STEREOPHILE, MAY 1995
Solid-state stereo power amplifier. Power output: 300W into 8 ohms (24.8dBW), 600W into 4 ohms (24.8dBW), 1200W into 2 ohms (24 8dBW}. frequency response: 20Hz-20kHz +/-O.5dB. Full power bandwidth: 10Hz-20kHz. Chanel separtion: > 60dB at 1kHz. Damping factor >200, equivalent to an output impeince of less than 0.04 ohms. THD: <0> 50V/us. Output impedance: <0.04 ohms. Input sensitiviy: low gain, 1.74V RMS; high gain, 430mV RMS. Input impedance: 50k ohms. Power consumption per side: 60W at idle, 250W with musical program, 1440W nominal continuous. Dimensions: 19" W by 6.9" H by 17.8" D with handles. Weight 72 lbs (net). Warranty: 5 years, parts and labor. Serial number of unit reviewed: 947202005. Price: $3995. Approximate number of dealers: 25. Manufacturer Carver Corporation, 20121 48th Aw. W., Lynnwood, WA 913036 Tel: (206) 77S-6245. Fax: (206) 775-9180. When it comes to amplifiers, ya gotcher tubes, yer solid-states, and yer hybrids. Although amplifier manufacturers would have you believe otherwise, the majority of designs within each category are variations on a few fairly-well-known themes. Everyone agrees that the power supply is extremely important. Most designers try to obtain the amplifier's desired frequency response and distortion characteristics with a minimum of negative feedback. It's also agreed&emdash;at least among designers of solid-state amps&emdash;that the ability to drive a variety of speakers, including those that present a low-impedance and/or reactive load, is an important priority.
In the spirit of high-fidelity, the amplifier's output should bear a close resemblance to the input signal ("straight wire with gain"), and to whatever extent the output is different from the input, the difference should be in a musically pleasing direction. Once priorities are set&emdash; and decisions have been made about such practical matters as cost, weight, size, heat dissipation, power consumption, etc.&emdash;designing an amplifier is often little more than applying tried-and-true principles and techniques. With careful attention paid to such factors as parts selection, circuit layout, and vibration control, such amplifiers can sound quite good; but to the audiophile who wants something different, they're a bit, well, boring. As Miss Peggy Lee put it, "Is That All There Is?"
I first found out about the Carver Research Lightstar Reference amplifier while reading Peter W. Mitchell's 1994 ACES Report (Vol.17 No.4). Judging by Peter's description, the Lightstar was not your standard could-have- been-designed-by-a-computer amplifier. "A remarkably clever, innovative, and cost-effective design that promises to deliver huge power levels to any load ' quoth Mr. M. At Stereophile's Hi-Fi '94 High-End Show in Miami last April. I had a chance to listen to a system featuring the Lightstar, and 1, too, came away impressed with its potential. I was keen to check out how it would perform in my own system.
DESCRIPTION & DESIGN
To those familiar with the built-to-a-price offerings from the Carver Corporation, examining the Lightstar will be a surprise. It's heavy, and very well-finished&emdash;almost to a Krell/Mark Levinson standard. The 3/8"'-thick black- anodized aluminum chassis, which also functions as a heatsink, has a "wraparound" design, with no visible screws or bolts on the top or sides. The Lightstar's front panel is dominated by two illuminated power meters; at night, it looks as if you're being stared at by some sort of robot or monster with bright slits for eyes2 Whether you consider this the pinnacle of high-tech or simply gimmicky will be a matter of individual taste, but it certainly looks striking. Personally, I would have liked to have been able to turn off the power meters&emdash;or at least the lights&emdash;while listening late at night.
The Lightstar is described as dual-mono. To this end, it has two power cords and two power switches in the back. In normal use, these switches are left in the on position, and the amplifier is controlled by a single front- panel touchplate switch, which toggles from Standby to Active. The top of the amplifier chassis was somewhat warm even with the amp in the Standby mode, and became much warmer when playing music, but never to the point that I couldn't touch it comfortably.
