Oct 21, 2004 5:21 PM, By George Petersen

STUDIO REFERENCE MONITOR SYSTEM

There are lots of studio monitors on the market today, so when the founders of the popular consumer speaker company NHT formed a separate company to build professional products, they had something different in mind. At last year’s AES show, NHTPro unveiled its flagship A-20 studio reference monitoring system, and it was definitely different from the usual approach to powered monitors.

Essentially, the A-20 is a unique system comprising a two-rackspace control amplifier that connects to two compact near-field speakers via two 20-foot multi-pin cables. However, the key word here is system: The speakers must be used with the control amplifier and vice versa.

From a design standpoint, it is this system approach that sets the A-20 apart from other powered monitors. Rather than build the amplifiers into the speakers—which leads to larger enclosures, heat build-up within the enclosures, etc.—the amps/control electronics are in an external chassis. Therefore, the speakers themselves remain fairly compact, the amplification is precisely matched to the drivers/crossover for optimal reproduction and the system-adjustment parameters are accessible from the amp/controller front panel.

The hefty 34-pound control amplifer houses a 250-watt, dual-mono amplifier and the crossover for the woofer. (To reduce the effects of long cable runs on low-power HF signals, the tweeter crossovers are housed within the speaker enclosures.) The control amp also features three five-position rotary switches: Sensitivity is an input gain adjust, with stops at -10/-3/+4/+11 dB and a mute position. The boundary control provides LF shelving below 400 Hz to reduce the effects of LF build-up caused by nearby walls or corners. When this control is set at 0, the filter is bypassed; each of the four settings below that cut -1.5 dB of LF response. The position switch has five settings that select for near, mid- and far-field response, with two intermediate positions. The position switch is essentially an HF attenuator that adjusts the monitors’ very high-end response using a 20kHz shelving filter; each setting below far-field (a flat response) attenuates the highs by -0.75 dB to adjust for HF losses due to distance, air absorption, etc. The mid-field setting has a -1.5dB roll-off, and the near-field setting is a -3dB filter.

The control amp also has a headphone jack, but I found this of little use. More valuable were some of the control amp’s other features, such as power and clipping LEDs, and a numerical LED readout that’s switched to display incoming line voltage, temperature of the output heat sink or average system SPL. Calibrated at a 2-meter distance from a stereo pair, the SPL display has a 68 to 120dB range and offers a useful indicator of just how loud your ears are being assaulted after hours of studio sessions.

Although monitoring the heat sink temperature or line voltage isn’t something you’d do every day, its use is somewhat akin to oil pressure or engine temperature gauges on a car, where access to this data is preferred to simply having a “fault” light that glows once your system has failed. Also, high readings on the temperature display may simply indicate that the amp needs better ventilation, such as leaving an extra rackspace above the unit for airflow. However, after using the system for several months, even in a tight rack, I never encountered any situations in which heat was a problem, and the unit only ran slightly warm to the touch.

The control amplifier’s rear panel offers flexible interfacing. Inputs are electronically balanced 1/4-inch TRS or XLR (pin 2 hot); the outputs are also 3-pin XLR but carrying a common ground and separate feeds to the tweeter and woofer, and NHTPro supplies a pair of high-quality 20-foot XLR speaker cables that are impedance-matched with the system.

The enclosures are mirror-imaged, with the 8x14-inch front baffle slanted inward so the 6.5-inch woofers and 1-inch aluminum-dome tweeters face toward the listener. This arrangement also creates a cabinet with fewer parallel surfaces. Each speaker’s front panel also has a power-on LED, which is phantom-powered from the control amp. Due to space considerations within the cabinet, the woofer has only partial mag-shielding, but as long as the speakers are kept at least a foot or so away from video/computer displays, image distortion is not a problem.

In session, the A-20 system was impressive. Setup was simple—little more than plug and go. The 20-foot cables included with the system provided ample length for most controls rooms, but just to check it out, I tried extending the stock cables with two high-quality 20-foot XLR mic lines. The net effect was subtle, with a slight perceptible change in the HF reproduction and perhaps a slight loss of tightness in the bass. Obviously, it’s best to use the system’s stock cables, but it’s nice to know that alternate cables can be substituted in an emergency—such as leaving your cables at home after transporting your A-20 system to a studio across town. Thankfully, NHTPro had the foresight to base its system’s multi-pin cabling around readily available items as opposed to something like 17-pin Tuchel jacks.

