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 favourite audio amplification device for devoted music lovers worldwide.
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 analysis of common, fundamental assumptions about DHTs. Those assumptions are based on many decades of 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 A-5000 and L-5000 DHT amplifier and preamplifier are pure DHT, from signal input to output.
OVERCOMING THE DIFFICULTIES OF DHT PHONO-STAGE DESIGN
• Rarity of DHT tubes suitable for phono amplification. The essential DHT tubes for phono amplification should have high gain, low internal resistance and high mutual conductance. It is extremely difficult to make that kind of DHT tube. Among hundreds of kinds of DHT tubes, there are fewer than 5 that have the specific characteristics required for phono amplification. To find those tubes requires broad and deep knowledge of tube types and specifications and much time consuming experimentation.
• DHTs’ extreme sensitivity to micro-phonic noise. Due to their large, bulky and weak construction, DHT tubes tend to vibrate with the tiniest external stimuli, so it is almost impossible to use a DHT for phono reproduction without breakthrough engineering solutions to inhibit sympathetic vibration in the DHT tube. Allnic’s patented GEL tube damper socket, based on liquid elastomer, mitigate micro-phonic vibrations in the DHT tube.
• Difficulty of properly matching transformers to DHTs. There is no tradition of manufacturing DHT phono-stages, so there are no matching transformers available in the marketplace. It is, therefore, essential to have in-house capability to design and wind the transformers specifically required for production of a pure DHT phono-stage. Allnic has long experience with designing and winding custom transformers, so it is quite natural that a perfect DHT phono-stage could be made in the Allnic factory. In addition, Allnic has developed and produces a very small output transformer for DHT phono amplification. The smaller the transformer, the better sound will be reproduced due to the reduced size of the magnetic field of the core material, while still providing enough inductance for realistic low frequency reproduction.
• Designing a clean, strong but balanced power supply for noise-free DHT phono amplification. Since DHT amplification is so sensitive, and has the potential for extremely low noise reproduction, an all tube, high-speed, automatic voltage regulation(AVR) circuit is most appropriate for the power supply. AVR enhances the signal to noise ratio for noise free operation in dual mono mode. For the H-5000 phono-stage preamplifier, Allnic uses 300B tubes, a typical DHT, as a series regulator tube in an AVR circuit. This design results in a superior signal to noise ratio.
Key Features of the H-5000 Phono-stage:
• LCR TYPE PRE-RIAA AND RIAA EQUALIZATION: RIAA equalization is a specification for the correct playback of vinyl records, established by the Recording Industry Association of America. The purpose of the equalization is to permit longer playback times and improve sound quality. Before the establishment of the RIAA equalization standard, different production companies utilized different equalization curves. Vinyl record production equalization is a form of establishing a flat frequency response for the playback of recorded music. The necessity for this equalization process arises from mechanical difficulties inherent in record production. In order to prevent the cutting needle from over-cutting into the next record groove in the bass, as a record is cut, some bass frequencies are attenuated. In the treble region, in order for high frequency sounds not to be masked by the noise inherent in moving a stylus over and through a modulated vinyl surface, some treble frequencies are boosted. With the application of the correct filtering techniques on playback, the result is a flat frequency response with better signal to noise ratios. The H-5000 is equipped with newly developed Allnic “Multi-Curve” LCR units (one for each channel) which have four (4) Turn-overs (frequency options) and four (4) Roll-offs (gain reductions in dB). These new units provide for the reproduction of various recording curves (both for bass attenuation and treble boost) used by different companies prior to the establishment of the RIAA standard. There are 4 Turn-over options, at: 250Hz, 400Hz, 500Hz RIAA and 700Hz. There are 4 Roll-off options (at 10KHz): -5dB, -11dB, -13.7dB RIAA and -16dB. The rectifier tubes supplied with the H-5000 power supply are DHTs. Please note that in some electrical environments, the rectifier tubes in the H-5000 power supply may need to be changed to an indirect heated tube. While rectifiers can have audible effects, the rectifiers are not in the signal path: the signal path in the phono-stage remains pure DHT.
• Why LCR? There are four de-emphasis methods for equalization to that can be applied at playback:
A. Active filters (Negative feedback types): Different quantities of negative feedback are applied, with deeper feedback to the high frequencies and shallower to the low frequencies. The benefits of this method are improved signal to noise ratios, low cost and consistent operation. Some of the shortfalls are looser bass reproduction and, possibly, a pinched and compressed high frequency playback due to excess feedback ratios.
