Kenwood KA-7100 Integrated Amplifier Review

Kenwood's KA-7100 integrated amplifier is a moderate-price product with outstandingly low distortion specifications and ample control flexibility for a well-equipped hi-fi system. It is rated to deliver 60 watts per chanhel to 8-ohm loads from 20 to 20,000 Hz, with less than 0.02 per cent total harmonic distortion. The KA-7100 has separate power supplies for its two channels, fed by a common power transformer. The power-amplifier sections of the KA-7100 are direct-coupled.

The dominant feature of the front panel is the large volume-control knob near its center. It operates a thirty-two-position step attenuator whose detents are so light that one is hardly aware of their presence. The bass and treble tone controls are each eleven-position step controls, and the balance control is lightly detented at its center. Below these are level switches for tone defeat, subsonic and high filters, and a 20-dB audio attenuator.

To the right of the volume knob is the tape switch, with positions for dubbing from one tape deck to another (but not back the other way) while monitoring the playback output from either deck and for conventional tape playback from either tape deck or a selected program source. The input selector has positions marked phono, tuner, and aux. Below are the loudness switch, with three positions marked off, 1, and 2, and the mode selector, with reverse, stereo, and mono positions. Completing the front-panel controls are the speakers switch at the upper left (for driving either, both, or neither of two pairs of speakers), the power switch, and a headphone jack.

In the rear of the amplifier are the various signal input and output jacks, plus a DIN socket duplicating one set of the tape-recorder connections. The speaker terminals are insulated binding posts. The two switched a.c. outlets will take no more than 100 watts of accessory equipment; there is one unswitched outlet. The Kenwood KA-7100 is approximately 17 inches wide, 15 inches deep, and 6 inches high. It weighs 25.4 pounds. Price: $300.

Laboratory Measurements

The FTC preconditioning period made the amplifier only moderately warm. Afterward, its outputs clipped at 72 watts per channel (at 1,000 Hz) driving 8-ohm loads. The 4-ohm and 16-ohm clipping levels were 95 and 46 watts.

The claimed low distortion of the KA-7100 was confirmed by a considerable margin. At 1,000 Hz, total harmonic distortion was between 0.003 and 0.004 per cent for all power levels from 1 to 60 watts, and it was only 0.0056 per cent at 70 watts, just before clipping occurred. The distortion also rose slightly at very low power levels, to 0.008 per cent at 0.1 watt. The intermodulation distortion (IM) was 0.006 to 0.008 per cent over most of the power range, increasing to 0.017 per cent at 0.1 watt and 0.012 percent at 70 watts.

This low distortion was not limited to middle frequencies. At the rated output and also at lower power levels, the distortion was about 0.005 per cent or less from 100 to 5,000 Hz, increasing to 0.018 per cent at 20 Hz and between 0.011 and 0.018 per cent at 20,000 Hz. The distortion at 20 Hz and —10 dB (6 watts output) was 0.025 per cent, but this included inaudible noise components that were comparable to the distortion at that low level.

The input required for a reference output of 10 watts was 61 millivolts at the high-level inputs and 0.95 millivolt at the phono inputs. The corresponding unweighted signal-to-noise ratios were 79 and 76.4 dB, both very good figures. The phono preamplifier overloaded at a very high 260 millivolts.

The tone-control action was moderate but perfectly adequate, with a maximum range of ±8 dB at the low frequencies and ±10 dB at the high frequencies. The filters had gradual 6-dB-per-octave slopes. The subsonic filter reduced the 20-Hz output by 2.5 dB. The high filter response was down 3 dB at 6,000 Hz, and its effect was not much different from that of a tone control.

The loudness compensation used on the KA-7100 is one of the better circuits of its type. Instead of providing for independent adjustment of overall gain and the compensation (the ideal system), it offers two degrees of bass boost at any given volume-control setting (the highs are unaffected). With either switch setting, the bass boost is much less than is common with loudness circuits, and we found that it could be used without imparting excessive heaviness to the sound.

The RIAA phono equalization was literally ruler-flat, with less than ±0.25 dB variation from 30 to 20,000 Hz, and down only 0.5 dB at 20 Hz. The inductance of typical phono cartridges caused a slight high-frequency rise in phono response at a rate of about 0.7 dB per octave above 2,000 Hz. The rise time of the amplifier (including the preamplifier section) was 2 microseconds and its slew rate was 45 volts per microsecond, both figures representing very good performance for an integrated amplifier of this power rating and general price class.

Comment

We have been observing with interest during the past couple of years the appearance of amplifiers boasting almost un-measurably low distortion levels (anything much under 0.02 per cent certainly falls into that category). The phenomenon has so far been restricted principally to the higher power and price ranges. The technology responsible for this (we assume it to be the development of improved transistors) has now found its way down to the $300 class of amplifiers, as exemplified by the Kenwood KA-7100.

Although we doubt that there are any audible benefits to reducing distortion from 0.1 per cent (or so) to the 0.02 per cent or less of this amplifier, what is amazing is that it can be done at all, let alone in such a low-price amplifier. The really impressive thing about the KA-7100 is how effectively it does its job, with nary a sour note—mechanical or audible—to mar the performance of a beautifully engineered product. Someone with $300 to spend would have to look long and hard to find a better amplifier for the money.

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