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In Reply to: RE: Actually,... posted by ahendler on December 25, 2015 at 20:18:34
"Of course you are correct on all counts. I was thinking of the single ended OTL by Transcendent sound that uses a big output capacitor. The Berning just has a small input capacitor to keep DC out. Direct coupled the rest of the way"
Looking at the ZOTL patent, it seems that the MOSFETS in the output stage, which are actually switching the high currents that pass through the loudspeaker, are DC isolated from the previous stage (which is vacuum tube), by the high-frequency transformer.
I think one way one can view the ZOTL is that what is essentially a switching-mode power supply is producing a high voltage output, which is then loaded by the "output" tubes. As the anode current drawn by the output tubes increases, so too the current draw in the low-voltage MOSFET switching circuit side of the "switching-mode power supply" must increase. The change in current on the low-voltage side is correspondingly much bigger than the change in current on the high voltage side. The high current on the low voltage side passes through the loudspeaker, and thus a large current change through the loudspeaker is produced as a result of a much smaller current change through the "output" tubes. In effect, the "switching-mode power supply" acts as an impedance matcher between a high impedance "output" tube and the low impedance speaker.
It's an ingenious design, and although I don't think it is fair to call it OTL, it certainly has a novel way to impedance match a tube "output" stage to a low--impedance speaker, without requiring the tubes to pass the high currents needed by the speaker. So it achieves the impedance matching without a conventional OPT.
What is not clear to me, though, is why such a MOSFET/tube hybrid amplifier is likely to be any better than a more conventional type of MOSFET/tube hybrid amplifier, in which vacuum tubes are used in order to drive a conventional MOSFET output stage.
People say it sounds very good, and I'm sure they're right. But probably conventional MOSFET/tube hybrids can sound very good too.
Chris
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