The Audiophile Workshop

DXT Electro - Acoustics DXT-08
DXT 08 Valve Pre-Amplifier with separate Tube regulated power supply
$3,800 AUD.incl tax
Recreating the original sound!
In order to complement our excellent range of monoblock amplifiers, it was necessary to design an equally high performance Pre-Amp to match. It is well known that each additional stage of amplification adds a slice of distortion to the original sound. The design philosophy then was to put in only the necessary number of amplification stages just enough to fulfill the required amplification to drive our range of monoblock ampfliers to full power. It is to be emphasized that Phono reproduction is the forte of our design. Being an all-vacuum tube design, our difficult goal was to eliminate hum completely in order to achieve a full musical enjoyment. Achieving this goal in a Solid-State design is easier than an all-vacuum tube design. However, we chose an all-vacuum tube design in favour of the superior mid-range frequencies (e.g. vocals) and transparency offered by vacuum tubes. As hum is a major factor that negates the complete enjoyment of music, it was necessary to eliminate hum induction by the introduction of a separate power supply unit. Thus was born the DXT-08 Pre-Amp and its external power supply.
The DXT-08 is a stereo all-vacuum tube pre-amplifier devoid of any tone control. It is recognized that a Pre-Amp with tone controls do colour the sound to a certain degree depending on the design. The DXT-08 features a true tape-monitoring capability for those die-hard analog audiophiles! It also has Line and Phono selection capabilities and is carefully handcrafted and built to last a lifetime. Although the DXT-08 is built to match our range of monoblock amplifiers, other amplifiers with an input impedance equal to or greater than 300K Ohms and a sensitivity equal to 1.4V rms or better can also be used.
There are three circular knobs grazing the front panel viz:- Selector, Tape Monitor and Volume. The input selector options are :- either CD, TUNER, AUX 1 and AUX 2/PHONO. By default, the AUX 2/PHONO position comes with a phono option. However, if Line sensitivity, in place of Phono sensitivity is required, it must be specified at the time of order.
The Pre-Amp utilizes 5 vacuum tubes namely :- two EF 86, two ECC 83 (12 AX7) and one ECC 82 (12AU7). At the front-end is the EF86 (pentode) which provides the appropriate LINEAR equalization for the LINE inputs or RIAA equalization for the PHONO input.

The DXT-08 Pre-Amp is powered by a separate Power Supply Unit. The reason for an external Power Supply Unit is to minimize the effect of hum-induction on the Pre-Amp, this is especially true when operating in the Phono mode where susceptibility to hum-induction is at its worst. Power for the High Tension and the Heaters in the Pre-Amp are supplied via a heavy duty 4-wire connecting cord. This cable can be detached during transportation. The Power Supply Unit utilizes an EZ80 vacuum tube rectifier for its High Tension supply while the 6.3 V Heater supply is d.c. regulated. The purpose of the EZ80 tube is to give a soft start to the Pre-Amp, thereby prolonging the lifespan of the audio tubes. There is a switched 6-Ampere IEC a.c. mains outlet for the purpose of controlling an ancillary equipment.
Both the Pre-Amp and the Power Supply Unit are high quality handcrafted products. The front panels are 6mm and 3 mm thick, gold colour, anodized aluminum material, respectively, while the one-piece welded chassis and a one-piece welded box-cover are powder-coated 2mm and 1.2 mm thick galvanized steel material, respectively.
Point-to-point wiring and Tag Boards are being used throughout in favour of PCBs which have the tendency to warp when subjected to the high internal temperature. High temperature grade wires, with Telfon insulation are being used in the high temperature areas. Vacuum tube bases are made from high temperature ceramic material.
The high-tracking accuracy of the Volume Pot is supplied by Noble, Japan. All signal-path capacitors are of the highest quality polypropylene Italian-made variety. The amplification devices are high quality Australian, American or European vacuum tubes. The selector switches have a positive feel when rotated and they come with silver-plated contacts to ensure long life. Finally, only high quality gold-plated RCA sockets are used.
The DXT pre- amplifier is warranted for 3 years parts and labour. Tubes are warranted by the manufacturers’ guarantee of 3 months. This High quality pre-amplifier is made to your order. View Specifications

