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In Reply to: why is that no one seems to be running OTL amps? posted by soundquest on September 28, 2004 at 08:32:58:
Without trying to insult or irritate OTL fans look at the basic concept. OTL likes looking into higher impedances rather than lower.
What dynamic loads do most speakers present? Not simple resistance.
Most speakers have very reactive loads. Then there are the OTL schemes. Circlotronic and Totem Pole. Both need banks of output tubes to develop power. Look at the loudspeakers favored to bring OTL amps into superlative performance - electrostatic. These have transformers to match amp to load. So, although your amp may be OTL, the speaker itself has a transformer. Maybe there really is something to having just one output device SE in class A. Sensitive speakers ( horns ) allow you to hear what the amp is doing in the less than 1 Watt range. Perhaps weaknesses in the overall OTL scheme are more easily heard with horns.Anyway, some thoughts on the subject. I should clarify that the last OTL amps in my system were Futterman H3aa's, though I have heard lots of others: Transcendent, Fourier, Atma-Sphere. Bottom line: if they sound good in your system, enjoy them. Too me it always seemed a bit silly to use an OTL if it was recommended to use an "autoformer" or transformer between amp and speaker to give the amp an edge through impedance matching. Happy listening!
Follow Ups:
I thought the Electro Voice Circlotronic circuit was a transformer coupled design derived from the "unity coupled" concept (ie... a competitor with Mc).Furthermore you say that "Both need banks of output tubes to develop power." I thought the banks of "output tubes" were used to lower the output impedence, not to develope the initial voltage swing in the output section.
Finally, though I totally agree with your autoformer statement, I think it is misleading to state that SE topologies have a single output device because this power tube is coupled through a transformer and the amp is is suceptable to any vagueries in the output tube, transformer, speaker interactions.
Check out Atma-Sphere's web site and note the Circlotronic references.
I refer to the basic topology or circuit notion not to actual Electro-Voice Circlotron amplifiers.Banks of output tubes are the mechanism to effect lowering working impedance. They also deliver power. Voltage gain in such schemes comes with the input tube.
A transformer whether on an amplifier chassis or sitting on the floor behind ( or in ) your loudspeakers is still a transformer.
I did a little reading at the Atma-Sphere site. The "bridge circuit" which was transformer coupled in the EV amp is generally refered to as Circlotronic, no matter the speaker coupling topology. I guess if I had thought about the power issue for a few minutes, it would have been obvious. The power being developed on through the bank of output tubes refers to the lowering of impedence and a cocomitant increase in current. This would be a linear relationship (I=V/R). However, power is the the square of current multiplied by resistance. Thus the effective power of the amp is developed across the output tubes that lower resistance.I guess this brings me to the last point which is that no matter what method of speaker coupling is used, it is still doing essentially doing the same thing, be it a transformer in a SE amp or a bank of tubes in an OTL. As to which is the best method, I would suspect each is prone to certain vagueries.
Thanks,
Rodney, hi. Yup, you got it. There is a certain magic that can be had with OTL in the right set up with the right speakers. I am a fan of the OTL notion. I have heard them do wondrous things. The reality is that in the "real" world there is always a downside to every upside. It is only opinion, born of experience, that any time you tie together a whole bunch of output tubes you are creating a situation where the signal can be degraded: micro subtleties, the edge on transients, etc. Think about any dynamic devices tied together to perform as one. You can do it, but something may be lost or fuzzed up in the process. The Futterman scheme runs the audio through a whopping capacitor at the output to block the B+. The Circlotronic does not. Cordially.
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