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In Reply to: But then why have an OTL? posted by Dr. Philosophy on October 22, 2004 at 05:55:33:
This subject was discussed ad nauseam on ASOG, and the texts can be found in the archives I am sure. First of all, it's NOT a transformer, it's an autoformer. In an autoformer there are no discrete primary and secondary windings, no gap. Also, the Zero is stepping up the impedance by only 2X to 4X in most applications (can go above 4X but not recommended); this is a much lower ratio than that which must be achieved by an output transformer on a transformer-coupled tube amp, where the output impedance of the tubes is often hundreds or thousands of ohms. Therefore, the Zero could be and was designed for much higher bandwidth and much lower distortion than typical output transformers can achieve. (The numbers are on the Zero website, but I believe the Zero to be flat from below 10Hz to well beyond 100kHz and to have v low distortion.) Moreover, the Zero is placed at or nearest to the speaker, not at the amplifier output side, so the amp drives the cable and "sees" the autoformer/speaker as a unit. All that said, basically you are correct; if there were a plethora of top quality 16-ohm speakers available to us, there would be little need for Zeros. But there ain't. The Zero enables one to use an OTL on a wide variety of speakers and is quite transparent in my own system, set at 2.7X.
Follow Ups:
I should have noted that the "need" for Zeros is really related mostly to the output impedance of one's OTL. I am running my Atma-sphere amps with no negative feedback (NFB), so the output impedance is purely determined by the plate resistances of the output tubes (and their mu), and it's relatively high. I do need Zeros to drive my Sound Lab M1s, which have an impedance minimum of about 4 ohms at 500Hz. If one is using an OTL with significant NFB and a resulting low output Z (the Transcendent comes to mind), then perhaps you do not need a Zero to drive a nominal 8-ohm speaker.
.. I just assumed 'autoformer' meant 'adjustable transformer' or some such. My amps are Transcenent Sound T16s; I don't know whether anyone has tried decreasing the NFB on those but adding autoformers.
I think I read on the Transcendent website that the T16 has a relatively low Z out, equal to 1 ohm or much less, due to the stock application of NFB. You wouldn't "need" a Zero to drive an 8-ohm speaker, but you might want the Zeros if you are driving ribbons or any of the high end notoriously inefficient 4-ohm types, like Thiel, older Avalons, etc.
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