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Deja Vu Audio History

After PRS Guitars dropped the amplifier project, Pritchard remained convinced that a solid state amplifier could sound like the classic amplifiers. With this conviction, Deja Vu Audio was born. The PRS effort was marketed by a fateful decision, whether to emulate the tubes or try to do a black box equivalent to sections of the amplifier. With the estimation that emulating the tubes would make a much more complex amplifier, the work began with the engineering standard approach of the black box equivalents. Much later, Pritchard read between the lines of an engineering text that this approach would have never worked because non-linear circuits and reactances have to be in the same relationships in order to be recreated with real components. However, at the beginning of Deja Vu Audio, Pritchard guessed that the detailed requirements of a musician's critique needed the tube characteristic.

One of the major problems in creating a solid state circuit to emulate 12AX7's with operational amplifiers was the limited gain-bandwidth product of the operational amplifiers. A new operational amplifier from Burr-Brown solved that problem plus operated at higher than normal voltages. Eventually, this amplifier became the gain element of a new emulator, an advancement over those disclosed in the application for U.S. Patent 5,434,536.

One problem with diagnosing the amplifier was knowing just where to attack the problems. This problem had a solution in a paper by Russell Hamm, "Tubes Versus Transistors - Is There An Audible Difference?", Journal of the Audio Engineering Society, May 1973, and reprinted in Glass Audio and now on several web sites. Russell Hamm had provided data and a means of measurement. Pritchard designed and built a tube emulator version of a generic microphone preamplifier then measured it according to Russell Hamm's techniques and compared with Hamm's data. The nearly identical results were published in db Magazine, the July/August 1994 as "The Tube Sound and Tube Emulators". This work revealed that the research had to be focused on the output stage.

The output stage was substantially more complex for three reasons, vacuum pentodes, push-pull architecture, and the lack of further guidance. The tube emulators were not suited to the quite standard differential, long-tail phase splitter because the operational amplifiers became unstable. An article in Vintage Guitar pointed to a variation on a vintage phase splitter that worked even better. The differential phase splitter is quite harmonic-free because the two halves compensate each other. By this time, Pritchard had strong suspicion that artistic concepts that would be desired by engineering concepts would not be desirable in an instrument amplifier. The vintage phase splitter consisted of a hardworking, full-bodied triode and second stage with equal resistors as the plate and cathode loads. This circuit went beyond the proportional capability of tubes as the need for more output level became evident. While 12AX7's are limited to 300 volts, this circuit was proportional to 500 volts. So for the first time, an amplifier circuit could not be reversed and implemented in vacuum tubes.

An important change in the amplifier design from the PRS HG amplifiers also was in the output stage. The HG amplifier used discrete power transistors. To counter the large potential for shorts in the standard output jack connections, this amplifier used nearly 800 watts of transistor dissipation potential. Even though the amplifier used a thermal switch on the transistor heat sink, this huge wattage could not be reduced. (Fold back current limiting oscillated and created a buzz on certain notes.) The answer would be smart transistors that included transistor junction temperature measurement. Although they never materialized, National Semiconductor began producing a monolithic output chip with over-voltage, over-current, and over-temperature protection - an answer to a reliability prayer.

An inquiry into the use of these tube emulators in a new design for an inexpensive amplifier, led to the parallel development of the "simplified" amplifiers. The dual track development provided two views of the same problem. These two views were particularly obvious in the preparations for the video demonstration production. There were simplified amplifiers and exact amplifiers to show the target of the video tape, potential investors.

During the lull between the shooting of the music portions of the video and the special effects, the theory of the transmission line speaker cabinet was tested again and again against guitar speakers. Eventually, the solution became a new cabinet theory, awarded U.S. Patent 6,411,720, and has since been dubbed Tunnel-Back. Tunnel-Back cabinets actually reduce the driver resonant frequency while both closed back and the ubiquitous ported cabinets both raise the resonant frequency. The open back cabinet rolls off the bass with the cancellation of the front wave by the rear wave. In the larger open back cabinets, the roll-off is compensated by the resonance to produce good tone. But smaller open back cabinets suffer a substantial loss of bass not found in the new tunnel-back cabinets.

Along the Deja Vu Audio way, Pritchard drove to meet a Vintage Guitar writer, Paul Bechtoldt. He had real doubts about solid state amplifier being able to sound anything like a tube amp, but he rapidly became convinced as he ran through his amp testing technique. He started playing softly and gradually raised the gain until the amp was screaming. He "was totally mind numbed by the sound. Sensitivity to picking; reaction to power chording; tonal control, harmonic overtones...I was definitely playing a tube amp." (Vintage Guitar, May 1994) Unfortunately, not all people listen the same way and others found chinks in the design as Pritchard found in the ongoing musician testing and a disastrous showing at a summer NAMM show about the same time.

Many musicians have helped. Robert Fiester tested early amps when Pritchard lived Bowie and quite often later on in a club in northern Baltimore. He also became the video spokesman. After an article on the amplifier technology appeared in Design News, an engineering trade magazine, Dean Shumaker, helped out. Later, Phil Zuckerman joined the fray because he could not find an amplifier of any technology that really suited him. Fortunately, Phil has critical hearing and readily picked out unwanted characteristics. His great interest has kept him involved for years.

Eventually, Pritchard's desire to recreate the classic amplifiers faded into the recognition that the practice of exaggerating the good tube amplifier characteristics and the reduction or elimination of bad characteristics had produced a unique amplifier. Paul Bechtoldt's suggestion to change the name took hold and Pritchard Amps was born.

More history:

History summary page
Carver Stories
Phil Zuckerman
PRS History
Deja Vu Audio
Pritchard Amps

 

 
     
 
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