LEX - Hybrid Mono-block Power Amplifier
Nominal power: 100W RMS into 8 ohms
Single ended RCA (50k), balanced XLR (5k) input, 700mV RMS for full power output
Tube complement: 6EB8 (2 units, amplification), 6AL5 (power output indicator)
LEX is a hybrid power amplifier designed to offer a synergistic union of the respective benefits of tube and semiconductor amplification technology. Unlike typical hybrid designs, where tubes deliver voltage and semiconductors current amplification, LEX uses a topology where semiconductors are conceptually in parallel with tubes, the latter offering a full chain of voltage and current amplification based on tube OTL topology (OTL standing for Output TransformerLess). The tube portion of the circuit is thus connected to the loudspeaker and has control over it. The semiconductor portion is notionally in parallel with the tube output stage and operates as an impedance multiplier, creating hundreds of virtual copies of the tube output stage all connected in parallel with the original in order to deliver the required output power. This approach creates a minimalist tube based amplification core, where the signal passes through only two stages (the first delivering all voltage amplification), with advanced circuit techniques supporting the core.
* Inherently balanced operation gained by using OTL topology enables flexibility of choice of input signal type. The concept has also been extended to the second (output) stage of the tube core, avoiding a typical problem present in most OTL designs, that causes output stage imbalance.
* The semiconductor portion is substantially overbuilt for the rated output power and based on original Hitachi metal cased lateral MOSFETs, which were specifically designed for audio use. They are selected out of a very large contingent for use in LEX, with a 3% maximum tolerance band both for same type and complementary type units.
* The critical ‘First Watt’ is guaranteed to be in full class A for loads down to 3 Ohms. The tube amplification core operates strictly in class A with an overhead far above the maximum power the amplifier can deliver. Together, with a carefully chosen operating point of the semiconductor portion, this guarantees maximum linearity for signals outside strict class A operation.
* The power bandwidth extends into the MHz region due to the use of OTL topology as the basis for the core. However, to insure immunity to noise and RF interference, it is limited by a low order filter to 250kHz, still insuring maximum fidelity of amplification of all currently used sources including those using digital formats with 384kHz sampling.
* LEX does not use global negative feedback, not even to set up circuit operating points. This avoids phase errors due to coupling time constants and the extremely undesirable effects of harmonic and intermodulation component multiplication, while maintaining maximum signal fidelity. Semiconductor current sources and a servo system insure precise establishment and maintaining of all operating points without any user adjustment.
* A precise average output power indicator without an initial dead zone is implemented using tube rectification. Unlike semiconductor rectifiers, tubes have no treshold voltage below which the indicating instrument would show no signal.
* Sophisticated power supply impedance control methods are employed throughout the 6 independent power supplies for the signal amplification circuits, including the completely separated semiconductor portion supply with a separate transformer and non-resonant 2-section filter bank. Since LEX does not use global feedback, the behavior of it’s power supplies under audio signal modulated load currents must be precisely predictable, which is why no feedback is used even in voltage regulation circuits.