Transcendent T8 OTL monoblock power amplifier Futterman Redux
We come now to the pièce de résistance of the Transcendent T8 design. Mr. Rozenblit is justifiably proud of US Patent #5,604,461, which he received on all 20 points of his application. (The last OTL patent awarded was to Julius Futterman back in 1964!)
Here's the gospel according to Rozenblit. With Futterman-style driver circuitry, the overall forward voltage gain through the amplifier is unequal due to the "parasitic" or "degenerative" effects of negative feedback. Futterman took the output signal and coupled it to a phase splitter to compensate for the "degenerative" feedback of the output stage in the "upper" tubes (upper tubes being one half of series-connected push-pull pairs).
But Futterman applied same compensation to the "lower" tubes as well, which don't require it. The net result is that the "lower" tubes deliver a lower voltage gain through the system than the uppers, thus unbalancing the system. The signal amplitudes may be correct, but the gain is not. It is this unbalanced forward gain that produces DC offset in the output stage proportional to output level. In fact, Rozenblit attributes to this imbalance the weak bass and poor sound typical OTLs make into dynamic speakers.
The Transcendent patent provides for equal forward voltage gains throughout the system. Rozenblit: "A floating unity-gain buffer—a cathode follower—is referenced to the output line. This connection adds the correct amount of signal boost at the grids of the 'upper' tubes to overcome the effects of degenerative feedback. No compensation is applied to the drive signal on the 'lower' tubes, where it's not needed. The lower tubes have a follower/buffer stage as well for symmetry, to equalize the impedance and time constants. In this way the voltage gains of the two drive signals are truly equal, verified by the absence of any DC offset in the output as the amplitude increases. That improves the bass and overall linearity, which reduces distortion and allows the output stage to be biased with a very low idle current, thus reducing waste heat."—Jonathan Scull