THE SQUARE TUBE® :
The Square Tube® system is a new way of assembling electronic tubes to amplify an audio signal, and is a registered trademark. The Square Tube® is the result of a long process of maturation. The tubes – an old technology – are used in a totally modern way.
TECHNICAL :
The circuit is built around a differential stage, which is symmetrical in essence, and an output stage that could be likened to an OTL push-pull. The whole assembly forms a complete operational amplifier, ‘passing’ DC. ‘Passing DC’ is of no interest from an acoustic point of view. On the other hand, this characteristic allows the Square Tube® assembly to be self-stable, and to maintain the perfect polarisation of the tubes independently of external elements, temperature and the effects of time. And that guarantees that the system will always work in the same way, with constant quality listening.
Four tubes are used to make a single amplification stage, hence the name! The Square Tube® provides a signal that is symmetrical in relation to the reference (earth). In other words, this signal alternates between positive voltages (to push the loudspeaker membrane) and negative voltages (to pull the membrane). This unprecedented capability with vacuum tubes makes it particularly well suited to the design of audio amplifiers.
The Square Tube® operates in a highly sophisticated way. The operating point of the tubes is imposed on them. For the input triodes, the current flowing through them is determined by a current source. Their polarisation is therefore known and invariant. They are given 2 mA and whether they like it or not, they have to ‘get on with it’, whatever their brand or characteristics!
Proprietary technology – Square Tube® is a registered trademark.
TUBE AMPLIFICATION :
In your Kora, the Square Tube® provides all the amplification functions.
If the Square Tube® is used in a power amplifier, the output signal from the tubes is identical to that presented to the loudspeakers. Complementary audio and hyper-linear power transistors will provide the current required by the loudspeakers, but will not be involved in amplifying the message. This is a tube amplifier.
WHY ATYPICAL :
The Square Tube® is a tube amplifier. Tubes amplify little, but well. They are generally more linear than transistors. To compensate for their low gain, the Square Tube® uses 4 tubes in a single stage, hence the name. As a result, the distortion rate obtained is lower than with a traditional assembly.
The Square Tube® is powered by two high voltages, of the order of 2 x 250 V (depending on the device). We use only a few per cent of this high voltage. At most 20% when the Square Tube® is used in an amplifier, and the latter is at its maximum power, for example. In other words: the assembly is always in its ‘comfort zone’, never exploited close to its limits. All the components are adjusted for nominal operation, there is no ‘room for manoeuvre’ for the tubes – whatever they may be – and so the acoustic and technical qualities of the Square Tube® are invariant over time.
For input triodes, the current flowing through them is determined by a current source. Their polarisation is therefore known and invariant. The currents in the output tubes are set lower than their nominal current by reference voltages. This is the designer’s intention. This guarantees exceptional component life. What’s more, there’s no need to change tubes, because the sound will not be improved. Square Tube® keeps tubes polarised, whatever their brand or age !
A final note: a Kora amplifier designed around the Square Tube® is not a hybrid amplifier with transistors preceded by tubes, but a tube amplifier assisted by transistors. This structure preserves the qualities of tubes and avoids the current limitations of power tubes. It also avoids the need for an output transformer, with its phase rotations…
LISTENING :
The neutrality of the circuit ‘rids’ the signals of micro-distortions, giving a much more natural listening experience. Sometimes an ‘a capella’ voice seems to emerge from nothing. The absence of artificial brilliance reveals a great deal of detail, giving the impression of precision in the message. The great respect for phase will bring great spatial precision to the location of each instrument. The message appears to have been cleaned up.
The result is particularly striking in the reproduction of large orchestral ensembles: all the instruments are present, in their place, and can be followed individually without difficulty. In power amplifiers, the ‘assisting’ transistors have no current limitation, so your amplifier gains in loudspeaker stability, which explains the limitless dynamics and the bass that is present but always without emphasis. Like all tube amplifiers, you never get the impression of clipping.
Do I need to talk about it? Unfortunately, all the qualities of the Square Tube® have a downside !
The Square Tube® will crudely reveal the bare content of the signal. Delivered without the frills of distortion, some content may be a little uninteresting ! Sorry, you’ll have to upgrade your music library !