There are balanced and single-ended inputs, two sets of high-qua]ity binding posts for bi-wire capability, and switch-selectable high-gain operation for use with a passive preamplifier or for direct connection to a CD player. I used the standard-gain mode. The Lightstar is protected in three ways: a thermal switch activates a mute circuit if the heatsink temperature gets too high; similarly, short-circuiting the output engages a mute circuit until the condition is remedied; and finally, if the rated current is exceeded for more than 10 seconds, the power switches on the rear panel, which are actually resettable magnetic circuit breakers, turn the amplifier Off. For once, I did nothing to engage any of these protective devices!
So, what's so special about the Light-star's design? Carver's literature on the Lightstar states that the design team had two basic goals: first, to restructure the signal path for a more neutral amplification ofthe audio signal; and second, to develop a high-current power supply capable of driving virtually any loudspeaker. They also wanted the amplifier to be impervious to AC-line noise, spikes, and line-voltage variations, and for signal amplification to be unaffected by reactive loads. I'll leave the technical description of how they actually did it to Peter W. Mitchell (see Peter's sidebar, taken from his 1994 WCES Show report); interested readers can write to Carver Research for the Lightstar's white paper. Carver's Jim Croft told me that the development of the Lightstar was very much system-onented&emdash;selecting what worked best in a particular context. For example, they found that there was an optimal power-supply capacitance value&emdash;higher or lower values didn't sound as good.
SYSTEM Analog sources were the Linn LP12 with all the accoutrements (Lingo, Cirkus, QR/DNM Design 330 Mk.II Ringmat), Linn Ittok, and AudioQuest AQ7000nsx. Digital sources were PS Audio's Lambda transport, Sonic Frontiers' UltraJitter-bug, PS Audio's UltraLink II, and Sonic Frontiers' SFD-2. The preamp used was mostly a Sonic Frontiers SFL-2, with some use also made of a Convergent Audio Technology SL-1 Signature.
Interconnects, digital links (AES/ EBU), and power cables were mosdy the iatest-generation TARA Labs RSC. The Lightstar manual warns against using power cables other than those supplied, which are specially shielded and able to handle high current, but I obtained better results with TARA Labs RSC. I have The Original CableJackets, grounded to a single wire, on all the power cables. My listening room has two dedicated AC lines; the Lightstar was plugged directly into two receptacles (remember: it has two power cables) fed by one of the lines. On the other AC line, the digital components were plugged into a Tice Series II Power Block line- conditioner, the pre-amp directly into AC. In connecting the SFL-2 to the Lightstar, I tried both balanced and unbalanced connections. The sonic differences (once I compensated for the higher gain in the balanced mode) were fairly minor, the unbalanced mode sounding a touch more open.
Speakers were Dunlavy Audio Labs SC-IVs&emdash;first the original pair that formed the basis of my review in April '94 (Vol.17 No.4), then the revised version discussed in my Follow-Up to that review in March '95 (Vol.18 No.3). The SC-IV has above-average sensitivity (9ldBlSV/m) and does not present a reactive load. However, its impedance, while extremely even, is in the 5 ohm range, so it is somewhat demanding of current.
Initial impressions ofthe Lightstar, using the PS UltraLink II processor and the CAT SL-l Signature, were quite positive; but before I could do any serious listening, I had to complete a variety of other tasks, including some surround-sound-processor reviews for the Stereophile Guide to Home Theater, and the Follow-Up of the latest version of the Dunlavy SC-IV.
Then I got a call from Carver, who told me that they had discovered a potential problem with the Lightstar that surfaced if the line voltage was too high. Apparently, one of the design briefs was to allow the amp to work with lower-than-normal line voltages, but they didn't look at the effects of voltages that are higher than normal. It was subsequently discovered that this can cause some subtle high-frequency distortion.
