OTL or SET? Differences?

With my limited knowledge, I know that OTL is transformerless and SET is single-ended. But what are the main advantages and disadvantages of these two designs? How to choose matching speakers for these two different designs?

Currently I have an Atma-Sphere OTL amplifier.

Thanks in advance!

You are one lucky and/or smart fella. Not many of us are blessed with OTLs. (Lifetimes ago, I had a pair of NYAL 'Futtermans'.)

OTLs, I think, are noted for their cleanliness and neutrality. SETs are noted for their musicality. I know this is a vague description, but for some reason SETs seem to retain more of music's harmonies; what they produce simply sounds more like music. SETs generally are lower powered, having by definition only one output tube, and hence are well suited to driving high-efficiency speaker systems. OTLs, because of their relatively high** output impedance, are not well suited for driving speaker systems that are lower than around 8 Ohms impedance or that demand high currents. High-ouput SETs have to use relatively large and exotic output tubes that often use VERY* high rail Voltages. Even my relatively common 50-Watt ASL Explorer 805s use over 900-Volt rails.

I wish I could find an affordable 150-Watt SET*** to use with my Eminent Technology 8s; meanwhile I continue to love the sound of ASL Hurricanes in triode driving the MR and tweeter panels of those 8s. Maybe I need a pair of all-triode 140-Watt Atma-Sphere MA-1 MkIIIs with their 5-octal frontends and FOURTEEN-6AS7-triode output stages.

* as is lethally high.
** sand-amp lovers probably would call that 'astronomically high'.
*** If, and I do mean 'if', an SET is the ultimate amplifier of music, and if that glorious rightness is due to several factors including 1. the use of indirectly heated tubes, 2. the use of only triodes, 3. the use of no paralleled tubes, and 4. certainly no push-pull operation, then my ASL 'canes, even running in triode, commit sin numbers 1, 3, and 4. The Atma-Spheres, too, commit those same 3 sins but have the benefit of not requiring output transformers. Hmm...
I own and sell both OTLs and SETs, and would say that they have many of the same qualities. At the risk of overgeneralizing, I'd say that OTLs tend to be a bit more lively and dynamic and SETs tend to be a bit sweeter and warmer (I can comment some on why if you'd like). I've heard superb presentations from systems with both types of amplifiers.

Since it sounds like you have a single Atma-Sphere amp, I presume it's an S-30. As long as you don't ask it to drive a speaker whose minimim impedance dips below 7 ohms you should be fine. An SET would be much more forgiving of lower speaker impedance, as would the larger Atma-Sphere amps. On the other hand if you pair the S-30 with a 12 or 16 ohm speaker, it sounds even better and will put out more power (roughly 45 watts into a 16 ohm load).

The S-30 is definitely a specialty amplifier when it comes to loudspeaker matching. Are you looking for speakers, and if so what are your requirements and ballpark budget?

I owned both the Lamm ML2 (SET) and the Tenor 75wi (OTL) for a while. I compared the amps with many speakers I had ranging from Kharma to Von Schweikert to Horns to Druids. I found the Tenors to be more sensitive to impedance and the Lamms more sensitive to sensitivity. This is by no means a scientific explination, but based on the limited combinations I've listed above. The Lamms could drive just about any speaker full range at reasonable volume levels. Based on the efficiency of the speaker, the Lamm would start to harden at higher volume levels. The Tenor however could play very loud even with inefficient speakers. However, they would loose control of the bass even at moderate volumes with speakers that had impedance dips. In general the distortion with Lamms was a hardening of the overall sound. The distoriton with Tenors was a boomy bass response. I'm not sure if this is OTL vs. SET or just the difference between Lamm and Tenor amps...hope it helps.
Duke, you are right. What I have is Atma-sphere M-30 and it is part of my second system which is for chamber music listening. The project is for my main system as I am experiencing slightly hearing loss on my left ear. That is the reason for me to change my Thiel 3.6 to speakers with high efficiency. I DO love the bass Thiel 3.6 brought me for the past 5 years. Yet I put lots of effort to tame its metal tweeter with little success. I also owned Silverline Sonata I around 4 years ago but the bass from Sonanta I just not enough for me while I listen to Mahler's 2nd symphony or some organ music. Please provide your suggestion.


