Tube Amplifier Kit
An Excellent Single-Ended Audio Component
Classical Design, 2 x 9 W, Class A Ultralinear
|We can still remember the times when acoustic devices based on traditional technologies using audio valves were replaced by new ones based on semiconductors. It is a fact that tube amplifiers differ from the semiconductor ones considerably as far as sound handling is concerned. They provide a particular, warmer sound that is so highly appreciated by audiophiles all over the world. There has always existed a large group of tube lovers who prefer to design and build their own tube amplifiers instead of simply buying them in a shop. Money has never been problem for them. It's the warmth of the soldering iron and the odour of soldering which attracts them. There is no greater happiness than to see tubes start glowing and to hear the warmly sounding music from your amplifier for the first time.
It has been our aim to help such audio enthusiasts by offering either a complete kit for a tube amplifier or only some vital components and parts. In this way we help them to avoid problems in finding these parts and still retaining the thrill of making something personal and unique.
|The ASV STA-8 is a classical tube amplifier with a single-ended circuit topology. It has several advantages, principally an excellent sound characteristic of this type of tube amplifiers. It is true, that the output power is rather low but enough for normal listening. Also, it can be compensated for by connecting more sensitive speakers.
The ASV STA-8 has been designed as a stereo 2 x 6W (2 x 9W) amplifier in the housing containing two power stages, a power supply and a passive preamplifier stage with inputs for CD, tuner and tape. A quality input switch and a stereo input potentiometer are fitted at the input. Thanking to a relatively high input sensitivity, no separate preamplifier is needed, except for phono cartridges. The housing is made of quality varnished steel sheet. The output transformers in metal cases, array of tubes and power supply capacitors on top panel contribute to an overall elegant look of the tube amplifier. Rear panel is fitted with all input and output terminals and the power switch.
In fact, the housing of STA-8 is very similar to PP amplifier KTA-50, so they are some kind of "brothers". In the basic model the front and the rear nameplates are printed on special durable polyester self-adhesive foil. However, black anodized and laser engraved nameplates will be available too.
|All components built in the ASV STA-8 were carefully selected: metal oxide resistors, toroidal power transformer, C-core choke, quality capacitors and electrolytics. The output transformers are wound on high quality EI cores. In the basic design the single-ended topology with three active elements per channel was followed. In the input stages, well-known dual triodes 6SN7 (or Russian 6H8C) are used. In the output stage, the power pentodes EL34 work in cathode bias, class A circuit. Power pentodes driver the output transformers in ultralinear mode of operation with a maximum power of up to 9W. For sophisticated constructors it wouldn't be difficult to add a switch for selection between ultralinear and triode modes.|
|The output transformers|
|A high quality output transformer is the heart of a tube amplifier. A poor output transformer fitted in a well designed amplifier will spoil its performance. It must deliver adequate and constant power without core saturation at wide bandwidth and minimum distortion.
Due to reasons dealt with so far, the most delicate point of single-ended and other tube amps are output transformers. The core in a SE output transformer is also magnetized by DC current of output valve, which requires an appropriate air gap between the core halves in order to prevent magnetic saturation of the core. Good linearity can only be achieved with a carefully calculated air gap.
The sound quality of the present SOT-8 transformer is very good. It is carefully wound on high quality EI lamination core, impregnated and tested.
|The final results are very good and also confirmed by measurements. The amplifier operates without overall negative feedback. This provides a clean, airy performance and is capable of delivering truly classic and warm tube sound.
The ASV STA-8 amplifier has been designed for assembling in the classical manner of manual soldering of all components and joints. All elements and connections are entirely hard-wired on the circuit assembly. No printed circuits are used in this amplifier. We believe that only such a design can, together with great degree of skill, know-how and enthusiasm, provide the result: warm, natural sound without disturbing, sharp transients.
Finally, in addition to comprehensive and detailed "Assembling Manual" you will receive an address of a special web page with a set of pictures. These photos show the most important steps of the assembling process.
|Output power at THD = 10%||2 x 9,5 W|
|Nominal load impedance||6 - 8 ohm|
|Input impedance||100 kohm|
|Input sensitivity for full output power||500 mV|
|Frequency response (-3 dB, at 1 W)||10 Hz - 27 kHz|
|Linearity in the hearing range
(20 Hz - 20 kHz, at 1W)
|Harmonic distortion (THD) at 1 kHz, 1 W||1.8%|
|Signal to noise ratio||> 95 dB(A)|
|Overall negative feedback||0 dB|
|Tube complement||2 x 6H8C
2 x EL34
|Energy stored in rectifier||44 J|
|Measures (w x h x d)||400 x 210 x 300 mm|