It is not in every situation that one finds a high efficiency speaker with a large woofer in a large cabinet to couple an amplifier to. With modern-day speakers they tend to be smaller in size and thus is less efficient. While the DXT-12 monoblock amplifier works extremely well with high efficiency speakers , the demand for more power is required to meet the needs of medium efficiency speakers. Thus, there was a need to conceive an amplifier capable of producing more power and higher damping factor than the DXT-12 Amp while still adhering to very similar design principles embodied in the original DXT-12 Amp. This new amplifier shall be designated the DXT-17 AMP, a monoblock amplifier.
For a start, as in any good amplifier design, only vacuum tubes with very linear characteristics are chosen. The Push-Pull Technology was chosen over Single-Ended design to yield the most efficient way of transferring power from the Vacuum Tubes to the loudspeaker. The design principle of Push-Pull design dictates that balanced signals are required for minimum distortion to occur. The key to the design is to understand how to achieve balanced signals all the way from the phase inverter to the output transformer.
Next comes the Phase Inverter. As with most types of phase inverter, the a.c. imbalance mainly occurs at this section of the amplifier.
One of the most common methods is to use a potentiometer to balance the a.c. signals coming out from the phase inverter. While this is good at the point of adjustment, it however has the disadvantage of a.c. imbalance appearing as the vacuum tubes age. Conventional long-tailed phase inverter uses a high resistance value on its tail-resistor to minimize a.c. imbalance. This technique is never good enough because the ideal situation of having a tail-resistor with infinite resistance will never be feasible and thus is a compromise. Our design overcomes this problem by replacing the tail-resistor with a constant current source. As a result, the output signals (in anti-phase) from the phase inverter will always be in perfect balance even as the power supply fluctuates or when the vacuum tube ages.

The DXT-17 Amplifier is designed to be a Monoblock, rated at 17 watts. For stereo operation, two monoblocks are required. The DXT-17 Amp uses a Push-Pull configuration that operates in Class AB mode. The overall topology of the design utilizes triodes and triode-connected vacuum tubes for a warm, smooth, accurate and life-like sound. In one word, the listening experience could be termed as ‘musical’ to describe the overall sound.The output vacuum tubes are based on a pair of very linear triode-connected EL34s. Vacuum tube rectification is in the form of the rectifier tube, the GZ32 and a Pi capacitor/choke/capacitor filter. The vacuum tube rectification provides a soft-start to the rest of the vacuum tubes which in turn prolongs the life span of the rest of the tubes. However, although this is more expensive, it is preferred to solid-state rectification which exhibits hard-starting and noisier high tension voltage supply due to the higher harmonics ripple caused by the switching characteristics of solid state rectifiers. In our view, vacuum tube rectification is superior to solid-state rectification for the reason just stated.
The DXT-17 is a three-stage amplifier. The front-end tube, the EF86 provides amplification as well as impedance matching between a Pre-Amp and this monoblock amplifier. It is interesting to note that the EF86 is triode-connected for its excellent linear characteristics which in turn brings out the EF86’s full musical potential. There would be a lesser musical potential, had it been connected as a pentode. The input sensitivity is designed to match the DXT-08 Pre-Amp or any high output Pre-Amp capable of supplying at least 1.4V rms (least sensitive mode; see specs below) with an output impedance of 74 Kilo Ohms or lower. The DXT-17 amplifier has a low susceptibility to hum. As a result, there is no audible hum even when placing one’s ear next to a high sensitivity 100dB speaker.

Proceeding into the second stage, the amplified signal from EF86 is then capacitively-coupled to a Long-tailed Phase Inverter , the 6CG7 vacuum tube. There is phase inversion as well as further amplification from this twin-triode valve. The perfectly balanced anti-phase signals then finally drive a pair of output vacuum tubes, the EL34s.The two EL34s are cathode-biased as opposed to using Fixed-grid bias. The advantage is the better auto-control of each tube as it ages without the need for bias re-adjustment. The downside of Cathode-bias is the higher supply voltage required to power the EL34 tubes because of the additional voltage drop across each cathode resistor. This means that more windings are required in the Mains Transformer leading to a slightly higher cost.

At the onset of the design, the goal was to achieve as low a distortion as possible without first applying negative feedback. It is with special selection and close matching of components that even without feedback, the distortion at 1KHz , mainly odd-harmonics is just under 1.5% at the rated output of 17 watts. In practice, it was necessary to strike a balance by applying the right amount of feedback to reduce the THD further without sacrificing the musicality. There is an overall or global negative feedback being applied from the secondary of the output transformer to the cathode of the first tube, the EF86.

The ‘right amount’ would then be a personal preference and hence the birth of the Variable Feedback Control (VFC). The distortion figure then drops further from just over 1.5% to these percentage figures depending on the setting of the continuously Variable Feedback Control e.g. :-
Fully anti-clockwise : 0.45% THD with 14.0 dB negative feedback,
Center position : 0.99% THD with 7.4 dB negative feedback and
Fully clockwise : 1.30% THD with 5.0dB negative feedback.

Higher amounts of feedback give better frequency response but a narrower soundstage. On the other hand, lower feedback has more details and a wider, less compressed soundstage amidst a slightly mellower sound. Obviously, the choices are plentiful and the best way would be for the listener to experiment with the VFC to find an optimum spot. In essence, it is like utilizing the VFC to match the monoblock amplifier to a particular type of loudspeaker.

The output impedance to match the speaker is factory-wired for 8 Ohms. For matching to a 1 Ohms, 4 Ohms or 15 Ohms speakers, this requirement must be specified at the time of order and can be factory-wired. The amplifier is a high quality handcrafted product that is built to last a life-time. The only parts that will ever need replacement are the vacuum tubes and no adjustment is needed thereafter. The time interval required to replace the tubes will depend on the usage but it would be typically in excess of 5 years for the small signal tubes and 3 years for the large signal tubes, assuming a daily usage of about 5 hours.
The output transformer, mains transformer and the choke are custom-designed and manufactured to exact specifications. The overly-sized and overly-rated iron-cored mains transformer and choke are deliberately designed to have cooler operation for long-term usage. The output transformer uses grain-oriented silicon steel (G.O.S.S.) 0.3 mm laminations to maximize flux-transfer for a given iron-core size.

