Oct 1, 2001 12:00 PM, BY MICHAEL COOPER


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Speaker manufacturers have endeavored for decades to create a reference monitor that could reproduce complex, broadband audio without imposing a sonic signature of its own. But the immutable laws of physics demand that speakers must exert some physical and electrical influences on whatever signals they transduce, thereby skewing the sound. Which prompts the question: If you can't prevent something, then why not simply undo it? That's what German speaker manufacturer KS Beschallungstechnik GmbH has done with its new ADM line of active, digital studio monitors.

KS had the brilliant idea to use high-powered DSP to program a unique inverse transfer function equalization curve for each of the ADM Series monitors, thereby neutralizing any inherent coloration effects. (For an in-depth look at the technology behind the KS ADM 2, see the sidebar “Thinking Inside the Box.”) In theory, the compensated monitors would then offer virtually perfect impulse and frequency response.

KS's ADM 2 Studio Monitor lies in the midrange, both in terms of price and size, of the ADM product line. An integrated system, the ADM 2 consists of a two-way monitor with internal amplifiers and AD/DA converters, onboard DSP (including 250-band equalization) and an optional remote level controller. As one would expect, all this costs a pretty penny: $4,950 list per monitor, or $9,900 for a pair. (The remote costs $295 extra.) But, you also get what you pay for. What I heard blew me away like a hurricane. The ADM 2 represents a paradigm shift for studio reference monitors.


The ADM 2 can accept analog or digital input via connections on the cabinet rear. A female XLR jack routes analog input to 24-bit sigma delta, 64× oversampled A/D converters. An AES/EBU connector accepts digital input at 32 to 56 kHz (continuously variable) sampling rates. Wordclock inputs are noticeably absent. Sixty-MHz, 32-bit floating point DSP provides the FIRTEC digital processing (see sidebar for more info) and crossover and protective limiting functions for the monitor's two drivers. Latency is specified at 6 ms for digital input, 7 ms for analog.

An internal 24-bit DAC feeds two MOSFET power amps for each monitor; a 100-watt amp powers the high-frequency driver, while a 200W amp drives the woofer. The slew rate for the power amps is a respectable 80 V/ms or better. A pair of ADM 2s produces a maximum peak SPL of 122 dB and continuous SPL of 116 dB at one meter. The ADM 2's frequency response is specified as 50 to 22k Hz, ±0.5 dB, and 38 to 22k Hz, ±3 dB.

You'll want some heavy-duty monitor stands for the ADM 2s. Each monitor cabinet weighs approximately 43 pounds, and measures roughly 16½ inches high by 12 inches wide by 12¾ inches deep. KS needs to provide more substantial packing for shipping these beauties — the cardboard inserts on the inside of the box were totally inadequate and did not prevent two units from arriving damaged. (KS has indicated that it will rectify this shortcoming, possibly substituting returnable flight cases for cardboard shipping boxes.)

KS's literature specifies that the ADM 2 incorporates an 8-inch radiator driver and a 1-inch compression driver. But the high-frequency driver is actually closer to five inches in diameter. It's mounted directly behind an extruded, exponential waveguide that provides a 1-inch-diameter opening for the driver (hence, the 1-inch specification). The woofer cone is made from lacquered paper and is coupled to a rubber surround. A front-firing, elliptical bass reflex port, tuned to roughly 42 Hz, graces the gray-black, textured, 22mm MDF cabinet. Mounting hardware is provided on both side panels, for horizontal orientation.

A small toggle on the ADM 2's rear panel switches between analog and digital audio input modes. In addition to the female XLR for analog input, there is a male XLR output. This provides audio pass-through to KS's optional ADM W subwoofer ($3,250). There are also both male and female XLRs for digital audio connections. The female jack accepts AES/EBU input, and the male serves as a digital signal pass-through to a second speaker. It doesn't matter which speaker gets the input first; simply patch an AES/EBU cable from, say, your digital mixer to either ADM 2 in a stereo pair, and then patch another AES/EBU cable from the first speaker's digital pass-through (male XLR) connector onto the second speaker's digital audio input.

