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Original Message

Re: Another recent exchange between David and Ralph...

Posted by twl on October 21, 2004 at 06:11:47:
Hi. I heard my name mentioned, and decided to come in for a chat.
I own a custom ZOTL that Dave made for me last year.
It is a SET-ZOTL which uses Type 45 output tubes driven by 6SN7 tubes, choke loading, 12v battery power only. The chokes were hand-wound by Dave, and the entire unit is hand-wired point-to-point and is considered a "prototype unit". I use Sylvania 6SN7GT tubes and Emmission Labs 45 mesh plate tubes.The output is rated at 2 watts into 8 ohms.
I find the discussion about the ZOTL being an "OTL" or not, to be something that has come up numerous times since Dave invented the ZOTL. I feel he intended to make some distinction between the two, because he used the name ZOTL instead of OTL. Whether any particular individual wishes to apply the moniker "OTL" to it, or not, is rather immaterial. Rather, this circuit truly is a unique design, which actually exceeds the capabilities of both output transformers and traditional OTL, so I would classify it as a "new generation" of circuit.The ZOTL provides the impedance matching as good or better than output transformers, without the drawbacks, and provides the sonic attributes as good or better than traditional OTL, without the drawbacks. This combination of superior impedance matching and lack of output transformer, provides some things that have never been available before. For example, my SET ZOTL has only one output tube per channel, and no output transformers. This is impossible to do with traditional OTL circuits, due to the inability to match the high impedance of the tube to the low output impedance needed to drive the speakers(unless a gang of output tubes is employed).
Regarding the presence of certain transformers in the output stage, they are not output transformers, and only perform impedance matching functions at the 250kHz level(out of the audio range), and pass the signal within the audio range. This is a unique method used in audio, and derives its origins from industrial power-transfer technology. It is obvious to see that these transformers could never be output transformers, from a cursory inspection.