I began my listening to the A-20s with the monitors mounted on stands behind the console. At a mere 17 pounds each, these aren’t liable to crush most meter bridges and present no challenge to most speaker stands. I used the A-20s in an equilateral triangle arrangement with the speakers about five feet apart and seating about five feet back from the monitors. In this particular space, the rear wall of the room was about 15 inches behind the speakers and there was a bit of overzealous LF build up; switching the boundary control one click back (-1.5 dB) from its flat position brought the bass into balance. Here, I liked the ability to make such changes from the sweet spot and I could quickly make A/B comparisons between settings. Of course, in a “traditional” powered speaker (with controls on the back panel), the notion of tweaking without leaving the listening position or making quick A/B changes is simply impossible.

My net impression is that the A-20s offer a huge sound from small boxes. They are accurate and smooth, especially through the critical 2.1kHz crossover region. The top end is flat and extends well past 17 kHz, and I liked the HF response best with the system set in the far-field position, even when I was listening mainly in the near- and mid-field.

Unlike most small studio monitors, the A-20s have sealed (non-orted) enclosures, which reduces their efficiency somewhat, but given the punch of the system’s 250W amp, the A-20s are capable of ample (110 dB-plus) SPLs for most small control rooms. The low end is nicely defined—never tubby or overblown—and as one would expect from compact 6.5-inch woofer design, and the LF response drops off sharply below 60 Hz, which is adequate for most applications. Ayone dealing with LF-heavy material—whether rap, rock or pipe organ—may need to consider adding a subwoofer to the system.

Overall, the A-20s reproduced instruments, taped tracks and all kinds of program material with clarity and accuracy. The off-axis response was free of undue coloration, creating a usable sweet spot that was wide enough so that the sound remained constant even when reaching off to the side to tweak a kick drum EQ on channel 1. The speakers’ low-distortion reproduction translated into low ear fatigue over extended listening sessions, and the system’s tight phase response offered excellent imaging throughout the soundstage. And no adjustment period was necessary: You can sit down in front of the A-20s, track and mix that same day, and sleep at night knowing your mixes will sound good on other systems ranging from boomboxes to huge stereos.

At $2,000 a pair (including cables and 250W amplifier), the NHTPro A-20s aren’t exactly in the low-budget league, but any serious listener looking for a well-crafted, compact system should give these an audition.

NHTPro, www.nhtpro.com.

Lab Analysis: NHTPro Model A-20
By Ron Horowitz, John Schaffer and Rob Baum

Speaker Mechanical Description
The NHTPro Model A-20 cabinet is constructed of double-laminated 3/4-inch miter-folded MDF. The finish is a black laminate on the outside with an inner liner that appears to be of an impregnated-fiber construction. Double lamination reduces warping, raises cabinet stiffness and increases the box strength.

Both drivers are flush-mounted, reducing early reflections and related comb filtering. The front baffle is canted at 21 degrees, which reduces the severity of internal standing waves. The speaker is of the acoustic suspension type and stuffed with polyester fiber.

The speaker connector is a locking XLR mounted in a recessed terminal cup on the back of the cabinet. The connector utilizes a common ground for the tweeter and woofer circuits, with phantom power on the tweeter circuit for the power status LED. A pair of XLR cables are included with the A-20 amplifier and were used for all tests.

Vifa of Denmark makes the cast-aluminum-frame, 6.5-inch woofer. The woofer has a treated-paper, curvilinear cone; treated-paper dust cap; and a butyl-rubber half-roll surround. The spider is flat, measuring approximately four inches in diameter and has four broad pleats. The motor structure uses a 13/4-inch-diameter copper-clad aluminum voice coil; a vented, staked pole piece; and a bumped back plate. A bucking magnet is used for shielding without any shielding can. Electrical connections are via 0.110-inch and 0.205-inch male connectors.

