FOR SALE: DXT Electro - Acoustics DXT-17

 

Item: Pair of DXT 17 Power amplifiers
Location:
Brisbane
Price:
$1200
Item Condition:
Perfect like new
Reason for selling:
I need only one pair of dxt 17
Payment Method: Pickup -
Cash, Paypal,money transfer...
Extra Info:
This is a very cheep sale 600 per monoblock > tubes are worth

$-+400

NOTES:

I Want to sale these amps locally only

Buyers are welcome to try before hand

I don't need to sell it  but  I won't need it anymore and don't want them to sit  here unused

 

Pictures:

 

SOLD

 

Vacuum Tube 17 watt Monoblock power amplifiers
$5 800 AUD per pair plus10%GST


Recreating the original sound!

 

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 GZ32/5V4 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 double primary windings are required in the Mains Transformer leading to a much 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 16 Ohms speaker, 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 2 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 except the Vacuum Tubes which are warranted by the manufacturers’ guarantee of 3 months.

 

DXT-17 AMP Specifications

 

Rated power output into a 8 Ohms speaker load :

17 Watts

 

Distortion @ 1KHz :
14.0 dB Feedback : better than 0.45% at rated output
7.4 dB Feedback : better than 0.99% at rated output
5.0 dB Feedback : better than 1.30% at rated output

 

Frequency response :

14.0 dB Feedback : 10Hz – 62kHz (-1.6, -3.0dB,respectively)
7.4 dB Feedback : 10Hz – 28kHz (-3.0, -3.0dB, respectively)
5.0 dB Feedback : 10Hz – 20.5kHz (-3.7, -3.0dB, respectively)

 

Input sensitivity at input impedance of 740 Kilo Ohms:

14.0 dB Feedback, control at minimum

(fully ACW) : 1.67 V rms
7.4 dB Feedback, control at midway : 0.80 V rms
5.0 dB Feedback control at maximum

(fully CW) : 0.67 V rms

 

Hum and Noise :

Better than - 80 dB

 

Power Supply Consumption :

125W

 

Dimensions :

Width :312 mm excluding protrusions

(add 35 mm to include protrusions)
Height :195 mm
Depth :260 mm

 

 

 

Finish :

Chassis : one piece welded, 2mm thick Gold Colour

powder-coated galvanized steel
Cover : one piece rectangular, 1.2 mm thick Gold

powder-coated galvanized steel

Weight : 17 kg

 

Specifications are subject to improvement

changes without notice.
Vacuum tube brands may vary depending

on available supplies.
Colour of chassis may vary depending

on available paint supplies.

 

 

 

 Rev#03 DXT Electro-Acoustics Pty Ltd
The DXT LOGO is a registered Trade Mark


 

 

 

 

Contact:

jiri_ch@yahoo.com