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What about bass "speed"? Some pundits have branded this term technically inaccurate, because, as they point out, the steepness of the transient's leading edge is determined by its upper harmonics, which may after all not be reproduced by the woofer but rather by the midbass or midrange drive-units. Yet, I think that we have all observed slow, sluggish, or woolly bass response. So the real question is: Does a woofer have to be fast? An excellent figure of merit for woofer speed is its acceleration factor, defined as the ratio of the woofer's magnetic force to its moving mass. This is nothing…
I think that most listeners would be perfectly satisfied with a 40Hz bass extension provided that there is sufficient excursion capability to reproduce the bass octaves cleanly and effortlessly at peak SPLs of around 110dB at the listening seat. Too often it's the lack of dynamic power rather than frequency extension that disappoints audiophiles. Take a good 50Hz horn, for example. Despite its modest frequency extension, such a horn invariably elicits an instinctive positive reaction because of its effortless and dynamic impact.
Once the SPL requirement at a particular frequency is…
The limits for Doppler distortion were also investigated by Fielder and Benjamin. To refresh your memory, Doppler distortion is frequency modulation of the higher frequencies by the lower frequencies being reproduced by a single driver; something like tape flutter except that here the music or cone excursion provides a constantly varying modulating signal. The bottom line is that, for practical cone excursions, and with the bandwidth limited to 12.5Hz to 100Hz, the FM distortion due to the Doppler effect appears to be far below the threshold of audibility. Thus the harmonic distortion…
A few years ago I reviewed the ProAc Tablette (Vol.7 No.4). When J. Gordon Holt, the champion of tonal accuracy, first heard them he was violently put off by their tonal imbalance. "Violins sound like children's toy violins, celli are emasculated," and on and on he went. My review was less than a rave, yet he castigated me as "having gone off the deep end" for my failure to trash them. Yet in my system, side by side with my reference speakers at the time—the helium-driven Hill Plasmatronics—the Tablettes were superior in defining the underlying bass lines and highlighting the pulse or…
Based in the Czech Republic, KR Enterprise is headed by an occasionally gruff Dr. Riccardo Kron and his American-born wife, Eunice, who operate the company out of a partially abandoned factory that was once part of the state-owned Tesla High Vacuum Technology facility in Prague. The Swiss-funded company is unique in that it manufactures both amplifiers and the tubes that power them. KR's tubes have found favor with other amplifier makers as well—especially the 300BXS, electrically identical to a standard 300B but rated at 25W in class-A.
Dr. Kron is driven to build tubes that…
Hookup is straightforward, with a single-ended RCA jack input and two pairs of hefty, custom-made, nicely machined speaker terminals (though it appears that only one tap is available from the output transformer). Flip the on/off switch and a red front-panel LED signals warm-up. Within a few minutes of turn-on, the unit reaches operating temperature and the LED changes color from red to green. The amp's sound begins to "develop" after about an hour's warm-up. There's also a three-position input-sensitivity knob that one should not turn with power on. How nice it would have been to have been…
Sidebar 1: Mikey's Praguenosis
When you see how vacuum tubes are made, you immediately understand why the transistor had to be invented: when done correctly, the process is expensive, time-consuming, labor-intensive, and messy. OSHA and the EPA would have big problems with it. I saw how it's done when Dr. Riccardo Kron (footbnote 1) and his wife, Eunice, invited me to the Czech Republic to visit the KR Enterprise facility, in Prague. I'm glad they did—had they just told me about it, I might not have believed them.
Located in a semi-abandoned building that was once part of the…
Sidebar 2: Associated Equipment
Analog sources: Simon Yorke, Kuzma Stabi S, turntables; Graham 2.0, Immedia RPM2, Rega RB600 tonearms; Crown Jewel SE, EMT TU, Lyra Parnassus D.C.t, Grado Statement phono cartridges.
Preamplification: Ayre K-1; Audio Research Reference and Musical Fidelity X-LP2 phono sections.
Power amplifiers: Mark Levinson No.335, Ayre V-1.
Loudspeakers: Audio Physic Virgo, Sonus Faber Amati Homage.
Cables: Yamamura Millennium 6000, Cardas Golden Reference, Electra-Glide interconnects; Yamamura Millennium 6000 speaker cables; Yamamura Quantum, Electra-Glide…
Sidebar 3: Measurements
The KR Enterprise VT8000 was warmed up for one hour at a third of its rated maximum power. For an amplifier with a tube output stage, it runs relatively cool.
The VT8000's input impedance measured 62.6k ohms. Its measured minimum output impedance is 4.8 ohms at 20Hz; the maximum measured value is 6.6 ohms at 20kHz; and there was some dependence on the exact figure and the load impedance used to make this measurement. This amplifier's frequency response will be very dependent on the loudspeaker with which it is used. Voltage gain into 8 ohms was 26.8dB in…
Sidebar 4: Specifications
Description: Hybrid monoblock power amplifier with solid-state input and driver stages, and "vacuum transducer" output stage. Maximum power: 75W RMS (18.75dBW) into 4–8 ohms. Frequency response: 15Hz–35kHz, –3dB; 30Hz–20kHz, –0.5dB. Input sensitivity: switchable 350mV, 1V, 2V, 5V. Input impedance: 100k ohms.
Dimensions: 13.75" W by 10" H by 19.75" D. Weight: 92 lbs.
Serial numbers of units reviewed: 150R, 150L.
Price: $25,000/pair (1999); no longer available (2008). Approximate number of dealers: 5. Warranty: 3 years, parts & labor.
Manufacturer…