Preparation supervised by: Robert Long, Peter Dobbin, and Edward J. Foster.
Laboratory data (unless otherwise noted) supplied by: CBS Technology Center or Diversified Science Laboratories.
World's First "Holographic" Preamp
New Measurement Standards: In making comparisons between current reports and those published in the past, readers are cautioned to pay particular attention to the reference levels and similar test criteria cited. S/N ratios for electronics, in particular, are measured very differently now that we have adopted salient features of the new IHF amplifier-measurement standard. While we believe that the new technique (which also implies a saner approach to loading of all inputs and outputs) will result in measurements that more perfectly reflect audible, in-use effects, they cannot be compared directly to the numbers resulting from the former, more conventional lab measurements.
Carver Model C-4000 Sonic Holography/Autocorrelation control preamplifier, in metal case. Dimensions: 19 by 6 3/8 inches (front panel), 8 1/4 inches deep plus clearance for handles and connections. AC convenience outlets: three switched, three unswitched (1,200 watts max. total). Price: 5867. Warranty: "limited," three years parts and labor.
Manufacturer:
Carver Corp., P.O. Box 664, Woodinville, Wash. 98072.
It was with a great deal of anticipation that we awaited the first production samples of Carver's original approach to more believable sound reproduction, the C-4000 preamp. We had heard prototypes as far back as the fall of 1978 and had been very much impressed. But how would the manufactured product, the one the public could buy, behave?
The Carver preamp is the sum of more parts than any model within memory. It has all the usual control and preamplification functions (though with a few twists). It includes autocorrelator and peak-un-limiter circuitry similar to that in the Phase Linear Model 2000 preamp (a product of Bob Carver's tenure there, before he founded Carver Corp.) and later offered separately in its Model 1000 (see: "The Many Paths to Noise Reduction," HF, March 1976). And there is an ambience simulation system that includes both line outputs for the delayed signals Ito be amplified and reproduced on a back speaker pair) and three sets of speaker terminals from a built-in low-power (11 1/2 dBW, or 14 watts, per channel at clipping) back amplifier: the normal left and right pair plus a CENTER set that combines both signals and cal be used to redirect the ambience toward the front if, for example, you don't want to use back speakers. But the element that has created all the stir is, of course, what Carver calls the sonic hologram generator.
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Carver C-4000 preamplifier
OUTPUT AT CLIPPING 6.4 volts
HARMONIC DISTORTION (THD; 20 Hz to 20 kHz) phono input .0.031% aux input .0.047% FREQUENCY RESPONSE +0,-1/4 dB, 13 Hz to 22 kHz
RIAA EQUALIZATION phono 1 ± V. dB, 20 Hz to 20 kHz phono 2 ± <A dB, 20 Hz to 20 kHz both -11 dB at 5 Hz
INPUT CHARACTERISTICS (IHF loading; A-weighting) sensitivity S/N ratio phono 1 1.15 mV 70 dB phono 2 uS mV 71 1/4 dB aux 64 mV 83 1/2 dB
PHONO OVERLOAD (clipping at 1 kHz)
PHONO IMPEDANCE phono
phono 2 1 35 mV 48k ohms; 40/200/390 pF
48k ohms; <20 pF
INFRASONIC FILTER -3 dB at 16 Hz; ca. 18 dB/oct.
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While he isn't saying exactly what the holographic circuitry does or how it works, it's clear that the fundamental intent is to create acoustic wave fronts at the listener that-at least in some key respects-simulate those that would be present had the recording been made binaurally (not in stereo) and reproduced on headphones (not speakers). In our review of the Sennheiser dummy-head binaural recording setup (January 1975) we exclaimed over the spatial verisimilitude it created: Recorded sounds seemed to come from specific spots within the listening room-or, rather, the simulated aural environment around the headphone listener--with such palpability that even good stereo imaging seemed blurred and diffuse by comparison. A detailed comparison of stereo and binaural recordings of a given source would turn up not only differences in absolute response, but extremely complex differences in interchannel phase and amplitude relationships. Subjectively, the basic difference is that, with headphone reproduction of the binaural recording, the listener has the feeling of being transported to a specific spot in the recording venue, while the stereo/loudspeaker version transports the recording venue, with less exactitude, into the listening room. The holographic generator is so called as a metaphor based on two similarities to visible holograms: It works with degrees of cancellation and reinforcement due to phase and amplitude differences in two related and "interfering" information channels to create its effect, and it produces a kind of three-dimensional illusion unavailable with more conventional reproduction techniques.
Carver says that at least six hours of tuning the system (including the room) are required for really good holographic imaging. Among basic considerations that may influence the process are the type and placement of the loudspeakers, the size and acoustics of the listening room, and the setting of the preamp controls. The latter include the holographic generator's on/off switch in the separate group of six pushbuttons, the THEORETICAL/NORMAL (essentially, two-mike/multimike-plus mixdown) switch just below the main selector, a NORMAL/NARROW switch (whose latter position compensates electrically for restricted inter-speaker spacing) on the back panel, and compensation for a room-dependent apparent increase in bass that may occur when the holographic generator is turned on. In general, it appears, the smaller the room-and, in particular, the lower its ceiling-the greater the required bass cut. Here three options are available in the left-hand row of pushbuttons. The -2 dB TRIM button introduces a bass shelf that drops (actually by about 4 dB) as frequency descends between 4 kHz and 300 Hz. The bass control itself has two modes seemingly named after their boosting action: The 40-Hz position reaches its maximum boost (up to about 15 dB) only at and below that frequency; LOUDNESS moves the hinge frequency upward by approximately one octave and does, indeed, produce better subjective loudness compensation than the 40-Hz mode. But as the bass controls are rotated counterclockwise past their "flat" positions, their character changes abruptly to shelving controls with higher turnover frequencies and more limited (6 dB maximum) range.
