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INTRODUCING THE L-8000 DHT PREAMPLIFIER
Allnic Audio and the Direct Heated Triode Breakthrough
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.
Allnic’s Mr. Kang Su 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 exceptionally low distortion 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
DHTs, 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
DHTs are only amplifying a signal already affected by the indirectly heated triodes in the prior
stage or stages.
Mr. Park, though, created a preamplifier that uses DHTs at each amplification stage. In his
design, the output stage DHTs are not merely amplifying the signal from previous, indirectly