The DXT amplifier is warranted for 3 years parts and labour. Tubes are warranted by the manufacturers’ guarantee of 3 months.
View Specifications

The DXT-25 Amplifier is designed to be a Monoblock, rated at 25 watts. For stereo operation, two monoblocks are required. The DXT-25 Amp uses a Push-Pull configuration that operates in Class AB mode. The overall topology of the design utilizes triodes and triode-connected vacuum tubes for a warm, smooth, accurate and life-like sound. In one word, the listening experience could be termed as ‘musical’ to describe the overall sound.
The output vacuum tubes are based on a pair of very linear triode-connected KT88s. Vacuum tube rectification is in the form of the rectifier tube, the GZ34 and a PI capacitor/choke/capacitor filter. The vacuum tube rectification provides a soft-start to the rest of the vacuum tubes which in turn prolongs the life span of the rest of the tubes. However, although this is more expensive, it is preferred to solid-state rectification which exhibits hard-starting and noisier high tension voltage supply due to the higher harmonics ripple caused by the switching characteristics of solid state rectifiers. In our view, vacuum tube rectification is superior to solid-state rectification for the reason just stated.
The DXT-25 is a three-stage amplifier. The front-end tube, the EF86 provides amplification as well as impedance matching between a Pre-Amp and this monoblock amplifier. It is interesting to note that the EF86 is triode-connected for its excellent linear characteristics which in turn brings out the EF86’s full musical potential. There would be a lesser musical potential, had it been connected as a pentode. The input sensitivity is designed to match the DXT-08 Pre-Amp or any high output Pre-Amp capable of supplying at least 1.4V rms (least sensitive mode; see specs below) with an output impedance of 74 Kilo Ohms or lower. The DXT-25 amplifier has a low susceptibility to hum. As a result, there is no audible hum even when placing one’s ear next to a high sensitivity 100dB speaker.
The two KT88s are cathode-biased as opposed to using Fixed-grid bias. The advantage is the better auto-control of each tube as it ages without the need for bias re-adjustment. The downside of Cathode-bias is the higher supply voltage required to power the KT88 tubes because of the additional voltage drop across each cathode resistor. This means that more windings are required in the Mains Transformer leading to a slightly higher cost.
At the onset of the design, the goal was to achieve as low a distortion as possible without first applying negative feedback. It is with special selection and close matching of components that even without feedback, the distortion at 1KHz , mainly odd-harmonics is just under 1.5% at the rated output of 25W. In practice, it was necessary to strike a balance by applying the right amount of feedback to reduce the THD further without sacrificing the musicality. There is an overall or global negative feedback being applied from the secondary of the output transformer to the cathode of the first tube, the EF86. The ‘right amount’ would then be a personal preference and hence the birth of the Variable Feedback Control (VFC). The distortion figure then drops further from just over 1.5% to these percentage figures depending on the setting of the continuously Variable Feedback Control e.g. :-


Fully anti-clockwise : 0.35% THD with 15.0 dB negative feedback,
Center position : 0.80% THD with 8.0 dB negative feedback
and Fully clockwise : 1.05% THD with 5.7dB negative feedback.

A bit of information about the materials used. The welded chassis is made from 2 mm thick galvanized steel material which is further powder-coated to enhance its appearance. Point-to-point wiring and UK-made Tag Boards are being used throughout in favour of PCBs which have the tendency to wrap when subjected to the high internal temperature. High temperature grade wires, with Telfon insulation are being used in the high temperature areas. Vacuum tube bases are made from high temperature resin material. Only high quality Australian, American or European-made vacuum tubes are being used. All signal-path and power supply filter capacitors are of the highest quality Italian-made polypropylene dielectric type. They are virtually indestructible when operated within its specified operating voltage. There is a total elimination of electrolytic capacitors as they generally have a limited life span and do deteriorate electrically over time. The only disadvantage of non-electrolytic capacitors is its higher cost and bigger size. The majority of resistors used are at least 1 Watt carbon-film resistors which are generously power-rated while the higher power 10 Watt resistors are generally wire-wound. Carbon film resistors produce less audio noise than the metal film variety and hence their obvious choice. The output transformer, mains transformer and the choke are custom-designed and manufactured to exact specifications. The overly-sized and overly-rated iron-cored mains transformer and choke are deliberately designed to have cooler operation for long-term usage. The output transformer uses grain-oriented silicon steel (G.O.S.S.) 0.3 mm laminations to maximize flux-transfer for a given iron-core size.
The DXT amplifier is warranted for 3 years parts and labour. Tubes are warranted by the manufacturers’ guarantee of 3 months.
View Specifications

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