Carver C-1 and M-500 Preamplifier and Power amplifier Review

The Carver C-1 Sonic Holography preamplifier is basically a conventional high-quality control center, but its nomenclature gives a clue to its major point of difference from any competitive products: it incorporates the latest form of the signal-processing circuit introduced by Carver in the C-4000 control console. Omitted from the C-1 are the “Autocorrelation” noise-reduction and peak-unlimiter features, as well as the time-delay system and its power amplifiers (all part of the C-4000).

The C-1, in addition to being much more compact than the C-4000, is considerably less expensive. Like the C-4000, it has separate bass and treble tone controls for each channel, with switchable turnover frequencies (8 or 2 kHz for the treble, 40 Hz or "loudness” for the bass). The control contours have been designed to provide maximum utility to the listener, with a limited amplitude of control range and asymmetrical boost and cut characteristics. The Carver C-1 is 19 inches wide, 3-1/2 inches high, and 10 inches deep. It is available in two versions: in an all-black finish with the panel edges slotted for rack mounting and in a pale-gold finish without the rack-mounting provision (both units have handles). The C-1 weighs 6 pounds.

In addition to the four tone-control knobs on the front panel, there is a headphone jack (driven by its own amplifier), an input-selector knob, a small balance knob, and a large volume control. The program sources include two magnetic-phono inputs (phono 1 for conventional moving-magnet or moving-iron cartridges, and phono 2 with 25 dB additional gain for moving-coil cartridges). The phono 1 input impedance is 47,000 ohms, with additional amounts of input capacitance (0, 180, or 390 picofarads) selectable by a slide switch in the rear of the unit. This makes it easy to match the input of the preamplifier to almost any phono cartridge. The phono 2 input is terminated by 39 ohms. The other input sources are identified as tuner, aux 1, and aux 2.

The other front-panel controls are small rectangular pushbuttons. A group of six controls the Sonic Hologram, silences the speakers for headphone-only listening, and switches the power to the C-1. The Sonic Hologram controls, besides turning the circuit on and off, select different internal operating conditions whose significance is explained in the instruction manual.

The other front-panel buttons form a row below the tone-control knobs. They include tone bypass, low- and high-frequency turnover selectors, two tape monitor switches, and two dub switches that cross-connect the tape decks for copying from either one to the other. There is also an external processor button that can be used to put a signal-processing accessory (equalizer, expander, etc.) in the signal path.

On the rear of the C-1 there are phono jacks for all signal inputs and outputs, including two pairs of main outputs. In addition to the switchable phono capacitance, there is an infrasonic filter button that inserts a 12-dB-per-octave rolloff below 15 Hz in the phono-preamplifier section. Three of the six a.c. convenience outlets are switched, with a total maximum rating of 500 watts. Price: $500.

Another Carver product (an ideal companion to the C-1, though not related to it) is the M-500 Magnetic Field power amplifier. Employing the same basic design as the cube-shaped M-400 amplifier, it is styled more conventionally, with a front panel 17-3/8 inches wide, 4-1/4 inches high, and 13-1/2 inches deep. Optional rack-mount adaptors are available, and these increase the width of the panel to 19 inches. The M-500 weighs 23 pounds, more than twice as much as the M-400, but that is still very light for an amplifier of its power rating (about 25 per cent greater than that of the M-400).

Dominant on the all-black front panel are the two output-level meters calibrated from — 40 to +3 dB (0 dB being the rated power output of 250 watts into 8 ohms). The meters are driven by a fast-acting peak-following circuit that produces an accurate and easily read power display. The M-500 has no operating controls.

The power rating of the Carver M-500 is 250 watts per channel from 20 to 20,000 Hz into 8-ohm loads with no more than 0.05 per cent total harmonic distortion. The 1HF A-weighted output noise is rated at better than 100 dB below rated power. As in the M-400, the output stages of the M-500 operate out of phase so that the two speakers cannot share a common ground. The same feature makes it very easy to bridge the two channels to make a 600-watt mono amplifier when driving an 8-ohm load; it is only necessary to connect the load between the two “hot” speaker binding posts in the rear and drive the inputs in parallel (through a “Y” connector). A small slide switch in the rear varies the brightness of the meter.

Unlike the M-400, the Carver M-500 has separate heat sinks for its output transistors as well as a larger mass and radiating surface to dissipate their heat. Although this has little effect on the maximum power output for normal listening, it enables the amplifier to deliver higher power for a much longer time without overheating to the point of being shut down (through overheating) by its comprehensive protective circuits. Those circuits have been extensively revised (compared to those of the M-400), but we have no detailed information on them at this time.

As might be expected, the inside layout of the M-500 is much more open than that of the M-400, and its encased “magnetic-field coil” (taking the place of the usual power transformer) is less prone to emit the occasional buzzes and groans that were sometimes heard from the unencased coil of the M-400. The price of the Carver M-500 is $550. It carries a three-year warranty, including parts, labor, and shipping costs one way.

