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JULY 2022

To "tube" or not to "tube"? That is the question.

To “tube” or not to “tube”? That is the question.

But to “square-tube”? Just when you think that you have seen it all, Kora comes up with one heck of a new circuit… and it is a Real Jewel… read on!



The Square Tube amplification circuit is a new way of using electronic tubes to amplify an audio signal. This innovation is significant, and as a consequence, it has been patented by Kora. Square Tube© is a now also a registered trademark. Kora high fidelity components are designed and built in France by cutting-edge electronics technology professionals. Led by chief designer Bruno Vander Elst, Kora’s design teams are also involved in the high-precision instrumentation field for civil, aeronautical and military measurement laboratories. As a consequence, the criteria and technical standards for Kora’s specific schematics and components meet the same exigent requirements.

The concept behind the SQUARE TUBE has been on Bruno’s radar for some time, and its final realization is the result of a long process of R&D. In a nutshell: tubes — an old technology — are used in a totally modern way : the circuit is organized around a differential stage which is inherently symmetrical — and an output stage which could be compared to an OTL push-pull design. This combination constitutes a complete operational amp, and can be described as «passing» the direct current… but not «Passing the DC», which is well understood to be acoustically detrimental. Further, this characteristic allows the SQUARE TUBE circuit design to be “self-stabilizing”, and to maintain a perfect polarization of the tubes, and this independent of other external factors..i.e. temperature and the effects of timing / phase. This innovative topology guarantees a constant operating behaviour as well as a constant listening quality.

Four tubes are used to make a single amplification stage, hence the name! The SQUARE TUBE «outputs» a symmetrical signal with respect to the reference (Ground). In other words, the signal alternates between positive voltage (to push the diaphragm of the loudspeaker) and negative voltage (to pull the diaphragm). This unique capacity with vacuum tubes makes it particularly suitable for the design of audio amplifiers. In your Kora amplifier, the SQUARE TUBE provides all the functions of amplification and control of the audio signal. At the output stage of the SQUARE TUBE, the signal is identical to the one which is input to the speakers. Complementary audio and hyper-linear power transistors, will provide the current required by the speakers. These are linear from 10mA to 10A. This is exceptional thanks to ON Semiconductors™! This current will be provided however without interfering with the amplification of the signal. This is because these are mounted in a “voltage follower mode”, and thus have a gain slightly lower than 1. We are definitely talking about a tube amplifier here!



The operation of the SQUARE TUBE is very sophisticated. The Operating Point of the tubes are “imposed on them”. For the pair of input triodes, the current flowing through them is determined by a “quasi current source”: Two mA are injected into the two cathodes, and then each input tube must work with 1 mA, regardless of its Brand or specifications. In the same way, the currents in the pair of «output» tubes are fixed by reference voltages. The currents in the tubes are much lower than their rated current. This aspect has been intentionally built into the design. This guarantees an exceptional lifetime for these components.

The SQUARE TUBE circuit is powered by two highly regulated voltage supplies. One positive and the other negative. As regards the heating of the tube filaments, and to scrupulously respect the specifications of the tubes, the filaments are powered by a «floating» mode supply through small modules called «DC-DC». All tubes are heated in direct current (except for the TB 140 output tubes).



Why does SQUARE Tube present unusually contradictory auditory qualities? First, let us stress again that the SQUARE TUBE is in fact a tube amplifier. Tubes inherently, and compared to transistors, have a relatively low amount of amplification, but they do it very well. They are generally more linear than transistors. To compensate for their low gain, the Kora SQUARE TUBE circuit topology uses 4 of them for each single stage.

The SQUARE TUBE is driven by voltages between 400 V and 500 V depending on the model. This supply voltage corresponds to an amplifier of 2500 Watts or more, under 8 ohms. Do not forget that the output voltage of the SQUARE TUBE is the one applied to the amplifier’s loudspeaker outputs ! Of this voltage of 400 V or 500 V, we actually utilize only 20% when the amplifier is at its maximum output. As for the afore-mentioned theoretical power capacity potential of 2500 W, we exploit only a few percent of it. In other words: the amplifier is always in «its comfort zone» and never operates near its limits. To reiterate — and this is important to remember: the SQUARE TUBE is not a transistor amplifier preceded by tubes, it is a tube amplifier assisted by transistors.

This design structure: makes it possible to preserve the very euphonic sound qualities of vacuum tubes whilst overcoming the current limitations of Power Tubes. It also avoids the use of an output transformer, with its phase-rotation issues. Given that there is no current limitation, your amplifier can drive the loudspeakers more “tightly” which explains the very good performance of the lower frequency spectrum — excellent Bass, but one that is never unnaturally emphasized.

Another advantage of the technology is also very evident in the amplifier’s ability to drive capacitive loads. The square signal’s behavior on a test load made up with a capacitor of 2.2 uF placed in parallel with a resistance of 8 ohms is simply remarkable. This also explains the good performance of the Kora’s amplifier circuit with speakers equipped with so-called “demanding” crossover filters! In summary,  there is plenty of current drive to deliver great Bass… and then, of course, there are those startling and limitless tube dynamics!



​The Square Tube is characterized by some striking sonic characteristics:
— A great precision in portraying the exact placement of instruments and vocals. This, with the greatest respect for Tone, in all its colors.
— An enlargement of the sound stage… in width, but also in depth. You will also often notice a gain in the height of the sound image.
— The sound “image’ (or images), within the soundstage, is (are) absolutely stable.
— An excellent level of dynamics, which equals that of the best transistor amplifiers.
— A great sense of softness and smoothness that provides a very natural listening experience… one that absolutely ensures the absence of any listening fatigue, even at high SPL levels, and which is the equal of that of the best tube amplifiers.
— The signal is rich and detailed, precisely shaped. The cymbals find a purely analog sound (you should discover a lot of unsuspected details in your music collection).

