SET vs OTL

"In a nutshell you have to be really careful about making broad stroke statements; when comparing SETs and OTLs it can go both ways insofar as to which comes out on top."

Agree and it absolutely does go both ways. This topic has been discussed numerous times on this and other forums. I’ve compared SET and OTL directly and would choose SET just as @antigrunge2 did.

@audition_audio has done similar comparison and finds OTL superior. Okay fine, his experience and good for him. We don’t need proclamations, just choose what you conclude is better. I’d never declare SET universally superior to OTL, NOR would I do the converse.

Plenty of music listeners have made the choice for OTLs and plenty have done so for SET. Why do we need to draw lines in the sand and have turf battles when it comes to audio components and listening preferences? People have moved on from SET to OTL and from OTL to SET.
Charles

My speakers are Duevel Bella Lunas with 6 ohms impedance and 93db efficiency. To run the Graaf I used Paul Speltz Autoformers to step them up. I used Bybee Golden Goddess Speaker Bullets in both instances and Auditorium23 speaker cable. 

In a nutshell, hypothetically, if the speaker is a "very easy drive without any hindrance" at all to either amp, the OTL in class-A will be the better amp full stop.

BUT!! for an OTL not to be hindered IN ANY WAY at all by todays "better sounding speakers loads" that have NOT had "their sound compromised" (because they were designed "firstly" too give a very easy load), is a big ask.
And those type of "very easy to drive speakers" usually have big other glaring problems to many listeners that are susceptible to their colorations,  so it’s a bit of a catch 22.

Cheers George

The autoformer used as an impedance matching device is, in effect, an output transformer.  It is doing exactly what an output transformer does.  To me, if something sounds good, I don't care how it got there.  I've heard many great tube amps, and I would not dare to say if any particular topology beats out another.  My personal favorite happens to be an OTL amp, but, it is an utterly unique, one-of-a-kind design.  A very close second (maybe not even second) is a pushpull amp that is extremely rare and costs six figures for a pair (Western Electric 59A).  I own a pretty good SET amp (Audio Note Kageki) and I've heard many other good SET amps, but the amp I currently run in my system is a low-powered pushpull amp.  

I know the Duevel line fairly well. What I would like to know is how low the impedance actually goes and with what type of swing. I agree that using an auto-former somewhat defeats the purpose of an OTL and would not be an option for me.

For the record I have heard Atma-sphere, Joule Electra, Berning quasi-OTLs, the older Tenor, Transcendent Sound and Einstein. 

I also have auditioned/owned several P.P. designs from Thor Audio, VTL, EAR, VAC, French company I cant remember that was famous for CD players and SETs using 845s, 2A3s, 45 and 300Bs. Granted no SET design that was over $ 20K. 

I believe that each type of amp share many similar sonic characteristics. But removing the transformer is always a plus if you can feed the amp its entre of choice. 

The best SET amp I have heard was an old Mastersound made 32B integrated amp running only an an amplifier.

@audition_audio,

the higher the impedance, the better for OTLs. On the Graaf, the autoformers gave a marked improvement to going without. Old speakers used to be 16 ohms which would be ideal for OTls, however most modern designs have impedance dips under 4 Ohms with which OTLs have a hard time The Duevels in this regard are relatively benign and an easy load.

P.S. Push pull designs and SETs are very different, The overtone characteristics of SETs make for a much more nuanced and colourful sound. The differences tend to be most pronounced via horn designs. Obviously pp is higher output.

 I agree that using an auto-former somewhat defeats the purpose of an OTL and would not be an option for me.

 Correct using a "Zero" type autoformer is a "bandaid fix" for an OTL amp that is not suitable for that speaker loading, far better off getting the right amp.
OTL's that tend to suit speakers without having to use autoformers, then speakers then are mostly not that accurate, like I said it's a "catch22" 
If you still want to use P/P tube, to use a P/P tube amp that has a "proper output transformer" as part of the amps design and feedback configuration.

Cheers George

P.P.S

The French company might have been Jadis or Audioaero, both of which build good amps.

