NAD & Rotel budget power amplifiers (Nov. 1992)

POWER FOR A SONG

T. J. N. reviews power amps for penny-pinchers from NAD & Rotel

Rotel RB-980BX. Stereo power amplifier, bridgeable to monoblock.

Continuous power output: 120Wpc (20.8dBW), 20Hz-20kHz.

DIN power output (1-khz, 4 ohms, 1% THD): 220Wpc (20.4dBW). Bridged power 360W minimum RMS driven into 8 ohms (25.6dBW), 20Hz-20kHz, no more than 0.05% THD. Peak current: 80A (0.1 ohm, 10µs, I pulse). Frequency response: 4Hz-100kHz, +0.5dB, -3dB. Total harmonic distortion: no more than 0.03% at continuous rated power. IM distortion: no more than 0.03% at continuous rated power. Damping factor 1000 (10Hz-20kHz, 8 ohms), equivalent to an output impedance of 0.008 ohms. Input sensitivity/impedance: IV/33k ohms.

S/N ratio (IHF/A-weighted): 120dB. Dimensions: 17.3" W by 4.75" H by 12.75" D. Net weight: 22.4 lbs. Serial number of sample reviewed: 161456.

Price: $599.90. Approximate number of dealers: 125. Distributor Rotel of America, P.O. Box 653, Buffalo, NY 14240. Tel: (416) 771-6610. Fax: (416) 882-8397.

NAD Monitor Series 2400THX. Stereo power amplifier, bridgeable to monoblock. Continuous power rating: 100Wpc into 8 ohms (20dBVV). Clipping power 130W (21.1dBVV). IHF dynamic headroom: 5.7dB. IHF dynamic power (max short-term power per channel): 370W into 8 ohms (25.7dBW), 400W into 4 ohms (23dBVV), 440W into 2 ohms (20.4dBW). Bridged continuous average power into 8 ohms (20Hz-20kHz with rated distortion): 300W (24.8dBW). IHF dynamic headroom, bridged: 5.7dB. IHF dynamic power, bridged: 800W into 8 ohms (29dBVV), 880W into 4 ohms (26.4dBW). Frequency response (THX input): 3Hz-100kHz, +0, -3dB. Slew rate: greater than 30V/µs. Damping factor (8 ohms, 50Hz): 100. Input impedance: 20k ohms. Input sensitivity for rated power IV. Voltage gain: 29dB. Infrasonic filter -3dB at 10Hz, 12dB/octave. Ultrasonic filter -3dB at 80kHz, 12dB/octave. SIN ratio, A-weighted: 98dB ref W. THD (20Hz-20kHz from 250mW to rated power): less than 0.03%. SMPTE IM distortion: less than 0.03%. Dimensions: 17.1" W by 4.75" H by 15" D. Net weight: 22 lbs.

Serial number of sample reviewed: 1080718. Price: $599. Approximate number of dealers: 250. Distributor Lenbrook Industries Limited, 633 Granite Court, Pickering, Ontario L1W 3K1, Canada. Tel: (416) 831-6333. Fax: (416) 831-6936.

I was on my way out of Audiophile's offices one evening, stumbling around in the dark while trying to close the last window and pondering some profundity or other, when my Rockport-protected toe nailed the side of a hard object. The sound was a rather tinny clank-certainly very un-Krell-like. Had it been a Krell, I would have been undoubtedly found the next morning, sprawled across the copy machine. As it was, the object was the side of the Rotel RB-980BX amplifier- small, unassuming, certainly very un-Krell like-sitting on the floor after having just been run through our lab tests.

It got me to pondering what makes one amplifier more expensive than another. Sheer mass has something to do with it-those super amps are brute-force designs. Fewer compromises means more sheet metal. More sheet metal means more money for sheet metal. And packing crates. And shipping. And on and on.

Not that I have any objection to sheet metal. If you have a huge transformer-a main contributor to an all-out design--you'd better support it properly. But $600 would be barely enough to buy the sheet metal alone in a monster amp. When the whole amplifier has to sell for that much, something's got to give.

Fortunately for most of us, there are still companies that feel that clever engineering can reduce costs across the board. A light weight chassis is just the end result of such engineering; a tinny sound when you rap it does not necessarily relate to a product's sound quality.

NAD, founded in England in the '70s, began life as a conglomerate of sorts. (NAD originally stood for New Acoustic Dimensions, but evolved rapidly to simply NAD.) Basing its operations on international design expertise and Taiwanese manufacturing, NAD made its name with a series of giant killer integrated amplifiers. Its core design principle was then, and remains today, large dynamic headroom-about which more in a moment. Cosmetics have always been deliberately low-key. The original concept was to avoid the bells and whistles profile rating on the market-dominating Japanese electronics, get back to basics, and use primarily European, and some American, design expertise to provide quality sound.

The concept was remarkably successful.

Although NAD remains small compared to the biggest Japanese manufacturers, it has nevertheless carved out an important niche for itself.

Rotel has done the same. Beginning in the '70s as a Japanese firm with product manufactured in Taiwan, Rotel made the same sort of products turned out by the major players- receivers and the like. But it really didn't compete with them on a volume or name recognition basis. Then, about 1980, Rotel introduced a line of small integrated amps in the UK, these designed with the help of UK engineers, and began building a reputation for itself as a manufacturer more interested in sound than cosmetics-just like NAD. Both companies, though they manufacture in the Far East, design and market their products very differently from the major consumer electronics companies based there.

NAD and Rotel today vie for a similar market, treading the middle ground between the High End and the Tokyo-by-night mass marketplace.

DESIGN & CONSTRUCTION

The Rotel BB-980BX is laid out in a twin mono configuration. Output stage operation is class-AB, using a Darlington triple parallel, fully complementary push-pull circuit. Its power transformer is a magnetically shielded toroid with separate windings for each channel's power supplies. The filter capacitors are British-designed and -built BHC electrolytics. The thermal protection circuit shuts down the amplifier in case of overload, illuminating a front-panel light when triggered.

Other than a rear-panel switch which selects normal or bridged operation, and a front-panel light which indicates the bridging position of this switch, hookup and operation of the 980BX is intuitive. There are no gee-whiz features which demand reference to the manual. The inputs are unbalanced.

About the only difficulty I encountered was the stripping of one of the output binding post nuts on the left channel, forcing me to revert to a banana plug instead of my preferred spade connectors. (When it came time for comparisons with the NAD, which does not accommodate spade connectors at all, bananas were used on all of the connections.) The NAD 240 is a considerably less straightforward design. In order to respond to the dynamic demands of music, for brief periods its high dynamic headroom produces substantially more output than its continuous rated power. The arguments relating this type of design to cost savings are compelling.

Continuous power is expensive-power supplies increase dramatically in size and price as the power rating goes up-and most of that big-bucks power is used for bursts of far less than a second's duration. Design for power-on-reserve, to be used only for short bursts, and your costs drop substantially. This idea got out of hand in the marketplace back in the '70s with the breakout of the IHF "Music Power" wars. But the IHF rating, even today, is based on a dynamic headroom test with a test-pulse duration of20 milliseconds.

