Kenwood C-1 / Kenwood M-1 Preamplifier and Power amplifier Review

Kenwood's Basic C-1 preamplifier and Basic M-1 power amplifier are part of its “Audio Purist” series of high-fidelity components. The C-1 and M-1 are matched in size, appearance, and function, forming a very high-quality amplifier system of moderate cost.

The C-1 preamplifier, 17-5/16 inches wide, 12-7/8 inches deep, and about 3 inches high, weighs 9-1/2 pounds. The M-1 power amplifier, the same width, almost exactly the same depth, and 4-7/16 inches high, weighs 20 pounds. The units are finished completely in flat black, with controls of the same color.

The C-1 has inputs for two high-level sources (aux and tuner) and a single phono cartridge with front-panel switching for either a moving-magnet (MM) or a moving-coil (MC) type. There are input and output facilities for two tape decks, and selecting both simultaneously connects them for dubbing from deck A to deck B. All signal selection is by means of narrow rectangular pushbuttons (mechanically interlocked) with red LED’s above them to show when they are engaged. A smaller button turns on the subsonic filter, which rolls off the response at 6 dB per octave below 18 Hz. The power switch is a large square button at the left of the panel.

The other front-panel controls of the C-1 are a large volume control, small center-de-tented knobs for bass and treble tone controls and the balance control, and a loudness knob. At the loudness knob’s farthest counterclockwise setting the loudness compensation is off, and turning the knob progressively clockwise adds a low-frequency response boost. Unlike most loudness controls, this one acts independently of the volume setting. On the rear of the C-1 are phono jacks for the various signal inputs and outputs and for the two preamplifier output jacks. There are three a.c. convenience outlets, one of them unswitched. There is no headphone output. The performance specifications of the C-1 are impressive, especially in view of its modest price. They include distortion of less than 0.005 per cent at a 1-volt output from any signal source and a frequency response down by 3 dB at 1 Hz and 250 kHz. The rated output is 1 volt, with a maximum output of 5 volts before clipping sets in.

The companion M-1 power amplifier carries a rating of 105 watts output into 8-ohm loads from 20 to 20,000 Hz with no more than 0.005 per cent total harmonic distortion. Although the M-1 has no formal power rating for lower load impedances, Kenwood does specify its 1,000-Hz clipping-power and dynamic-power outputs into both 8 and 4 ohms, as well as the corresponding headroom ratings. The dynamic-output rating of 210 watts per channel into 4 ohms implies a good current-delivery capability. Like the preamplifier, the M-1 is a wideband design. Its response is rated at — 3 dB at 1 Hz and 300 kHz, with a slew rate of ± 100 volts per microsecond.

The M-1 has no controls on its panel other than the square power button (with an adjacent pilot light) and two narrow pushbuttons for independent (not simultaneous) operation of two sets of speakers. It also has a front-panel headphone jack. The M-1 features the Sigma Drive circuit introduced by Kenwood a couple of years ago. The technique essentially extends the negative-feed-back path of the amplifier to the speaker terminals themselves instead of only to the amplifier output terminals, as is usually the case. This allows the negative feedback to compensate for any losses or other undesired effects in a long speaker cable, and it requires either a special four-wire cable or the use of two wire pairs per speaker.

On the rear of the M-1 are large insulated binding posts for the speaker outputs (including an extra pair of sigma sensor terminals for each of the speaker-A outputs), the input phono jacks, and three a.c. outlets, one of them switched. The Sigma Drive circuit operates only on the speaker-A outputs, and the speaker-B terminals are two sets of conventional binding posts.

The suggested retail prices of the Kenwood C-1 and M-1 are $225 and $330, respectively.

Laboratory Measurements

The C-1 preamplifier had a frequency response flat within 0.5 dB overall from 20 to 20,000 Hz, and the subsonic filter reduced the 20-Hz output by about 1.5 dB. Its 6-dB-per-octave rolloff rate is too slow to be very effective at reducing infrasonic signals, however. The tone controls had conventional characteristics, with a moderate control range of about ± 10 dB at the frequency extremes.

The loudness control, as specified, also had a modest effect on the overall response. At its maximum setting, the maximum boost below 100 Hz was only 8 dB. We noted that the turnover frequency of the loudness compensator was approximately the same as that of the bass tone control, making the two somewhat redundant (except in the unlikely case of using both simultaneously). The loudness compensation did not affect the high-frequency response of the amplifier.