B. Passive filters (CR type): The frequencies are filtered to fit the RIAA specification by varying the amount of attenuation at different frequencies through a complex capacitor-resistor network. This technique results in no voltage overload, purer reproduction (because there is no feedback), and more accurate RIAA compensation. However, there are problems because the system provides no gain, and insertion loss and impedance matching issues arise.
C. Hybrid filters (use of both CR and negative feedback types): In this method, both types of filters applied separately; an active filter is applied to the low frequencies and a passive filter to the high frequencies. Unfortunately, both the advantages and disadvantages of each of these two types of filters, already discussed, affect the playback system at the same time.
D. LCR filters, used in the H-5000: Two pieces of a linear reactor (a kind of choke coil) comprise the main part of these filters, assisted by precise CR filters, in order to lower impedances and insertion loss. In vacuum tube circuits, active and passive filters usually are operated on one hundred plus kilo-ohms of impedance. An LCR equalization filter’s impedance is a constant 600 ohms. Furthermore, an LCR equalization filter’s series resistance is less than 13 ohms (as a comparative, some famous ones are 31 ohms). The lower the impedance, the more dynamic is the sound reproduction, with better bass response and speed. But LCR equalization units have drawbacks as well. These drawbacks are high cost and the difficulty of impedance matching; the latter has been the primary hindrance to the commercialization of this superb method in the construction of phono-stage amplifiers. However, Allnic Audio manufactures high quality LCR equalization units in-house and has developed a 600 ohms impedance matching method.
• The H-5000 Phono-stage is transformer coupled, with the exception of the first gain stage, the HL-2 tube.
• No negative feedback design with three gain stages
• A power supply unit separate from the pure DHT phono-stage itself
• High quality MC Step-up Transformers with Permalloy cores are used for the H-5000’s dual MC inputs.
• 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 micro-phonic noise propagation in the tubes. The Allnic Audio Absorb Gel damper technology effectively solves a problem that plagues most tube amplification systems. Provided other tube components do not introduce micro-phonic noise into your system, with the Absorb Gel damping system, you will enjoy a degree of transparent sound that will surprise and please you.
• Pure Class A operation
• Pure balanced operation
• As are all Allnic Audio products, the H-5000 is fully RoHS (EU Reduction of Hazardous Substances regulation) compliant in construction and materials.
• Moving Coil (MC) × two (2) pairs unbalanced (RCA)
• Moving Magnet (MM) x two (2) pairs unbalanced (RCA)
• Two (2) x screw type terminals
• One (1) pair x unbalanced (RCA)
• One (1) pair x balanced (XLR)
Frequency Response (RIAA):
• 30Hz ~ 20KHz (±0.5dB)
• MM +44dB (1KHz)
• MC +74dB (1Khz)
• MC – up to 200 Ω
• MM – up to 47 KΩ or, upon request, up to 85 KΩ
Maximum Input Voltage:
• MM non-clipping:
• 30 Hz –10mV
• 100 Hz –45mV
• 1KHz – 200mV
• 10 KHz –600 mV
Total Harmonic Distortion (THD):
• Less than 0.3% (1KHz, Output 1V)
• 300 Ω (Constant)
Signal to Noise (S/N) Ratio:
• – 80db (CCIR, 1KHz)
• 70W at 220 V / 110/120 V / 50 / 60 Hz
• Two (2) x HL-2 – first gain stage: no equivalents
• Four (4) x KR242 – second and third gain stages: equivalent is Telefunken RS242 only
• Two (2) x 5Y3 – Rectifiers
• Two (2) x 5654 – Voltage error detectors: equivalents are 6AK5W, E95F, M8100, 6096, CK5654, 5654W, GL-5654, CV4010, CV850, 5654RT
• Two (2) x 300B – Voltage Regulators – no equivalents
• AC 2A, 250V
• 430mm (16.9 inches) x 360mm (14.3 inches) x 250mm (10 inches) (W x D x H)
• 430mm (16.9 inches) x 360mm (14.3 inches) x 250mm (10 inches) (W x D x H)
• 15 Kg (33 lbs) unpacked
• 15 Kg (33 lbs) unpacked Both units in original packing:
• 17.2 Kg (38 lbs.)
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