NAD, it's important to note, designs its amplifiers to provide substantial reserve power (they claim two to three times the continuous rating) for up to several hundred milliseconds. They call this their "Power Envelope" design.

The NAD 2400THX's output circuit is fully complementary DC-coupled class-AB. The output devices are designed to handle high peak currents and a large peak-to-peak voltage swing-far larger than for a typical amplifier of the NAD's continuous power rating.

To make use of these large current and voltage output capabilities, the 2400THX employs a dual power supply-though not dual in the common sense of "separate for each charmer A low-voltage supply operates up to the NAD's rated continuous power; a high-voltage supply furnishes the extra juice required-through the opening of an electronic gate-for short bursts of power above that level. The high-voltage supply is designed to smoothly disengage if high power is demanded for longer than a few seconds, to prevent overheating.

Rotel RB-980BX power amplifier.

The 2400THX is also somewhat more complex in operational features than most power amplifiers. Two sets of loudspeaker terminals are furnished, with front-panel switching between them. (The terminals themselves are nonstandard, allowing for the use of bare wires or bananas, but not spade connectors.) Two sets of (unbalanced) inputs are provided, normal and so-called "THX" (the latter appear to be merely a re-labeling of what NAD used to call their "Lab" inputs prior to their licensing of the THX designation). The THX inputs are more direct; the normal inputs provide a degree of ultra- and infrasonic filtering, and also run through a set of front-panel level controls (out of the circuit in THX mode). A rear-panel input-selector switch is said to optimize the amplifier for use with either high-impedance (8 ohms) or lower-impedance (4 ohms and below) loads.

A front-panel overload indicator illuminates whenever the 2400THX is driven into clipping. A switchable "soft clipping" circuit is designed to gently round the corners of NAD 2400THX power amplifier clipped waveforms, thus minimizing the harshness resulting from overdriving the amplifier.

The NAD 2400THX is claimed to be the first power amplifier ever approved by Lucas film (the developer of theater and home THX) for home theater THX use. The specific requirements for an amplifier to be so licensed, however, do not appear to be public information. Certainly a major requirement is dynamic power capability--a requirement that the 2400THX would appear to have no trouble fulfilling.

Internal inspection of both the NAD and Rotel revealed nothing to keep high-end manufacturers awake nights. Both do make extensive use of discrete components and have roughly equivalent-sized transformers.

But the Rotel's interior has a mini, if not quite high-end, look to it; the NAD appears to have no pretensions in that direction. The Rotel has by far the neater build-quality. The NAD has more point-to-point wiring of the "rat's nest" variety, including some rather long input-to-circuit-board cabling, which, combined with a more tightly packed circuit board, gives it a far less tidy appearance.

Tidiness does not necessarily translate into better sound, of course (I've even heard the reverse suggested). The Rotel's edge in this area appears to be due to a layout which inherently provides for a neater build, not any carelessness in NAD's assembly.

ASSOCIATED EQUIPMENT

Equipment used in conjunction with this review included the MSB CD player, the SOTA Cosmos turntable with Graham tonearm and Clavis cartridge, the Rowland Consummate preamplifier, and the Apogee Stage, Vandersteen 2Ce, Sonus Faber Electa Amator, and Clearfield Metropolitan loud speakers. Interconnects were AudioQuest Lapis from CD player and turntable to pre amp, Cardas Hedink from preamp to power amp, and Symo loudspeaker cable (bi-wired).

SOUND

Rotel RB-980BX: $599.90: I found the Rotel RB-980BX a very pleasant surprise.

Well, not a surprise, actually. Rotel is developing a reputation for coming up with good sounding, moderately priced equipment. I knew from the first time I hooked up the 980 to our Apogee Stages-not the easiest loud speaker to drive-that something special was going on.

I'll dispense with the Rotel's weaknesses immediately. Present on a wide range of recordings was a slight crispness and grain at the very top, the remnants of an all-but slain solid-state dragon. Never hard, edgy, or zippy with good program material, the sound of the 980 nevertheless had a trace of the analytical. Sibilants on typically close miked pop vocals were not subdued. Strings didn't shriek or grate, but sounded less sweet than with top-class amplifiers. I never found the Rotel's top end in any way irritating through any of the four loudspeakers I tried it with, but a degree of emphasis and a fine but noticeable grain was present with all of them. It was least evident on the Vandersteens, most evident on the Stages.

On the low end of the scale, the Rotel did not control events as well as the powerhouse amplifiers. It was no Krell or, to come closer in price, Aragon either. The extension was fine, but the vise-like grip was not there. The bass of the Stages can be difficult for even the best amplifiers to control; the Rotel held on, but the strongest LF transients were more soft and cushy than tight and attention grabbing. Some of this, I suspect, was due to the Rotel's performance into low-impedance loads-note that the Stages have an impedance of under 4 ohms. Matters were better with the Sonus Fabers (which have a surprisingly satisfying low end but still never quite transcend the lower-octave limits of a good mini monitor) and the Vandersteens. With the latter in particular, though the bottom was still slightly softened, the Rotel dearly had an easier time than with the Stages.

Beyond a degree of emphasis at the top end and softness at the bottom, however, the RB 980BX proved to be one of those products reviewers dream of. We blather on endlessly about how high-end sound is not about price, but more often than not the products we end up raving about cost an arm and a leg.

The Rotel, whether by design or accident, turned out to be more than just another modestly priced amplifier, competent but uninspiring. If the NAD 2400THX never offended, neither did it ever really excite on music sources. The Rotel, on the other hand, got up on the high-end high-wire without a net-and performed. At times it tottered- as when its bottom-end or top-end limitations combined with the wrong program material and/or loudspeakers-but it never fell. And when its footing was secure, it was thrilling to watch. I mean, hear.

No, the Rotel is no super-amp. (If I ever find one for $600, you'll be the first to know.) But the test of a really solid performer is how long you want to continue to listen to it. With the Rotel, I only rarely wondered if the sound would be improved by substituting a Krell or other super-amp. I simply enjoyed the sound too much to be distracted by such thoughts.

I took over 50 pages of listening notes in my evaluations of the Rotel and NAD, yet in the case of the Rotel, the score was clear in the first few pages. Beginning with the female vocal and small-scale instrumental work on Sara K.'s Closer Than They Appear (Chesky JD67) over the Apogees, I noted the slight grain and somewhat prominent sibilants, along with the rather loose, warm bass (qualities also characteristic of the Apogees under some circumstances). But the sound was lively in the best sense of that term, not dark, veiled, dosed-in; or (conversely) hard, edgy, or zippy. The soundstage was secure, the overall perspective just about perfect-neither laid-back nor in your lap. The Stages' essential qualities-a vibrant, live, dimensional sound with a striking midrange-were clearly evident. There was no sensation of spatial squashing or a front-to-back squeeze-play in the soundstage. If only the Rotel could continue to keep this up, I noted, it couldn't lose.