The RIAA phono equalization was perfectly flat from 100 to 20,000 Hz, rising only 1.5 dB at 20 Hz. The phono (MM) input termination was 47,000 ohms in parallel with 30 picofarads, an unusually low value that makes the C-1 compatible with virtually any combination of cartridge and tone-arm wiring. The phono preamplifier overloaded at a relatively high input level of 200 to 217 millivolts (mV), depending on the frequency, over the entire audio range. The input sensitivity for a reference output of 0.5 volts was 84 mV for the high-level inputs and 1.4 mV for the phono (MM) input. The sensitivity of the phono (MC) input was approximately 20 dB higher, or 0.12 mV. The A-weighted noise level, for any input, was below our 100-microvolt measurement “floor” (-76 dB referred to 0.5 volt). The isolation between inputs of the C-1 was total, with no cross-coupling detectable, even at 20,000 Hz, down to —140 dB.

The distortion of the Kenwood C-1, at most frequencies and output levels, was nearly unmeasurable. From 0.0003 per cent at 1 volt output (1,000 Hz), it rose smoothly to 0.0008 per cent at 4 volts and 0.0028 per cent at 5.7 volts, just before the output clipped at 5.75 volts. (All measurements were made with a standard IHF load of 10.000 ohms in parallel with 1,000 picofarads.) At a 1-volt output, the distortion was 0.0013 per cent at 20 Hz, 0.0003 per cent at 1.000 Hz, and 0.007 per cent at 20,000 Hz. At 5 volts it was slightly higher, 0.0045, 0.001, and 0.022 per cent, respectively.

During its hour of preconditioning at one-third power (35 watts), the M-1 power amplifier became only slightly warm over the heat-sink ventilating slots. Later, our high-power tests, especially with 4-ohm or lower load impedances, caused the top of the amplifier to become quite hot, although its thermal protection system never shut it down. In normal use, its top plate became only slightly warm.

We made all our measurements on the M-1 at its speaker-B outputs (without Sigma Drive). With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 132 watts per channel, for a clipping-headroom rating of 1 dB. With 4-ohm loads, the clipping power was 177 watts (it is specified as 160 watts). Although the M-1 is not rated for driving 2-ohm loads, we also tested it in that way and found that the 6-ampere power-line fuse blew before we reached clipping (it is an internal fuse, not intended to be user-replaceable). Dynamic power measurements produced maximum outputs of 178 watts and 284 watts for 8- and 4-ohm loads, respectively (the ratings are 170 and 210 watts). Thus, the 8-ohm dynamic-headroom rating is a very good 2.3 dB. We also found that the M-l delivered a considerable output (200 watts) into 2 ohms under dynamic-test conditions.

The M-1 was stable with reactive loads, although we observed a tendency for its protective relay to shut it down momentarily when we drove a 10,000-Hz square wave into a load whose impedance fell to about 2 ohms at about 70 kHz. This is highly unlikely to occur with any reasonable combination of speaker load and input signal. The input sensitivity of the M-1 (for a reference output of 1 watt) was 100 millivolts. The output noise was less than our measurement floor of 100 microvolts, or —90 dB referred to 1 watt. The frequency response of the amplifier was flat from 10 to 20,000 Hz, falling to —0.3 dB at 5 and 50,000 Hz, and to — 4 dB at 500,000 Hz (our upper measurement limit). The slew factor exceeded our measurement limit of 25.

The distortion of the M-1, like that of the C-1, was difficult to measure. At 1,000 Hz, it was 0.00045 per cent at 1 watt, 0.0034 per cent at the rated 105 watts, and 0.0045 per cent at 120 watts. With 4-ohm loads, the readings were very similar, 0.00045 per cent at most listening levels, 0.0025 per cent at 120 watts, and 0.0045 per cent at 160 watts. The IHF intermodulation distortion with test signals of 18 and 19 kHz was —82 dB for second-order (1,000-Hz) components and —69 dB for third-order (17- and 20-kHz) components referred to 105 watts.

Comment

Our measurements on the Kenwood Audio Purist Basic components speak for themselves. For all practical purposes (and most impractical ones!) they are distortionless and noiseless. The C-1 preamplifier offers all the control flexibility most of us will ever need, yet it is simple to use and free of any “bugs” that we could find. The combination of the C-1 and the M-1 power amplifier was audibly completely noise-free at maximum volume on any input except phono (MC); the high gain of this input produced an audible hiss at maximum volume, but the amplifier was totally quiet at any normally usable control setting.

Naturally, these amplifiers have no sound of their own. We did not attempt to use them with Sigma Drive, but we have previously tested Kenwood amplifiers having that feature and have found no effects, good or bad, from its use. The ability of the M-1 to drive almost any speaker load under program conditions is a strong plus (the blown fuse during its high-power steady-state operation into 2 ohms is a sign of the effective protection designed into the amplifier).

It is satisfying to find this caliber of performance at what must be considered very moderate prices in today’s market. At higher prices one can probably find more or different control features or added signal-processing flexibility as well as higher power (though we doubt that many people will find the power capabilities of the M-1 inadequate). We are quite certain, however, that more money will not buy better-sounding amplifiers.

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