Laboratory Measurements

The Carver C-1 preamplifier required 55 millivolts (mV) at its high-level inputs for a reference output of 0.5 volt. The phono i sensitivity was 0.9 mV (phono 2 was not measured). The A-weighted output noise from any input was less than our measurement limit of 100 microvolts, or 74 dB below 0.5 volt. The phono preamplifier clipped with 93 millivolts input at 1,000 Hz, and the equivalent clipping voltages at 20 and 20,000 Hz were 90.5 and 81.6 mV, respectively. The phono i input impedance was 46,000 ohms in parallel with a capacitance of 20, 175, or 370 pF, depending on the setting of the switch in the rear of the unit. The output waveform from the preamplifier clipped at 5.5 volts.

The preamplifier frequency response measured through the high-level inputs was down 0.4 dB at 5 Hz and 0.5 dB at 20,000 Hz, with a — 3-dB point of 50,000 Hz. The infrasonic filter reduced the response at 20 Hz by only 0.3 dB, but it was down 2.9 dB at 15 Hz and 21.2 dB at 5 Hz. As claimed, the tone-control response curves were highly asymmetrical, with a maximum low-frequency boost of 17.5 dB but a cut of only 5 dB. At high frequencies, the maximum boost was 8 to 12 dB at 20,000 Hz, with a maximum cut of about 5 dB. The turnover-frequency selector buttons had the stated effect. The loudness position of the bass button enables the bass tone control to give approximately correct loudness compensation curves unaffected by the volume-control setting. The 40-Hz turnover actually gives a sliding turnover frequency affecting the response below 200 to 400 Hz near the maximum boost setting. The tone controls in their cut settings provided a flat shelf-shaped curve, with each control affecting almost half the audio bandwidth.

The RIAA phono equalization was flat within ±0.5 dB from 50 to 20,000 Hz, and it was +1.5 dB at 20 Hz. It was barely affected by the inductance of typical phono cartridges, which boosted the output above 12,000 Hz to about +1 dB at 17,000 Hz and +2 dB at 20,000 Hz.

The harmonic distortion of the C-1 was extremely low—well under 0.01 per cent for outputs up to 3 volts and at frequencies up to 10,000 Hz or even higher. It reached a maximum at 20,000 Hz, where we measured 0.013 per cent at 2 volts and 0.018 per cent at 3 volts. Although we measured the effect of the Sonic Hologram circuit on frequency response, it can be properly evaluated only through critical listening.

The Carver M-500 power amplifier is, by any standards, impressive. Although its top cover (over the heat sinks) became very hot during the preconditioning period and even hotter during full-power tests, the amplifier was able to withstand this treatment without the excessively frequent shutdowns (by its protective system) that made testing the earlier M-400 such a frustrating experience.

With both channels driven to clipping level, even into 8-ohm loads, the 15-ampere line fuse of the amplifier blew (our 15-ampere bench fuses would certainly have blown if the amplifier fuse had not blown first). As we noted in reviewing the M-400, it is characteristic of the Carver “magnetic-field” amplifier design that it draws very large currents from the power line at high power outputs. Since this current is largely 90 degrees out of phase with the voltage, it does not represent power consumption (the kind you pay for!), but it can blow fuses very easily. Because of this, we were forced to depart from our regular test procedure and complete our high-power tests with only one channel driven.

An input of 97 mV drove the amplifier to a reference output of 1 watt, and its A-weighted noise output was 86 dB below that level. The 1,000-Hz clipping power into 8 ohms was 288 watts. Into 4 ohms it was 400 watts, but with 2-ohm loads the protective relay shut the amplifier down at 85 watts. The 8-ohm IHF clipping headroom was 0.61 dB. Using the 20-millisecond tone-burst signal of the IHF dynamic-power test, we measured a maximum output of 290 watts into 8 ohms, 403 watts into 4 ohms, and 142 watts into 2 ohms, giving an 8-ohm IHF dynamic-headroom rating of 0.64 dB. (In a conversation subsequent to our test, Carver stated that the amplifier did have a reasonably large headroom, but the frequency of the test signal we used did not disclose it.)

The 1,000-Hz harmonic distortion into 8-ohm loads was less than 0.0004 per cent up to 10 watts, rising smoothly to 0.0014 per cent at 100 watts, 0.002 per cent at 200 watts, and 0.003 per cent at 280 watts. It was generally similar when driving 4-ohm loads, the distortion rising from 0.0003 per cent (our test-equipment residual) at 10 watts to 0.0022 per cent at 300 watts. Although the amplifier was obviously not meant to drive 2-ohm loads, even under that severe condition the distortion was only 0.0018 per cent at 1 watt, 0.0066 per cent at 50 watts, and 0.4 per cent at the clipping point of 85 watts.

