Kenwood KA-828 Integrated Amplifier Review

Kenwood’s KA-828 integrated stereo amplifier, despite its modest size and weight, is rated to deliver 130 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.007 percent total harmonic distortion. It employs a number of design techniques typical of limited-production high-end amplifiers, such as elimination of internal ground loops and common ground impedances through extensive use of one-point grounding within the amplifier’s key circuit blocks.

The KA-828 also features several unusual circuit configurations. Its output stages employ what Kenwood calls Dynamic Linear Drive (DLD). The output stage for each channel consists of separate low-power and high-power amplifiers, each with its own power supply. The low-power section operates when the amplifier is called on to deliver up to about a third of its maximum power, and the high-power section takes over when more output is needed. The rationale for the DLD system is provided by tests conducted by the Japanese broadcasting system, NHK, which showed that the difference between peak and average power levels required to reproduce various types of recorded music was about 15 to 16 dB, a ratio of roughly 20 to 1. This means that an amplifier should be able to put out peaks twenty times higher than the average power used.

The choice of the transition level between the low- and high-power amplifier sections in the KA-828 was based on Kenwood’s own experiments, which indicated that a power output greater than one third of the peak level was required less than 2 percent of the time. It is obviously wasteful and inefficient for an amplifier to be designed to deliver a high-power output on a more or less continuous basis when that much power is required less than 2 percent of the time. But that is how most amplifiers are built, and it entails large penalties in size, weight, and cost. Kenwood’s DLD circuit makes it possible to use smaller heat sinks and power-supply components, which nevertheless are very conservatively designed for a 40-watt amplifier—the approximate rating of the low-power section in the KA-828—in order to be able to support the full output of the amplifier for the relatively brief periods when it is required.

Kenwood contends that the DLD system enables an amplifier to provide superior reproduction of low-level sounds, with less heat dissipation, than a conventional amplifier, yet also to deliver the high peak-power levels that are becoming increasingly important in the age of digital audio. The separate power supplies used for the two sections of the KA-828’s power-output stage are said to have better regulation than those in a typical Class B or Class AB amplifier, another factor Kenwood believes improves the sound quality of the amplifier. Although the KA-828 is fully protected against incorrect output loading or excessive drive levels, with a relay to disconnect the speakers if necessary and thermal devices to shut the amp off entirely if it becomes too hot, it does not use conventional current-limiting circuits and is thus able to drive low load impedances without loss of power or undesirable side effects.

Other features of the Kenwood KA-828 include a phono-input amplifier whose gain and input impedance can be switched from the front panel for either moving-magnet (MM) or moving-coil (MC) cartridges. The tone-control section is designed to have very small changes in overall negative feedback (and distortion) as settings are varied.

Although the KA-828 has all the control features most people will ever need or want in an integrated amplifier, it is relatively free of the meaningless or purely cosmetic frills found on many of today’s hi-fi components. Flush-mounted pushbuttons switch the a.c. power and select the input source. Oddly, the audio-muting button is about three times the size of the input selectors—for quick use if the telephone rings, perhaps. Smaller buttons activate the loudness-compensation and tone-control circuits, select the high-level video/aux input (whose jacks are on the front panel), and switch the phono stage between its MM and MC modes. Knobs are used for the bass and treble tone controls and channel balance control, with a larger knob for volume. Other front-panel knobs select the speaker outputs and control tape-recorder monitoring or dubbing.

The Kenwood KA-828, finished entirely in black, has its heat sinks inside the cabinet, which measures 13-3/8 inches wide, 14-1/2 inches deep and about 4-1/4 inches high. The amplifier weighs about 20-1/4 pounds. Price: $450.

Lab Tests

The FTC-required preconditioning at one-third rated power into 8 ohms for 1 hour made the top of the KA-828 quite hot, but no hotter than most amplifiers under those conditions. Although the KA-828 is only rated for 8-ohm operation, it gave an excellent demonstration of its hidden capabilities by driving 4-ohm and 2-ohm loads to impressive levels. The 2-ohm tests heated the output stages (presumably their high-power portions) sufficiently to trigger the thermal protection system, which shut down the amplifier. After 10 to 15 minutes, it had cooled enough to return to operation. To avoid such interruptions, we made our 2-ohm distortion measurements with only one channel driven (it delivered 300 watts at the clipping point!). The dynamic headroom was better than usual.

The total harmonic distortion (TFID + noise) increased smoothly at lower power levels because of the relatively greater contribution of the small amount of noise present in the amplifier’s output, reaching 0.02 percent at 1 watt. The distortion curves for 4-ohm and 2-ohm loads were similar, differing only in their slightly higher maximum outputs and distortion levels. But even under worst-case conditions (2 ohms), the maximum 1,000-Hz THD + N was only 0.04 percent at 1 watt. Driving 8-ohm loads, the amplifier’s distortion was 0.004 to 0.01 percent over most of the audio range at power outputs from 13 watts to the rated 130 watts, reaching 0.017 percent at 20,000 Hz and 13 watts output. The amplifier was stable with simulated speaker loads at low and high frequencies.

In the preamp section, the phono RIAA equalization was not affected by cartridge loading. The tone controls had conventional response characteristics, with the bass turnover frequency sliding between 300 and 500 Hz and the treble curves hinged at 2,500 Hz. The loudness compensation introduced only a moderate boost of the low frequencies as the volume was lowered.

Comments

While we cannot comment on any identifiable sonic contributions of the DLD and other design features of the Kenwood KA-828, since it sounded fine under any drive or load conditions at our disposal, we can say that it delivered an audible “punch” out of all proportion to its size, weight, and price. It looks and feels much like any of a number of good 50- or 60-watt amplifiers, and it doesn’t cost much more either, but it surely didn’t behave like any of them when it was driving low- to medium-efficiency speakers at high volume. Its power is demonstrably real, whether judged by test-bench performance or by listening. We turned the volume high enough to cause acoustic feedback from some of our speakers to turntables and even CD players (!) across the room, but there was never a sign of clipping or other distortion.

While the precise method by which the KA-828 splices the low-and high-power sections of its output stages is not particularly obvious, even from its schematic (it appears to be done by a rather complex system of diode or transistor switching), we certainly have no doubts that the DLD system works as claimed. The KA-828 is an eloquent witness to its effectiveness, being a very impressive amplifier in a modest and unassuming package.

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