Try sopme other speakers.Thiels are way to brite and I like a brite sound.Not sure if you are commited to them that one model is more and offender than another but have thought read the best speaker they made was the CS6.I say difernt because I knwo that my freind had CS 7's (huge with cement bafles-what a bitch egtinbg those to a secopnd floor!!)If going SET I see there are some used Von Schweikert DB99 derived fromsome aburdly priced $60K speaker used for $10K (ouch!).But they got very good reviews have bass covered by 300 wat bass amp but deliver 99DB from only one watt!!!But the price and size maybe more suited for main system not a smaller chamber one.
Me I've thought of inexpensive Omega's for bedroom system with maybe a Fi using low power 2A3 while my main rig will be high powered SS or higher powered tubes (wish I never sold that Mesa Baron.While not SET you could swith it's 12 tubes in banks of four from pentode to triode "on the fly" suiting the amp to different music geting 60/90/120 in full pentode you selected.
I bought a pair of German Odeon Tosca's which is a semi horn and dealer sold Atmasphere and said he had many happy customers match them up (horns like wonderfull Avantegardes and others tend to be very efficinet and might be worth hearing but again size/expense of many may not suit and subs can be difficult.Only REL and other subs which are VERY fast usually using multiple small drivers can even try to blend with horns,electrostsic or planar speakers like Maggie.Everything sub wise is mainly to sluggish.
Not fammilar with sound of OTL but from above even though I like dynamics and speed sounds like I'd prefer the SET sound.Nothing beats lot's of EL34 power for a great midrange in medium efficncy and B300,2A3,845 etc in high effciency speakers.

The Atma-Sphere MA-1 and MA-2 amps are far more flexible in handling a wide variety of speaker loads than the lower output Atma-Sphere amps (S-30 and M-60). With the increased number of output tubes, they have much lower output impedance (as well as higher power) making them much more flexible partners. The MA-2 will drive just about any speaker whose minimum impedance doesn't drop materially below 3.5 ohms. With the MA-1, you need to be a bit more careful in speaker matching, but still a broad selection. With either the MA-2 or MA-1 and appropriately matched full-range speakers, you will get bass impact, resolution and agility that is as good or better than you can get from the best solid state amps I've listened to. Plus, you get that magical reproduction of timbre and harmonic overtone structure that only tubes provide.

In contrast, SETs, as mentioned above, deliver a magical quality through the midrange that can be quite addictive. SETs are often described as sweeter and more "musical;" never quite "accurate," but making the music more beautiful than real -- all of which can be a good thing given all the trade-offs we work with. But, a big trade-off for a classical music listener is that I've yet to hear any SET/speaker combination that can begin to deliver on complex orchestral music (like the Mahler 2nd) or large organ. (There may be a combination of SET and horns out there that does this, I just haven't yet heard it.) If your primary listening preference is small ensemble jazz or chamber music or singer songwriter, SET can be a wonderful way to travel. But be prepared to trade-off on more complex music and on music with substantial mid- and low-bass information.

On the flip side, the Atma-Sphere OTLs will be very neutral, very detailed, highly resolving -- in the words of a listening partner here: pristing like a clear glass onto the music. They won't make the music more beautiful, you'll simply get what's coming from your front-end source without editorial.

N.B., I'm an Atma-Sphere MA-2 and Avalon Eidolon owner.
Hi Otto,

If I understand correctly, you're driving the Thiel 3.6's with your S-30. The Thiel's impedance is way below what the S-30 is comfortable with. Apparently you tried the Silverline Sonata also but it didn't have enough bass to do justice to Mahler ("Mauler"?).

High efficiency, ultradeep bass is the most expensive commodity in audio, both in money and in real estate (due to box size requirements). I'm sure you've thought of this, but one possibility might be easier-to-drive but bass-limited main speakers plus a subwoofer or two.

Another possibility is for you to keep the Thiels and use an autoformer to make life easier on your Atma-Spheres. Pauls Speltz's "Zero Autoformer" has saved the day for many an Atma-Sphere owner, and I have customers who use it with more conventional transformer-coupled tube amps. In the majority of cases the Zero is a significant net improvement, but not always, so it's best to try before buying if possible. I do think it would be in your case. I just sold my demo pair or else I'd offer to let you try them.