It could and it didn't. Britten's The Young Person's Guide to the Orchestra (Neeme Jarvi, Bergen Philharmonic, Bis CD-420) demonstrated a fine sense of depth combined with lusciously rendered hall sound. The imaging on this recording was real-not pinpoint, but much as you might hear it from the mid hall perspective of the recording. Mary Black's Babes in the Wood (Dara/Gifthorse/Curb D2-77528) was tightly focused, open, and detailed. The inner detailing was impressive, the midbass clear. Only the vocal sibilants intruded-and a lot of the blame here belonged to the recording engineer.' As big a cost mismatch as the Rotel driving the Apogee Stages may appear to be, the Rotel driving the $4500 Sonus Faber Electa Amator is even bigger. Yet the combination did not sound at all ridiculous. My first impression of the Amators after switching over from the Stages was of a dramatically reduced bottom end and a less immediate and more colored (though by no means seriously so) midrange. But Hiked the Amators almost immediately, and the more I listened, the more they impressed me. Chief among them was a truly excellent soundstage with a decidedly more obvious sense of depth than that managed by the Stages. All this, remember, with the Rotel driving them. Though the lowest bass was still a bit loose (far less obvious than with the Apogees), the midbass was tightly detailed. The overall layering could be striking-listen to the instrumental depth in the complex mixture of orchestra and small Irish band (the Chieftains) on the Far and Away soundtrack (MCA MCAD-10628), or the even more complex mix in Andreas Vollenweider's Book of Roses (Columbia CK 48601). The unlikely Rotel/Amator combi nation did not disappoint on either of these recordings; nor did it on many others.

The Vandersteen 2Ces are less open sounding than the Sonus Fabers, less palpable than the Stages. They also cost about $3000

[1 This is a far better recording, in my opinion, than Ms. Black's earlier effort, No Frontiers (Dara/Gifthorse/Curb D2 77308), considered by some to be of reference quality. Were it not for the sibilants, however, I could almost be convinced that Babes in the Wood belongs in that category. Nor does it disappoint in music or performance.]

less than the former, $1000 less than the latter, stands excepted--an eminently practical match for the Rotel. The life, soundstage, spaciousness, and detail of the Rotel were evident on the Vandersteens as well.

The Rotel also had a trace of midbass lean ness hardly worth mentioning but for the fact that, combined with the amp's slight top octave emphasis, it made for a tonal balance which could not be confused with tube sound.

In light of the fact that Vandersteens are often demonstrated with tubes, one might wonder if the Rotel can actually work with them.

First, you're unlikely to find any new tube amplifier for $600. Further, the Rotel's slight leaning-out of the midbass, at least in my listening room, and compared with the NAD 2400THX, definitely opened up the 2Ce's sound.

I'll have more to say later about the Roters sound vs that of the NAD, including their use with video sound sources. Suffice it to say here that the Rotel offered a tantalizing taste of the High End. The NAD, though a solid performer with strengths of its own, on music sources never quite illuminated the overall sound as the Rotel often did. The Rotel won't make you forget Krells, Rowlands, Audio Researches, and other super-amps. But for the rest of us who can't afford (or who might be barely able to afford, but not justify) the high-priced stuff, the Rotel RB-980BX is unlikely to disappoint-especially at a price which, in today's high-end market, is pocket change.

NAD 2400THX: $599: Gutsy and weighty.

That's the sound that I got from the NAD 2400THX from the start, and that's the sound I continued to get from it throughout the evaluation. At its best, the NAD will make you wonder why you wanted a more powerful amplifier. Its combination of moderate continuous power and large dynamic headroom, though no longer a novel approach (even if NAD is still one of its primary proponents), really does seem to work. Whether or not it's responsible for the amplifier's weak nesses (which I'll get to shortly), there's no doubt that the 2400 sounds more powerful than any 100Wpc amplifier has a right to.

Even without its soft-clipping circuit engaged (which I never found any reason to use 2 ), I never felt the 2400THX was running out of steam. But readers demanding more sheer SPL capability than my preferred "plenty loud but short of immediate eviction" levels (if I'm not getting too technical) may want to look to even more powerful amplifiers (of which NAD itself makes at least two). The most convincing test of the NAD's dynamic capability was with video-based sources (see below). Next to its sense of unrestricted power and unflappability, however, the NAD's most positive sonic quality was its full-bodied weight and timbral balance through the midrange. While I did feel that its immediacy through this region was at least partially the result of a somewhat forward (but not pushy) quality, there was no doubt about the palpable quality of voices and instruments Kenny Ranldn's voice on Because of You (CheskyJD63) was up-front yet intimate. Jennifer Warnes was also very much "there" on her fine new (though rather bass heavy) album The Hunter (Private Music 01005-82089-2). At this point, however, the 2400THX began to get a rather mixed report card. Considering its price and unflappability, the NAD is certainly a good value for those who simply must have an amplifier which will grab a loudspeaker and tell it who's boss. But it never really convinced me that I was listening to something other than reproduced sound. It never got irritated or lost its composure, but at the same time it never quite opened up and came alive.

Nowhere was this more evident than in the 2400's top octaves and soundstaging. The lower treble was a bit forward, lending a trace of edge on some but not all program material.

Further up the scale the sound was rather rounded. Not dull, by any means-it just lacked that last margin of openness. This softness reduced the soundstage's overall focus and spatiality-none of these factors are independent. The imaging on Vollenweider's Book of Roses, which can be quite striking with the right system, was slightly diffused, enlarged, and two-dimensional with the 2400. And the Rankin and Warnes albums combined an immediate midrange with a certain degree of front-to-back image compression and reduction in high-end air and transparency.

[2 Soft clipping was first incorporated in NAD's original, low powered integrated amplifiers. With power ratings of 20Wpc or not much more--even with good dynamic headroom-soft clipping makes more practical sense.]

In the bottom end, the NAD was potent if a trace soft compared with the best-and far more expensive-amplifiers. The Stages tend to be under-controlled at the bottom end (for me, their own serious weakness). The NAD pumped as much bass through them as any amplifier at anywhere near its power, though the low end remained a bit under damped. This was also true through the Vandersteen 2Ces, but less so through the Sonus Fabers--the latter having less bass extension in any case. Still, I wouldn't rate the NAD's bottom end as less than good; it's certainly competitive with anything else near its price.

And that may be the bottom line for the NAD in any event. Had I not heard it in conjunction with the Rotel, my reaction might well have been more favorable. The Rotel about equaled the NAD in useful bass output and bass quality (both were good, but less than hair-raisingly tight), and fell a bit short of it in upper-octave sweetness, the Rotel being a shade crisper with more evident grain. But the Rotel had a see-through transparency and a feeling of correct dimensionality which offer more than a trace of the High End. While listening to the Rotel, I never felt a strong urge to switch to another amplifier. When listening to the NAD, I always wondered what this or that recording would sound like through the Rotel.

You would be excused at this point for thinking that the NAD might be best left off your list of potential amplifier candidates. So you might be more than a little puzzled to hear that I'm grappling with the urge to buy one or both of the review samples. Life is never simple. There's more than mere hype to that little THX logo on the front panel.