The line voltage where I live tends to be on the high side&emdash;I've measured as high as 129V&emdash;so this could have presented a problem. Not to worry&emdash;they had a mod that would fix things, so I shipped the review sample back to Carver. By the time I got the modified, ready-to-cope-with-any-voltage unit back, I had changed my digital front-end to a Sonic Frontiers SFD-2, and was using a Sonic Frontiers SFL-2 line stage with a balanced connection between the SFD-2 and the SFL-2.
I was ready for some serious listening. I went through the usual test pieces (Chesky, Reference Recordings, etc.), and also listened to some "ordinary" records that I just happened to feel like playing. The sound was impressively neutral, the amp doing little to call attention to itself&emdash;which isiust what you want it to do. With the Lightstar in the system, I was content to listen to the music, and fclt little urge to switch over to one ofthe other amps I had on hand (all of them more expensive). This, too, is a sign that an amplifier is doing something right.
Dynamics and a sense of power held in reserve werc among the Lightstar's major strengths. The ebb and flow of large-scale music&emdash;such as the dramatic first movement of Nelhybel's Trittico, track I on the Reference Recordings HDCD'F sampler&emdash;played at a realistic level (ie, loud) had the sort of effortlessness that only muscle amps seem to be able to deliver. At the same time, the Lightstar didn't have that rough-around-the-edges quality that typically accom-panies muscle amps. The critical upper midrange and treble were smooth and refreshingly free of hardness or edginess.
Timbral accuracy was excellent, voices and instrumental textures sounding quite lifelike. The opening of Turandot (London 4l 4 274-2) was crisp without being overly etched. If everything isn'tjust so, this recording can easily start sounding harsh; the fact that it didn't is a tribute to the quality of the reproduction chain, including the Lightstar. If anything, the Lightstar's presentation was on the soft, forgiving side, with perhaps a bit of added warmth. (Did someone say "tube-like".)
In my opinion, most non-audiophile recordings are balanced to bc too bright and forward, so the Lightstar's slight tendency to counteract this wasn't such a bad thing. To put it in comparative terms, the Lightstar's tonal presentation reminded me of that of the Krell KSA-100S I reviewed in September '94 (Vol.17 No.9), but with better dynamics. The Bryston 7Bs were more dynamic still, but they had a touch of brashness when dealing with the orchestral/vocal com-plexities of Turandot.
The contrast between the Brystons and the Lightstar was also brought out by Beachcomber, a new CD of wind- band encore pieces by Frederick Fennell and the Dallas Wind Symphony (Reference RR-62CD). "Russian Sailor's Dance" had natural timbres, excellent dynamics, and very good depth and ambience through the Lightstar; switching to the Brystons resulted in a sharper, crisper sound that better communicated fine details and depth/ambience, but it sounded a bit more "electronic."
With this recording, the Threshold T-200 (see my review last month) provided the best overall balance between dynamics, clarity, sweetness, and bass extension/tightness; but the Lightstar wasn't far behind. This was the case at loud-but-sensible levels. At very high levels (peaks measuring about 103dB, C-weighting, "fast" response on the Radio Shack sound-pressure-level meter), the Lightstar surpassed the Threshold T-200 in the ability to drive the Dunlavy SC-IVs in a way that sounded dynamically more open and effortless.
One of the claims for the Lightstar is for excellent bass performances so I spent some time checking this out. The Dunlavy SC-IVs have twin 10" drivers in a large sealed box: the bass they produce has the potential of being both extended and tight. The extension was fully there through the Lightstar, but, compared to the Threshold T- 200 or the Bryston 7Bs, the bass wasn't quite as tightly controlled. The bass drum in the opening of Trittico certainly didn't lack weight or power through the Lightstar, but neither was it as crisp and focused as it was through the other two amplifiers. With speakers that have a more rolled-off bottom end than the Dunlavy SC- IVs, the Lightstar's added warmth would likely be more of an asset.
