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Power Output: 40 watts continuous power per channel, 8 ohm loads, from 20 Hz to 20 kHz.
THD: 0.2 percent at all power levels from 0.25 watts to rated output.
IM Distortion: 0.2 percent at rated output.
Damping Factor: 50. Input Sensitivity: Phono, 2.5 mV; Aux and tuner, 200 mV. S/N: Phono, 76 dB referred to 5 mV input; High Level Inputs, 90 dB.
Bass & Treble Control Range: ±8 dB @ 100 Hz and 10 kHz.
High Filter Cut: -6 dB @ 10 kHz.
Dimensions: 14 1/2 in. (36.8 cm) W x 5 1/2 in. (14 cm) H x 10-3/8 in. (26.35 cm) D.
Weight: 16 1/2 lb. (7.48 kg).
So much emphasis has been placed, of late, on those super-performance, super-powered integrated and basic amplifiers (with their 0.002 percent distortion figures and almost-infinite stewing rates) that we tend to overlook what, to many, are products which offer a great value for their price and, in fact, represent greater value in today's hi-fi marketplace than anything that was available in their categories just a few short years ago. Such a product is Ken wood's little integrated amplifier, Model KA-3500. If you crave 100 watts plus per channel, ne-plus-ultra measured specs, and bandwidth from d.c. to channel 5, read no further and save your pennies. But if an honest 40 watts per channel with reasonably low distortion, intelligent layout, and good indications of reliable long-term performance are what you are after and you have less than $200.00 to spend on your preamp/amp components, read on.
The most prominent control on the front panel of the KA 3500 is a giant master volume control. It is a conventional potentiometer, of course, but one which is mechanically coupled to no less than 40 "click-stop" positions for easy resetability (augmented by evenly spaced calibration marks from 0 to 10). At the upper left is a rotary speaker selector switch, with Off, A, B and A+B positions, along with Bass and Treble controls, each screened with + and dB calibration marks and fitted with 11 click-stop detent positions.
Three interlocked push-buttons at the upper right select Phono, Tuner or Aux program sources, while just below them are a pair of three-position toggle switches which take care of either of two tape monitor circuits, plus A-to-B and B-to-A tape deck dubbing. Controls at the lower left include a toggle power On/Off switch, a balance control (with an easily defined center position) and yet another three-position toggle switch with positions for loudness compensation, Off or High Filter. This combined arrangement precludes the possibility of selecting both loudness circuits and high-cut filtering at the same time.
The rear panel of the KA-3500 is equipped with the usual phono-tip input jacks for all program sources, tape record output jacks for two tape decks, plus a DIN socket for one of the two tape out/in circuits. Two sets of speaker terminals are of the thumb screw type, with polarizing notations screened nearby. Two switched and one unswitched a.c. receptacles are located as far away from the input circuits as possible, while a chassis ground terminal is positioned just below the phono input jacks.
Internal Construction and Layout
While Kenwood does not supply a schematic diagram with this amplifier, they do indicate in their literature that the power amplifier section is a direct-coupled, pure complementary circuit with an FET differential first-stage amplifier. In addition to time-delayed turn on, the amplifier is equipped with protection circuitry which Kenwood calls ASO (for Area of Safe Operation) and which the company uses in its more expensive, more powerful amplifiers and receivers.
Circuitry is essentially distributed between three p.c. modules, with the largest containing the power amplifier section. A U-shaped heat sink structure runs almost the full width of the chassis and is coupled directly to the output devices which are plugged right into the main circuit board.
Preamp-equalizer parts are on the small p.c. board up front, while tone and voltage amplifier circuitry is mounted on a third, vertically oriented module also up front. Power supply parts, mounted on the main amplifier board, include a pair of 6800 /IF filter capacitors for the required positive and negative voltage supplies of the direct-coupled output circuitry. Judging by size alone, the power transformer, well isolated from input circuits, seemed adequate for the power rating of the unit and, during the course of our tests, remained cool enough to be touched.
At mid frequencies, the KA-3500 delivered 45.3 watts per channel before reaching its rated THD level of 0.2 percent.
At 40 watts per channel output into 8 ohms, THD measured a low 0.043 percent THD and 0.075 percent IM. Distortion versus power output into 8-ohm loads is plotted in Fig. 1 and, at all power levels below rated output, THD and IM were well below 0.1 percent. The slight rise in the THD curve of Fig. 1 was occasioned more by the influence of wideband noise than by actual harmonic distortion components, good evidence that the amplifier has no cross-over or notch distortion at such low listening levels.
Were it not for high frequency power limitations, this amplifier might well have been rated at a few more watts than the 40 specified, but, as shown in the graph of Fig. 2, the unit barely delivered its rated 40 watts per channel with a test frequency of 20 kHz, and distortion climbed rapidly beyond that power level or at higher test frequencies. We measured a damping factor of 40, short of the 50 claimed by Kenwood, but certainly high enough for all practical purposes. Residual hum and noise (with the volume control at minimum) measured 92 dB below rated output, unweighted.
Frequency response measured via the Aux or tuner inputs extended from 19 Hz to 54 kHz for the-1 dB rolloff points, from 11 Hz to 65 kHz for a-3 dB drop-off. Tone control range of bass and treble controls was "graphed" using a spectrum analyzer, and the resulting scope photo is reproduced in Fig. 3. Note that Kenwood wisely "shelved" the boost characteristics of both the treble and bass controls, so that they do not increase extreme low- or high-frequency response by more than about 9 dB, even beyond the limits of the audio spectrum. Figure 4 shows the attenuation characteristic of the high-cut filter as compared with the attenuation of the treble control when the latter is set to its maximum cut position. Notice that despite the fact that the high-cut filter has a slope of only 6 dB per octave, it would be more effective in moderately reducing high-frequency scratch and noise than would the tone control, because its turnover or cut-off point is set at a higher frequency. The action of the loudness circuitry is graphed in the scope photo of Fig. 5 and involves bass boost only at progressively lower volume control settings, with no treble emphasis employed.
Phono input sensitivity measured 2.8 mV for full output and, with a signal input frequency of 1 kHz, there was no evidence of overload distortion until the amplitude of the input signal reached a high 225 millivolts. RIAA equalization was accurate to within 1 dB, with that deviation occurring at the extreme 30-Hz test frequency. At all frequencies above 50 Hz, RIAA accuracy was better than 0.5 dB. Measured S/N in phono (unweighted) was-67 dB referred to rated input sensitivity (2.5 mV). Kenwood quotes the phono S/N referred to 5.0 mV, but translated to that reference input level, we would still come up with a reading of-73 dB compared to the-76 dB claimed. Our measured figure is certainly not a poor one, but simply falls short of their claims by 3 dB.
Use and Listening Tests
We found that the KA-3500s rather simple front panel control arrangement offered as much flexibility and adjustment capability as most hi-fi listeners might require. Control action was smooth and repeatable, rivaling that of many more costly integrated amplifiers. Kenwood was wise in sacrificing power bandwidth at the high end of the spectrum, if they had to sacrifice anything, for they have retained good tight bass all the way down to below audible frequencies, and that honest power reserve at the low end makes the amplifier suitable for use with a variety of medium-to-high efficiency speakers, some of which might be expected to require more nominal power to produce clean, loud levels. As we mentioned at the outset, all of us tend to overlook some of the inexpensive products that still abound in this rapidly expanding industry. It is nice to occasionally come back down to earth and take a good hard look at a low-priced integrated amplifier that can be counted on to form the control and amplifying center for a very respectable, yet low-cost high fidelity component system.
(Source: Audio magazine, Dec. 1976)
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