Pardon me now Otto, but since I've mentioned and suggested something I sell, there's a matter that I have to take care of. I have a stalker who sends me threatening e-mails when he perceives my posts as being overly commercial - according to his criteria, rather than Audiogon's. This morning I got one from him because of my first post in this thread, and I'm sure my mentioning the Zeros will cross the line he has drawn.

Stanhifi, I am tired of your threatening e-mails. Please stop sending them. Don't e-mail Otto as you threatened to either. If you have a problem with my conduct on Audiogon, let's get it out in the open. Go public and take your best shot, or leave me alone.

And Otto, if Stan does e-mail you to attack me, I would very much appreciate your letting me know. I'm not trying to hijack your thread so I invite you to let me know via e-mail if you'd rather, but I'll leave that up to you.

Okay getting back to Otto's situation, note that if you do go for a high efficiency, easy-to-drive fullrange speaker that has genuine deep bass you're talking about a very big enclosure. Without an idea of your price ballpark I hesitate to make specific recommendations among new speakers. In used speakers I'd suggest the big Altec Model 19's which show up occasionally for ballpark a grand or so used (more for a pair in very good condition, but still less than new speakers of comparable performance).

Finally, is building your own enclosures a possibility? If so, prhaps I can offer a suggestion or two that would work well with the S-30.

Hello Otto...Greetings from Australia !!!
I have tried several good Amplifiers on the Horning Alkibiades 99db,which have very deep bass and the appropiate big enclosure...Rushtons ,comments of the Atma-sphere S30 are spot on "On the flip side, the Atma-Sphere OTLs will be very neutral, very detailed, highly resolving -- in the words of a listening partner here: pristing like a clear glass onto the music. They won't make the music more beautiful, you'll simply get what's coming from your front-end source without editorial."
The Tom Evans "Soul S15" SET Cass A falls somewhere in between my current amplifier,the KR Audio Stereo Kronzilla and the Atma-sphere...The "Soul" controlled the bass on the Hornings better and being a KT88 valve,you could tweak the sound a bit...I would say it was a bit more involving,but it needed a active pre-amp ,where the Atm-sphere and KR are happy with a passive...the KR Audio Kronzilla was in another league...also many would argue that it is not a "true" Set amplifier ,as it uses,like the "Soul",a SS front end...the Kronzilla simply was a step above all of the amplifiers I have heard in control,air and musicality...the PRAT is just great...the only amplifier ,I reckon would compete ,would be the VAC or Canary 300B push/pull...hope this helps...
FWIW, one thing that OTLs and SETs have in common is that neither has any hysteresis loss in the output transformer.

SETs do not reverse the magnetic field in the transformer, and OTLs don't by definition.

Removing the transformer also removes the argument against push-pull. You don't have hysteresis loss, so you don't get the attendant loss of detail that is an effect hysteresis loss. This means that push pull need not be a 'sin' as mentioned above.

Of course our stuff is all-triode too.

Hysteresis loss is the property of magnetic devices wherein it takes a little bit of energy to reverse the magnetic field in the device. In the case of output transformers, this energy comes from the signal and is responsible for a loss of low level detail and an attendant increase in low signal distortion.

Duke, sorry to hear you are having worries. So far in the years I've been on this forum, no-one has challenged me in the way you mention. For that matter I've not had any posts that could not be approved (that I remember anyway...), but I try to be really careful not to cross the line. Too much:)
Excellent post Rushton, I completely agree with your comments, especially with reference to full range music and the typical listening impressions when listening through OTL vs SET, its true, its true.
Tubegroover, thanks.

Audiokinesis/Duke, you're consistently one of the most balanced and helpful people posting on Audiogon. I'm amazed someone has made a point of attacking you; that's unconscionable. Thanks for all that you've added here over the years, and I hope the nonsense doesn't put you off.

Ralph, I've never heard that hysterisis loss explanaton before. Thank you.
Of course those of us who have been paying attention are well aware of his tt/arm/cart/preamp/amp/spekers. Time to post pictures?