THE SOUND OF VIDEO

Since the NAD is certified to wear the THX logo into battle with the slings and arrows of outrageous video sound sources, it seemed appropriate to listen to it with such program material, at least briefly. I don't at present have an audio/video system up to the task, much less a home THX setup? Instead, I brought my Panasonic LX-1000 laserdisc player to Audiophiles listening room, along with a couple of choice videodiscs--Terminator 2 and The Rocketeer. Looking around for loud speakers able to handle the assault about to be thrown at them, I wrestled out the new Clearfield Metropolitans. Jack English is slated to review these brutes in an upcoming issue, and the pair here in Santa Fe definitely needed additional breaking-in. Still, I soldiered on.

Remember, the setup used with these video sources was strictly two-channel stereo-no surround-sound in sight. Or TV, for that mat ter. This was audio playback only of a video laserdisc. The limitations of video soundtracks played back in this mode were very apparent- they have a certain bloated, overprocessed quality which can be mitigated somewhat by a good surround setup. I first played some familiar audio recordings to adjust to the Metropolitans' sound. It was quite obvious that the sonic characteristics of both the Rotel and the NAD could be discerned through the Clearfields in a head-to-head comparison. The Rotel continued to sound more open and transparent.

Furthermore, the NAD's extra warmth was not a plus through the rather warm-sounding Metropolitans.

Video sources are not noted for refinement and subtlety. Most are big-boned, rather raw, and in-your-face. Frankly, to me the greatest benefit of good surround-sound for video is the way it acids realism to the few subtleties that are present on most laserdiscs-the chirping of insects in a night scene, the rustle of leaves or the sound of the wind. While such sounds are hard to find in the general din of 12, they do exist in Rocketeer, primarily as musical cues.

The band "Jenny Wakes Up in Sinclair's Home" opens with a plaintive horn solo in the far distance, and the music remains subdued through much of the cut. On this the Rotel scored with its ability to convey depth and space. The NAD did well, but it was less three dimensional.

On the other hand, when it came to sheer power, there was no question that the NAD outgunned the Rotel. The latter certainly acquitted itself well, and never went to pieces even at antisocial levels. But careful listening revealed it to have a spitchy quality on hard, high-level transients-indicative of clipping which was entirely absent with the Mane NAD's extra low-end weight was also more evident than it had been with audio sources.

[3. I currently use the little, self-powered Pattern surround sound system from Atlantic Technologies. It has a certain limited ultimate output capability-though better than you might expect-and decidedly tubby midbass and restricted lower octave-and-a-half (my current video room-not the same room I use for listening evaluations-does not permit much experimentation with placement of the bass module). But otherwise this may well be the video surround-sound system for audiophiles who don't want to get heavily into audio-video systems but would like at least some semblance of the movie-theater experience at home. I may have more to say about it later, but perhaps nothing more needs to be said.]

In 72's main title band, the bottom end was more solid. Gunshots, which cradded slightly through the Rotel and were robbed of some of the fullness which follows the initial transient, sounded cleaner, with more resultant weight and impact, through the NAD. And, when the going got heavy, and despite what I've said above, there were actually times when the NAD appeared to beat the Rotel in its sense of depth with these high-power sources. That 72 title cut was a case in point.

With the NAD, the sheer impact and power of the truck/motorcycle chase scene was over whelming. And while the Rotel continued to win out on The Rocketeer's musical cues, the NAD showed its superior overload characteristics here as well. You wouldn't expect the clash of steel swords (in Racketeer's soundstage fencing scene) to be much of a challenge, but they rang out clearly with the NAD while sounding slightly fuzzy with the Rotel. The slight softness of the NAD in comparison with the Rotel was in some ways a plus with this slightly bright soundtrack-not at all atypical of otherwise good soundtracks in general.

Some of the NAD's power-delivery benefits may be less important when used in subwoofer-equipped video systems. But, contrary to popular opinion, there is a great deal of power in the upper-bass/lower midrange region of both music and natural sounds.

I could not imagine a video system, even in a fairly large room, needing more power than that provided by the NAD with loud speakers comparable in sensitivity to the Clearfields. That sensitivity is fairly high, however-several dB higher than the Stages, for instance. And the Clearfields' impedance is moderately low at the bottom end, if not unusually so-comfortably above 4 ohms at most frequencies, but dipping to about 3.5 ohms at 300Hz. A fairly insensitive 8 ohm loudspeaker, drawing less power from the NAD, might not be quite so ready to knock down the walls.

It's just possible that the NAD was operating with one power supply tied behind its back during this audio/video comparison. As I describe in the NAD measurement section below, it's possible for the amplifier to remain fully functional up to its continuous power limit-a substantial 200W with a 4 ohm load-with the high-voltage supply's fuses blown. Just before doing the audio/video comparison, I completed the primary measurements on the NAD. After the comparison, I rechecked a few measurements-and found that the high-voltage supply fuses were, indeed, blown. It was not possible to determine for certain, but there's a possibility that the fuses blew before comparison.

There is no visible indication of blown high voltage supply fuses, or any audible one short of continuous power clipping. Time did not permit re-running the audio/video comparison, but what point would that have served? Whether handicapped or not, the NAD still put in a superb showing in that portion of the listening tests.

MEASUREMENTS:

ROTEL RB-980sx

The chassis of the Rotel RB-980BX was hot following the 60-minute, 1 / 2 -power preconditioning. Input impedance measured just over 34k ohms and the amplifier was non inverting. S/N ratio measured 86.9dB, 82.4dB bridged (both unweighted at 1W into 8 ohms). The Rotel's output impedance was consistently low, never rising higher than 0.015 ohms (0.016 ohms bridged) from 20Hz to 1-khz, 0.02 ohms (0.025 ohms bridged) at 20kHz. DC offset at the outputs measured 10mV in the left channel, 2.7mV in the right.

Like the NAD, the Rotel had its lowest chassis AC voltages (with no other equipment connected) with the line cord reversed from its normal position-in the case of the Rotel a surprisingly high 72.5V in the normal hookup, 24V reversed It is, of course, possible that other samples of the Rotel are wired differently.

Fig.1 shows the frequency response of the Rotel at 1W into 8 ohms. The response for 2W into 8 ohms is virtually identical, as is the bridged response into either impedance The Rotel's reproduction of a 10kHz squarewave is shown in fig.2, with a fast risetime, a very slight rounding of the top of the leading edge, and no overshoot.

The crosstalk plots in fig 3 show nothing of note except marginally better left-to-right

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4. Defeating the wide blade/narrow blade AC cord requires a cheater; readers are not advised to do this without the ability to measure for minimum voltage and a knowledge of what they are doing.

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Fig.1 Rotel RB-980BX, frequency response at IW into 8 ohms (right channel dashed, 0.5dB/vertical div.).

Fig.2 Rotel RB-980BX, 10kHz squarewave at 1.4I4V peak into 8 ohms.

Fig.3 Rotel RB-980BX, crosstalk: R-L, top; L-R, bottom (5dB/vertical div.).

Fig.4 Rotel RB-980BX, THD+noise vs frequency at, from bottom to top: IW into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed).

separation, and the typical increase in cross talk at higher frequencies-almost always the result of capacitive coupling between channels. The distortion curves in fig.4 (scale expanded for clarity) indicate an increase in THD +noise at higher frequencies and lower impedances, and also a noticeable increase in distortion into 2 ohms at lower frequencies.

