Sidebar 3: Measurements
I use a mixture of nearfield, in-room, and quasi-anechoic FFT measurement techniques (using primarily DRA Labs' MLSSA system with a B&K 4006 microphone, but also an Audio Control Industrial SA-3050A 1/3-octave spectrum analyzer with its calibrated microphone) to investigate objective factors that might explain the sound heard. The speakers' nearfield low-frequency responses and impedance phase and amplitude were measured using the magazine's Audio Precision System One.
There are no surprises in the Infinity's measured impedance (fig.1). The sealed-box…
Reviewing a vacuum-tube power amplifier is like having your pants pulled down in front of a large crowd of people. I don't know how else to describe the feeling of spending a month or two luxuriating in fabulous sound, then writing a glowing review, then receiving a copy of the review as it will appear in the magazine, complete with John Atkinson's assessment of the amp's test-bench performance, which is usually miserable.
Based on my reviewing track record and my addiction to vinyl, it would not be unfair for some readers to conclude that my preferences are for noise, high…
Modjeski claims more than 100Wpc from each pair of tubes, and says that, unlike other tube designs, as the load impedance drops, the RM-200's output actually increases. The manual claims that, from its 8 ohm taps, the amplifier will deliver 112W into 8 ohms, 145W into 4 ohms, and 125W into 2 ohms. According to Modjeski, "typical" tube amps yield 100W, 65W, and 40W into those loads. However, the manual also states that the claimed power represents the amplifier's "True Music Power, not the typical RMS rating." As I write, how the RM-200's True Music Power numbers will translate on JA's test…
Within the instrumental groupings, the individual players also appeared unusually well-converged and placed in space, especially from front to back. Only Belafonte and his audience—most of whom have since probably gone to that great banana boat in the sky—know how accurate was the placement I heard, but it was impressive. The RM-200's rendering of space and dimensionality was therefore exceptional—similar to what the Kora Cosmos delivered, but with more bottom-end weight, and perhaps somewhat less in the way of air and shimmer to cymbals and brass. The overall tonal picture was "classic…
Changes
As the RM-200 came set up for them, I did most of my listening with the Chinese KT88s—or, as Modjeski described them to me in an e-mail, "the much-maligned KT88s." I also used my reference Harmonic Technology Pro-Silway II interconnect and Magic Woofer speaker cable, as well as Synergistic Research's ultra-detailed Designer's Reference Coupler AC cord. Before trying the more expensive 6550s, I experimented with some cables. I need to digress for a minute. Recently, a press liaison for a mainstream speaker manufacturer paid a visit, bringing with him a new $1500/pair full-…
Sidebar 1: Specifications Description: Hybrid solid-state/tube stereo power amplifier. Tube complement: two 6BQ7, four KT88 or 6550. Output transformer taps: 8 ohms, 4 ohms, 1 ohm. Output power: 100Wpc (20dBW). Frequency response: 6Hz-20kHz, ±0.5dB; -3dB at 2Hz and 60kHz. Half-power bandwidth: 13Hz-65kHz. Full-power bandwidth: 20Hz-35kHz. Harmonic distortion: <0.5% at 1kHz/100W. Signal/noise ratio: not specified. Input sensitivity: 1.1V for full output. Input impedance: 15k ohms, one leg driven; 30k ohms, balanced.
Dimensions: 17¼" W by 7" H by 14½" D. Shipping weight: 52 lbs.
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Sidebar 2: Associated Equipment Analog sources: Simon Yorke, VPI Aries Extended turntables; Graham 2.2, Immedia RPM2, VPI JMW 12.5 tonearms; Lyra Helikon, Helikon LE, Helikon mono, Benz Micro Glider L2, ZYX R-100FS phono cartridges.
Digital sources: Musical Fidelity Nu-Vista 3D CD player, Marantz SA-14 SACD player.
Preamplification: Hovland HP-100 preamplifier, Manley Steelhead and Zanden 1200 phono sections.
Power amplifier: Musical Fidelity Nu-Vista 300.
Loudspeakers: Audio Physic Avanti III.
Cables: Hovland Music Groove DIN/RCA, RCA/RCA phono. Interconnect: Discovery…
Sidebar 3: Measurements One amplifier, three separate transformer outputs, balanced or unbalanced drive, and two choices for output tubes—to measure every combination offered by the Music Reference RM-200 was not practicable, so I used balanced drive and what I assumed were the Chinese output tubes MF preferred (they were only identified as "RAM Labs" tubes).
The amplifier was non-inverting, with pin 2 of the XLR jack wired as positive. Its single-ended input impedance measured 15.4k ohms at 1kHz, this rising slightly at the edges of the audioband and doubling when driven…
The audibility of any distortion depends on its order. Low-order harmonics are preferable because they're musically consonant—see my essay on this subject in the Stereophile Test CD 2 booklet—and more easily masked by musical fundamentals. Figs. 9 and 10 reveal that the RM-200's distortion is predominantly third-harmonic. Fig.9 was taken with a 1kHz sinewave at a low level (1W into 8 ohms from the 8 ohm tap), with just 0.011% THD. Fig.10 was taken about 3dB below clipping into 4 ohms from the 4 ohm tap, with a low-frequency tone. The third harmonic lies at -50dB (0.3%), with the second,…
Manufacturer's Comment Editor: Being both a degreed electrical engineer and an audiophile designer is walking the razor's edge. Too many "engineers" design poor-sounding products when they become slaves to the test equipment. On the other hand, too many "seat-of-the-pants" designers commit faults in the mundane engineering aspects of serviceability, stability, and knowing the limitations of the parts. With the quality of today's parts, there is no excuse for poor reliability. When I hear a manufacturer blaming a part for his reliability problem, it is all too often that the designer…
