NHT Pro A-20 Speaker System

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Professional studio engineers check the details of their mixes on huge monitor speakers built into their control-room walls but then switch off to “near-field monitors,” small speakers balanced precariously atop the mixing console, to check how those mixes will sound on typical small speakers. For decades, two particular near- fields dominated the studio scene. Yamaha’s NS-10M, an unremarkable small bookshelf type, stood in for the typical home hi-fi speaker, sometimes with Kleenex draped over its tweeters to soften a somewhat frisky top end. And almost always, a pair of tiny Auratones sat adjacent, 4-inch whizzer- cone cubes uncannily able to mimic the sound of cheesy car stereos, clock radios, and low-rent boomboxes.

above: The foam strip near the A-20s tweeter presumably cuts down diffraction effects and side-wall reflections.

But over the years, a funny thing happened. Engineers and producers began noticing that they often heard “more,” “better,” or “deeper” via their cheap near- fields than from their hulking, big-dollar, horn-loaded bruisers. Today, in the production of music both pop and classical, it’s quite common to mix and master using near-field monitoring predominantly or exclusively.

The reason is simple. When you sit with in a meter or so of the loudspeakers, your ear is swamped by direct sound, taking the profound effects of room acoustics (even in “scientifically treated” rooms like big-time studios) pretty much out of the picture; you hear what you put on the tape (or disc), with less influence from your listening room’s acoustics. Pro audio manufacturers have therefore been quick to produce small, high-performance two-ways specifically targeted at this market. Some of these are almost identical to “hi-fi ” two-ways, distinguishable only by their by top-grade, high- power-handling components, careful voicing for accurate response, and Unit costs many times the price of similar-looking home models. Many others, however, are “active,” having built-in amplifiers and electronic crossovers.

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Rated Frequency Response:

48 Hz to 20 kHz, ±2 dB; —6 dB at 40 Hz (in room).

Rated Amplifier Power:

250 watts channel continuous, 400 watts/channel for 1 00-millisecond pecks.


Speaker, 7 in. W x 14 in H x D (19cm x 35.0 cm x 30.2 cm), amplifier/controller, 19 in.

W x 3 in. H x 10’/ in. D{48.3 cm x 8.9 cm x 27.4 cm)

Weight: Speaker, 17 lbs. (7.7 kg), amplifier/controller, 42 lbs. (19 kg).

Price: $2,000/pair, including amp Company Address: Vergence Technology, 555 First St., #302, Benicia, Cal. 94510; www.NHTPro.com.

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The advantages of active loudspeakers are well known and should require little rehearsal here: Improved efficiency and reduced distortion (thanks to active electronic crossovers, which eliminate the inherently dirty, watt-sucking passive di viding circuit); ditto, improved dynamic potential and peak output; superior transient behavior, double-ditto; flatter top octave and smoother inter-driver transition response, triple-ditto; and so on. Better still, powered speakers are a natural for custom-tailored equalization to smooth response, improve accuracy, and extend bass response, so most designers are happy to include it.

And so we come to NHTPro, a new company born of the peregrinations of NHT, a home-fl brand originated by longtime audio designer Ken Kantor, previously of Acoustic Research and elsewhere. NHT quickly established a prominent rep for high-value, low-silliness designs capable of excellent sound; Kantor, a lifelong musician and recording semipro, seems to have set out to steer his new company along the same path in the pro world. NHTPro’s charter is to bring to the studio the level of reproduction quality familiar to high-end fanatics the world around, via small, afford able monitors that make such quality easily repeatable from session to session and from studio to studio.

above: NHTPro A-20 speaker system amp.

The A-20 is NHTPro’s “statement” de sign, an active two-way with a difference:

The 250-watt/channel stereo amplifier and associated electronics (interestingly, NHT Pro chose to go with a conventional passive crossover rather than an active line-level circuit) reside not in the speaker cabinet itself but in an ordinary-looking rack-width chassis. Placing the electronics away from the enclosures re moves their heat, electromagnetic influence, and noise potential from the speakers themselves, reduces the speakers’ bulk and weight, and puts the processing controls within easy reach in stead of on the backs of the two speakers. Yet this dedicated “amplifier/processor” retains all the custom- engineering advantages of active design.

