Harman Kardon HK6900 Stereo Amplifier Review

The HK6900, rated at 170 watts per channel into 8-ohm loads from 20 to 20,000 Hz with less than 0.08 percent harmonic or intermodulation distortion, is Harman Kardon’s most powerful integrated amplifier. Among its design and circuit features are the use of discrete (as opposed to integrated) electronic components throughout, low overall negative feedback (only 12 dB), and a wide bandwidth, extending far beyond the audio range. It is also designed to deliver high instantaneous peak currents into low-impedance loads. According to the manufacturer, the HK6900 can supply as much as 90 amperes of instantaneous output.

The HK6900 is a large, heavy amplifier, measuring 17-3/8 inches wide, 16 inches deep, and 6-1/4 inches high and weighing 37-1/2 pounds. Its full-size control knobs operate with a strikingly smooth and positive feel, and their functions are clearly labeled in gold characters on a black panel. Bar knobs select the speaker outputs for one or two pairs of speakers, monitor the playback from either of two audio or video tape recorders, and connect the amplifier to any of five input sources: phono, CD, tuner, a/v, and auxiliary. The tuner input can be switched to either set of tape outputs independently of the listening selection, and the two decks can be connected for dubbing in either direction.

At the bottom of the panel are two small pushbutton switches. One turns off the amplifier’s video-preamplifier circuits (the manual says, without elaboration, that this is “for purest possible sound quality”), and the other selects between the moving-magnet (mm) and moving-coil (mc) phono-preamplifier inputs (there are separate rear-panel jacks for the two types of cartridges).

There are four flat, rectangular switches at the lower left of the control panel for power, tone-control bypass, loudness compensation, and the subsonic (infrasonic) filter. Another switch, main direct, bypasses the preamplifier stages, routing the selected signal directly (but through the volume control) to the power-amplifier input. All of the front-panel controls use high-quality electronic switches.

According to Harman Kardon, the HK6900’s loudness-compensation circuit has been designed to correct for the midrange phase errors normally associated with low-frequency equalization. It is said to be free of the coloration or stereo-imaging degradation said to occur with conventional loudness-compensation circuits.

In addition to the various signal input and output phono jacks, the rear apron of the HK6900 has video input/output jacks for the two tape loops, which are designed to switch VCR’s as well as audio tape decks.

There are also monitor outputs as well as audio and video inputs for use with a laserdisc player. The speaker terminals are large, heavy-duty binding posts; unfortunately, they are spaced too widely to accept dual banana-plug connectors.

A somewhat unusual feature is a pair of jacks labeled “pre-out”— without the usual accompanying main-amp-in jacks. After some investigation, we determined that these jacks simply provide a separate line-level feed from the preamp section. They could be used to drive an external amplifier, although with no provision for disconnecting the HK6900’s own power amplifier that would be a waste. A more likely use for these jacks is to feed a signal to a surround device, a tape deck, or a mixing board.

A rear-panel pushbutton labeled SPEAKER OPERATING MODE adjusts the power-supply voltage on the output transistors for safe operation with either 4- or 8-ohm speakers. The instructions suggest normal use of the 4-ohm setting, which insures the coolest and safest operation of the amplifier. If only a single pair of 8-ohm speakers is used, however, the 8-ohm setting provides the maximum possible power output. Two of the three AC outlets are switched. Price: $1,299.

Lab Tests

During the 1-hour preconditioning at one-third rated power (57 watts) into 8-ohm loads, the top of the amplifier (over the internal heat sinks) became quite warm, though never too hot to touch. The unit’s thermal-protection system shut the amplifier down a number of times during this period. During extended normal operation, the top of the cabinet became only slightly warm.

We measured power levels at clipping and distortion levels using several combinations of load resistance and speaker-mode settings. With the recommended setting, the 1,000-Hz output clipped at 225 watts per channel with both channels driving 8-ohm loads and at 227 watts into 4 ohms. The respective clipping-headroom numbers were 1.22 and 1.26 dB. When 8-ohm loads were used with the amplifier’s 4-ohm switch setting, the clipping level was reduced to 140 watts, but with a 2-ohm load (not recommended) the output clipped at an impressive 300 watts per channel. Finally, using 4-ohm loads with the 8-ohm switch position produced a clipping output level of 312 watts.

Dynamic power measurements were made with the same combinations of load resistance and switch settings. Using the recommended setting, the 8-ohm power was 272 watts (dynamic headroom of 2 dB). The 4-ohm output was 190 watts, slightly lower than the clipping power under the same conditions, for a dynamic headroom of 0.5 dB. The combinations of 2-ohm loads with the 4-ohm switch position or 4-ohm loads with the 8-ohm switch position produced truly enormous dynamic outputs, 462 and 465 watts, respectively.