The THD + noise waveform is shown in fig.5 (2W into 4ohms). This is mostly random noise. (The primary frequency of the distortion+ noise waveform actually appears to be below the frequency of the signal, not a harmonic of it. Perhaps it's hum.) A plot showing the output resulting from a high-level 50Hz output (147W into 4 ohms) is shown in fig.6. The harmonics are all well down in level, the highest being the third harmonic at 150Hz (-92.6dB, or about 0.0024%) and 350Hz (-91.6dB). There is a strange artifact at 170Hz at -86dB (about 0.005%), which is dose to but not quite at the right frequency to be power-line-related. Fig.7 shows

Fig.5 Rotel RB-980BX, !kHz waveform at 2W into 4 ohms (top), noise and distortion waveform with fundamental notched out (bottom).

Fig.6 Rotel RB-9808X, spectrum of 50Hz waveform, DC-I kHz, at I47W into 4 ohms.

Note that the seventh harmonic at 350Hz is the highest in level at -91.5dB, or 0.0025%. Fig.7 Rotel RB-980BX, HF intermodulation spectrum, DC-22kHz, 19+20kHz at I14.7W into 4 ohms (linear frequency scale).

Fig.8 Rotel RB-980BX, distortion vs output power, from bottom to top at IOW, into 8, 4, and 2 ohms

Fig.9 Rotel RB-980BX in bridged mode, THD+noise vs frequency at, from bottom to top: 1 W into 8 ohms, 2 W into 4 ohms, 4 W into 2 ohms (right channel dashed). Fig.10 Rotel RB-980BX in bridged mode, distortion vs output power, from bottom to top, into 8 and 4 ohms.

a similar spectral plot, this time the result of a high-level 19 +20kHz signal, showing the intermodulation (IM) between these two tones. In this case the output was 114.7W into 4 ohms-just short of visible clipping. The largest IM artifacts are at 18kHz and 21 kHz, at about -80dB (0.01%) each. The rest of the artifacts are very low in level, including the 1-khz difference tone at -88.6dB (0.004%). The spectral plot for the same two frequencies with an 8 ohm load and 67.7W input (again, just prior to clipping, and not shown) had even lower artifacts (0.007%). The THD +noise vs level sweeps (at 1 kHz) are shown in fig.8. The outputs show consistently low distortion up to the breakpoint of the distortion "knee." Note that the curve for a 2 ohm load ends abruptly; at the point where it ended, the power-supply rail fuses blew. The Rotel will work into 2 ohms, but it is clearly not at its happiest with that load.

The discrete measurements for clipping (1% distortion) show an output of 125.5W (21dBW, left) and 120.4W (20.8dBW, right), both channels driven, 137.2W (21.4dBW) for one channel driven, into an 8 ohm load. With a 4 ohm load, the corresponding figures were 1933W (19.85dBW, left) and 191W (19.8dBW, right), both channels driven, and 240.4W (20.8dBW) with a single channel driven. The line voltage ranged between 110V and 112V for these readings. Discrete clipping measurements into a 2 ohm load were not attempted because of the marginal 2 ohm performance noted in fig.8.

Finally, the THD + noise vs frequency for the Rotel in the bridged mode is shown in fig.9-with slightly higher levels than in the unbridged mode, as I would have expected.

The THD + noise vs level curves in fig.10 indicate a strong performance into 8 ohms.

(I measured a discrete clipping, 1% THD level of 415.9W (26.2 dBW) into 8 ohms.) The 4 ohm curve, however, does not overlay the 8 ohm curve above 350W as the curve might imply. Instead, the power-supply fuse blew at 350W into a 4 ohm load. This is consistent with the Rotel's performance in the unbridged mode into 2 ohms. I do not recommend using the Rotel bridged into much below an 8 ohm load. While it is unlikely to run into trouble with music sources at least down to 4 ohms, the design is clearly not optimized for continuous use into low impedances-and 4 ohms is a low impedance for a bridged amplifier. Aside from this, how ever, the Rotel's test-bench performance was fine for an amplifier at this price.

MEASUREMENTS:

NAD Monitor Series 2400THX

With its normal and THX inputs, switched 4 or 8 ohm load-impedance selector, soft or normal clipping, and potential for bridging- not to mention a dual-rail power supply, the upper rail of which is only designed to pro vide a power boost for a fraction of a second- the NAD 2400THX promised to be complex to measure. Every possible measurement for every possible setting could not be made, but a large cross-section of them were. The results here concentrate on the THX mode with the 8 ohm load switch position and soft clipping off. Other conditions, where presented here, are noted.

The 60-minute, 1 / 3-power preconditioning test left the NAD's heatsink very hot-too hot to touch for more than an instant. Its input impedance measured just under 18k ohms through the THX input, and a considerably higher value of just under 208k ohms through the normal input with gain controls on full. Gain was 29dB from the TI-DC input and a maximum of 35.8dB from the normal input. The amplifier was non-inverting. Its S/N ratio measured 80.9dB from the THX input, 78.1dB from the normal input (both measurements unweighted at 1W into 8 ohms). The 2400THX's output impedance was 0.11 ohms below 1 kHz, rising to 0.19 ohms maximum at 20kHz. (In bridged mode, it rose marginally to 0.15 ohms up to 1-khz, 0.3 ohms at 20kHz.) DC offset at the outputs measured a rather high 49.9mV in the left channel, 37mV in the right. As was the case with the Rotel when connected only to the line and not to other equipment, the AC volt age on the chassis measured lowest with the line cord reversed from its normal position 33V in the normal hookup, 16.8V reversed.

(Again, defeating the wide blade/narrow blade AC cord requires a cheater; readers are not advised to do this without the ability to measure for minimum voltage and a knowledge of what they are doing.) Fig.11 shows the 240C/THX's small-signal frequency response. (The curves which remain nearly flat to 10Hz are for the THX input, the low-frequency rolloff’s from the normal input.) While the THX input gives the slightly flatter response at high frequencies, the differences in the upper octaves are not significant. The curves shown are for the 8 ohm output taps at 1W into 8 ohms. At 2W into 4 ohms, the response (not shown) is down approximately 0.1dB more at 20 kHz, 05dB at 50kHz. Changing to the 4 ohm output tap leaves the frequency response unchanged; bridging the amplifier increases the high frequency rolloff slightly (another 0.1dB at 20kHz). The 10kHz squarewave with its slightly rounded leading edges, shown in fig.12, reflects the ultrasonic rolloff in the frequency response.

The crosstalk in fig.13 shows that the left to-right and right-to-left cross-talks for the THX input (middle two curves referenced at the 1-khz point) are practically identical.

---

5. There are, strictly speaking, no 8 or 4 ohm "taps" at the out puts. However, since it becomes cumbersome to constantly refer to the "8 ohm output switch position" or the "4 ohm output switch position" of the NAD, I will hereafter refer to these settings as the 8 ohm or 4 ohm output taps.