Finally, there's thc matter of overall transparency/clarity. J. Gordon Holt, in whose definitions we trust, defined transparency as "freedom from veiling, texturing, or any other quality which tends to obscure the signal. A quality of crystalline clarity." I had some difficulty getting a handle on this aspect of the Lightstar's performance. There was no doubt that the Lightstar was good, but exactly how good was it? Comparable to the Threshold T- 200, which I had thought worthy of Class A rating?
I did some controlled listening, comparing the Lightstar with the T-200 at matched levels, using track 1 of All- Star Percussion Ensemble (Golden String GS CD 005) and track 10 of Sylvia McNair's Sure Thing: The Jerome Kern Songbook(Philips 442 129-2). Conclusion: a close call, but I'd say the Lightstar falls short of the overall transparency offered by the T-200. In terms of the familiar visual analogy, it was as if the view through the Lightstar was slighdy obscured&emdash;veiled, if you like&emdash;in comparison to the "crystalline clarity" of dhe Threshold. The difference was fairly small&emdash;smaller than, for example, the difference between the Mk.I and Mk.lI versions of the Sonic Frontiers SFD-2 digital processor. (I had an early sample ofthe Mk.ll on loan for one tantalizing weekend.) I should also note that the comparisons were done at moderate levels (peak sound-pressure levels just below 90dB); the outcome might be different at very high levels and/or with speakers that have a lower sensitivity.
MEASUREMENTS FROM TJN
A full set of measurements of the Carver Lightstar was made in its unbalanced mode, with selected measurements repeated for the balanced configuration. Unless otherwise noted, the results below are for unbalanced operation. Also, al] measurements (except for the gain check) were made with the normal gain setting.
Following the 1/3-power, one-hour preconditioning test, the Lightstar's heatsink/chassis was too hot to touch comfortably for more dlan a few seconds.
The Lightstar is non-inverting in the unbalanced mode. Balanced, pin 2 is configured as the positive leg, pin 3 the negative. The input impedance measured 54.2k ohms (unbalanced) and 115,.2k ohms (balanced). The output impedance of 0.05 ohms or slightly less (depending on frequency and load) up to lkHz, and 0.27 ohms or less at 20kHz, should have no practical effect on the amplifier's per-formance with different loudspeaker loads. Voltage gain into 8 ohms mea-sured 28.8dB unbalanced, and virtually the same, 29dB, balanced in the normal gain setting. Balanced gain in the high gain setting was 40.8dB. DC offset measured a negligible 3mV in the left channel, 2.2mV in thc right.
Signal/noise (ref. and output of 2.83V; ie, a power level of 1W into 8 ohms) measured 73.2dB (75.7dB, balanced) over a 22Hz-22kHz bandwidth, unweighted; 79.6dB (78.9dB, balanced) over a lOHz-500kHz bandwidth, A-weighted. Unweighted over the latter bandwidth, it measured a rather uninspiring 64dB, suggesting the presence of considerable power-supply noise outside the audible range. Within the audible range, a high-frequency mechanical whistle emanated directly from the Lightstar at various times during the tests. It was well below the irritation threshold for me (though was more annoying for Copy Editor Kristen Weitz, whose desk is in the same room as our test bench, and who frequently grits her teeth at the innumerable tortures we inflict on innocent amplifiers). This whistle became louder the harder the amplifier was pushed. I wouldn't expect it to be a problem in normal operation, however.
Fig.1 shows the frequency response of the Lightstar driven from both its balanced and unbalanced inputs. Note that, in the unbalanced mode, there's a slight peak at the top end (with a max-imum of just under 0.5dB at 20kHz into 8 ohms). in the balanced mode, there's no peak here; instead, the response has begun its high-frequency rolloff. Neither deviation should be audibly significant, though the difference (if you compare the balanced and unbalanced modes) may be subtly noticeable.