The total harmonic distortion plus noise (THD + N) was comfortably within the amplifier’s ratings. The 8-ohm load-and-switch combination produced readings that decreased from 0.05 percent at 1 watt to 0.013 percent in the 40- to 60-watt range, rising to 0.023 percent at 170 watts and 0.053 percent at 200 watts. The corresponding 4-ohm readings were 0.08 percent at 1 watt, 0.018 percent at 100 watts, and 0.056 at 200 watts.

At the rated 170 watts into 8 ohms, the distortion measured 0.025 percent from 20 to 3,000 Hz, rising to a maximum of 0.06 percent at 20,000 Hz. At half power and one-tenth power, the distortion curves were similar to the full-power measurements but with lower readings: in the 0.013 percent range up to 3,000 Hz and 0.04 to 0.046 percent at 20,000 Hz.

The amplifier’s frequency response was flat within ±0.2 dB overall from 10 to 30,000 Hz, and down 1 dB at 100,000 Hz. A power-bandwidth measurement, at a constant distortion of 0.1 percent into 6 ohms, showed an output of 200 watts at 20 Hz and 210 watts from 100 to 20,000 Hz. The tone controls had conventional response characteristics, with a sliding bass-turn-over frequency (from below 100 Hz to about 300 Hz) and treble curves hinged at 2,000 Hz. The infrasonic filter, specified as having a 15-Hz cutoff with a 6-dB-per-octave slope, had no significant effect within the audible frequency range.

The loudness compensation was the same at all level settings from — 10 to —60 dB, providing a mild bass boost (about 4 dB between 20 and 100 Hz) and no effect on frequencies above 600 Hz. Because of Harman Kardon’s claims for the phase accuracy of this loudness circuit, we also measured its phase shift versus frequency. Phase shifting was, as claimed, extremely low, ranging from —0.2 degree at 20 Hz to +0.7 degree at 20,000 Hz.

The phono-preamplifier section’s equalization response was within ±0.03 dB of the RIAA characteristic from 20 to 8,000 Hz and only — 0.1 dB at 20,000 Hz. The amplifier’s sensitivity, for a standard reference output of 1 watt, was 19.3 millivolts (mv) through the CD input, 0.2 mv for mm phono, and a very good 0.01 mv for mc phono. The respective A-weighted noise levels (referred to 1 watt) were —78, — 77, and —75 dB. The phono-preamplifier input overloaded at 145 millivolts (mv) at frequencies of 1,000 and 20,000 Hz and at 130 mv at 20 Hz. Its impedance (mm) was 47,000 ohms in parallel with a 112-picofarad capacitance.

Comments

The Harman Kardon HK6900 is unquestionably one of the most powerful integrated amplifiers on the market, and its overall quality is as impressive as its sheer power. Despite the operating flexibility of the HK6900, its front panel presents a relatively simple, uncluttered appearance. We would have preferred to have visible index lines on the tone-control and balance knobs, whose settings were very difficult to see or to feel (their center detents are much too light).

For all of its formidable size, weight, and power, the HK6900 is a comfortable amplifier to use. It doesn’t have to be coddled or treated with kid gloves. Its bulk and weight encouraged us to throw caution to the winds in our power and distortion measurements, during which we violated just about every one of the manufacturer’s recommendations. Nothing untoward happened, not even a blown fuse (we even ventured to drive both channels during the 2-ohm measurements).

The HK6900 does have a few peculiarities, though. The pair of “pre-out” jacks are puzzling, and the owner’s manual incorrectly describes their function (we had to call Harman Kardon’s engineering department to find out what they are for). A conventional combination of preamp-out and main-amp-in jacks would have been more flexible and less confusing. Harman Kardon’s engineers defend the unusual design, however, which they say provides the HK6900 with the shortest, cleanest possible internal signal path. The record OUT selector provides only two choices of a recording source (source and tuner) in addition to the tape-dubbing positions. A completely independent choice of listening and recording program sources, a feature of many good amplifiers and preamplifiers, would have been more useful. Finally, we cannot understand why the speaker binding posts have been spaced to exclude the use of dual banana plugs, which we find to be by far the most convenient method of connecting speakers to amplifiers.

Aside from these quibbles, the HK6900 proved to be as fine an amplifier as one could wish for. The loudness compensation was mild enough to be useful without excessive coloration of the sound. And though we had never been concerned about the phase shifts associated with conventional loudness-compensation circuits (which we rarely use in any case), it is undeniable that this one is free of phase-shift effects.

The noise level of the amplifier was very low, especially in view of its very high gain. Its input sensitivity was unusually high, yet even when phono was selected, only the faintest hiss could be heard from the speakers at a maximum volume setting—and there was no crosstalk from the tuner input.

If you are one of those who believe that every amplifier has its own special sonic qualities, you will have to judge the HK6900 for yourself. I do not find such special characteristics in most amplifiers, especially not in any as good as this one, and have no reservations about my enthusiasm for this versatile, ruggedly constructed, and handsomely styled amplifier.

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