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Kenwood KA-900

New High Speed Integrated Amplifier (1980-82)

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Kenwood KA-900


Power output: 80 watts per channel into 8Ω (stereo)

Frequency response: 20Hz to 20kHz

Total harmonic distortion: 0.005%

Damping factor: 500

Input sensitivity: 0.2mV (MC), 2.5mV (MM), 150mV (line)

Signal to noise ratio: 66dB (MC), 86dB (MM), 105dB (line)

Output: 150mV (line), 30mV (DIN)

Speaker load impedance: 4Ω to 16Ω

Dimensions: 440 x 123 x 375mm

Weight: 10kg

Year: 1980


service manual   English - MBG

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Reviewed Sep 17th, 2015 by raven31

I purchased a Kenwood KA-900 from one of our esteemed members (thank you Galdiator335/Vlad) and it just came in on Friday. The KA-900 is the next step down from the KA-1000, and is rated for 20WPC less into 8-ohms, or 80WPC. Most of its design is carried over from the KA-1000 including dual power (two secondaries on a single power transformer, with separate rectification and filtering for each channel), the Fader control, Sigma drive (explained later), and non-magnetic design. The concept behind non-magnetic design is that you use less sheet metal and more plastic to reduce distortion. How, exactly, this is supposed to work is beyond me, but the design is not without its merits. First off, the structure is very non-resonant. Tapping on the sides, top, or bottom produces a simple “thud” instead of the usual “ting”. The unit is also comparatively light, weighing in at a paltry 24-lbs; 6 less than my comparable h/k PM660. And the structure is not altogether without metal: the top cover is largely made of sheet metal as is the rear-panel , and there is a thicker gauge sheet metal skeleton within. This leaves the bottom, side panels, the forward section of the top panel, and some of the front baffle making up the “non-magnetic” design.

The styling is somewhat of a mix of 1970's “silver iron” and 1980's post modernism. The lower portion of the front panel is actually made of aluminum, with plastic knobs and buttons. Rarely used controls are hidden away behind an elegant real-glass cover. The more useful and frequently used functions (power, input, volume, and fader) are located on the top 1/3 of the front baffle. Controls are logical and well laid out. My one little gripe is that the tone controls do not have center detents; this is negated to some effect by having the controls bypassed. Turn over frequency is switchable between 200Hz and 400Hz in bass, and 3kHz and 6kHz in treble. Additionally, there is a so-called “Loudness” compensation, but it is the worst one I have ever encountered (I will comment on this below), and a DC/LC filter.

Internally, the amp is a bastard child of a good 1970's integrated amp and a 1980's discount catalogue receiver. It is obvious that the engineers and marketing/accounting probably butted heads on some things. Fortunately, all of the important parts are high quality. However, there is an abundant amount of small-gauge ribbon cable running all over, and the switches and pots are all of quite mediocre quality, much like you would find in a bargain-basement Scott or Sherwood receiver in 1985. Such things would have been unheard of in a KA-8100 but were foreshadowing the near future in the KA-900. The power transformer is on the smallish side for a quality 80WPC amp, but it is not undersized. The dual power supplies use discrete diodes to rectify the voltage and there are two banks of 2x 63VDC/10,000uFd main filter caps (one for each channel). The power amp section is almost a clone of the Mitsubishi DA-A30, retooled to fit in the KA-900 chassis. Each channel is treated to 2 pair of 2SC2837 and 2SA1116 high speed output transistors, and is cooled by a heat-pipe solution. The amplifier is fully direct coupled from any high-level input to the output, as long as the low-cut filter is disengaged. Unlike later fully direct coupled designs, there is no servo or other method of dynamically controlling DC, except a simple DC detection/output cutoff circuit. In an era of buzzwords, the KA-900 is not lacking; from “New High-Speed Amplifier” to “Sigma-Drive”, every “feature” has a trade-marked name. Sigma-drive is a little different, in that it was somewhat unusual (dare I say, novel?) and it performed a measureable function that separated it from the competition. Sigma Drive effectively incorporated the speaker cable and, to some extent, the speaker itself into the negative-feedback loop. This effectively increased the dampening factor 10-20 fold over the same design not using Sigma Drive. The results were, auspiciously, tighter, more well defined bass and midrange, with precise, extended highs. The more practical results were a little different, as I will explain below. The pre-amp section is mostly unimpressive, except for the direct coupled design. FET's are used where appropriate though. The phono preamp/EQ does deserve some attention, though. In 1980, when the amp was released, LP's and turn-tables were still the predominant source of choice for high-quality sound reproduction. This is reflected in the KA-900, which features a high-quality MM and MC pre-amp, with adjustable input impedance, and high-grade capacitors and other components. The phono preamp even gets its own board, and the user adjustable controls are set right on the board to minimize signal path length.

So, how does she sound? Not too bad at all; a little different, but not bad. The first thing I noticed was that the overall tone favored the highs, though not to the attenuation of the lower registers. I left the tone controls defeated and loudness off, but I did turn down the tweeters and mids on my zero9's to -4dB in order to even it out a bit. Bass is incredibly tight and detailed, almost forceful. Kick drums are sharp and very live sounding. Unlike many amps that I have encountered, though, it does not seem to sacrifice deep bass response for cleaner bass. The highs are very extended, without being shrill or glaring, while mids and vocals are very clear. It actually has a sound similar to that of my QSC PLX-3402 pro power amp. This is perhaps the most technically accurate amp I have ancountered yet. Imaging with this amp is barely acceptable, though, as there is little difference between Stereo and Mono, with only the overt stereo effects leaving the boundary of the speakers. This can make the unit sound a little cluttered at times. This is a trait shared with my h/k PM660, although the PM660 is a lesser offender. I mentioned the very poor loudness control before; now let me explain my dislike. It severely muddies up the bass and brings it up to about 100 (on a scale of 10), then adds some glare to the highs, just for good measure. This is a control best left in the “off” position. It is hard to understand what the thought process behind it was; it is certainly not the subtle loudness compensation of old.
I did try using the speakers without the Sigma Drive via the “B” outputs, and it does make a noticeable difference. To further test its effectiveness, I tried two, 3-meter pair of Monster Cable XP in both the Sigma and non Sigma configuration and found that aside from the perceived difference in sound, there was also a difference in how the woofers moved and how quickly they did so. Sigma drive had them starting and stopping on a dime, whereas the non-Sigma had a little bit of overhang.
I think I will have to try the KA-900 with several different speakers to find the ones it works best with. I am imagining some under-damped ported system of the Kabuki variety, but I've just started.

It has been said that the KA-800/900/1000 were Kenwood's last gasp at quality. While I don't think that it was their last one, I can see that the bean counters were beginning to win the battle with this one. Still, it is a more than decent amp, and, if you leave the loundness button alone, perhaps one of the most accurate you will find.



re: KA-900

Thank you very match.

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