With unrestricted current capacity, the bass speakers are firmly driven, giving an ample and rich bass, always controlled, and free of disastrous harmonic “wobbles and exaggerations” (aka the “boom-boom” syndrome). As such, you will have the feeling of having gained an additional octave in the lower frequency spectrum. An uncanny ability to “transcribe” the musical message, whatever the level of its complexity. Instruments and voices are reproduced with aeration. They will be “cleanly delineated”, without halo. They benefit from a great “readability”.



• AC Supply Technical Aspects
The Kora amplifiers are designed to operate at a rated main voltage in the range of +7% and -10%. In order to minimize the disturbances present on the AC, the Kora amplifiers are equipped with a double-filter input:
— A first filter processes the so-called «common mode» noise (between the ground and the two neutral and phase wires),
— A second circuit filters the so-called «differential mode» disturbances (between neutral and phase).
It is recommended that one always use a good quality power cord, plugged into a proper AC outlet!

The output power available on the speaker terminals is directly dependent on the mains voltage: we draw attention to the fact that an increase of the AC voltage of 10% leads to an increase in the available power of more than 20% at the output. The Kora amplifier has a huge current capacity: it will not falter! Therefore, it is important that one always be very careful if using low-power speakers. Given the natural and life-like sound of the Kora amplifier, one may have a tendency to want to listen at a higher level than usual.

• Speaker Output Protection
For the power supply section, the Kora amplifier has a generously dimensioned “double mono” power supply, with — depending on the model — one or two toroidal transformer(s). The output transistors are powered by as many fuses as transistors. The fuses are placed at the “collectors” of the transistors; a so called “high impedance zone”. This way, the non-linearities of fuses have no effect on the signal. The ratings of the fuses used are far below the limits of the ratings of the transistors. This, so as to actually provide protection against mishandling. A resettable circuit “disconnects” the output transistor which would end up without a fuse. The DC output voltage is thus kept close to zero, to protect the speakers. As a result, the amplifier continues to operate, but with reduced current capacity.

• Pre-charging of the Power Supplies
The Kora range is equipped with generously sized power supplies. Positive thermal coefficient power resistors work as limiters to inrush current at power on. This avoids overloading unnecessarily the filter capacitors which are essential power reservoirs to ensure flawless operation. These components owe their performance to a thin film of aluminum oxide. To preserve this precious film, Kora power supplies are equipped with a device which will insert very gently, and for a few minutes, a slight current in order to permanently maintain a residual voltage at the Capacitor’s terminals. Once they are pre-loaded, the current is canceled. First advantage: maintained in this way, the lifetime of the components can be counted in decades, while maintaining their characteristics. Second advantage: in the start-up mode, the Capacitors are close to their operating voltage, and thus the inrush current will be indeed reduced.

• Voltage Monitoring
A microcontroller constantly monitors the important supply voltage of the amplifier, the tubes’ outputs, and the outputs to the the loudspeakers. The SQUARE TUBE is a tube amplifier. This technology is characterized by “passing DC”. During the warm-up process, the output voltage is uncontrolled, as a tube warms up always faster than its neighbor. As long as the resulting voltage is not stabilized at a corresponding value (Less than 1 Volt in general), the SQUARE TUBE remains disconnected from the output transistors. An identical phenomenon occurs while setting Standby. A tube cools faster than the other, the output transistors are, in this case too, disconnected from the tubes. If the correct voltage is not reached after the normal heating delay, a message reports the problem. In this case, no signal is transmitted to the outputs. lf a DC voltage appears on the speaker output terminals, the amplifier is placed in Stand-by mode and an alert message appears.

• Polarization of the Output Transistors
To avoid sending the speakers a completely useless, even harmful DC current, the output voltage is constantly monitored and held close to 0 by a dedicated circuit (@ less than 50mV). This circuit is not involved in the amplification chain and has no effect on the audio signal. The Kora amplifier works in class A/B. The Zero Signal Current — which is approximately 30. mA per transistor — is thermally stabilized in a traditional way. The output transistors being only «Followers» of the SQUARE TUBE output stage, the bias circuit is powered by a small floating power supply. Some floating power supply noise (tube warming, zero signal current) is generated by small modules called «DC-DC» operating at a very high frequency, above 100 kHz. Examining the output signal at the oscilloscope can reveal traces of these oscillators. This is far beyond the technical capabilities of the speakers to reproduce, and of our ears to hear, and therefore has no influence on the musical signal.



Notwithstanding the very elaborate circuit design, the general and fundamental design of the Kora product line follows simple precepts:
— To make a High Quality Product in all respects
— To ensure that this product is highly satisfying on the musical level, of course
— To ensure that the product provides the “quality experience” for very many years

As such, all components are carefully selected to provide an extremely high and precise level of performance that is typical of products with military specification requirements. Further, Operating Points are determined and set to be far below specification limits, thus ensuring the longevity of the amplifier.

And finally, as concerns the actual manufacturing approach of the amplifiers: all Kora amplifiers are built in a modular way. Each function is provided by a dedicated and independent electronic board. These boards are designed to be easy to replace in case of failure. All boards use easily sourced components. All of this to ensure that service will be always possible, even in a few decades.

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