Home Theater Geeks 29

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Home Theater Geeks
Episode 29

Contents

Home Theater Geeks 29: Audio Legend Bob Carver

Guests

Robert W. (Bob) Carver is an American designer of audio equipment based in the Pacific Northwest, currently the Chief Technology Officer of Sunfire.

Educated as a physicist and engineer, he found an interest in audio equipment at a very young age. He applied his talent to produce numerous innovative high-fidelity designs since the 1970s. He is known for designing the Phase Linear 700, at 350 watts per channel the most powerful consumer audio amplifier available in 1972. He went on to found the Carver Corporation in 1979 and Sunfire in 1994. Although his 35 years in the industry have produced over a dozen industry milestones in consumer audio, his work at Sunfire has changed the way consumers and the industry listen to audio systems.

Sunfire Revolution

Bob started Sunfire in 1994 to design, develop, manufacture, and market the smallest, most powerful speakers, subwoofers, and amplifiers in the world. Sold in over 65 countries, Sunfire combines unconventional thinking with precision engineering to create innovative theater products that deliver exceptional performance.

Carver has approached his projects with the intellect of a physicist and the passion of an enthusiast, with more than 35 years of scientific research that have resulted in numerous patents, including the Tracking Downconverter power supply used in the company's receivers and amplifiers, and High Pressure, High Back-emf technology found in the company's subwoofers and speakers.

Amplifiers. Bob has developed many amplifier designs over the years, but Sunfire's Tracking Downconverter is so efficient it "runs cool" and in many cases eliminates the need for large wired transformers. This allowed for smaller more powerful amplifiers which helped drive Sunfire's credo – small box, big power.

Subwoofers. Bob was the first to think "out of the big box," introducing the small-box, high-power subwoofer category that is now an industry standard. He was able to do this by hot-rodding speakers in the same way '50s teens used to hot rod cars, by increasing displacement and creating an efficient driver.

Speakers. Bob's Cinema Ribbon speakers continued the smaller-is-better concept by creating a small-box form factor with enormous large-box pressure, High Back-emf drivers and, a controlled sound wavelaunch that is free of unwanted diffractions, refractions, and reflections. Bob's first "Amazing Loudspeaker" featured a 6-foot-tall ribbon that could handle the output of a 400-watt amplifier without distortion. That technology was applied to later speakers by looping ribbon strips from the original huge speaker for the same performance in the small box. The Early Years

Carver started working with speakers as a graduate student. He caused a stir in the industry in the mid-1980s when he challenged two high-end audio magazines to give him any audio amplifier at any price, and he'd duplicate its sound in one of his lower-cost (and usually much more powerful) designs. Two magazines took him up on the challenge. First, The Audio Critic chose a Mark Levinson ML-2, which Bob acoustically copied (using transfer-function duplication) and sold as his M1.5t amplifier (the "t" stood for the modified transfer function).

In 1985, Stereophile magazine challenged Bob to copy a Conrad-Johnson Premier Five amplifier at their offices in New Mexico within 48 hours. The Conrad Johnson amplifier was one of the most highly regarded amplifiers of its day, costing in excess of $12,000. In both cases, the challenging amplifier could only be treated as a "black box" and could not even have its lid removed. Using null-difference testing, Carver successfully copied the sound of the target amplifier and won the challenge. The Stereophile employees failed to pass a single blind test with their own equipment in their own listening room.

He marketed "t" versions of his amplifiers incorporating the sound of the Mark Levinson and Conrad Johnson designs, which drew some criticism from those who failed to understand the true nature of the challenge—that it was possible to duplicate an audio amplifier's sound in two completely dissimilar designs. Based on this research and in light of this criticism, Bob Carver went on to design the Silver Seven, the most expensive and esoteric conventional amplifier up to that time and then duplicate its sound in his M 4.0t and later models that sold for about 2.5 percent of the price (around $600–$1500).

This also started Carver's departure from the M-series amplifier to the more robust and current-pushing TFM-series amplifiers. The TFM amplifiers were designed specifically to drive the demanding load of the Amazing ribbon loudspeakers. The apex of Carver's amplifier line was the Lightstar, which is now a collectors' item. Only about 100 of them were made. The original Lightstar amplifier, called the Lightstar Reference, featured a dual-monoblock design, with separate power cords for each channel. A later version, called the Lightstar 2.0, featured one power cord and other cost-saving measures to shave approximately $1800 off the retail price. The two are reported to be sonically identical.

