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Educate me if you don't mind? I have zero experience with OTL amps. I'm guessing there are advantages, disadvantages over push pull, and set? Where do OTL amps fall in line?
Lots of ideas and input. Thanks for all the help! There is an Atmosphere dealer in my city, but I'm not going to waste his time as I don't have that kind of cash right now. I may go down the OTL trail eventually though.
OTL appears to be a very intriguing type of amp! I presume the OTL preamp would be of the same design. What is the lest expensive brand of these amps?
Most preamps ARE "OTL" - but they are not driving low impedance Amps - most amps have an input impedance of 47K Ohms - a tad higher than most speakers...
Wouldn't it be more correct to say that most amplifiers have an input impedance ranging from 10K to 100K ohms, with solid-state amplifiers clustered at the bottom of that range and tube amplifiers up around 50K and above? "47K" is the standard load resistance for an MM phono stage, as I am sure you know.
Most preamps are OTL in that they are Output TransformerLess.
IOW, some preamps do indeed have output transformers.
OTOH, a preamp that does not also have an output coupling cap is a bit different. Since our output circuit for our amps allowed for a direct-coupled circuit, we applied the same circuit to the output of our preamps, so no output transformer and no output coupling cap.
I only know of one company that makes "OTL preamps", Atma-sphere. Because the output stage requires an entirely separate circuit, such a preamplifier cannot be inexpensive. But on the other hand, I have no idea what your limitations are for expense.
If I wasn't rushing off to the dentist, when I wrote the above, I would have fixed it sooner.
When the OP mentioned "OTL preamplifiers", I was thinking of output capacitor-less preamplifiers. Most preamplifiers do not use output coupling transformers. But by far most do use capacitor-coupling. Atma MP1 and MP3 are direct-coupled, which is rare; there is no output coupling capacitor.
Actually, Transcendent is the lowest price, but those generally start out as kits so either DIY or get lucky and find a competent builder. Output power tends to be on the low side.
If you need more power, a good value (IMHO) is a used Atma-Sphere M50/M60 (any version), but you need to find one that is maintained (or at least has power supply caps that are not too old). I am still running a beautiful gold face original pair of M50s (with the original transformers!) with only upgraded coupling caps, and I would not trade those for anything. That pair has been going so long that some of the front panel lights are almost faded out!
Speaker choice will be an issue, but you get a lot more flexibility with a pair of zero autoformers.
By going OTL the amp is instantly wider bandwidth: deeper bass and more extended highs. Its also instantly more transparent. This is easy to demonstrate regardless of the competition.
The flip side is you have to be more careful about the loudspeaker. But in practice, that is true of any amplifier.
Generally speaking, you want a speaker that is at least 8 ohms in the bass or better yet 16 ohms, unless you have one of the really big high power OTLs, for which 4 ohms is not so challenging.
Another aspect is that OTLs often have more power tubes. But heat does not seem to be a problem because of that. Some OTLs run cool and others do not, depending on the class of operation and the power tubes used.
"OTL" is an abbreviation for "Output Transformer-Less", meaning there is no transformer between the output tube stage and the speaker. Some OTLs are direct-coupled, like Atma-sphere products, and some are capacitor-coupled to the speaker, like Transcendent amplifiers. Burning ZOTL amplifiers are sort of in a class by themselves. An OTL amplifier can be push-pull (PP) or single-ended (SE), but by far the vast majority are PP types.
The advantage of getting rid of the output transformer is to get rid of nonlinearities, bandwidth limitations, and distortions introduced by coupling transformers. The disadvantage of not using a coupling transformer is that the output impedance of an OTL tends to be fairly high, which may introduce issues with driving low impedance speakers. (The output transformer converts voltage to current and allows the output tube therefore to transfer energy into the speaker. Most speakers have much lower input impedance compared to the output impedance of the average power tube used in transformer-coupled amplifiers.) To ameliorate that issue for OTL amps, one uses a plethora of output tubes in parallel. Very high current, low plate resistance, high transconductance output tubes are also to be preferred. That's not all there is to it, but that's the basic scoop.
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