The Lightstar's small-signal IkHz squarewave response in fig.2 is unlike any I have ever seen. The risetime is fast, with a small, damped overshoot at the end of the rising (or falling) edge of the wave-form. But just before the rise&emdash;or drop&emdash; there's a single cycle of ringing. This same behavior is visible in the 10kHz squarewave response of fig3 This is very poor squarewave performance. Observed more closely, however, it shows a square-wave with a slow risetime (the probable result of the rapid roll off in the amplifier's response above audibility observable in fig.1) with that pre-rise, post-drop ringing cycle superimposed on it.
The crosstalk shown in fig.4 is also slightly worse than we're accustomed to observing. Since this is a dual-mono design, I found this result puzzling, though it could relate to circuit-board proximity, noise, or something unique to the unusual design of the amplifier. With a nunimsn crosstalk of about -49dB above 10kHz, however, the smaller-than-normal channel separation in abso-lute terms should not affect the audible performance. The Lightstar's THD+ noise vs frequency results are plotted in fig.5. While the balanced mode shows a slightly higher result, all of the measurements are very closely clustered, with a typical increase at higher frequencies. Whether this result is due to noise or distortion is not apparent.
The Carver's distortion waveforms are unlike any I have ever seen. The results varied considerably with load impedance and power output. The results in fig.6 show periodic positive- and negative-going spikes&emdash;probably power supply-related&emdash;which occur at each half-cycle of the source signal. The "noise" between the spikes appears to follow the source waveform, at least in a general fashion. The distortion waveform in fig.7 is the most distinctive one I obtained during testing. The spikes are still visible, as is a positive-going burst that occurs at each cycle of the source. Again, it appears to "ride" the modulations of the source itself I wouldn't attempt to predict the audibility of this type of distor-tion; since it occurs at the samc frequency as the source, it may be masked by it.
The actual distortion produced by the Lightstar is low compared with the noise level. The low-frequency spectrum ofthe ampliher's output whilc reproducing a 50Hz sinewave at 400W into 4 ohms (2/3 rated power at that load) is shown in fig.8. This is an excellent result; all of the artifacts are below -89dB (about 0.0035%), with the third harmonic being the highest in level. Fig.9 shows the amplifier's output spectrum repro-ducing a combined 19+20kHz signal&emdash; the intermodulation products resulting from an input signal consisting of an equal combination of these two frequencies&emdash;at 164W into 8 ohms. The largest artifacts here are at 18kHz and 21kHz (-66dB, or about 0.05%), with the next largest at lkHz (-76.5dB, or about 0.015%). The artifacts from this test signal at 303W into 4 ohms (not shown) were very similar both in distribution and in level. In both cases, this was the maximum power obtainable with this signal prior to visible clipping &emdash;not unusual with a 19+20kHz input. Very few amplifiers will approach their rated power with this signal, which has a peak level twice its RMS level.
The Carver's discrete clipping levels (at 1% THD+N) are shown in Table 1. The lkHz, THD+N % vs output power curves are shown in fig.10. This amplifier is clearly a powerhouse While the power does not quite double with each halving of frequency, it comes close&emdash;except at 2 ohms, where it still comes within 50W of meeting its rated 1200W output (the 118V line voltage may have been enough to account for the difference here). The Lightstar certainly ranks among the most powerful amplifiers we have tested, including some which are dramatically more expensive.
The test-bench results of the Lightstar were generally good (spectacular in the case of its power output). Only two results really gave me pause: the strange squarewave performance, and the lower-than-expected S/N ratio coupled with the pulse-type noise signature. Apparently, neither affected RD's positive listening experiences with the amplifier.&emdash;Thomas J. Norton
RD SUMS UP
Whenever I hear of a new technology, I get excited by its potential, but wonder whether the technology, while perhaps solving one set of problems, will create another set. The Carver Lightstar Reference does represent new technology, but as far as I can tell, one with no significant sonic downside. The Lightstar performed with perfect reliability, without any operational quirks or untoward side effects deriving from its novel design.
