand. All the loudspeakers designed by Kloss, first for KLH and later for Advent, were closed-back. These were the KLH's famous 1960s hits: the KLH Model Five three-way 12-inch bass speaker with a 12-inch woofer, the Model Six two-way 10-inch bass speaker, and the Model Seventeen two-way 10-inch bass speaker.

The woofer element's cone is made of polypropylene, and it has a large magnet inside.

KLH Model Two

The original KLH Model Two from 1957 was also a true acoustic-suspension speaker with one 11-inch bass driver, either a JansZen Model 130 electrostatic speaker or later a KLH Model Five group speaker.

The Model Two, of which we are talking about, namely the KLH Model Two Analog Computer Loudspeaker, is a completely different beast than the original KLH Model Two. The KLH Computer loudspeakers date from the years 1978–1980, when KLH had already transferred ownership to Infinity.

Unlike the original Model Two, the KLH Model Two ACL speaker has a 200mm woofer in a bass reflex housing rather than a closed one. Despite this, and even though Kloss had already left KLH 10 years earlier, his legacy lives on in this speaker as well. Namely, to preserve the acoustic-suspension speaker’s advantages – precise bass response, low distortion, and compact size – the KLH developed a device that it calls an analog bass computer.

The computer is connected between the audio source and the main amplifier. Through the feedback loop, it monitors in real time the signal fed to the jack pins of the amplifier while also controlling the movement of the bass cone so that the movement deviation of the bass cone below a specific bass frequency determined for each model never increases too much and does not cause an overload situation.

By controlling the movement of the bass element’s cone, the computer allows the speaker to operate at bass frequencies and volumes that it would not be able to do without additional equipment. According to the manufacturer, the Model Two is capable of reproducing 38 Hertz (–3 dB) at a sensitivity of 26 x 53 x 21 centimeters with a housing of 102 decibels. Without a computer, these readings would hardly reach 200 millimeters of bass sound in a housing of less than 30 liters.

The analog bass computer can be kept active as a frequency corrector/limiter that controls the signal fed to the speakers. The fact that it has a sixth-order –36 dB per octave filter indicates that the corrector is not intended to interfere with the performance of each speaker above the operating point frequency set for it.

The bass cone is made of polypropylene, the suspension is reversed and the body is cast aluminum. The sufficient movement tolerance of the cone is guaranteed by a large magnet. Without a computer box, such a magnet might break the element when receiving a suitable signal.

The mid-range is replicated by KLH’s own 100-millimeter polypropylene cone element, and the tweeter has the manufacturer’s 25-millimeter dome. The enclosure feels solidly built.

The internal damping and the split diffuser follow traditional style.

Competent voice

The soundstage of the speaker turned out to be surprisingly neutral, perhaps slightly warmer than neutral. There are no distinct irregularities in the balance of the sound. The speaker does not sound modern in a bright way, but it also does not attenuate the mid-range when the rich bass tone often associated with 1980s American speakers is too subdued and dark. The mid-range is brightly illuminated in a healthy way, and the treble is mostly imperceptible.

And yes, the bass from these speakers really comes out! There’s a button on the front of the computer that lets you lower the bass level when the speaker is closer than a meter from the wall behind it. You can immediately hear the difference. There’s also an indicator on the front that shows when the computer is participating in the speaker’s playback and when it’s not. Not very sensitive, apparently, which tells you that the speaker’s bass reproduction alone is quite decent. It’s only when there’s very low-frequency audio material in the music track and the music is played at a significant volume that the computer gets involved in the game.

The KLH Model Two Analog Computer Speaker cost 660 dollars at the time, which is a little over 2,000 dollars in today's money. Today, you can get one used for about a couple hundred euros.

The speaker's woofer is 200 millimeters, the mid-range is reproduced by a 100 millimeter polypropylene cone, and the tweeter has the manufacturer's 25 millimeter dome. The enclosure feels solidly built.