From a distance, the A-20 monitors look much like NHT SuperOne home two-ways, but they have higher-quality drivers (6½- inch treated-paper woofer and 1-inch aluminum-dome tweeter) and a slightly larger acoustic-suspension enclosure. A strip of foam, rather like weatherstripping, borders each tweeter’s outer edge, presumably cut ting down diffraction effects and side-wall high-frequency reflections. The box has the signature NHT slash-cut baffle that aims inward some 30° or so, obviating any need for toe-in and giving the enclosure one fewer pair of parallel internal surfaces. The speaker’s cabinet is made up of heavy, ¾- inch MDF and sounds very dead to knuckle-raps. A tiny green LED, located between tweeter and woofer, glows when the controller is on.

The A-20’s rather gorgeous finish de serves a mention: It’s a subtly sumptuous, “Steinway-black” ebony-toned wood-grain with tight, excellent joinery and slightly chamfered edges. The A-20s do not have grilles. However, when I think of the cigarette (and, er, other) burns, beer rings, and carved initials to which these speakers will be subjected in the typical studio, I want to weep.

Each A-20’s rear panel presents only a recessed female XLR jack, which accepts the swanky red, dedicated 25-foot cables from the A-20 Control Amplifier. These multi-conductor cables are necessary because the amp delivers discrete tweeter and woofer signals (and has to power those spiffy baffle-board LED5). Besides the corresponding male XLR outputs, the amp’s back panel holds balanced XLR and unbalanced ¼- inch inputs (with pin-configuration info screened right on the panel—yay a fused IEC AC-cord receptacle, and a user-accessible AC voltage selector behind a clear panel.

On the control amp’s face are a big power key, a large green numeric display, and three controls. There’s also a headphone jack, which mutes speaker output whenever something’s plugged in—a very handy feature, as you can never have too many good- sounding headphone outputs in the studio.

---The A-20s’ sonics are so accurate that commentary seems almost superfluous.---

The three controls, all five-position rotary switches, are marked “Sensitivity,” “Boundary,” and “Position.” The first has four gain settings, 7 dB apart (+11, +4, —3, and —10), plus muting (“M”); the discrete steps make level-set repeatability a literal snap. The “Boundary” knob manages out put in the bottom octaves, compensating for the reinforcement of—you guessed it— boundary surfaces; its five settings range from “2” (suggested for quarter-space, in- corner locations) to “0” (for full-space, free-standing setups). It sounded to me like this knob was a low-frequency “tilt” control, hinged at about 400 Hz and with 1 or 2 dB per step; I suspect many users will simply set it to taste, matching their expectations (heavily influenced by earlier experiences with predecessor speakers), regardless of physical setup. “Position” does much the same thing, but over the top two octaves or so; its marked settings are “NF,” “MF,” and “FF,” for near field, mid-field, and far field, plus two unmarked intermediate positions. This control is said to compensate for power response changes due to room influences and air absorption. It, too, sounded like a simple tilt filter, this one hinging at maybe 4 kHz and with perhaps 1 dB per step.

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above: The rear-panel inputs are professional XLR female jacks; the standard connecter for recording studios.

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Figure 1 shows the NHTPro A-20’s on-axis frequency response with its amplifier’s “Boundary” and “Position” controls at several different settings.

Responses were taken with each switch at three of its five possible positions: full counterclockwise, midway, and full clock wise. These correspond to indications of “2,” “1,” and “0” for the “Boundary” control and “NF” (near field), “MF” (mid-field), and “FF” (far field) for the “Position” control. The curves combine ground-plane measurements in the bass range with measurements taken in a large anechoic chamber. The test microphone was located halfway between the woofer and tweeter.

Fig. 1—One-meter, on-axis frequency response.

Fig. 2—On-axis phase response and group delay.

Fig. 3—Horizontal off-axis frequency responses.

The flattest frequency response (which is very flat indeed) resulted with the “Boundary” switch set to “0” and the “Position” switch set to “NF.” This is as it should be, since these settings are the ones most appropriate for the test conditions. Higher “Boundary” control values yielded progressively greater rolloff below 600 Hz, to compensate for the acoustical low-frequency augmentation that will occur when the speaker is placed close to one or more room boundaries. With the control set to “2,” the axial output was reduced by about 5 to 6 dB below 100 Hz. Each click of the “Boundary” control provided about 1.5 dB of bass adjustment at 50 Hz. The “Position” control provided gentle treble lift above about 3 kHz, with a maximum boost of about 2.5 dB at 10 kHz and 4 dB at 20 kHz. Each click of the “Position” control altered the output about 0.6 dB at 10 kHz and 1 dB at 20 kHz. With the controls set to give flattest response under the test conditions, a 2.83-volt output from the amplifier (equivalent to 1 watt into 8 ohms) generated an average sound pressure level of 86 d from the speaker between 250 Hz and 4kHz.