At rated power, the distortion of the M-500 was typically between 0.002 and 0.01 per cent from 20 to 5,000 Hz, rising to 0.037 per cent at 15,000 Hz and 0.11 per cent at 20,000 Hz. At half power and one-tenth rated power, the characteristic was similar but with lower distortion readings (respectively, 0.026 and 0.013 per cent at 20.000 Hz). It should be noted that the 0.11 per cent distortion reading at full power and 20.000 Hz was made with a null-type distortion analyzer and includes a number of low-level distortion and noise components above the audio-frequency range. The other readings were made with a spectrum analyzer that allowed us to measure only the harmonic distortion components. Since we noted that the full-power, high-frequency operation of the amplifier generated a number of ultrasonic components, we found it more convenient to make a “THD + N” measurement at that point.

The actual high-frequency linearity of the M-500 was very good, as shown by its IHF intermodulation-distortion measurement with 19,000- and 20,000-Hz signals. The third- and fifth-order distortion products were, respectively, 90 and 94 dB below 250 watts, and the second-order (1,000-Hz) distortion was completely undetectable down to our measurement “floor” of —95 dB. The IHF slew factor was 2.5 because the amplifier’s protective relay shut it off at 50 kHz when it was driven by a “full-power” input signal. The low-level frequency response was flat from 10 to 1,000 Hz, down 0.2 dB at 20,000 Hz, and reached —3 dB just below 100 kHz.

Although it has no bearing on the performance of the amplifier, we also checked the accuracy of the power meters (on most amplifiers a cosmetic feature of dubious accuracy). Both meters were essentially alike and surprisingly accurate, reading within 10 per cent of the actual power output delivered to an 8-ohm load from their minimum reading of 0.01 watt to a full 200 watts or more. At least as important was the rapid response of the meters, which read correctly on 20-millisecond tone bursts with a 0.5-second repetition period. Their decay time was relatively slow, so the meters gave a constant and informative display of the amplifier’s power-output variation with complex-program waveforms.

Comment

The Carver C-1 offers the audiophile an opportunity to experience the effects of the Sonic Hologram at about half the price of the C-4000 control center by sacrificing a few signal-processing abilities (which might well be duplicated elsewhere in a well-equipped system). Its general signal-modification abilities (the tone controls and infrasonic filter) are relatively subtle though highly effective in comparison with the sometimes heavy-handed approach taken by the designers of some amplifiers and receivers. Used in moderation, they can adjust the perceived frequency response and balance of a music system to one’s taste with little risk of doing violence to the overall integrity of the sound.

In essence, the Sonic Hologram operates by cross-feeding a portion of each channel’s program to the other, with appropriate time delays and phase shifting, in such a manner that (ideally) each ear hears only the contribution of its corresponding speaker. This is not unlike the situation existing during headphone listening, although the subjective effect is completely different. Essentially, it takes the form of a widening of the apparent sound source so that it extends beyond the spatial limits set by the speakers (both laterally and in depth) and in some cases causes the sound to wrap around the sides of the room and extend well toward the rear.

The effect is both impressive and exciting to experience, but it is obtainable with full effect only at the price of extremely critical speaker location and listener positioning. In its original form it could be heard properly by only a single person at a time, although a diluted effect was present over much of the room. The “second-generation” system used in the C-1 (and in current models of the C-4000) is not quite so critical with respect to listener location, but it still requires careful setup to obtain best results. We also noted that the tendency toward bass heaviness in the early Sonic Hologram has been effectively eliminated.

The instruction manual for the Carver C-1 presents a most informative discussion and explanation of the Sonic Hologram. Although the procedure to be followed in setting up the system may seem discouragingly complex, it is well worth the effort! Aside from this feature, the C-1 is a superb preamplifier and control center, priced quite competitively even without consideration of its special features. In view of the growing popularity of moving-coil phono cartridges, the ability to use one without a sizable investment in a separate head amplifier or transformer is a strong “plus.”

The M-500 power amplifier carries on the tradition of the original M-400 by offering an unprecedented number of clean watts per dollar. The earlier unit proved to be extremely rugged and reliable in our severe testing program, and the M-500 is, if anything, even better protected. It will turn itself off before any damage can occur, whether due to overdriving, insufficient load impedance, high temperatures, or any other incorrect operating condition. Even if it is played at high levels, the M-500 is one of the coolest-running amplifiers we have used.

The Magnetic Field amplifier can be considered a conventional amplifier with a very fast, smart power supply that can adjust its output voltages almost instantaneously to the requirements of the signal waveform. Since even at high home listening levels most power amplifiers are called upon only for a few watts of average power, the M-500 is virtually “loafing along” most of the time, using very little power from the a.c. line and dissipating almost no heat. When more power is required (usually for very brief periods), it is instantly “on tap” for demands of up to hundreds of watts without the usual penalty of a large, heavy, hot, and expensive amplifier most of whose capability is hardly ever used.

Although we could hear traces of buzzing from the power supply of the M-500 under certain test conditions (this is caused by vibrations of the core laminations in its magnetic-field coil), it was never in evidence in normal use, and we doubt that it would ever be heard in an operating music system.

In summary, the new Carver products reflect an evolutionary process by which a new technology is gradually applied to a variety of products, usually with cost and/or performance benefits to the consumer. They are obviously excellent values for the money, and our tests confirm that they are top performers by any standard.

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