A front panel LED serves as both a power status indicator and as a guide for L/R speaker setup. The LED for the left speaker is situated in the bottom-left corner below the woofer (when the speaker orientation is vertical); the right speaker's LED is in the bottom-right corner.

A rear panel stepped control knob attenuates the ADM 2's level up to 32 dB in 4dB increments. This works for both analog and digital audio input modes and, because the attenuator follows the onboard DAC, no reduction in wordlength occurs. An additional knob setting, marked Off, provides infinity attenuation (muted output).

The ADM 2 can also be attenuated via the optional Model 01091 handheld, wired remote box ($295). This is connected to one speaker cabinet's rear panel via a captive 7-meter cable, fitted with a D-type connector. You then patch a companion cable of equal length from the first speaker to the next, using a pass-through D connector on the first monitor's rear panel. The remote allows the user to adjust levels simultaneously for all ADM 2s in a system (including multichannel surround setups); level adjustment is post-DACs in order to maintain bit depth. While a mixing console control room monitor pot will also provide control over analog levels, the remote's control over digital levels makes it especially useful. The remote also provides a convenient toggle for switching between analog and digital audio input modes. Furthermore, the remote box's level attenuator knob provides much smaller increments of level change than the speaker's rear panel control. Nevertheless, I was pleased to find that the rear panel attenuators remain active with the remote hooked up.

Also found on each monitor's rear panel is a low-frequency room equalization knob. This steps through the two FIR equalization presets (see sidebar), plus eight other filter settings that modify the presets. The eight modified filter settings provide varying degrees of boost or cut at 20 Hz, with four settings modifying the inverse-transfer function preset curve, and another four modifying the preset that also counters room modes.

A power switch, detachable AC cord and heat sink (for the onboard amplifiers) round out the ADM 2's rear panel features.


For my critical listening tests, I listened to some of my favorite CD releases that I consistently use as a reference, spanning rock, pop, country, techno, folk and Celtic music styles, plus several of my recent stereo mixes. My review units only had the inverse-transfer function FIR curve and its four modified filter settings stored in memory, as I did not request additional DSP to correct for room modes. Although the modified filter presets could be very useful in some control rooms, the ADM 2s sounded best in my room using the unmodified FIR preset (i.e., no additional boost or cut at 20 Hz), so that's what I used to review the speakers' performance.

I could immediately tell that the ADM 2s' FIR curves were calculated from test signals passed through the monitors' A/D converters, because analog input mode sounded so much better than digital input mode. (It's a real testament to KS's FIRTEC technology that, even with one additional conversion, the analog inputs eclipsed digital in sound quality.) Compared to digital, analog input exhibited noticeably better transient response, more extended and crystalline highs, slightly smoother high mids, more open low mids and tighter bass. The pitch of bass guitar notes was also better defined with analog input. In comparison, digital input sounded a tad flabby in the bass, less open and detailed, and exhibited a slight glare in the upper mids. It's not that digital input mode sounded bad, however; it actually sounded quite good. But analog mode sounded so spectacular that digital suffered by comparison.

In fact, I'd never before heard two-way monitors achieve such accurate spectral balance and extended response. Particularly impressive was the reproduction of low bass frequencies. You won't need a subwoofer with these babies. When the subterranean bass on Paula Cole's “Tiger” (from her This Fire album) kicked in during my listening tests, I literally whipped around to turn off my subwoofer — it was already off! The ADM 2s reproduced low bass so effectively that the bottom of my seat vibrated, with only 85dB SPL at the mix position. The rest of the spectrum, all the way up to airy highs, was so proportionally balanced and coherently reproduced that I just sat there slackjawed, listening in awe. I couldn't hear any smearing of transients, as is common with other speakers.