The extremely powerful Lightstar has exciting dynamics and impressive bass response, and also possesses a considerable degree of finesse and subtlety. Having said that, I must admit that I ultimately preferred the superior transparency of the Threshold T-200 (which costs about 40% more and has only 30% of the power); but the solid, highly competent Lightstar may prove a particularly good match with speakers that are difficult to drive and which can use a bit of extra warmth. &emdash;Robert Deutsch
CARVER RESEARCH LIGHTSTAR REFERENCE: DESIGN
The Carver Lightstar Reference is a power amplifier whose ratings suggest that it is an ideal voltage source&emdash; meaning that its rated power doubles with each halving of load impedance (300W into 8 ohms, 600W into 4 ohms, 1200W into 2 ohms). That's difficult to accomplish with conven-tional power transistors, which operate safely only within a restricted range of voltage/current combina-tions. (The graph of this range is called the transistor's safe operating area.) Many amplifiers deal with this limitation by using several transistors wired in parallel to share the output current. This works, but multiple devices usually don't have exactly identical characteristics, and some designers feel that a single set of out-put devices per channel can sound slightly better. Delivering high power into nor-mal impedances requires high output voltages. To provide these without violating each MOSFET's 30V limit the positive and negative power-supply voltages track up and down together with the audio signal, while the voltage difference across the MOSFETs is kept within a safe 10V range. In effect, all of the voltages in the output stage "float" up and down with the audio signal, while the high-current MOSS ETs are free to provide virtually limidess current to drive a loudspeaker's voice-coil. While dhe incoming audio signal is fed to the MOSFETs to produce the output current, it also drives a pair of PWM (pulse-width modulation) amplifiers that modulate the power-supply voltages up and down. The result is a purist output stage that can provide both high curTent and a wide voltage range regardless of the speaker's impedance.
One potential difficult remained: Music is full of transients, and speakers respond to transients by generating "back-EMF" pulses that must be absorbed by the amplifier without affecting its sound or its safety. Feedback diodes in the Lightstar provide a return path for back-EMF energy, which is fed back into the power supply. &emdash;Peter Mitchell
I For the benefit of those Stereefhile readers who aren't fully up on industry scuttlebutt, here's the real scoop (or at least as much as I know) about Carver the Man and Carver the Corporation: Bob Carver founded Carver Corporanon in the early '80s and was its CEO for a number of years, eventually taking the company public. About two years ago, he left the Corporation on a year-long sabbatical, and ended up resigning in March 1994. There were suits and countersuits about financial matters, non-competition clauses, and patent rights. These legal issues have now been resolved&emdash; see "Manufacturers' Comments," March '95, p.195&emdash; and Carver Corporation is proceeding with its product development and marketing.
Carver Research, the high-end division of Carver Corporation, is devoted to the "introduction of leading-edge technologies in their most advanced form" According to Carver Research, Bob Carver was the person who came up with the onginal idea for the Lightstar, but the final product is the result of extensive devdopment by a team headed by VP of Research and Development Jim Croft, an engineer with a long list of credits in audio and Compact Dics design.
Bob Carver has a new company, which has introduced its own amplifier, the Sunfire. The Sunfire is conceptually similar to the Lightstar, but has followed a different path of development. In this review, subsequent references to "Carver" win be to the Corporation, not to the Man.
CARVER RESEARCH LIGHTSTAR REFERANCE POWER AMPLIFIER
Has Carver Created The First True "Amp For All Speakers"?
BY JULIAN HIRSCH Stereo Review, November 1994 Some fourteen years ago, Carver developed something called a "Magnetic Field" amplifier, a revolutionary change from conventional power-amplifier design. It was small (a cube less than My 7 inches on a side), weighed only a few pounds, delivered 200 watts per channel into 8-ohm loads, and sold for $349. Over the years, other Carver amplifiers have employed the same principles, some of which were adopted in various forms by other manufacturers.
