Turning to the Musical Fidelity kW750, I preconditioned the amplifier by running it at 250W into 8 ohms for about 35 minutes, by which time my dummy load was too hot for safety. (It was also when the amplifier's illuminated feet changed from orange to blue.) The kW750's chassis and heatsinks were very hot, implying that they are only just large enough to pass the FTC heat test. The THD+noise at this power level was 0.02% with the amplifier cold, dropping to 0.013% after five minutes, 0.01% after 15 minutes, and returning to 0.017% after 25 minutes. The THD…
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Sidebar 4: kW750 Measurements
Pass Laboratories' X amplifier series represents the efforts of designer Nelson Pass to prove that simple linear amplifier topologies can be scaled to provide high-quality audio performance at very high power levels. The handsome X1000 monoblock under scrutiny here, the largest and most powerful amp in the Pass stable, makes 1000W into 8 ohms and a mighty 2000W into 4 ohms. The amplifier has no global negative feedback, and only two gain stages: the front-end provides all the voltage gain and feeds a high-current follower stage.
Pass's Supersymmetry Balanced Single-Ended class-A…
Interestingly, as far as bass is concerned, the X1000 wasn't the deepest-going or BIGGEST-sounding amp I've heard. Its sheer power was expressed more noticeably in the control, subtlety, and tonal shading at the bottom of the frequency range. It not only succeeded in driving along many a trip-hop and jazz session, but also fleshed out the acoustics and space of the recording venues. Transitioning up through the mid- and upper bass was smooth, seamless, and free of aberration, if a touch dry. That same lack of bloat, however, contributed to a high degree of detail and transparency. The lower…
Sidebar 1: Specifications
Description: Solid-state monoblock power amplifier. Maximum output power (1% THD, 1kHz): 1000W into 8 ohms. Maximum output voltage: ±45V. Maximum output current: ±30A. Frequency response: -0dB at DC, -3dB at 60kHz. Input impedance: 20k ohms balanced. Damping factor: 60 ref. 8 ohms nominal. Voltage gain: 30dB. Slew rate: >40V/µs. Output noise: 500µV unweighted, 20Hz-20kHz. Random noise floor: approximately 2µV. Dynamic range: 155dB (random noise floor to peak output). Balanced Common-Mode Rejection Ratio: 85dB 1kHz. DC offset: <100mV. Power consumption:…
Sidebar 2: Supersymmetry Explained
No negative feedback circuits might be more accurately described, in most cases, as local feedback only designs. Some might argue that local feedback is still feedback, but according to Nelson Pass, "there is always some amount of feedback locally around any gain device by the nature of the device." He thinks that "no feedback" describes a circuit in which negative feedback doesn't extend further than a single stage or gain device.
Conventional negative feedback, local or otherwise, is used to make the circuit's output look more like its input.…
Sidebar 3: Associated Equipment
I drove the X1000s with a variety of front-ends. This included running direct from the analog outputs of the dCS 972 Sample Rate Converter and Elgar D/A processor at 24/192, and from the balanced outputs of the fully differential BAT VK-5i/VK-P10 preamp/phono combo. (The X1000 manual is quite explicit that the output impedance of the associated preamp is of little consequence, and that balanced is better.) I also used the new Classé Omega line-level preamp, from both its balanced and single-ended outputs. I got fabulous sound with the single-ended Nagra PL…
Sidebar 4: Measurements
Because of the Pass X1000's power output and the restrictions of our test load, I couldn't perform our usual 1/3-power preconditioning test. Instead, the amplifier was warmed up for one hour at 50W into 8 ohms before any measurements were taken.
The X1000's input impedance measured 23.7k ohms. The output impedance measured between 0.18 and 0.20 ohms (the higher values were at 20kHz)—slightly higher than normally experienced with solid-state amplifiers, but not enough to be a serious concern. The voltage gain was 30.14dB, and pin 2 of the (balanced) input…
It's always tough to follow an award-winning act. Wes Phillips raved about the original EgglestonWorks Andra back in October 1997, and it was subsequently dubbed Stereophile's Speaker of the Year for 1997. The Andra won many other plaudits, and found its way into a number of top-shelf recording studios as the monitor of choice. Such a reputation for excellence is the stuff most speaker designers dream of. It also imposes the burden of expectation—the "new and improved" version of such a knockout product had better be good, or else.
Since the Andra's debut in Stereophile there have been…
The crossovers are kept as simple as possible, with 6dB/octave symmetrical filters applied at the woofer/mid transition [Albert von Schweikert says "12dB/octave (acoustic) slopes in his cover letter—Ed.] with Zobel impedance compensation, and 24dB/octave (acoustic) symmetrical filters used at the 1.8kHz mid/tweeter transition. According to EW's literature, the tweeter high-pass crossover filter consists of nothing more than a Hovland MusiCap and a pair of Vishay resistors as an L-pad, this working with the drive-unit's acoustic rollout to give a 24dB/octave slope.
After more than a year…
As much fun as mega-music was through the Andra, the speaker was just as wonderful with more intimate fare. Never had my wonderful old green-label first pressing of Van Morrison's Moondance (LP, Warner Bros. WS 1835) sounded so involving as through the Halcro dm58/Andra II combo. The lithe, mellow flute and sax, the warm, brown-toned throb of the bass guitar, and Morrison's ur-soulful voice ascended to the sublime. Paired with the Lamm ML2.1 amps, Oscar Peterson's A Tribute to My Friends (LP, Pablo 2310-902) simply sang. The Andra not only articulated Peterson's dizzying fingerwork and…