By driving the tweeter and woofer individually and then comparing their responses (not shown), I determined that the acoustic crossover frequency was 2.2 kHz. The outputs of both drivers were down 6 dB at that point, which indicates a good in- phase crossover design. Above this frequency, the woofer rolled of at 12 dB/octave; below it, the tweeter rolled off at a higher rate of 24 dB/octave. Between 100 Hz and 20 kHz, the outputs of the right and left speakers matched within a close ± 1 dB, with deviations spread equally over frequency.

Figure 2 shows the A-20’s phase and group-delay characteristics referenced to the tweeter’s arrival time. As with most other speakers I measure, the woofer acoustically lags the tweeter slightly. When averaged from 400 Hz to 2 kHz, the group delay indicates a difference of about 0.3 millisecond, due to both crossover and physical offset effects. The lag is reflected in the phase curve, which falls continually with frequency until about 6 kHz, where it levels out in the tweeter range. A separate plot of waveform phase (not shown) indicated that the A-20 will not preserve the waveshapes of signals it reproduces in any frequency range, which, again, is typical of conventionally designed speakers.

Fig. 4—Vertical off-axis frequency responses, upward (A) and downward (B).

Figures 3 and 4 show the A-20’s horizontal and vertical off-axis frequency responses, respectively. The horizontal responses, measured at angles of 0°, ±15°, ±30°, and ±45°, are quite well behaved. Only slight rolloff above 14kHz is evident at 15° off axis. At greater angles, the high. frequency response exhibits progressively more rolloff. In addition, some midrange and upper-midrange narrowing is evident, between 700 Hz and 2.5 kHz.

The upward and downward off-axis responses (Figs. 4A and 4B, respectively) are similar to the horizontal responses except for significant narrowing in the 2-kHz crossover region, evidenced by dips in the 30° and 45° plots. The up and down responses are fairly symmetrical, except for a deeper dip at 30° downward than upward. The symmetry is an indicator of good in-phase crossover behavior.

The A-20’s 50-Hz second- through fifth- harmonic distortion (Fig. 5) is referenced to the point at which the system’s amplifier overloads (at or near its power rating of 250 watts), as indicated by the red front- panel clipping LED. The drive levels are given in dB below the clipping point. At power levels above —10 dB (roughly 25 watts), the third and fifth harmonics rise swiftly, reaching a very high 107% third and 28.8% fifth. Odd harmonics indicate symmetrical overload of a driver’s suspension; it simply runs Out of excursion capability. Although high, the A-20’s distortion didn’t sound as bad as the numbers might suggest, and the woofer suffered no permanent damage. These distortion values also indicate that NHTPro has chosen not to include any form of limiting in its power amplifier.

Fed high-level sine-wave sweeps, the speaker exhibited no significant cabinet- wall vibrations and was quite rigid. The woofer had a generous excursion capability of about 0.4 inch, peak to peak, and over loaded quite gracefully. No dynamic offset was apparent at any drive level or frequency.

The A-20’s intermodulation distortion versus input level for equal-amplitude tones at 50 and 440 Hz (A is shown in Fig. 6, up to the amplifier’s clipping point. The amplifier controls were set to yield flattest response. The IM distortion rises smoothly up to the —10-dB point, then in creases more rapidly at higher power levels, reaching 20% at full power. Setting the amplifier’s “Boundary” switch to “2,” which produces the least amount of bass, reduced the IM distortion at full power to less than 14.9% (not shown). Note that this setting of the “Boundary” switch effectively reduced the 50-Hz power applied to the woofer in relation to the 440-Hz power, because in this test I maintained equal levels at the input of the amplifier.

Figure 7 shows the A-20’s maximum short-term peak output for a shaped tone- burst input signal. Three characteristics stand out: the curve’s overall smoothness, the low output in the bass range below 80 Hz, and the relatively low maximum Out put above 400 Hz. Larger speakers are usu ally capable of significantly more deep- bass output. However, the A-20 competes very well in this respect with other systems of comparable size.

Fig. 5—Harmonic distortion for 50 Hz.

Fig. 6—IM distortion for A (440 Hz) and 50Hz.

Fig. 7—Maximum peak sound output.