BTW, both on OTLs and on SETs low capacitance speaker cables are clearly preferable, think Shindo or similar

Do you guys that run low powered set amps prefer tube rectification or solid state diodes?

Thanks it was Audioaero. This was a good P.P. amp. that was flexible on which power tubes were used. I think I ended up using some old stock  6550s. This amp used 6SN7s as drivers and I was smitten by the Tung-Sol round plates in this application. Using P.P or S.S. designs sure makes life easier in many ways. 

Oh and so you know, much of the auditioning of amps was done on a pair of DeVore 0/96s which responded very well to changes in amplification. A speaker which is better than the sum of its parts and I mean this in a very positive way, although this was not my personal speaker. 

👌 on the DeVores!

The one thing that clearly differentiates an SET from any Push-Pull (OTL or not) is the crossover distortion which occurs due to splitting the wave and joining it back. Once I heard and lived with an SET, that distortion was so clearly audible in Push-pull that it was no more acceptable as "right". It is a fundamental compromise.

The sound without the crossover distortion is a very continuous flow like a inner tissue connecting the whole spectrum of music. It feels like a "one" sound. Whereas Push-pull just feels disconnected pieces playing together "in comparison". It is not a triode or tube thing. I have heard the same effect even in single ended solid state amps (Sugden, Valvet).

In real life all sounds are single ended hence it is recognisable as natural when that aspect is preserved. Surprising that no one points to this.

@pani,

touché: your description is dead-on. That said all but the best SETs suffer from their transformers: it usually sounds like there has been a cushion put on the music; yes, the flow is there but it somewhat sounds muffled and damped. Other than my Wavac I have only heard Kondo, top end Audionote and Reimyo not severely affected by this problem and none of them are cheap. Transformer design is the real differentiating factors on SETs.

@pani wrote,
'The sound without the crossover distortion is a very continuous flow like a inner tissue connecting the whole spectrum of music. It feels like a "one" sound. Whereas Push-pull just feels disconnected pieces playing together.'

@pani, "it feels like a one sound". Yes true, and this has been pointed out before by other listeners. This purity and natural sound character is what makes SET unique and special in my opinion. I agree that high quality output transformers are paramount to achieve this.

This is why SET will always have its devotees. OTL will always have its own devotees as well for what it provides. It’s been stated many times that it all depends on what one is seeking. Both SET and OTL clearly have their specific strengths. What hidg quality SET does so well are exactly the sonic attributes I want. @panic you nailed it.
Charles

BUT!! for an OTL not to be hindered IN ANY WAY at all by todays "better sounding speakers loads" that have NOT had "their sound compromised" (because they were designed "firstly" too give a very easy load), is a big ask.

There are plenty of them out there in high end audio. The Sound Labs are a good example, although you can’t run them with SETs to do a fair comparison. But usually any speaker that works well with SETs will work well with OTLs too.

I agree that using an auto-former somewhat defeats the purpose of an OTL and would not be an option for me.

There is an enormous difference using an autoformer like the ZERO with OTLs! In most tube amps the output transformer defines the bandwidth of the amp. Because of its very low turns ratio and consequent low inter-winding capacitance, the ZERO has full power bandwidth from 2 Hz to over 1MHz!! -much wider than **any** tube amp. So with an OTL you would have the possibility of the widest bandwidth tube amp (which OTLs are anyway), plus the ability to drive extremely low impedances. We used to offer such a device (called the Z-Music Autoformer) but Paul Speltz started making one too and we felt it easier to sell his than ours.

The one thing that clearly differentiates an SET from any Push-Pull (OTL or not) is the crossover distortion which occurs due to splitting the wave and joining it back. Once I heard and lived with an SET, that distortion was so clearly audible in Push-pull that it was no more acceptable as "right". It is a fundamental compromise.

@pani @charles1dad , sorry, he didn’t nail it; here’s why.


The statement is false. You can easily demonstrate on the bench and in listening that an OTL need not have any crossover distortion whatsoever, at **any** power level. This is one of the traditional arguments for going class A of course (which we’ve been pushing for decades- you don’t get crossover distortion in class A circuits...). The simple fact is there isn’t any crossover distortion in our amps (I can’t speak for other OTLs, but assuming competent design I doubt they have it either).


