KEF Model 104.2 Speaker System Review

The KEF 104.2 combines many of the familiar qualities of KEF speakers—a low degree of coloration, stable stereo imaging, a handsomely finished cabinet—with a novel design that’s intended to provide the high efficiency needed to reproduce wide-dynamic-range program material without excessive amounts of amplifier power. The three-way columnar system features a unique bass-driver configuration consisting of two upward-firing 8-inch drivers mounted entirely within the cabinet. The drivers operate in opposite phase in separate enclosures about 18 inches apart. The air volume between them, which loads the front of the bottom driver and the back of the top driver, opens to the outside through a frontal port about 4-1/2 inches in diameter.

The 104.2’s bass system operates only up to about 150 Hz, and it appears that the internal air volume works with the port size and the crossover network to optimize its performance in that range. In addition to the considerable stiffening imparted to the cabinet by its internal partitions, the frames and magnet structures of the two bass drivers are rigidly joined by what appears to be an aluminum bar about 1/2 inch in diameter. The velocity of the air pumped through the port by the two woofers is considerable, but air turbulence is minimized by a smoothly rounded plastic bezel in the port. The internal volume is too small to support resonant modes at frequencies of 150 Hz or lower, and higher modes are suppressed by several collars of plastic foam surrounding the vertical rod that joins the driver frames.

The KEF 104.2’s other drivers are inside a separate sealed enclosure mounted on the front of the cabinet above the woofer port. In a vertical line on its front surface are two 4-inch Bextrene-cone drivers with a 1-inch dome tweeter between them. The tweeter crossover frequency is not specified, but KEF does state that the speaker’s frequency response is 55 to 20,000 Hz ±2 dB measured at 2 meters distance on the reference axis. The company also gives the directional characteristics of the system in terms of the upper (-2-dB) response limits at several angles to the reference axis in both the horizontal and the vertical planes. The 104.2 is a high-efficiency speaker, by high-fidelity standards, being rated to deliver a sound-pressure level (SPL) of 92 dB at 1 meter on axis with a pink-noise input of 2.83 volts. It is recommended for use with amplifiers rated to deliver between 25 and 200 watts to 4-ohm resistive loads.

It is this last rating that points up one of the most unusual qualities of the KEF 104.2. The speaker has been designed to have a virtually constant impedance of 4 ohms across the audio range—in other words, to look like a 4-ohm resistor to the driving amplifier. It is commonplace to contrast the performance of amplifiers under laboratory conditions, where they are required to drive resistive loads of 4 or 8 ohms, with their performance in the “real world,” where they drive very complex loads that bear little resemblance to an electrical resistance of any value. The constant 4-ohm impedance of the KEF 104.2 enables an amplifier to perform as well in its intended application as in a laboratory test. In addition, since most amplifiers will deliver considerably more power to 4-ohm loads than to 8-ohm loads, the 104.2 can develop the maximum possible program sound level from a given amplifier.

There is yet another bonus from a constant-low-impedance design: the frequency response of the system will not be affected by the resistance of the speaker cable used with it, which can impress the shape of a speaker’s impedance curve on an otherwise flat amplifier response. KEF’s instructions indicate that any kind of speaker wire should be perfectly satisfactory with the 104.2 as long as the total resistance in series with each speaker is less than 0.2 ohm (up to 12 feet for 14-gauge wire or 20 feet for 12-gauge wire).

The 104.2’s constant resistance was achieved with the aid of KEF’s extensive computer-aided design and engineering facilities, which were able to design impedance-correcting networks for each driver as well as the frequency-shaping and level-matching networks needed to perform the crossover functions. According to KEF, an electrical time delay in the crossover network tilts the optimum listening axis up about 10 degrees from the horizontal in order to give optimum sound quality for listeners seated about 10 feet from the speakers.

The KEF 104.2 has no user-adjustable level controls. Its only external feature, aside from the midrange and high-frequency drivers and their black grille cover, is a pair of heavy-duty insulated binding posts on the rear of the cabinet. These posts will accept stripped wires, lugs, or banana plugs (but not dual banana plugs since the terminals are spaced 1-1/2 inches apart). The cabinet is handsomely veneered in walnut and measures 36 inches high, 11 inches wide, and 16-3/8 inches deep. Each speaker weighs 70-1/2 pounds. Small, removable rubber cabinet feet are supplied. The KEF 104.2 is sold in matched pairs for $1,600 per pair.