Basically, the Carver Magnetic Field amplifier had a signal-sensing power supply that rapidly increased the supply voltage to the amplifier circuits to accommo-date signal peaks exceeding normal listening requirements. The design enabled a relatively low-power amplifier (and power supply) to handle high-level program peaks without distortion. Only a couple of steps of voltage increase were practical at the time of the original Carver M-400 amp, but they were sufficient for it and its successors to provide the essential performance of a heavy, expensive power amplifier in a much more economical fashion. Over the years technology has advanced, especially in digital circuits and devices, and Carver Research (a division of Carver Corporation) has been working for some time on a vastly improved amplifier carrying the basic concept of the original Magnetic Field amps to its ultimate potential. The Carver Research Lightstar Reference amplifier, as the first commercial product using the new technology is called, invites (actually, demands) superlatives. It is very large and heavy, and correspondingly expensive, with a list price of $3,500. It is also probably the first true voltage-source amplifier to reach the hi-fi market, certainly the first one I have seen or heard of. I suspect it will not be the last.
The Lightstar develops most of the required output voltage in its power supply, which is built around what the company calls a Digital Transformer, and the role of the output devices is merely to funnel the necessary current to the load. Although the power-supply (signal) voltage can be as high as i170 volts, the output stage uses only two devices, operating in Class A, and the amplifier has no global negative feed-back or conventional gain stages in its signal path.
There are many other distinct differences between the Lightstar and ordinary amplifiers, some of which are explained more fully in "The Lightstar Advantage" (box). And for those seeking an in-depth exposition, Carver Research has a white paper on the design of this remarkable amplifier.
Basically, the most unusual operating feature of the Lightstar is that it is a virtually ideal voltage source with enormous power capabilities. The manufacturer's specifications tell the story: The Lightstar is rated to deliver 300 watts into 8 ohms, 600 watts into 4 ohms, and 1,200 watts into 2 ohms, from 10 Hz to 20 kHz, with less than 0.2 percent total harmonic distortion (THD). It can also safely drive even lower load impedances (down to 1 ohm or less), although its performance is not rated or specified below 2 ohms.
The Lightstar's physical appearance is as novel as its electrical characteristics. All black, it is made of thick (3/8-inch) aluminum and measures 19 inches wide, 7 inches high, and 173/4 inches deep. It weighs 72 pounds. The sculptured front panel has two illuminated level meters and an oval touch-plate standby on/off switch. The amplifier remains on in standby mode, but with reduced power consumption.
The Lightstar is actually a dual-mono amplifier. Not only are the two channels entirely separate from each other, both physically and electrically, but they even have separate heavy-duty line cords and circuit-breaker/power switches on the rear panel. Equipped with sturdy handles, the rear panel also has conventional single-ended signal inputs, via gold-plated phono jacks, and balanced inputs with gold-plated Cannon-type connectors. A small toggle switch near each input section switches it from standard-gain mode to high gain for use when the amplifier is being driven directly from a CD player or "passive preamplifier."
The speaker connectors are gold-plated binding posts usable with single or dual banana plugs, lugs, or stripped wire. There are two pairs of outputs for each channel, providing added versatility when driving a multiple-speaker array or when biwiring speakers.The Lightstar is protected against shorted outputs by a muting circuit, which operates when it is driven hard into an impedance of 0.5 ohm or less. The magnetic circuit breakers in the power-cord inputs are each designed to trip if the power consumption in that channel substantially exceeds its rated maximum value of 1,440 watts. The two channels together can draw between 2,000 and 3,000 watts in full-power operation, so the power cords should be plugged into outlets with a suitable rating&emdash;and never through an extension cord! Since the Lightstar's Digital Transformer has the potential to generate radio-frequency interfer-ence (RFI), the amplifier is tightly shielded by its cabinet and has RFI filters in its line and speaker circuits.
