Kenwood KA-1000 Integrated Amplifier Review

The top-of-the-line integrated amplifier in the Kenwood “Purist” Series is the KA-1000, rated at 100 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.005 per cent total harmonic distortion. The most unusual feature of the KA-1000 (and of the two less expensive amplifiers in the Purist Series) is Sigma Drive, an auxiliary negative-feedback arrangement that includes the speaker cables within the loop. The result, as Kenwood puts it, is to “effectively place the amplifier across the speaker terminals.” Special 13-foot speaker cables are supplied with the amplifier. In addition to two heavy conductors for the speaker drive, these cables include a pair of lighter-gauge wires to carry the feedback signals from the speaker terminals to special Sigma Drive binding posts in the rear of the amplifier.

Among the benefits claimed for Sigma Drive are a very large increase in effective damping factor (rated at a minimum of 600 at the speaker end of the cable), compared with any conventional output system, and a considerable reduction in the distortion of the electrical signal supplied to the speaker. Kenwood states that an amplifier—even one as advanced as the KA-1000—produces far more distortion at the speaker than is measured directly at the amplifier output because of the impedance of the connecting cable and the nonlinear impedance of the speaker. Sigma Drive makes the extremely low distortion of the amplifier more readily available to the speaker.

Kenwood engineers also believe that the presence of magnetic materials in the immediate vicinity of signal circuits can introduce measurably significant amounts of distortion. To eliminate such effects, they have made the KA-1000 entirely of nonmagnetic materials—aluminum top cover and front panel, molded plastic sides and trim. The power supply is a separate unit, connected to the amplifier through a 3-foot cable. Separate power transformers are used for the left and right channels, and, according to Kenwood, there are actually five separate power supplies in the KA-1000.

In addition to Sigma Drive, the Kenwood KA-1000 has wide-band circuitry with full d.c. coupling from the preamplifier’s high-level inputs to the speaker outputs (a low-frequency filter can be switched in to provide d.c. isolation as well as some attenuation of turntable rumble). The output stages use a “zero-switching” bias system to minimize crossover distortion. The heat from the plastic-cased output transistors is conducted to a refrigerant which circulates in a closed system to heat-radiating fins. The entire cooling system is contained within the amplifier cabinet.

From the front, the Kenwood KA-1000 presents a distinctive appearance, with the lower portion of its panel covered by a smoked-glass panel behind which can be seen a number of bar-knob and pushbutton controls. The glass panel hinges down for access to these controls, most of which are used infrequently. The upper part of the panel, which is silver grey like the rest of the amplifier, contains a flat rectangular power switch at the left and a large rectangular transparent plate (fader) at the right. To the left of the fader plate is a horizontal-slider volume control, below which are five input-selector pushbuttons.

The volume slider sets the listening level when the fader plate is illuminated from the rear by a pale blue light. Touching the plate lightly causes the volume to fade smoothly down to zero over a period of a second or two, and another touch returns it to the preset value over the same time period. This feature, which replaces the “audio muting” feature of many amplifiers, accomplishes much the same result, but Kenwood’s system drops the volume to zero instead of making the usual 20-dB reduction, and it changes with a smooth fading action that encourages its use. As the signal fades, the blue light behind the touch plate fades with it, so the status of the system can be seen at a glance. In addition, the position of the volume slider gives an indication of how loud the sound can be expected to be when the signal is fully restored.

The program-selector buttons offer a choice of tape a. tape b, aux, tuner, and phono inputs. Their identifying markings are below their buttons on the hinged glass panel. Red lights next to smaller markings on the metal panel behind the door also show the program selection. Behind the hinged panel is a rec OUT knob switch that selects the recording-output program independently of the program being played. It duplicates the phono, aux, and tuner sources, plus two dubbing positions for transferring programs from one tape deck to another. In its off setting, the tape outputs are completely isolated from the signal path.

The bass and treble tone controls can be bypassed by a pushbutton that completely removes all active tone-control circuits from the signal path (this also permits a d.c. response through the entire amplifier). In addition, there are turnover-frequency buttons for the tone controls, giving a choice of 200 or 400 Hz for the bass and 3 or 6 kHz for the treble control.

