Kenwood KA-900 Integrated Amplifier Review

The Kenwood KA-900 integrated amplifier is one of that company’s “Purist” line of audio components. It is rated to deliver 80 watts per channel to 8-ohm loads from 20 to 20,000 Hz with no more than 0.005 per cent total harmonic distortion. Its other specifications are of similar high caliber, establishing the KA-900 as one of today’s more refined amplifiers.

Except for its power rating and the use of an internal power supply, the KA-900 is almost identical in design and appearance to the top-of-the-line KA-1000. Its styling is strikingly distinctive, with the lower half of the “panel” being a smoke-glass, bottom-hinged door that partially conceals the controls behind it. With the glass in place, the only accessible controls are a large rectangular power pushbutton at the left, a horizontal-slider volume control near the center, five thin pushbuttons below it that select TAPE A, TAPE B, AUX, TUNER, or PHONO sources for listening, and a large clear-plastic fader panel at the extreme right. The latter operates an electronic circuit that makes a smooth transition from the volume set by the slider to a fully silenced condition over a period of a couple of seconds. Simultaneously, the blue-green lighting of the fader panel goes from fully on to off as the volume changes.

Swinging down the glass door reveals slim rectangular bar knobs for the bass and treble tone controls (non-detented) and a switch for selecting speakers A, B, or A + B (or off for headphone listening through the adjacent jack). Another knob is the rec out selector, which channels any of the input sources to the tape-recording outputs independently of whatever input is being heard through the amplifier. The remaining knob is the balance control.

The other concealed controls are pushbutton switches. Between the tone controls are a small tone-defeat switch and separate bass and treble turnover selectors (respectively offering a choice of 200 or 400 Hz and 3 or 6 kHz). The rec out control has positions marked aux, tuner, phono, and off and two dubbing positions for copying tapes from one machine to the other. The remaining small buttons activate the loudness compensation, the subsonic filter (with the latter out, the amplifier is direct-coupled from its high-level inputs to the speakers), and the stereo/mono mode switch. Larger buttons select either of two phono inputs (1 or 2) and switch the phono-preamplifier gain and input resistance for moving-magnet (MM) or moving-coil (MC) cartridges.

On the rear panel of the KA-900, in addition to the various signal-input and -output jacks and a DIN socket for tape B, there is a small slide switch that selects a MM phono-input resistance of 33,000, 47,000, or 100,000 ohms. Heavy-duty insulated binding posts are used for the two sets of speaker outputs. A second set of speaker A binding posts, marked 2 sensor, gives a clue to an unusual feature of the KA-900.

The Kenwood Purist Series amplifiers all feature what the company calls Sigma Drive, an auxiliary feedback path taken from the speaker terminals themselves. This enables the amplifier to deliver its rated output and distortion levels at the speaker instead of at its output terminals, compensating for any possible losses in the speaker cables. For this purpose, the KA-900 is furnished with a pair of heavy-duty, 13-foot, four-conductor speaker cables. Two conductors are heavy wires that go from the speakers to the amplifier outputs. The others are lighter-gauge wires that carry the feedback signals to the Sigma-Sensor terminals on the amplifier.

Sigma Drive operates only on the speaker A outputs. Ordinary speaker wires can be used from speaker B to a second set of speakers. If a single pair of speakers is used, connected to speaker A terminals, changing the front-panel switch from A to A + B will disable the Sigma Drive feedback so that its effect can be evaluated in a listening test.

Kenwood engineers believe that eddy currents, induced magnetically in a metal chassis and covers, can create magnetic fields that in turn generate distortion in the amplifier circuits. This effect is very small and entirely unmeasurable in most amplifiers, but, with the ultra-low distortion of the Purist series, Kenwood felt the need to minimize even this minute source of distortion. Therefore, they have made as much as possible of the structure of the KA-900 of plastic, leaving only the bottom and rear of the chassis, plus the top cover, to be made of aluminum. The plastic portions are satin-silver in color and look exactly like the metal portions.

Within the amplifier, the output transistors are mounted on heavy aluminum blocks and heat is conducted from them through fluid-filled tubing to large internal heat-radiating fins. The entire cooling system operates by convection and does not require a fan or external fins. The Kenwood KA-900 is 17-1/4 inches wide, 4-7/8 inches deep, and 14-3/4 inches high. It weighs 22 pounds. Price is $520.