Subwoofer it ain’t, but why doesn’t the maximum level at high frequencies rise into the low to mid 120-dB range, as with most of the speakers I test, instead of top ping out at about 110 dB above 3 kHz? Be cause I normally use an amplifier much more powerful than the 250-watt unit sup plied with the A-20s. My amplifier is some 10 dB, or ten times, more potent. But in the A-20s’ designed application, as near- field monitors, this is scarcely a concern. Placed within 3 to 5 feet of your listening position, they will play quite loud indeed.

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My favorite front-panel feature, however, was the A-20 controller’s three-digit display. Via an adjacent “Mode” key, this cycles between showing speaker output in dB SPL, line voltage (if this goes to 0, your power is out!), and heatsink temperature in Celsius (the amp shuts down at about 1000 C). The dB SPL display will, I suspect, be welcomed in pro studios, as it furnishes a quick-and- easy reference for actual monitoring loudness. This promotes consistency from session to session and even across multiple projects, which can be important, since overall monitoring loudness profoundly influences mixing balance. (The display is, in fact, quite accurate, since the designer had foreknowledge of listening distance and speaker and power amp sensitivities, the key factors affecting output. My trusty Radio Shack sound level meter’s readings matched the A-20 controller’s setting with in ±0.75 dB SPL at the listening position.)

I set up the A-20 system on the workstation rack-desk that occupies one end of my 21 x 16-foot studio/listening room. The speakers sat about 40 inches apart and a like distance from my head. This positioning is more or less free-space: The room’s front wall is 12 feet distant, and the nearest side wall is 4 feet to one side, slanting away at 45° to become the back wall about 9 feet be hind me; the nearer portion of the angled wall is modestly treated to reduce focused reflections. I played DATs with my old (but hale and still really great-sounding) JVC deck routed via an Apt/Holman preamp to the A-20 controller’s unbalanced inputs; hard-disk audio at a 48-kHz sampling rate arrived via a Korg l68rc (16-bit) digital mixer. I also auditioned many CDs via a direct-connected Sony ES player.

After so lengthy a preamble, I actually have embarrassingly little to say about the NHTPro A-20’s sonics: They are so accurate that commentary seems almost superfluous. Tonal balance is flat, flat, flat; the A-20s sounded as if they would prove to be the most ac curate speakers, measured anechoically, I’ve ever had in-house. This makes them, of course, entirely ruthless. A rather nice two-piano concert (Brahms and Messiaen) I recorded with a simple spaced omni setup years ago sounded great: airy and extremely detailed. I could hear—and vividly recall—all the flaws in each piano’s setup, such as a mild “crink” in the mechanism of one’s soft pedal (which always gets a workout in Messiaen) and a subtle buzz from one A-flat of the other. I also got an excellent representation of individual piano tonalities, of hall sound and air, and of back-wall bounce. But, boy, was this recording noisy by modern standards. It was recorded live to 15-ips stereo on a half-track Revox A77, wonder fully quiet by the standards of the day, then later dubbed direct to DAT. But the virtually horizontal top octaves of the A-20s’ response brutally showcased what we would today consider unacceptable noise.

The A-20 system’s bass sounded unrestricted down to about 50 Hz. There was useful output to perhaps 40 Hz, and then it fell off pretty steeply below that point. So, for example, on Natalie Merchant’s Ophelia, an excellent-sounding CD (Elektra 62196), the very loose and deep bass drum on “Thick As Thieves” sounded weighty and solid, with easily discerned bloom from the very soft beater, but it did not have nearly the bottom-octave weight and sternum-thumping impact as on my “big” system (Platinum Solos supported below 40 Hz by a B&W ASW 800 subwoofer) at the other end of the room. Overall, despite my more-or-less free-field setup, I had a clear preference for a “Boundary” setting of “0.5” or even “1,” a click or two counter clockwise from the “0” position; these yielded a tighter, more transparent feel that I felt outweighed their mild sacrifice in low- frequency output. But even set at “0,” the NHTPro A-20s delivered what most hi-fi listeners would characterize as a somewhat lean bottom end, very easily listened through for low-midrange sounds, edits, flaws, hums, and the like.