We began with a setup that already met several of Carver's desiderata:
a fairly large room with a moderately high ceiling in which the floor-standing speakers are at least four feet from the side walls and from that behind them and are angled inward toward the listeners' seating. We were aided as well by previous experience of the holographic effect. When we followed the manual's explicit instructions for initial setup, we found that the room acoustics already were acceptable for the purpose, without applying the sound-treatment measures it suggests if nasty reflections--which, it says, can hamper holographic imaging should show up at this stage. Thus, in well under Carver's six-hour time span, we came up with sonic holography that closely approximated what we had heard in his demonstrations and that was little improved by further tweaking.
The ambience system also involves choices and, in a sense, fine tuning.
Next to the TIME DELAY on/off is a button that sets the basic delay time: 50 or 80 milliseconds. In the group of four knobs to the left of these pushbuttons are controls for the level of the time-delay output (either to its line-level jacks or to the amplified speaker terminals) and for echo density, or what is termed "regeneration" on some ambience units. As with any ambience device we've worked with, the success of the effect depends in large part on your ability to tune it to the ambience already present in the signal you are seeking to enhance and on keeping the magnitude of the enhancement to no more than bare perceptibility. Turning everything all the way up is patently foolish with any such device and delivers an offensive, spring-like reverb sound with the Carver circuit. While it evidently is intended as a further enhancement of the holographic circuitry, it can be used with normal stereo; when we did so, we considered its effect madly pleasant after careful adjustment.
The peak un-limiter and autocorrelator can be switched in individually.
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Equipment reports are based on laboratory measurements and controlled listening tests. Unless otherwise noted, test data and measurements are obtained by CBS Technology Center, a division of Columbia Broadcasting System, Inc., and Diversified Science Laboratories. The choice of equipment to be tested rests with the editors of HIGH FIDELITY. Samples normally are supplied on loan from the manufacturer. Manufacturers are not permit ted to read reports in advance of publication, and no re port, or portion thereof, may be reproduced for any purpose or in any form without written permission of the publisher. All reports should be construed as applying to the specific samples tested; HIGH FIDELITY, CBS Technology Center, and Diversified Science Laboratories assume no responsibility for product performance or quality.
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The un-limiter (which, in essence, provides some upward expansion of signal levels above a manually set threshold) is like Phase Linear's: capable of providing a little more "punch" if judiciously used but also of intrusively arbitrary expansion depending on the nature of the incoming signal and the threshold setting. The autocorrelator is said to be more complex and "gentler"-that is, subtler in its action, with less maximum noise filtering and less chance of unwanted side effects-than its already gentle forebear, and direct comparison confirms this. Hiss and rumble fade noticeably when it is turned on and have less tendency to reappear in time to the music and, therefore, to sound like extramusical noises such as over-blowing in flute or pedal action in piano. We sometimes preferred our manual settings to the automatic-threshold mode, which is new in this incarnation.
Both on the Diversified Science Laboratories test bench and in our listening room, the basic functions of the preamp are impressive. The design of the two phono preamps (note that each has full circuitry, though they cannot be used simultaneously to dub one record while you listen to another) is not identical, even ignoring the back-panel switch for capacitive loading on PHONO 1. PHONO 2 proved a hair quieter (by up to 2 dB, depending on which functions were switched in during the test) and a split-hair more accurate (within 1/10, rather than 1/4 dB) in RIAA compensation, but some capacitance-sensitive pickups may give PHONO 1 the edge in sound quality. Distortion, while not super-low, was below our threshold of audibility and could not be distinguished even in A/B tests with a model measuring considerably "better." But the overriding question raised by the unit is how its holographic generator sounds, since this is its unique claim to fame. Our answer: Terrific. With the system set up right and the listener ensconced in the preferred position, the stereo image-even with recordings that, because they are multimiked, depart from Carver's theoretical ideal-generally is crystalline in a way that almost beggars normal stereo reproduction. If that norm can be likened to a curtain of sound extending between the two speakers, the holographic generator seems to open the curtain and reveal a deployment of musical forces extending behind, between, and beyond the speakers, which then appear to be somewhere within the fiddles (left) and cellos (right) of the standard symphonic setup. If the time delay is also turned on, some of the auditorium walls as well as the stage of the presumed concert hall seem to be carried into the listening room, though we find this illusion far less convincing or desirable than that provided by the Carver holography. And, to carry the simile one step further, turning on the autocorrelator is like lifting a thin residual scrim of noise from between the auditor and the stage.
Even peripatetic listeners (like most of us at HO will find that sonic holography offers imaging advantages; though the effect is admittedly best in axial listening-where, among other things, bass anomalies due to standing waves are minimized in the setup-much of its quality holds up as you move about the room.
Those who consider the use of stereo equipment to be essentially a spectator sport may, however, be less inclined to enthusiasm. Not only does sonic holography (at least in its initial form) require an activist approach if you are to get it right, but so do the time-delay, autocorrelator, and peak-un-limiter elements of the design. And the formidably complex front panel does not encourage use by the uninvolved.
Simpler formats may follow; for the time being, sonic holography is a glorious hobbyists' toy with a promising future, and the C-4000 is the most fascinating hands-on preamp we know of.
(High Fidelity, Jan. 1980)
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