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With the normal input (top and bottom curves referenced at 1-khz), left-to-right crosstalk is noticeably less than right-to-left-though the difference is probably inaudible It is likely caused by a difference in the capacitive coupling between the channels when in the normal mode, perhaps related to the extra wiring required for the front panel controls.

The distortion curves through the THX inputs in fig.14 (8 ohm output tap) show a good result, with some increase into lower impedances at high frequencies, a common amplifier trait. The distortion measurements through the normal inputs (not shown) were slightly higher. The high-power distortion (133W into 4 ohms from the 8 ohm tap) from a 50Hz input signal at the THX input is shown in fig.15. These and later figures indicate that the NAD is a low-distortion-and probably relatively high-feedback-design.

(Neither specs nor manual claim low feed back.) The harmonics are very low in level, the highest (at 150Hz, 300Hz, 550Hz, and 850Hz) down 93dB (below 0.0025%) or more. Fig.16 shows a similar spectral plot, here the result ola combined 19 +20kHz signal. The residual indicates the intermodulation between these two tones-the sum-and difference tones created by the amplifier with these frequencies at the input. The output of the amplifier in fig.16 was 133W into 4 ohms (8 ohm tap). The 211cHz 1M component was -72dB (about 0.025%); the 18kHz -74.5dB (about 0.02%); elsewhere it never exceeded 0.02%, and was generally much lower. The IM spectral plot for an 8 ohm load at 66.67W (not shown) indicated an even lower distortion, with all artifacts below 0.01%. The swept curves for THD + noise vs level (at 1-khz) are shown in fig.17 and 18. Fig.18 is the same data, but shown on an expanded scale above 100W for clarity. Notice the complex structure of; for example, the 8 ohm load curve. Just above 100W it begins to break, the classic knee leading to the onset of clipping.

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Fig.11 NAD 2400THX, 8 ohm output, frequency response at IW into 8 ohms for normal and THX (more extended at LF) inputs (right channel dashed, 0.5dB/vertical div.).

Fig.12 NAD 2400THX, 10kHz squarewave at 1.4I4V peak into 4 ohms.

Fig.13 NAD 2400THX, crosstalk through THX inputs (middle two curves at 1 kHz) and normal inputs: R-L, bottom; L-R, top (5dB/vertical div.

Fig. 14 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, THD+noise vs frequency at, from bottom to top: 1 W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (right channel dashed).

Fig.15 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, spectrum of 50Hz waveform, 8Hz-1 kHz, at 133W into 4 ohms. Note that the third harmonic at 150Hz is the highest in level at -93dB, or 0.0025%.

Fig. 16 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, HF intermodulation spectrum, DC-30kHz, 19+20kHz at 133W into 4 ohms (linear frequency scale).

Fig.17 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, distortion vs output power, from bottom to top at 100W, into 8, 4, and 2 ohms.

Fig.18 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, distortion vs output power, from bottom to top at 100W, into 8, 4, and 2 ohms.

Fig.19 NAD 2400THX, THX inputs, 8 ohm output, soft clip off, high-voltage rail fuses removed, distortion vs output power, from bottom to top at 100W, into 8, 4, and 2 ohms.

Fig.20 NAD 2400THX, normal inputs, 8 ohm output, distortion vs output power into 8 ohms with soft clip on (left) and off (right).

But at about 140W the break levels out, keeping the distortion low until about 260W, at which point it begins a rapid ascent, reaching the 1% point at just over 300W. What's happening here is the action of the dual-rail power supply. The first rail carries the amplifier to moderately high power, probably something in the region of 100W. The second rail then engages, carrying the output briefly to the 260W point, after which it, too, reaches its limit. That limit is quite brief, as designed.

In fact, the precision of the THD + noise vs level curves above the initial knee, after the second rail enters the picture, is open to question. Note that the THD +noise vs level sweep at higher power may last more than long enough for the second rail to run out of steam. NAD states in their literature that the high rail disengages after "a few seconds." This is a rather indefinite term, but I suspect, based on our curves, that it is enough to sustain the output through the sweep. What the curves do make clear-and why I've chosen to present them-is that the second rail does provide a significant power boost; with an input of shorter duration it might well be even more than that shown by our data. The reader should keep this limitation in mind, however, when interpreting all the THD+noise vs level data and curves which follow.

Note also that no discrete clipping measurements are shown for the NAD due to the same limitations. The action of taking a discrete reading, while it may appear to be less stressful on the supply than a sweep if the signal is terminated immediately after the reading is made, is actually more stressful. It takes a minimum of 1 second, and usually more like 2, to turn on the generator, wait for the THD reading to stabilize, take the reading, and shut off the generator. I made two attempts to do this. On the first attempt, the upper power-supply rail fuses blew. On the second they did not, but the output power saturated (increasing the input merely increases the distortion without increasing the power output) well before the power levels shown in these figures. The THX inputs were used for figs.17 and 18. With the normal inputs (results not shown), the results were not significantly different, the distortion increasing slightly (by 0.001% to 0.003%) up to the first, thereafter following the TI-DC distortion curve quite closely. In both cases I used the 8 ohm output tap.

Blowing the upper power-supply rail fuses uncovered a quirk of the 2400THX's operation that I might not have noticed otherwise: The upper-rail fuses blew but those on the lower, or normal rail, did not. The amplifier still sailed along, happy as a clam, though with a slightly reduced power output, per forming precisely as it might if it had no upper rail at all. There is no visible indication of this condition, and likely no audible one either except at very high playback levels (see "Sound" for more on this). I decided it might be of interest to run a few measurements on the amplifier in this state The gain was unchanged, as was the frequency response. The S/N ratio actually improved by 3-4dB, as might be expected with one less power-supply signal running around the chassis. Perhaps most interesting are the THD+noise vs level curves in this upper-rail less condition (fig.19). Note the differences between this and the previous figures. The enhanced power visible in figs.17 and 18 with the upper rail in operation leads me to believe that, even with the strain on the upper rail of the power supply caused by the duration of the sweep, figs.17 and 18 (and the other THD +noise vs level data presented in Table 1) are probably not that far off in their depiction of the NAD's maximum instantaneous power capability.

Table 1 shows the result on the output at 1-khz of using the 4 ohm output tap. Indicated are the final knee or break point in each THD +noise vs level curve (the point beyond which the curve makes its final rise to clipping), and the 1% distortion point in Wand dBW, both read from the appropriate measurement curve. Note that these results are also subject to the same potential upper-rail limitations in running a swept THD +noise vs level measurement as are the graphs. Still, they do provide useful information. Note that, in every case, the available output is higher when using the 8 ohm output tap, Table 1 Final break 1% THD 8 ohm load W (% THD) d BW W d BW 8 ohm tap 270 (0.007%) 24.3 295 24.7 4 ohm tap 210 (0.0072%) 23.2 230 23.6 4 ohm load 8 ohm tap 320 (0.013%) 22.0 400 23.0 4 ohm tap 270 (0.012%) 21.3 330 22.2 2 ohm load 8 ohm tap 400 (0.047%) 20.0 370 19.7 4 ohm tap 320 (0.028%) 19.0 290 18.6 even with a 2 ohm load. NAD may have other reasons for recommending the use of the 4 ohm tap other than power output; my own best guess is to actually reduce the power delivered, thus reducing the strain and heating on the amplifier. Remember that this is not a brute-force design. Trying to squeeze the last drop of power out of it into low impedances may not be the optimal mode of operation, though this should be less stressful with program material than with test signals.