The balance control is center-detented and, like the tone controls, can be completely bypassed by a button that removes the balance control from the amplifier circuit. Other pushbuttons activate the loudness compensation, select either DC coupled or subsonic filter operation, and choose stereo or mono mode.

The KA-1000 has two phono inputs, selectable from the front panel by a pushbutton. In the rear of the amplifier is a slide switch that provides phono-input terminating resistances of 33, 47, or 100 kilohms. Another front-panel switch converts the phono preamplifier from a moving-magnet (MM) cartridge input to a high-gain, 100-ohm moving-coil (MC) cartridge input.

The remaining front-panel control is a knob-operated speaker switch controlling two pairs of speaker outputs. Only the speaker a outputs include the Sigma Drive terminals, and a red light above the speaker switch indicates when it is in use. There is an A + B position that drives both sets of speakers simultaneously, but it does not use the Sigma Drive. If only the speaker a terminals are used, it is easy to check the audible effects of Sigma Drive by switching to A + B, which disables that circuit. A headphone jack is located next to the speaker switch.

The Kenwood KA-1000 amplifier is about 17-3/8 inches wide, 14-3/4 inches deep, and 4-7/8 inches high. Because its power supply is separate, the amplifier unit weighs only about 13-1/2 pounds. The power-supply unit, which matches the amplifier in appearance, is 5-1/2 inches wide, 14-3/4 inches deep, and 47/s inches high, and it weighs about 18-1/4 pounds. On its rear panel are three switched a.c. outlets and one unswitched outlet. The two units can be placed side by side or the power supply can be located anywhere within its cable length of the amplifier (it has no controls). The suggested retail price of the Kenwood KA-1000 is $795.

Laboratory Measurements. The 1-hour preconditioning period at one-third rated power made the top of the KA-1000 uncomfortably hot over the heat sinks, but elsewhere it was quite cool. Even in normal operation the top of the amplifier becomes rather warm, and proper ventilation is important. With both channels driving 8-ohm loads at 1,000 Hz, the waveform clipped at 114 watts per channel for an IHF clipping-headroom rating of 0.57 dB. The amplifier is not rated for loads other than 8 ohms, but its measured output at clipping into 4- and 2-ohm loads was 118 and 56 watts per channel, respectively. Dynamic-power measurements into these load impedances yielded clipping outputs of 128, 114, and 63 watts, giving the amplifier an 8-ohm IHF dynamic-headroom rating of 1.07 dB. The overload recovery time, when overdriven 10 dB with the pulsed test signal, was a good 10 microseconds.

Distortion measurements demonstrated the extraordinary linearity of the Kenwood circuits through most of the audio range. The distortion at 1,000 Hz was undetectable below 10 watts output into 8 ohms, being below the noise level and our measurement limit of 0.0003 per cent! It measured between 0.0005 and 0.001 per cent from 10 to 100 watts output. With 4-ohm loads the results were generally similar: levels of 0.0005 to 0.0007 per cent were measured in the 10- to 50-watt range and 0.008 per cent at 100 watts. Even when we drove 2-ohm loads (for which the amplifier is specifically not rated, being recommended for use only with speakers rated at 4 ohms or higher) the distortion was typically between 0.0015 and 0.002 per cent up to 40 watts, reaching 0.01 per cent at 50 watts.

At the rated 100-watts output, the distortion was under 0.001 per cent from 20 to 1.000 Hz, and it rose linearly at higher frequencies to the rated 0.005 per cent at 8.000 Hz and 0.01 per cent at 20,000 Hz. At lower power outputs the distortion was still lower. These measurements were made with the amplifier gain set to 20 dB below maximum, in accordance with IHF measurement standards. When the distortion is measured at maximum gain setting, the amplifier meets the published rating of 0.005 per cent at 20,000 Hz.

