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L-5000 DIRECTLY HEATED TRIODE PREAMPLIFIER
In the early years of the last century, the directly heated triode was invented independently, and almost simultaneously, in the USA and Austria. This simple device, an anode, cathode and a grid in a vacuum contained in a glass bottle, transformed the world. With all the applications in which it has been employed, and with all the changes in electronic design for audio reproduction that have occurred since its invention, it is remarkable that over one hundred years later, the very first triode, the simplest vacuum tube audio signal amplifier, despite the imperfections ascribed to it, remains a favourite audio amplification device of many devoted music lovers worldwide.
Debate, on both subjective and empirical bases, about the quality of the directly heated triode’s sound reproduction potential continues even today. One of the interesting aspects of these debates is the degree to which they ignore analysis of common, fundamental assumptions based on decades of circuit and mechanical design experimentation and compromise, about the use of these devices. Indeed, because of the difficulties in application of directly heated triodes in audio circuits, many issues arising out of circuit design problems related to them have evolved into assumptions about the audio characteristics of the valves themselves.
Kang Soo Park has spent a lifetime studying and designing circuits and transformers for application of direct heated triodes for audio reproduction. By questioning basic assumptions about the valves and re-examining the original and many historical variations of circuit designs for them, and experimenting constantly in a framework of rigorous application of theory, Mr. Park’s work has resulted in some remarkable audio power amplification devices. With the 300B, for example, his creations produce significantly more power than is expected (without shortening valve life because the power increase is not achieved by increased bias), in conjunction with exceptional distortion characteristics and wide bandwidth.
Now, Mr. Park has successfully combined elements of what he has learned and invented over decades to the development and production of a pure, direct heated triode (DHT) based preamplifier. The main breakthrough, which came after several years of work applying all he knows, was the design of a circuit, valve and transformer combination the measured results of which astounded him. The purity of its square wave reproduction – as perfect as anything he had ever seen from a DHT valve amplification circuit – had him questioning what he was seeing! Of course, he tested and re-tested, replicated and recorded these results many times before assuring himself that there was no mistake.
With such an unprecedented outcome for the circuit design, Mr. Park could not be satisfied merely with placing a DHT stage after one or two prior stages of amplification using indirectly heated triodes. This approach is used in a number of commercially available preamplifiers using DHT’s, but it is a compromise. It does not result in PURE DHT amplification; it necessarily introduces the characteristics of indirectly heated triode sound into the circuit, so the output DHT’s are only amplifying a signal already affected by the indirectly heated triodes in the prior stage or stages.
Mr. Park, though, has created a preamplifier that uses DHT’s at each amplification stage. In his design, the output stage DHT’s are not merely amplifying the signal from previous, indirectly heated triode stages. The L-5000 preamplifier is pure DHT - from input to output.
Of course, Mr. Park is not the only builder who has accomplished this, but to our knowledge no other pure DHT preamplifiers have been commercially available. There are serious and significant reasons why such preamplifiers could not be acceptable in the marketplace. First among them has been the enormous problem of microphony. One of the many special elements of Mr. Park’s design is the successful elimination of microphonic influence on the DHT’s. Mr.Park has succeeded in creating a circuit and mechanical design that eliminates this serious detriment to sound reproduction that others have sometimes resolved to accept because even with it, they prefer the incredible realism of the pure DHT approach. Like the Allnic L-3000, which is an indirectly heated triode preamplifier, the L-5000 has a noise floor that is effectively absent.
The Allnic L-5000 preamplifier is THE WORLD’S FIRST COMMERCIALLY PRODUCED, PURE DHT PREAMPLIFIER.
Currently, the L-5000’s large output tubes are very rare items, produced many decades ago. Allnic has been able to secure a limited number of these valves, enough to produce extremely limited number of L-5000 DHT preamplifiers. This allows for adequate valve stock to be retained for warranty replacements, should that ever become necessary, something that is unlikely for a very, very long time – unless one drops the tube! This is because the qualities of the valves combined with the circuit in which they function will extend their life to the utmost limits, very, very much beyond 10,000 hours.The L-5000 DHT is a fresh new beginning. With the wildly successful introduction and hunger for such a component, Mr Park has announced future Direct Heated Triode products. These will be a phono preamplifier, power amp and a more modest line stage to be designed and built towards the latter part of 2011 and early 2012.
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