Next I investigated the action of the soft clip circuit. Fig.20 overlays two curves, THD + noise vs level into an 8 ohm load with normal clipping, and the same with soft clip ping (8 ohm output tap). Both are identical up to about 140W. But note what happens at this point. The soft clip (the leftmost curve above 140W) rises more smoothly to its maximum. The penalty for this is a reduced maximum output and increased distortion above 140W. Figs.21 and 22 actually show the wave form ola 1-khz signal driving the amplifier just into clipping (top traces), along with the associated distortion waveform (bottom traces), without and with the soft clipping engaged into an 8 ohm load. Note the flattened tops and bottoms of the top waveform in fig.21. The shape of the distortion wave form indicates a heavy concentration of high-order harmonics. Note the rounding of the previously clipped waveform when soft clip ping is engaged (fig.22). While it is clearly not a pure sinewave, it now more closely resembles one. The distortion waveform is now predominantly third harmonic. What does not show in these oscillograms (but was noted in fig. 20) is actually an increase in the measured THD +noise and slightly lowered output power when the amplifier is operated above clipping in the soft-clip mode. But the nature of the distortion is more benign, with significantly lower levels of high-order harmonics. Despite this, however, I never felt the need to use the soft-clipping position during the listening tests; the 2400THX has such ample reserve power that pushing it into obvious clipping with program material would involve very high listening levels.

Fig. 21 NAD 2400THX, 8 ohm output, soft clip off, 'kHz waveform just above maximum power into 8 ohms (top), distortion waveform with fundamental notched out (bottom). Fig. 22 NAD 2400THX, 8 ohm output, soft clip on, 1 kHz waveform just above maximum power into 8 ohms (top), distortion waveform with fundamental notched out (bottom).

Fig. 23 NAD 2400THX, normal inputs, 8 ohm output, distortion vs output power into 2 ohms with soft clip on and off.

Fig. 24 NAD 2400THX in bridged mode, normal inputs, 8 ohm output, soft clip off.

THD+noise vs frequency at, from bottom to top: IW into 8 ohms, 2W into 4 ohms, 4W into 2 ohms

Fig.25 NAD 2400THX in bridged mode, normal inputs, 4 ohm output, soft clip off, distortion vs output power, from bottom to top at 100W, into 8 and 4 ohms.

Fig.23 is similar to fig.20, but for a 2 ohm load (8 ohm tap). Interestingly, in this case the difference between the soft-clip and nor mal modes is practically non-existent. With a 4 ohm load (8 ohm tap, not shown), the result falls midway between these two, with a slightly less pronounced, but still clearly apparent, difference between the normal and soft-clip modes.

Finally, I did some investigations of the bridged mode of operation. Here, the 4 ohm tap must be used; there is an interlock between the output impedance-set control and the normal/bridged operation control which prevents the 8 ohm setting from being used. Fig. 24 shows the THD +noise vs frequency for the bridged mode. Note the increased distortion over the normal mode, although the overall levels are still low.

Fig. 25 is the THD +noise vs level for the bridged mode. The lower curve is for an 8 ohm load-producing over 500W into an 8 ohm load (the specifications call for 800W, but remember again the possible time limits on the upper rail of the power supply with our swept tests). The upper curve is for a 4 ohm load. While it appears to follow the 8 ohm curve from about 390W up, actually the 4 ohm curve stops at 390W. At this point the amplifier failed. No fuses blew-it just failed, with the power-indicator light on the front panel going out and the amplifier refusing to perform further duties. Fortunately, all of the tests were finished by this point.

Normally, bridged amplifiers don't like being loaded with less than 4 ohms unless rated for normal operation into well under 2 ohms; the NAD apparently found 4 ohms not to its liking, at least on the test bench. It would probably work satisfactorily into 4 ohms, bridged, with music. But my recommendation would be to keep the load comfortably above that point. I'm not even sure why, from a power-output standpoint in any case, you even need to bridge the 2400THX. It has ample reserves in the stereo mode. If you want to use two of them to get the potential benefits of monoblocks, using them in a bi-amp mode might be more sonically beneficial.

CONCLUSIONS

There's no question that, both sonically and measurably, the NAD is the powerhouse of this duo. Its abilities were not always most evident on musical sources, where the Rotel kept up the pace in the gutsiness department, only falling a bit short in its overall sense of timbral weight. The real strengths of the NAD became evident only with the sonic mayhem of hyper video-sound sources.

There it was quite evident that the Rotel, although it performed valiantly and never really turned sour, could not match the NAD in sheer motive power.

But with music as a source-the sort of use I suspect most of our readers are primarily interested in-the Rotel had a magic about it that the NAD's more workmanlike sound could not match. The Rotel was measurably less at ease with low-impedance loads, yet it drove the under-4 ohm Apogee Stages with relative ease, only having some trouble in providing ultimate control of the bot tom end. Both amplifiers are good performers, and the Rotel is more than that. Unpack it, plug it in, and your first impulse will not be to start a savings account for a super amp, but to spend more money on recordings.

++++++++++

Manuf Comments

NAD 2400THX

Editor: I must thank Tom N. for the tremendous amount of work he has clearly put in to compile this detailed appraisal of the NAD 2400THX power amplifier. The 2400, as he says, is a complex product, and Mr. N. has shown an understanding of its intricacies unparalleled by any other reviewer. There are one or two technical points which he didn't describe quite correctly (or fully); I will come to these later.

The design brief for the 2400THX power amplifier was that it should drive over 100W (IHF continuous) while producing sufficient dynamic power on program material to drive even insensitive speakers in a large room (in this case, over 400W into 4 ohms IHF dynamic). In addition to this, it had to be able to drive "real" 4 ohm speakers (which dip to 2 ohms), and all this with a sound quality which would satisfy the audiophile. The THX licensing indicates that the amplifier is also intended for use in the highest-quality home theater installations.

Mr. N.'s review clearly suggests that the 2400 has succeeded in all its design areas. It produces plenty of power ("there's no doubt that the 2400 sounds more powerful than any 100Wpc amplifier has a right to"). And, most significantly on video material, "when it came to sheer power, there was no question that the NAD outgunned the Rotel." Now the technical points. First, regarding the requirements imposed by Lucas-Film on a "THX" amplifier. In fact, dynamic power capability is not a "major requirement," as Mr.

N. states. The THX spec calls for a minimum of 100Wpc into 8 ohms (continuous) and a minimum of only 1 dB of dynamic headroom.

However, NAD quickly realized the potential of its Power Envelope concept in handling the sometimes grossly "enhanced" dynamics of video sound. Mr. N. experienced the advantages of this time and time again in the course of his review, e.g.: "With the NAD, the sheer impact and power of the truck/motor cycle chase scene was overwhelming." This is exactly what good home theater should be.