Our measurements were made through the complete amplifier (aux input to output loads) with the tone controls and balance control bypassed. The sensitivity at maximum gain for a reference output of 1 watt was 16 millivolts (mV); through the MM phono input it was 0.23 mV. No measurements were made using the MC input. Under reference-gain conditions (0.5 volt into aux or 5 mV into phono producing a 1-watt output) the A-weighted signal-to-noise-ratio (S/N) readings were —80 and — 78 dB for aux and phono, respectively, referred to 1 watt. Relative to the amplifier’s rated power, the S/N readings were -100 and -98 dB.

The phono input overloaded at 270 to 290 millivolts depending on frequency (referred to equivalent 1,000-Hz values). The phono-input impedances measured 110, 50, and 34 kilohms for the positions marked 100, 47, and 33 kilohms. The phono-input capacitance was 150 picofarads. The MC input resistance was 130 ohms.

The amplifier was stable with capacitive loads as great as 3 microfarads across the resistive loads. We measured its IHF intermodulation distortion using equal-amplitude signals, at 19,000 and 20,000 Hz, whose combined peak value was equivalent to that of a sine-wave signal at 100 watts output. The third-order IM distortion at 18,000 Hz was —83 dB and the second-order distortion at 1,000 Hz was —84 dB, both referred to 100 watts output. The amplifier rise time was 6 microseconds at the IHF standard-gain setting but improved to 1 microsecond (approximately as rated) with maximum gain. The slew rate was about 100 volts per microsecond, quite close to the rated 120 volts per microsecond, and the IHF slew factor exceeded our measurement limit of 25.

With the 400-Hz and 3-kHz turnover settings, the tone controls caused considerable change of overall gain; halfway settings of the controls produced a gain change of 2 to 3 dB, with little alteration of frequency response. With the other crossover settings (200 Hz and 6 kHz) the gain was fairly constant and there was a good control effect at the frequency extremes (although the treble control had negligible effect in the first half of its rotation from center). The controls are not detented, but the bypass switch makes it easy to return to a flat-response condition whenever one desires.

The loudness compensation boosted only the lower frequencies, below about 1,000 Hz, as the volume was lowered. The “subsonic filter,” which has a gradual 6-dB-per-octave slope below its rated cutoff frequency of 18 Hz, reduced the response by 2.5 dB at 20 Hz with very slight gain reduction as high as 100 Hz. The RIAA phono equalization was extremely accurate, deviating only ±0.5 dB from 20 to 20,000 Hz, which is within our normal measurement-error range. There was a slight interaction between cartridge inductance and phono response at frequencies above 7,000 Hz, depending on the specific cartridge used. In one case, the output rose to +1.2 dB at 15,000 Hz and +2.5 dB at 20,000 Hz, while with another cartridge the total change was +1, —0.5 dB up to 20,000 Hz.

All the preceding measurements were made using the Sigma Drive circuit and with the test loads connected to the amplifier via its special four-wire cables. To evaluate the action of the Sigma Drive, it was necessary to simulate a speaker load (the feature has virtually no effect with purely resistive loads). The best simulation is a real speaker, so we connected the amplifier to a high-quality three-way system (the KEF 105.2) and measured the response and distortion at the speaker terminals. To exaggerate the effect of long connecting cables, we also made measurements with an extra impedance in series with the speaker line at the amplifier where it would allow the Sigma Drive to operate with full effectiveness. For some measurements we used a 4-ohm resistor, and for others we used a large series inductor measuring 0.7 ohm and 2.5 millihenries.

First, we measured the distortion at the speaker through the KA-1000 cables using a 10-watt drive level at 40 Hz. The only measurable distortions were a 0.013 per cent second harmonic and several higher-order harmonics at about 0.006 per cent. The distortion was the same with or without the Sigma Drive in operation.

