"Allnic Audio L-8000 DHT Preamplifier" OCTOBER 16, 2020, ANTHONY KERSHAW - Review Link
 
 
" There are no speakers; there are no walls." - A brief summary of Allnic's current Line Stage (preamplifier) product line. - Review Link
 
 
Description
 
In the early years of the last century, the directly heated triode (DHT) was invented independently, and almost simultaneously, in the USA and Austria. This simple device, an anode, a cathode and a grid in a vacuum contained in a glass bottle, transformed the world. Over one hundred years later, the very first triode remains a favorite audio amplification device for devoted music lovers worldwide.
 
The debate about the quality of the DHT’s sound reproduction potential continues even today. One of the interesting aspects of these debates is the degree to which they ignore the analysis of common, fundamental assumptions about DHTs. Those assumptions are based on many decades of the circuit and mechanical experimentation and compromise. Indeed, because of the difficulty of using DHTs for audio, many circuit design issues have evolved into assumptions about the audio characteristics of DHTs themselves.
 
Allnic’s Mr. Kang Su Park has spent a lifetime studying and designing circuits and transformers for use of DHTs in audio reproduction. Mr. Park has questioned basic assumptions about DHTs, re-examined variations of DHT circuit designs, and researched and experimented rigorously. His work has resulted in some remarkable audio power amplification devices. For example, Allnic’s 300B amplifiers produce significantly more power than normally expected. They do this without shortening tube life because the power increase is not achieved by increased bias. In conjunction with an increase in power, Mr. Park’s 300B amplifiers also have exceptionally low distortion and wide bandwidth.
 
Mr. Park successfully combined elements of what he learned and invented over decades to the development and production of a pure, direct heated triode (DHT) based amplifier and preamplifier: the A-5000 and L-5000, respectively. The main breakthrough was the design of a circuit, valve and transformer combination with astounding measured results. 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 replicated these tests many times and recorded them and their results before assuring himself that there was no mistake.
 
With such an unprecedented superior outcome for the circuit design, Mr. Park could not be satisfied placing a DHT stage after one or two stages using indirectly heated triodes. That 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 introduces the characteristics of indirectly heated triode sound into the circuit. The output DHTs are only amplifying a signal already determined by the initial stage indirectly heated triodes. In contrast, Mr. Park created an amplifier and a preamplifier that use DHTs at each amplification stage. In his design, the output stage DHTs do not merely amplify the signal from indirectly heated triode stages. The L-8000 DHT preamplifier is pure DHT, from the signal input to output.
Remote control included.
 
Key Features of the L-8000 DHT Preamplifier:
 
• Line output transformer coupling – The L-8000 DHT is “transformer coupled”. In tube amp circuitry, there are two coupling methods; one is capacitor coupling and the other is transformer coupling. Capacitor coupling is the traditional, low-cost method. It is somewhat stable but transfers only voltage, not wattage (i.e., not real energy). With transformer coupling, about 90% of real wattage is transferred (there is still a transformer loss of about10% of wattage–voltage is not affected). Transformer coupling is superior to capacitor coupling. However, all the advantages of transformer coupling depend on the quality of the transformer and on the choice of tubes. Allnic Audio manufactures its own transformers and uses what it views to be the best core material, Permalloy. Allnic Audio’s unprecedented, wide (16Hz ~ 75kHz, -3db), low distortion, and ultra-ultra-flexible (up to 50kHz square wave response) output transformer helps Allnic Audio to realize the ideal transformer coupled preamplifier. Of course, gain tubes are also carefully selected for three critical factors for function with the Permalloy output transformers: high gm, low internal resistance and high mu.
• Constant and low output impedance – One of the benefits of transformer coupling is that it facilitates constant low output impedance. Low output impedance is critical to the design of a good preamplifier. The L-8000 DHT has a 120 ohm (120Ω) constant output impedance at all frequencies. In capacitor coupling (C-R coupling), a “cathode follower circuit” is usually used to lower output impedance. Unfortunately, this method of lowering output impedance is accompanied by high distortion and has an “L” shaped output impedance curve. Please compare the two graphs below, especially for low-frequency response.
 
• No negative feedback design
• Advanced tube technology voltage regulation – For quieter and more dynamic operation, the L-8000 DHT has an ultra high-speed automatic voltage regulation circuit, utilizing vacuum tubes. This also protects the amplifying tubes from any change in the AC line supply and copes with any abrupt, internal current demand.
• New vacuum tube damping technology – Allnic Audio’s patented “Absorb GEL tube damper” technology prevents harmful vibrations from reaching the signal/gain tubes and, therefore, prevents microphonic noise propagation in the tubes. The Allnic Audio Absorb Gel damper technology effectively solves a problem that plagues most tube, and especially DHT, amplification systems. Provided other tube components do not introduce microphonic noise into your system, with the Absorb Gel damping system, you will enjoy a degree of transparent sound that will surprise and please you.
• Remote Controlled Precision attenuator volume control – The L-8000 DHT does not employ a digital IC volume control or a low-cost carbon film volume control with a motor. Allnic Audio has developed a precision oil clutched motorized attenuator; the L-8000 DHT has no (± 0db) channel unbalance at any volume level.
• Pure Class A operation
• Pure balanced output stage
• As are all Allnic Audio products, the L-8000 is fully RoHS (EU Reduction of Hazardous Substances regulation) compliant in construction and materials