Laboratory Measurements. The preconditioning period left the top cover of the KA-900 quite hot directly over the heat-radiating fins but comfortable elsewhere. In normal operation it ran only mildly warm. With both channels operating at 1,000 Hz, the output clipped at 100 watts per channel into 8 ohms, 125 watts into 4 ohms, and 120 watts into 2 ohms. The IHF clipping headroom (8-ohm) was 1 dB. With the pulsed IHF dynamic-test signal the maximum outputs were 120 watts into 8 ohms, 180 watts into 4 ohms, and 164 watts into 2 ohms for an IHF dynamic-headroom rating (8-ohm) of 1.76 dB.

The harmonic distortion at 1,000 Hz was unmeasurable (below the noise level) until the output exceeded 20 watts into 8 ohms. The distortion was 0.001 to 0.0013 per cent from 30 to 70 watts and 0.0016 per cent at 90 watts. The distortion was not very different into lower load impedances. Into 4 ohms it was 0.001 per cent or less up to 50 watts and 0.0013 per cent at 100 watts. Even 2-ohm loads did not materially increase the distortion, which was 0.002 per cent or less up to 80 watts and 0.013 per cent at 110 watts. However, very high-power operation into 2 ohms caused the thermal cutout to shut off the amplifier for a couple of minutes near the end of our high-power tests.

At rated power into 8-ohm loads, the distortion was very low over most of the audiofrequency range. It was under 0.003 per cent up to 3,000 Hz, increasing to a maximum of 0.013 per cent at 20,000 Hz. At lower power levels the behavior of the amplifier was similar, with slightly lower distortion readings. Below 50 Hz it was not possible to measure the distortion accurately because of the proximity of the distortion frequencies to power-line frequencies. However, it appeared to be no greater than 0.001 per cent even at the lowest frequencies. We noted that the distortion was mostly second-harmonic, the least objectionable type (not that this matters much when dealing with these nearly unmeasurable levels).

The IHF slew factor was greater than 25, and the amplifier was stable with a complex simulated speaker load. In the two-tone IHF intermodulation test, with inputs of 19 and 20 kHz, the second-order distortion (1,000 Hz) was 93 dB below 80 watts. Odd-order distortion products at 18 and 17 kHz were at —82 and —91 dB. The amplifier rise time was 5 microseconds at IHF standard gain and 1 microsecond at maximum gain.

A high-level input of 18 millivolts produced a reference output of 1 watt, and through the phono (MM) input the sensitivity was 0.25 millivolt. The A-weighted noise level referred to 1 watt was —79 to — 80 dB through either input. The phono preamplifier overloaded at an input of 300 millivolts at 1,000 and 20,000 Hz and 314 millivolts at 20 Hz (the latter two readings being converted to equivalent 1,000-Hz values). The phono (MM) input impedance was 50,000 ohms in parallel with 175 picofarads, and in the MC operating condition the input had a 135-ohm resistance. The maximum deviation from ideal RIAA phono equalization was ±0.25 dB from 20 to 20,000 Hz, and when measured through the inductance of a phono cartridge the response changed to a maximum of + 1 dB at 15,000 Hz.

The tone controls (especially the treble control) had very little effect in the first half of their rotation, but the maximum range was completely adequate. When the 200- and 6,000-Hz turnover frequencies were selected, the midrange level was not affected significantly at maximum control rotation. The loudness compensation boosted only the low frequencies at reduced volume settings, and the subsonic filter response was down 3 dB at 27 Hz.

Comment. Comparing our measurements and comments on the Kenwood KA-900 to those we made on the KA-1000 we tested last year, we find remarkably few differences. The distortion of the KA-900, like that of the KA-1000, is nearly unmeasurable and totally insignificant by any standards. Although our distortion readings at high frequencies exceeded the amplifier’s 0.005 per cent rating, this was because we used the IHF standard gain setting for the measurement. At maximum gain, the distortion met the published ratings (the KA-1000 exhibited the same behavior). Virtually every other measured characteristic was identical for the two units. Interestingly, the major difference was in favor of the KA-900, whose 2-ohm performance was outstanding. Few integrated amplifiers can drive 2-ohm loads to any reasonable power level without excessive distortion or without shutting themselves down.

The fader control worked smoothly and well, although the slider volume control on our test unit had some free play that was very apparent during our tests but not in use. Although the KA-900 is undeniably handsome, we found it difficult to determine at a glance where any of its controls were set or to identify their functions.

We were unable to confirm the practical benefits of non-metallic construction or the audible advantages of Sigma Drive. Perhaps because our speaker leads are adequate to start with, numerous A-B comparisons turning the Sigma Drive on and off failed to produce any audible change in sound quality. But, aside from its special design features, the Kenwood KA-900 is a fine, “state-of-the-art” integrated amplifier that can hold its own against competitors.

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