The Power Envelope circuitry, as applied to the 2400THX, employs two separate power supplies as Mr. N. describes, one normal supply and one high-voltage supply (of course, each actually consists of a plus and a minus sup ply). This allows the amplifier to drive very much higher output voltages than a conventional 100Wpc amplifier. The 4 ohm/8 ohm switch alters these voltages slightly so that the supplies can be re-optimized to get the best out of each type of speaker without over-stressing the amplifier.

The additional power available from the high-voltage supply is there for musical climaxes and transients-not continuous sine waves. It therefore has additional protection to guard against continuous operation in the form of fuses and PTC (Positive Temperature Coefficient) thermistors. These latter operate a bit like a solid-state fuse with automatic reset. They have a very low resistance (< <1 ohms) at nor mal temperatures, but if they pass too much current for too long (several seconds), their resistance increases dramatically. If the load is removed, they quickly cool down and return to normal operation. As Mr. N. discovered, the 2400 will operate perfectly normally with the high-voltage fuses open (or the PTCs open)-but simply with the power of a "nor mal" 100Wpc amplifier.

As Mr. N. also discovered, if the 2400 is operated well outside its specified operating parameters it will protect itself. Thus he found that the high supply fuses blew when he was driving sinewaves from the high supply into 4 ohms with the impedance switch set to 8 ohms-although no damage was done, of course. (Under these "abnormal" conditions, totally unlike normal music or home theater use, the fuses blew before the FTC protectors opened.) He also found it difficult to measure THD much above 100W, where the high voltage sup ply cuts in. These measurements are best carried out dynamically using an FFT analyzer and tone bursts. The measured 260Wpc into 8 ohms is essentially a continuous power figure. The difference between this and the 370Wpd8 ohm specification is caused by the additional 1.5dB dynamic headroom afforded by the 2400's high supply.

Finally, Mr. N. tried driving the amplifier bridged into a 4 ohm load with a continuous sinewave sweep (although it is clearly marked "minimum 8 ohms" in bridge mode). The result, again, was that the amplifier protected itself. Under these conditions, most of the power is drawn from the low-voltage supplies-which are not fused on the secondary side in the same way as the high supplies.

Their protection comes from a primary side fuse-which opened in this case, causing the symptoms observed.

My last point concerns "Soft Clipping." It is indeed difficult to test this on a Power Envelope amplifier-once again, dynamic signals should be used. Mr. N. was able to observe the more benign harmonic structure in soft clipping, but he missed its most critical advantage. When a conventional amplifier clips- particularly at lower-mid and bass frequencies --the main distortion heard is not harmonic, but power-supply intermodulation products--at the signal frequency ± 120Hz, 240Hz, etc. (assuming the amp has unregulated supplies). Soft clipping acts against a clean reference voltage and prevents this intermodulation- making the clipped sound very much more acceptable. Of course, with the power capability of the 2400, most users would never clip the amp. But it does offer the customer (possibly more particularly the video sound customer) the possibility of even higher sound levels on highly dynamic material without the worry of occasional offensive clipping.

I am sorry if this letter seems nearly as long as the review, but the 2400 packs a great deal of carefully designed electronics, all primarily in the service of music and the music listener, into a neat and low-cost package. I couldn't do it, or the review, justice in a couple of sentences.

CHRIS EVANS; Director of Engineering, NAD

+++++++++++++++++++++++

ROTEL RB-980BX

Editor: The fax machine whirred into life. I struggled out of my chair, glass of Tyrells Long Flat red vino in hand, and wandered into the other room, knocking over, in the dark, a large pile of papers.

Oh! A Audiophile fax. If it's from Ken Nelson it can stay until morning. No, it's from Tom N.. I wonder what he wants? Page after page unwraps itself from the printer; I begin to see that it's TJN's review of a Rotel amplifier.

Time to recharge the glass and settle back and read while I'm waiting for the fax to end . . . Memories of a wonderful dinner in an Indian restaurant in Santa Fe fill my head.

I was visiting John A., his lovely wife Laura, little Harry Jr., and Tom N.. Knowing my love for Indian food, Laura arranged a surprise dinner at a local restaurant. Hard to believe, but Audiophile really does have a superb Indian restaurant nearby, which I highly recommend! It seems strange to me that Santa Fe should have such a spot when other American cities are devoid of this exquisite culinary art form.

Anyway, it was a cold winter's day when I last ventured down to audiophile Mecca in Santa Fe. For those who don't know it, Santa Fe is a long way from anywhere. Audiophile's office isn't exactly easy to get to, since modern transportation doesn't venture close. It's easiest to fly, then drive.

A journey to Santa Fe from the East coast takes a while. It's a plane to Albuquerque, then a two-hour drive via highway-or go the scenic route (recommended by JA); it's a little longer, but well worth it! Once you've arrived from your journey, you notice immediately that you've entered a time zone that is unhurried and quite relaxed-time is on your side, and you must allow for it. You can't rush in and rush out like you can in New York. I think this relaxed pace comes across in Audiophile's editorial.

Once in the swing of things, you can visit a truly picturesque town. The capital of New Mexico has a beautiful square, filled with trees and bordered by adobe buildings. When I was there, you could still see the snow on the mountaintops above the low, flat-roofed buildings.

Lovely as it was, I was not in town as a tourist but rather to experience the Audiophile demonstration room and to listen to some of my products in their listening space. Rooms have a big effect on performance; regular readers of Audiophile will know the difficulties encountered in trying to maximize a room's potential.

And what about the listening session, you ask? What an experience it is to be in the same room auditioning a product with TJN, JA, and LA. Their interaction is energizing. All were quick to point out that this was not a review session. Quite the opposite-it was an opportunity for me to learn more about Audiophile and their criteria for evaluating product.

At Rotel, we care deeply about the sound of our products--a great deal of time is spent auditioning products before they are shipped into the marketplace You can talk about sound, but until you've experienced it in the listening room, you can't fully appreciate it.

So get to the point, Bartlett. After spending time with these talented gentlemen and under standing how they appreciate good-sounding equipment, I am more than flattered that TJN found the Rotel 980 amplifier to be to his liking.

Well, the bottle of Tyrells was well used by the time I'd digested Tom's words, certainly no reason for heartburn or even a vague hangover.

The Rotel piece did what it is supposed to do: namely, bring a little high-end sonic experience to those who can't afford the super stuff.

No, the RB-980 is not built for brute force, nor is it endowed with expensive front panels that do nothing for the music. It's designed for people to enjoy music, by people who enjoy music, and when teamed with an RC 980 $500 preamp, they're a masterful combination. And, if you want real power, a product strong enough to break your toe, with incredible sonic performance, then there's the RB 990, which is still a fraction of the price of a Krell. But that's another story.

Audiophile and Rotel do share another common ground, and that is to reinforce this message to your readers: When you are shopping for new audio components, it is extremely important to find a decent dealer who will let you hear for yourself the differences between products. Don't be lazy, and be prepared to form your own opinion.

MIKE BARTLETT; Rotel America

[adapted from Nov-1992 issue]

Also see:

AUDIO RESEARCH PH1 PHONO PREAMPLIFIER (review)

Forte Models 4 and 6 solid-state amplifiers (review)