Do you know France-based Kora? Based in Toulouse, the ‘Silicon Valley’ of France, the company’s principals work in the Aerospace industry specializing in power supplies and signal treatment while also building integrated amplifiers that utilize tubes and transistors in an unconventional manner.
The TB140, TB200, and TB400 employ Kora’s patented Square Tube Circuit and differ mainly in output power with the TB140’s 70 watts per channel, the TB200’s 100 watts per channel (I appreciate logical product names), and the TB400’s 200 watts per channel. The company uses Motorola transistors in the Square Tube circuit and the number of transistors grows along with output power from four in the TB140 to twenty in the TB400.
Here’s the company on their Square Tube Circuit:
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 ( earth ). In other words, the signal goes alternately in the field of positive voltages (to push the diaphragm of the loudspeaker) and in the field of negative voltages (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 power transistors, audio and hyper-linear will give the current required by the speakers, but without interfering in the amplification of the signal.
We are definitely talking about a tube amplifier here!
A traditional tube amplifier would replace the 12AX7s found in the Kora’s output side with power tubes, e.g. 300Bs, i.e. big tubes, to amplify the small signal tube’s voltage, while adding an output transformer to deliver the current required to couple the amplifier’s output stage to the loudspeaker. In the TB140 we have the 12AX7s providing voltage amplification and instead of an output transformer, we have transistors providing current delivery.
The company claims this approach avoids a number of issues found in transformer-based amplifiers by offering no phase shifts on low or high frequencies, no bad coupling on low end, and no current limit. This last bit, no current limit, should translate into an iron grip on any speaker load.
