Square Tube

Patented technology. The Square Tube technology is a new way of assembling the electronic tubes to amplify an audio signal.

The “SQUARE TUBE” is the result of long research and long-term assessment. The tubes – a well-known technology – are used in a modern and innovative way. Four tubes are used to make a single amplification stage, hence the name. Powered by two tensions, a positive and a negative, the Square Tube® delivers a signal which is symmetrical to the reference (the ground). That means that the signal alternatively reaches the positively charged domain (to push the membrane of the speaker) and the negatively charged domain (to pull the membrane). This innovative and original architecture along with the usage of vacuum tubes makes this scheme suits particularly well the staged amplification method.

Gamme KORA

« Square Tube » is a registered trademark.

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In the Kora products, Square Tube technology ensures every amplification and audio signal control function.

When the Square Tube is used in a power amplification device, at the end of the tube, the signal is identical to the one provided to the speakers. Complementary power transistors, audio and hyper-linear, provide the demanded current to the speakers, without interfering with the amplification signal. This makes this amplifier a true tube amplification device.

The operation of the Square Tube is very sophisticated compared to all other vacuum tubes technologies. More detailed information is provided in our “technical information” documents, which can be dowloaded here.


How does the Square Tube implement these otherwise incompatible audio features?

The Square Tube is a tube amplification device, and tubes are not used as pre-amp. The tubes have a very clean amplification capability, however it is quite limited. They are, generally speaking, more linear than transistors. To compensate their weak gain, the Square Tube uses four on each stage. With this method the distorsion is reduced, compared to a traditional assembly.

The Square Tube is powered by two high tensions rails, each one delivering around 250 V (2 x 250 V). These tensions provide more than the power required by the “audio” signal. Therefore, the power used when the amplifier is at full power is around 20% of the total provided. In short: The Square Tube always operates in its “confort zone”, and never close to its limits.

An amplifier built around the Square Tube module is not a hybrid (with transistors preceded by tubes), but a proper tube amplifier assisted by transistors. This structure allows to maintain the quality of the tube system and to avoid the current limitations of the output tubes. It also avoids the need for an output transformer, with all of its phase rotations…

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What are the listening experience results?

The listening experience is natural thanks to the neutrality of the system, canceling the signals that lead to micro-distorsions. Sometimes, an almost “acapella” sounding voice seems to arise from the void. The absence of artificial brilliance reveals a number of details, hence the impression of clarity of the message. The respect of the phases gives great spatial precision on the localisation of every (single) instrument. The message sounds cleaned.


The result is remarkably striking when an orchestra is played: all the instruments are present, perfectly focused and can be singularly followed with ease. In the power amplification section, the assistance transistors have no current limitations, resulting in the amplifier gaining to hold the speakers – which explains the limitless dynamics and the deep tones – but also in the lack of emphasis. Like in every tube-based amplifier, the feel of clipping is never perceived.

Do we need to insist on this? Well, all these qualities are balanced by a downside, unfortunately: the Square Tube® will harshly reveal the strict content of the signal! Built without the frills related to distortion (issues), some of your favorite musical tracks may suddenly seem very dull!