A Receiver That "Really Works"
Harman Kardon 680i AM /FM receiver, in metal case. Dimensions: 17.25 by 4.5 inches, 13 inches deep plus clearance for controls and connections. AC convenience outlets: two switched (150 watts max. total), one unswitched (150 watts max.). Price: $630. Warranty: "limited," two years parts and labor.
Manufacturer: made in Japan for Harman Kardon, Inc., 240 Crossways Park West, Woodbury, N.Y. 11797.
FM tuner section
MONO FREQUENCY RESPONSE; STEREO RESPONSE & CHANNEL SEPARATION
FOR A GENERATION NOW, Harman Kardon has had a reputation as a producer of perfectionist gear, generally for moderate prices. The not so moderately priced Citation line contributed to that reputation, but the company's unshakable loyalty to such criteria as broad band electronics-even when a criterion was out of style in the marketplace-has contributed more. Now that reputation has been enhanced by HK's liaison with Matti Otala. in whose acoustics laboratories (in Finland) the various forms of dynamic intermodulation distortion were first isolated and quantified. It is largely as an indirect result of his investigation of transient intermodulation. interface intermodulation, and the rest that use of negative feedback as a cure all for amplification circuits has fallen into disfavor in some quarters of the audio industry; a central article of current design faith is that open-loop performance (that is, with no feedback in use) should be intrinsically as good as possible and the lily he gilded only by the minimum feedback needed to achieve design goals.
The hk-680i, the top of the receiver line in which Harman Kardon has applied Otala's ideas, is the most impressive product we've reviewed from that company in quite some time. Not only is the amplifier section clean-as you'd expect. given the design objectives-but the digital tuner section proves to be at least its match.
Getting to know it was not as easy as with most receivers because of the multiple tuning modes and options. Digital synthesis is employed in both the FM and AM bands, and there is quartz lock as well. (When DSL tried to verify tuning frequencies on the FM hand, it couldn't. The lock followed the frequency of the lab's RF generator over a fairly wide swing, preventing even intentional mistuning within the FM channel.) Since not all countries share our 10-kHz AM station spacing-and since there has been some discussion of adopting the 9-kHz spacing here to permit more stations on the band-a back-panel switch gives you a choice of the two spacings. All this inner complexity actually works very simply in the manual tuning mode: from the user's point of view, the complexity becomes apparent when you start using the automatic tuning features, partly because the owner's manual is least clear in this area.
For example, the first users both at HF and at DSL began by assuming that the AUTO-FM button in the selector group had to do with automatic tuning and wondered why we couldn't get stereo reception when we punched up FM for manual tuning. "Auto," in this case, refers to automatic mono/stereo switching: the button marked "FM" is for mono reception only. To select the tuning mode, you must employ the MAN-/ AUTO button just below the tuning bars. The jazzy automatic mode steps in the direction you've selected on the tuning bars, pausing a few seconds at each station to let you hear what's going on there. When you hear something you like, you simply press SCAN STOP: if you don't get to the set before the scan has moved on, you reverse the scan direction and stop it when the interesting station reappears.
If you want to keep that station in one of the preselectors (there are six, each accepting one FM and one AM frequency), you press MEMORY and then the preselector button where you want the frequency stored. Harman Kardon has built in a ni-cad battery to retain the memorized information even through power blackouts and those annoying moments when the wrong plug is pulled to make room for the vacuum cleaner.
Another nicety is a back-panel muting threshold adjustment with a range of more than 10 dB and a minimum setting equal to that of the stereo threshold. The muting threshold also has a hysteresis "window" about I dB wide, meaning that slight fluctuations in already-weak signal strength (usually multipath due to a moving reflector like an airplane) won't easily flop back and forth across the threshold, making reception "flicker." And the signal-strength "meter" is unusually useful for an LED display both because of its range (16 1/2 to 64 dBf in ten steps, though there are twelve LEDs) and because of its continuity be tween steps, with each LED brightening gradually as signal strength rises through its range.
Both on the test bench and in the listening room, all this proved exception ally gratifying, and in ways that are difficult to pinpoint. DSL commented that it was an "easy" tuner to test in that it consistently delivered positive, unequivocal results in areas where many tuners generate what you might call yes-buts. (E.g.: "Yes, but if you tune it slightly high by the meter, stereo distortion drops a hair.") Selectivity, for instance, measured the same whether the upper or the lower channel was doing the interfering suggesting excellent IF-filter symmetry; normally, the two situations deliver somewhat different measurements, and the reported figure is the average be tween the two. Incidentally, the 680i's adjacent-channel selectivity, at 6 3/4 dB, proved as good (vis-à-vis competing tuner sections) as the alternate-channel figure. While a count of received channels can be misleading because propagation conditions may change between counts, it is our strong impression that we seldom have received so many so clearly.
By the relatively megalomaniac standards of recent years, Harman Kardon's flagship receiver is no blockbuster at its 60-watt rating. Its dynamic head room actually brings it to the equivalent of 85 watts per channel on music signals, but the real point is that so engaging a receiver should not be passed over for want of the more impressive power ratings that few of us really need. The priorities here are with quality rather than superabundance, coupled with a full complement of features. We welcomed the tape switching, for example (though it, like the tuner, took a little getting used to). But most important of all, it's a de sign that really works.
FM SENSITIVITY & QUIETING
-------- Mono sensitivity (for 50-dB noise suppression) 14 1/4 dBf at 98 MHz
Muting threshold 18 to 29 dBf (adjustable)
Stereo threshold 18 dBf
Stereo S/N ratio (at 65 dBf) 67 1/2 dB
Mono S/N ratio (at 65 dBf) 75 dB
CAPTURE RATIO 1 1/2 dB
ALTERNATE-CHANNEL SELECTIVITY 64 dB
HARMONIC DISTORTION (THD + N) stereo mono at 100 Hz
0.16% 0.22% at 1 kHz
0.068% 0.093% at 6 kHz
0.19% 0.13%
STEREO PILOT INTERMODULATION 0.10%
IM DISTORTION (mono) 0.78%
AM SUPPRESSION 63 dB
PILOT (19 kHz)
SUPPRESSION 58 1/2 dB
SUBCARRIER (38 kHz) SUPPR. >90 dB
Amplifier section
RATED POWER 17.25 dBW (60 watts)/channel
OUTPUT AT CLIPPING (both channels driven) into 8 ohms 18 3/4 dBW (75 watts)/channel into 4 ohms 20 1/2 dBW (112 watts)/channel into 16 ohms 16 1/2 dBW (45 watts)/channel
DYNAMIC HEADROOM (8 ohms) 1 1/2 dB
HARMONIC DISTORTION (THD; 20 Hz to 20 kHz) at 17 3/4 dBW (60 watts) 0.025% at 0 dBW (1 watt) 5_ 0.023%
FREQUENCY RESPONSE + 0,-1/4dB, < 10 Hz to 46 kHz;-3 dB at 245 kHz
RIAA EQUALIZATION ± < 1/ 4 dB, 20 Hz to 20 kHz;-0 dB at 5 Hz
INPUT CHARACTERISTICS (re 0 dBW; A-weighting) sensitivity
S/N ratio phono 0.30 mV 74 3/4 dB aux 18 mV 78 1/4 dB
PHONO OVERLOAD (at 1 kHz) 245 mV
PHONO IMPEDANCE 47k ohms; 140 pF
DAMPING FACTOR (at 50 Hz) 70
HIGH FILTER-3 dB at 6 kHz; dB/oct.
INFRASONIC FILTER -3 dB at 15 Hz; dB/oct.
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(High Fidelity, Jul. 1981)
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