Interestingly, vocal tones changed subtly over relatively small shifts in listening distance. Throughout the Merchant CD (and other vocal music), when I listened from close in, as I might while actually working the mixing board, I heard a hint of the slightly nasal, up-front tonality I associate with speakers whose response, measured anechoically, is ultra-flat. But rolling my chair back just 18 inches or so brought a subtle but audible change to a more traditionally “hi-fi ” balance—a little less nasal, with a little more warmth and “hoo” to the lower-midrange region. I quickly began responding to this as a virtue rather than a flaw, because it let me change from microscopic, headphone-like monitoring to a more real-world/end-user overall-music effect, merely by leaning forward or back. (You will hear a similar shift when you move super-close to ‘most any decent small two-way, but it’s unusual for a speaker to sound this good both ways.)

The “Position” knob did not really affect this phenomenon at all. It seemed instead to affect still higher frequencies, giving me a gradual increase/decrease of “air” or “tizz,” depending on how much of either was on the recording. I had a clear preference for the control’s near-field, fully rolled-off, setting. For instance, on acoustic-guitar recordings I’d made with a single large-diaphragm condenser, the more far-field settings emphasized the pick-attack “plink” more than I was accustomed to or desired.

A country-western-style tune I have on hard disk in multitrack form, not yet mixed, referenced very closely to my usual monitors (no great surprise; quite by coincidence, they’re NHT Super- Ones). The tune’s dominant instrument is a Telecaster guitar played through a Fender Deluxe reverb, recorded via a single, off-axis large-diaphragm condenser mike mixed with a phase-inverted Shure SM-57 dynamic mike hung behind the guitar amp’s speaker (via the pro-sanctioned method of wrapping the mike cable around the amp handle). This setup makes for a pretty danged spanky guitar sound: The A- 20 system delivered it with full, honking twang and lots of pick-attack click’n’scrape, with each individual slap-boing of the amp’s reverb springs clearly audible. My ear could easily pick out each mike’s contributions (even though that shouldn’t theoretically be possible unless I had the chance to fade from one to the other). The NHTPros’ even, extended treble also made the guitar amp’s inherent noise a bit more obvious than my usual setup. (You’re probably ask ing yourself why rock recordists spend thousands on high-end mikes and 100-dB recording systems when the 30-year-old guitar amps that anchor most sessions offer dynamic range of maybe 60 dB on a really good day. That’s an excellent question; can I get back to you on it? I gotta run out and spend $975 on a ‘58 Champ with a hum problem....)

The A-20s will not deliver the 120-dB SPL average levels old-school rock gods are thought of as desiring, but almost nobody monitors that way anymore. (What’s that? Speak up, sonny!) They did sound just great up to about 106 dB SPL (at 1 meter), retaining full definition and unchanged tonal balance; much beyond that point they began sounding a little forced to my ears. Then again, this might’ve just been my ears.

I found the A-20 system’s imaging, at least in terms of soundstage, to be unusually stable as I moved forward or back about a foot in each direction. This is highly desirable, since you can never really hold a single position as you work at mixing. I’ve always found near-field spatial sound a bit paradoxical, especially from good recordings. For instance, on Arvo Part’s Fratres for string orchestra, an excellent EMI CD (7243 5 55619 2), the NHTs spread out a lovely lateral soundstage, very cohesive and stable from the first desk of fiddles to the farthest- right bass; they also yielded an excellent illusion of depth, placing the percussion ensemble on an almost palpably inset back tier. But—and I’ve noticed this from all near-field monitors—the depth dimension seemed a bit cone-shaped, sounding decidedly less pronounced toward the outer edges of the sound field.

I’m not sure the NHTPro A-20 is a system on which I’d want to drop $2,000 for recreational listening; it’s too devoid of the euphonic colorations and sonic melanging that we routinely tolerate—or encourage— even from high-end loudspeakers. On the other hand, two large is downright cheap for the kind of precise, ultra-close vantage on incoming audio that the A-20 system provides. The result isn’t always pretty, at least at my place, as my productions are too often flawed, sonically and musically, for me to be entirely thrilled by hearing them quite so well. But if you’re serious about music production—as distinct from music reproduction—that queasy feeling on play back is something you must learn to live with, since perfection is but a theoretical state.

In short, NHTPro’s A-20 combo produces just the sort of reproduction you need for serious recording. A bit ironically, given near-field monitoring’s hi-fi origins, similar monitors are now appearing for similar money (or more—in some cases much more) from the major pro audio brands, names largely unknown in the hi-fi world. But several of these clearly don’t match the A-20 system’s resolution, and I’ve yet to hear one that seemed to deliver greater resolution in any meaningful way.

(Source: Audio magazine, Sept 1999)

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Updated: Sunday, 2018-05-06 7:44 PST