Imaginative product naming

Carver has used some names that have clearly defined technical meanings so oddly unrelated to his products' actual design and function as to be merely amusing rather than deceptive.

"Magnetic field coil power amplifier" -- Audio amplifiers of the vacuum-tube era used transformers incorporating coils of wire to convert the high-voltage, low-current output of the output tubes to a low-voltage, high-current output suitable for loudspeakers. Output transformers are no longer necessary with amplifiers, including Carver's, thanks to transistors. Carver's U. S. Patent 4,218,660 describes a power supply whose input switches on and off at an ultrasonic rate to adapt to the varying power demand of the amplifier. The innovation is in the power supply, not the amplification circuitry, and in the use of switching, not in the use of a coil or coils.

The "auto-correlator" is a multi-band, single-ended dynamic noise filter, as described in U.S. patent 3,989,897. The Phase Linear 1000 and the Model 4000 preamplifier, which incorporates the same circuitry, have noise reduction in four frequency bands, one in the bass intended to suppress turntable rumble and three in the treble intended to suppress tape hiss and the like. The noise filters are controlled by signals derived from bandpass filters, not from a correlation process. Correlation is the comparison of two or more signals, while auto-correlation is the comparison of a signal with delayed versions of itself to determine the degree of similarity at different delay times—useful, for example, to determine the amount of delay that produced an echo.

"Sonic holography," as described in U.S. Patent 4,218,585, is the enhancement of stereo imaging by introducing a delayed and equalized signal from the right channel at the left loudspeaker to cancel the signal from the right loudspeaker at the listener's left ear (and vice versa). The result is an enormous 3D soundstage. Sunfire incorporates Sonic Holography to this day in its 401-series receiver and pre/pro.

The "asymmetrical charge-coupled detector" is not the FM detector circuit of Carver's tuners, but rather, as described in U.S. patent 4,457,012, it is single-ended dynamic noise-reduction processing applied to the noisy left-minus-right component of a weak FM-stereo signal, along with artificial ambience generation to compensate for loss of ambience resulting from the noise reduction.


Patents

U.S. Patent 3,727,148 Amplifier with Protective Energy Limiter Circuit Components, filed January 1972, issued April 1973

U.S. Patent 3,989,897 Method and Apparatus for Reducing Noise Content in Audio Signals, filed October 1974, issued November 1976 (auto-correlator noise reduction)

U.S. Patent 4,218,585 Dimensional Sound Producing Apparatus and Method, filed April 1979, issued August 1980 (sonic holography)

U.S. Patent 4,218,660 Audio Amplifier and Method of Operating Same, filed November 1978, issued August 1980 (magnetic field coil power amplifier)

U.S. Patent 4,309,570 Dimensional Sound Recording and Apparatus and Method for Producing the Same, filed April 1979, issued January 1982

U.S. Patent 4,415,768 Tuning Apparatus and Method, filed May 1981, issued November 1983

U.S. Patent 4,4,450,95 Audio Amplifier, filed February 1982, issued April 1984

U.S. Patent 4,457,012 FM Stereo Apparatus and Method, filed June 1982, issued June 1984 (asymmetrical charge coupled stereo detector)

U.S. Patent 4,484,150 High Efficiency, Light Weight Audio Amplifier and Power Supply, filed September 1982, issued November 1984

U.S. Patent 4,586,002 Audio Amplifying Apparatus and Method, filed June 1984, issued April 1986

U.S. Patent 4,808,946 Lightweight, High Power Audio Amplifier and Power Supply, filed December 1986, issued February 1989

U.S. Patent 4,815,141 Apparatus and Methods for Removing Unwanted Components from a Communications Signal, filed December 1986, issued March 1989

U.S. Patent 5,748,753 High Power Audio Subwoofer, filed January 1996, issued May 1998

U.S. Patent 5,937,074 High Back EMF, High Pressure Subwoofer, filed August 1997, issued August 1999

U.S. Patent 6,166,605 Integrated Audio Amplifier, filed September 1998, issued December 20000

U.S. Patent 11,325,881 Anti-shake subwoofer technology, issued July 2010. Anti-shake technology (we call it StillBass) provides an input drive signal that is transmitted to both a speaker drive section and an inertial noise suppression section such that the drive signal causes the inertial mass to move synchronously with the movement of the diaphragm of the speaker, thus diminishing unwanted noise.

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Production Information

  • Edited by: Erik
  • Notes:
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