Since the amplifier has less than 0.0005 per cent distortion under the same conditions when driving an 8-ohm resistor load, it appears that the complex nature of a speaker load can increase the distortion by about thirty times, although in our view it is still completely negligible. When we inserted the 4-ohm resistance in the speaker line (with Sigma Drive off) the second-harmonic distortion rose to —47 dB (0.45 per cent). Though hardly likely to be audible under any conditions, this distortion level can no longer be summarily dismissed since it is not far below the levels attained by some good speaker systems at the frequency and power levels we used. There was also a third harmonic, at 120 Hz, at a —60 dB (0.1 per cent) level. Switching in the Sigma Drive reduced these distortion levels by 12 to 15 dB, certainly a very substantial improvement. The 80- and 120-Hz components were now, respectively, —63 and — 72 dB (0.07 and 0.025 per cent), which are truly negligible compared to any speaker distortions.

One would also expect Sigma Drive to correct for any frequency-response changes caused by the impedance of the speaker cables. With a purely resistive impedance, these effects would merely follow the impedance characteristic of the speaker; with series inductance they would also show up as a loss of high-frequency output. This was tested by using our spectrum analyzer as a frequency-response plotter, with the same series impedances used for the distortion tests. With the 4-ohm series resistance and Sigma Drive off, the voltage at the speaker terminals varied about 2 dB over the range of frequencies up to 2,000 Hz. Turning on the Sigma Drive reduced the total variation to about 0.5 dB.

The series inductance had a more pronounced effect, reducing the speaker-drive voltage rapidly above 1,000 Hz as well as introducing several smaller irregularities at lower frequencies. The Sigma Drive eliminated the high-frequency roll off, actually converting it to a slight increase in output. Finally, an overall sweep from 0 to 20,000 Hz without Sigma Drive showed a drop in voltage of about 9 dB in the upper part of that range, caused by the impedance of the series inductor. The Sigma Drive converted this to a nearly flat response, varying only ±2 dB over the full range.

Comment. In the KA-1000, Kenwood engineers concentrated on improving certain specific areas of performance according to their perception of the importance of those characteristics. Our measurements show that they have succeeded quite well in attaining their goals. However, we wish that the KA-1000's controls had been more versatile and effective and the control labeling more visible.

On the plus side of the ledger, the separate recording-output selector is a welcome feature, as is the smooth action of the fader, which mutes and restores the program so smoothly and unobtrusively that one is hardly aware of the action. The amplifier is dead quiet and, needless to say, utterly clean sounding.

Which brings us to Sigma Drive, the KA-1000’s most unusual feature. Our measurements confirm that it does just what is claimed for it, effectively compensating for the presence of the speaker cables. Despite Kenwood’s claim that it makes a “striking” difference in the sound, however, we never found more than the most subtle smoothing and “warming” of the frequency balance when it was switched in, and then only with certain program material. Its effect would also be quite dependent on the particular speakers used. The action of this circuit can be measured more easily than heard, which is an interesting inversion of the all-too-common case of subjective qualities that cannot be explained by any conventional measurements.

The longer the speaker leads, and the smaller their gauge, the more advantage will be realized from Sigma Drive. Thus, the special four-wire cables supplied with the amplifier are the least likely to disclose its merits. If they are not used and longer speaker leads are needed, it will be necessary to run a second pair of leads back from each speaker to the Sigma Drive feedback terminals on the amplifier, but they carry almost no current and can therefore be of very small gauge. Judging by the effectiveness with which the Sigma Drive compensated for a 4-ohm connecting resistance (corresponding to hundreds of feet of speaker wire) there should be little need for any user of the KA-1000 to invest in any of the special “low-loss” cables on the market.

The KA-1000 will not be at its best with very-low-impedance speaker loads, and if your speaker drops much below 4 ohms we would not recommend its use (nor would Kenwood). On the other hand, we cannot imagine a cleaner-sounding amplifier, not one which can come closer to equaling its test-bench performance when driving real loudspeaker systems.

Whatever the reasons, this was an amplifier we especially enjoyed using and listening to. On a subliminal level, which may be one of the best ways to judge a very highly refined amplifier or other component, we found the KA-1000 to be one of the best-sounding amplifiers available. Even if the full benefits of Sigma Drive probably can be realized only under rather special conditions, and even though it produced no spectacular results in our installation, we consider it to be a genuine step in the right direction and, unlike some other circuit developments of recent years, not at all in the “gimmick” category.

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