Lab Tests

We installed a pair of KEF 104.2 speakers as recommended, about 2 feet from the wall behind them and 4 feet from the side walls of the room. The room response was very smooth, varying less than ±4 dB from 150 to 20,000 Hz. The woofer response, measured at the port opening, reached its maximum at 70 Hz, falling at 18 dB per octave below 60 Hz. The output also decreased, to —5 dB, at 130 Hz and more steeply above that frequency. A close-miked measurement of the response of one of the midrange drivers showed a maximum at 250 Hz, with an 18-dB-per-octave decline below 150 Hz and about a 6-dB-per-octave rolloff above 250 Hz. Our close-miked tweeter measurements indicated that the crossover took place at about 3,000 Hz.

Splicing the close-miked bass and midrange curves to the room-response curve in an unambiguous manner was even more difficult than usual, and the composite curve showed an appreciable peak at the woofer’s maximum-output frequency. This peak did not appear in our quasi-anechoic FFT response measurements, nor was it particularly obvious in listening tests. The FFT response (at 1 meter) was flat within ± 3 dB from 180 to 20,000 Hz, with the major departure from flatness being a dip of about 5 dB at 3,000 Hz. The response dip was apparently the result of an interference effect between the midrange and high-frequency drivers; not only did it fail to show up in our room measurements, but it disappeared when we measured the speaker’s response 45 degrees off its reference axis.

The off-axis response diverged steeply from the axial response above 8,000 Hz, a result of the tweeter’s directivity. KEF suggests that angling the speakers toward the center of the room may improve imaging under some circumstances, although in our room this was not necessary. The phase linearity of the system was excellent at middle and high frequencies, as shown by the overall group-delay variation of about 0.1 millisecond from 4,000 to 20.000 Hz and of less than 0.5 millisecond as low as 1,000 Hz.

The 104.2’s sensitivity was almost exactly as rated, with a 91-dB SPL measured 1 meter from the speaker when it was driven by 2.83 volts of pink noise in the octave centered at 1.000 Hz. The impedance varied only between 3.3 and 4.6 ohms from 20 to 20,000 Hz. This was not only the most uniform speaker impedance we have ever measured, but its very uniformity attests to its almost purely resistive nature. The woofer distortion, at a constant drive level of 2.5 volts (corresponding to a midrange output of 90 dB), was less than 1 percent from 140 to 75 Hz, rising gently to 2.15 percent at 40 Hz and 4 percent at 30 Hz.

Finally, our pulse power-handling tests produced some of the most prodigious power figures we have yet come up with (in part the high figures resulted from our using a different amplifier than we usually have for these tests and operating it in the bridged mode). With a sine-wave signal on for 1 cycle, then off for 128 cycles, the 100-Hz woofer limit was 1,127 watts. At 1,000 Hz, the system withstood 1,806 watts, and at 10,000 Hz the limit was 1,670 watts. All these figures refer to the amplifier’s output at its clipping point; the speaker did not clip or otherwise show signs of overload in any of these measurements.

Comments

The KEF 104.2 speakers sounded as smooth as they measured, with a somewhat “warm” character resulting from a slightly emphasized bass output in combination with a gently declining midrange and high-frequency response. The kinship of these speakers to the other top KEF models (specifically the 105.2, with which we are quite familiar) was obvious. The sound was balanced, certainly without any obvious emphasis in the higher frequencies, yet not in the least dull or muffled. Of course, it is quite possible that in another room the bass output of the 104.2’s would be less prominent, but the weight of the speakers discouraged us from experiments in placement.

Since our tests suggested that the 104.2 could deliver extremely high sound levels without distortion, we put them to the test with a CD that opens with an enormous bang (Telarc CD-80098, whose first selection is the Strauss Explosions Polka). With the playback gain set so that a 0-dB (maximum) digitally recorded signal would drive the amplifier to clipping (well over 1,000 watts per channel into 4 ohms), we were treated (?) to a good simulation of a gunshot at close range. There was a true physical impact as well as a very loud sound, but no damage resulted to amplifier, speakers, or ears. The experiment reinforced our view that there is no such thing as having too much power for playback of a wide-range CD, assuming that the speakers can handle it. These can.

Priced at little more than half as much as KEFs renowned 105 series, the 104.2 delivers sound of very similar quality and quantity from a much smaller and visually less obtrusive cabinet. It demonstrates quite dramatically the ongoing progress in speaker design. Certainly KEF has realized well its stated goals of wide dynamic range, low coloration, stable stereo imaging, and pleasing appearance in the Model 104.2.

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