Because the Lightstar can consume so much line current, we drove only one channel at a time for high-power tests (both channels were driven for certain low-level measurements). The amplifier easily met its specifications, impressive as they are. Frequency response was better than 0.1 dB from 20 Hz to 10 kHz, rising to +0.4 dB at 20 kHz. A wide-range measurement from 10 Hz to 200 kHz (the limits of our Audio Precision System One test equipment) showed a response of +0.4, -0.2 dB from 10 Hz to 20 kHz; the response dipped to -1.1 dB at 60 kHz, rising again to +1.5 dB at 130 kHz before falling to below -2 dB at 200 kHz. These small (and insignificant) variations may have been due to the RFI filtering or to special aspects of the power supply.
Power output was, to put it mildly, prodigious (see "Measurements"). As would be expected from an ideal voltage source, the Lightstar's dynamic power output was virtually identical to its steady-state clipping-level power. I suspect that they were actually the same, but the dynamic measurement (reading the display from an oscilloscope and calculating the power) is inherently less accurate than a metered steady-state measurement.
The Lightstar's cabinet became extremely hot during the high-power tests. The top and side surfaces were too hot to touch for more than a sec-ond or two, although the front panel remained comfortable to the touch. After all, the top, bottom, and sides of the Lightstar are, in effect, the heat sinks for the most powerful amplifier most of us will ever see and touch, and a miniaturized watt has yet to appear on the scene.
Harmonic distortion at 300 watts in-to 8 ohms was about 0.05 percent from 100 Hz to 5 kHz, reaching 0.1 percent at 20 Hz and about I percent at 20 kHz. At 1 kHz the distortion was less than 0.1 percent from 5 watts to about 320 watts into 8 ohms. All of these readings were of total harmonic distor-tion plus noise (THD+N). A spectrum analysis (excluding noise) of the har-monics in the audio range showed a few components reaching as high as 0.01 percent and others typically around 0.001 percent or so.
Given the Lightstar's measured performance, one would not expect it to contribute any sound of its own in normal listening, and that was certainly our experience. Sonically, it was utter neutral and transparent. On the other hand, we were not willing to place the speakers in jeopardy by using more than a tiny fraction of this amplifier's power reserve. Carver Research recommends (and we heartily concur) that the speakers be protected by external fuses of the value suggested by the speaker manufacturer.
During our listening tests, we left the Lightstar energized, in its standby condition, so that it could be activated at a finger touch (and because it was not practical to reach behind it to turn the two mono amplifiers on and off). Although the standby mode of operation is undeni-ably convenient, we suspect that it could run up a sizable electric bill in time. The amplifier's top, after 24 hours in standby, was fairly warm to the touch and became slightly warmer during use at normal listening levels (averaging less than 1 watt). Based on our experience with the Lightstar, we recommend it highly for driving difficult speaker loads, particu-larly ones in which the impedance dips to 2 ohms or less. On the other hand, even if you use only a minute fraction of its capability, the Lightstar Reference is probably the most advanced power amplifier you can buy, and I doubt that any speaker has yet been made that it cannot drive with ease.
If its drawbacks (weight, power consumption, size, and so forth) are discouraging to consider, be patient. We understand that Carver Research is planning other, more affordable Lightstar components for the near future.
Output: at clipping (1kHz)
8 ohms 331 watts
4 ohms 650 watts
2 ohms 1,300 watts
Clipping headroom (re: rated output)
8 ohms 0.43 dB
4 ohms 0.35 dB
2 ohms 0.35 dB
8 ohms 315 watts
4 ohms 612 watts
2 ohms .. 1,223 watts
Dynelmk heendroom (re: rated output)
8 ohms 0.21 dB
4 ohms 0.09 dB
2 ohms 0.08 dB
Distortion at rated output O.045%
Sensitivity (1 watt into 8 ohms)
Standard 100 mV
High 26.5 mV
Noise (A-weighted, re: l-watt output)
Standard -82 dB
High -74 dB
Freequeney response...................20 Hz to 20 kHz +0.4, -0.1 dB