Unfortunately, the ADM 2's imaging was less than great. The monitors' stereo sound stage localization was a little ghosty. The depth of the image also could have been better. While the monitors are adequate in this regard, they do not offer the pinpoint imaging of D.A.S. Monitor-8 or Hafler M5 monitors, for example, both of which cost a fraction of the ADM 2's list price. But those monitors can't touch the ADM 2's extended bass frequency response.

I suspect that the addition of wordclock inputs or a digital link between the monitors might tighten up the imaging and improve depth. Another design improvement would be to use a common DSP card to clock both speakers; as it stands now, each monitor has its own card. I'd also like to see a FIR equalization preset for digital input so that it can sound as accurate as the analog path.

Nevertheless, the ADM 2s — at least in analog mode — are the ultimate reference for checking spectral balance and transient content in your mix. They are articulate yet sweet, clear yet warm, detailed yet non-fatiguing. Bottom line: The ADM 2s sound absolutely amazing.

CAP Audio Professional Corp. (U.S. distributor for KS Beschallungstechnik GmbH), Weaver Park, 1060 Cephas Dr., Clearwater, FL 33765; 727/447-9656; info@cap-audio.com; www.cap-audio.com.

Michael Cooper is a Mix contributing editor and owner of Michael Cooper Recording in beautiful Sisters, Ore.

Thinking Inside the Box

To understand how KS's technology transcends the ordinary limitations of speaker performance, let's first take a look at a speaker's inherent shortcomings. Every driver has an infinite impulse response; that is, inertial forces cause it to move both before and after a signal is reproduced. This unrelated movement distorts the original waveform's frequency, amplitude and phase components, causing skewed spectral balance, smeared transients and phase anomalies such as comb filtering. Multiple drivers, along with active circuitry and the cabinet's acoustical response, compound the distortion.

The composite result of these aberrations can be quantified in the monitor's transfer function curve. The transfer function is essentially the sonic signature — both electronic and acoustic — that the monitor imposes on the source signal. Put another way: If you were to subtract the input signal's frequency response curve from the signal at the monitor's output, then the curve of the remaining signal — the speaker's transfer function curve — would show the speaker's unwanted contribution to the sound.

After a 25-hour burn-in period, KS measures the transfer function of each new ADM 2 monitor, using 22ms bursts of full-bandwidth audio to excite the monitors. An inverse equalization curve is then computed and mapped into the memory of the ADM 2's onboard 250-band digital equalizer, making sure that the curve is properly aligned along the timeline with the monitor's transfer function curve. KS refers to this inverse equalization curve as a Finite Impulse Response (FIR) curve, and to the proprietary KS technology as FIRTEC (pronounced “Fire-Tech”) processing. In theory, FIRTEC processing should completely neutralize, or cancel out, the ADM 2's sonic signature, producing an accurate reproduction of the source material.

A unique, custom FIR equalization curve is applied to each and every speaker before it leaves the factory. That is, an average response curve is not computed from a sample set of units. Each monitor gets the exact curve needed to compensate for any manufacturing tolerances (or deviance in the physical properties of materials used) that may have affected that individual monitor. Once computed, this custom EQ curve is programmed into non-volatile memory inside the monitor and made available to the user via a preset selection knob on the monitor's rear panel. The FIR curve is also logged against the assigned serial number for future reference.

Upon request (and currently at no extra charge), KS will also program a second preset equalization curve that tweaks a particular ADM 2's response to compensate for control room's acoustics (in addition to compensating for the speaker's transfer function). In order to compute the needed curve, KS will need a recording of swept tones, taken at the mix position. KS will then return some PC software that contains the computed inverse EQ curve for the room in question. The software may be downloaded via the ADM 2's remote-control connector. The downloaded preset is stored in non-volatile memory inside the monitor, and can be recalled via the preset selection knob on the unit's rear panel.
Michael Cooper

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