The 1-inch aluminum-dome tweeter is by SEAS of Norway. The tweeter uses a shallow aluminum dome; a separate, treated-cloth half-roll surround; and a vented aluminum voice coil former. The voice coil is underhung, with 180-degree lead-outs to reduce rocking. The coil operates in a ferrofluid-filled gap. Electrical connections are via a pair of 0.110-inch connectors mounted at 180 degrees under the faceplate. The pole is not vented or chambered, and the driver has a small bucking magnet and can for magnetic shielding. The faceplate is black plastic, with a high-frequency diffuser for improved off-axis performance.

Amplifier Description
The NHTPro A-20 system is very unusual in comparison to other monitors we have tested in that the amp is external to the speakers and the crossover is passive. The two-rackspace A-20 amplifier houses the passive woofer crossovers, while the tweeter crossovers are in the loudspeaker cabinets. The A-20 amplifier and speakers are not usable with other speakers or amps.

The A-20 amplifier’s front panel has three rotary controls for sensitivity, boundary proximity and distance. The five-position sensitivity control has steps at -10, -3, +4, +11 and M for mute. All acoustical tests were made using the +11 position. The boundary control provides low-frequency equalization, according to the manufacturer, to offset the acoustical bass-boost effect of wall, floor or ceiling mounting, and is marked in the number of reflecting surfaces, i.e., 0-2. (Presumably, a “3” setting for corner mounting was not considered necessary.) The user adjusts the control to match the acoustical environment, yielding the flattest possible low-end response. The distance control, on the other hand, provides high-frequency equalization to offset the high-frequency effects of near- and far-field monitoring. This five-position control is calibrated from NF (near-field) to FF (far-field) and is adjusted accordingly for various listening positions.

The front panel houses a speaker-muting headphone jack, a rocker-type power switch and a momentary pushbutton-type switch that controls a three-function LED. The LED can be toggled among SPL mode to display the approximate sound pressure level at two meters, “VAC” mode to display the mains voltage or TMP mode to display the amplifier operating temperature. Although we didn’t test the VAC display with a Variac, the VAC display appeared to be running several volts high when measured against a Fluke 87. No calibration control is provided for the VAC display. We were unable to verify the amplifier temperature display, as the amplifier wasn’t bench-tested and never got hotter than ambient during acoustical tests.

All electrical connections, except for the headphone output, are made on the amplifier back panel. Either XLR or 1/4-inch stereo phone plugs (configured as a balanced mono connector) make low-level connections, while speaker outputs are by XLR. The amplifier uses star-type grounding and there is no “ground float” provision on the chassis. The amplifier has two large, heavy, EI-type power transformers positioned at either side of the chassis, which yields a balanced but heavy package.

Acoustical Description
The NHTPro Model A-20 has a flat frequency response and low distortion. The A-20 measures about 3 dB from about 100 Hz (the lower limit of the frequency response measurements) all the way out to 20 kHz. The exception is an octave-wide, 2dB shallow dip from 4 kHz to 8 kHz. The 30° off-axis response is also very smooth.

The A-20’s impulse response is very good. The transient response indicates the drivers’ acoustic centers are well aligned. The impulse is damped. There are slight ripples in the response, with a smooth and rapid decay afterward.

The distortion of the A-20 system is low. From 120 Hz on up, the THD remains at or below 0.5%, dominated by a third harmonic. However, at 50 Hz, the distortion is fairly high, more than 10%, which is about what can be expected from a 6.5-inch driver, even a very good one. Above about 5 kHz, the distortion drops even further to around 0.2%, in part an indication of the high quality of the drivers selected by NHTPro.

The spectral contamination measurement tests a speaker system’s nonlinear distortion, meaning the amount of nonharmonically related distortion products generated by the device under test. Spectral contamination measures the most objectionable type of distortion and is a good indicator of a system’s subjective “clarity.” The A-20 measured well on this test of self-noise, with distortion products approximately 50 dB below the input tones. The A-20 measured better than most monitors did, except below 100 Hz, and about as well as other low-distortion powered speakers.