Probably what you are hearing (because I’m not disputing that you might have heard a difference with a PP amp as opposed to an SET) *is* distortion, just not **crossover** distortion.


This comes from the simple fact that when people compare SETs to PP amps, there are a ton of variables that can cause false conclusions. The idea that its crossover distortion is one of them. So I should point something out: In SETs, the distortion generated is due to a quadratic non-linearity. This results in a fairly substantial 2nd and 3rd harmonic, which masks higher ordered harmonics from the ear (at lower power levels) resulting in smooth sound.


Now if the circuit is fully differential and balanced, you get a different non-linearity known as ’cubic’. This results in the 3rd being most prominent (at a level slightly less than seen in a single-ended circuit). It too masks higher ordered harmonics, also resulting in smooth sound. But the higher orders fall off at a faster rate (due to the cubic function, essentially distortion isn’t compounded as much from stage to stage in the circuit since even orders are cancelled) so the circuit is inherently lower distortion and more transparent, since distortion masks lower level detail.

Now when you **combine** the two non-linearities, as seen in a variety of PP amps with single-ended input, you get algebraic summing of the harmonic orders, resulting in a prominent 5th harmonic. This is well-known; Norman Crowhurst was writing about this 65 years ago. **That** is what you are hearing- not crossover distortion. But if the amp is fully differential you won’t be hearing either one.

In a nutshell, while I don’t doubt that you hear differences, its probably best if you know what those differences are as I’ve described above.

’In a nutshell, while I don’t doubt that you hear differences, its probably best if you know what those differences are as I’ve described above.’

For clarification my primary reason for agreement with @pani had to do with his finding, that single ended sound to be "it feels like a one sound" I get exactly what he means. This is what I recognized when I got my SET and listened compared to my otherwise very push pull amplifiers.

There’s an unmistakable purity and naturalness. If others haven’t had the same outcome I understand. I can only give an account of what I hear. So from my vantage point, subjectively  @pani "nailed it".
Charles

@atmasphere,

there were two factors that made me move from Graaf +Autoformers to Wavac:
Comparing bass performance, the Wavac was more tuneful and ‘dry’ whereas the Graaf was ‘fruity’ and more wallowing
In terms of attack on individual instruments the Wavac got woodwinds and cello right in terms of ‘reediness’, bow attack and reverb whereas the Graaf sounded beautiful but blooming and ebbing off.
The latter is probably why the Wavac seemed to me the faster amp.
Finally the Wavac can be better finetuned by changing rectifier and power tubes, choices for the Graaf were much more limited.
Thanks for your comments, I find them highly educational

"it feels like a one sound" I get exactly what he means. This is what I recognized when I got my SET and listened compared to my otherwise very push pull amplifiers.

Funny - people describe that about our stuff too.


As best I can make out, what seems to be important in any amplifier is the distortion signature, possibly more important than *how much* distortion it actually has, although the lower you can make the distortion the more detail you can reveal. The important thing in the distortion signature seems to be that the lower orders be in sufficient amount that they can mask the higher orders. Many solid state amps violate this idea by having very little of the lower orders, leaving only small amounts of the higher orders. Because the ear is so sensitive to those orders, they are easily heard, causing such amps to sound harsh and bright, especially at higher volumes. This is literally why tubes are still about 60 years after solid state was introduced to hifi.


I recently built a little 5W/channel tube amp using output transformers. The power tubes are EL95s which are a cute little pentode power tube. I wanted something that was low power, compact (as an integrated stereo amp this one can sit on a sheet of paper with full margins visible) and otherwise as high quality as I could come up with (it was for my bedroom system and also a high quality desktop). It uses a differential amplifier for the voltage amplifier/driver circuit (IOW no separate 'phase splitter'). The output section is ultralinear and class A. A high quality CCS circuit was used for the 12AT7 input differential amplifier, so its CMRR numbers are quite high. So literally two power tubes and a 12AT7 per channel. I gave it two inputs and a volume control.


The inputs are single-ended although this amp is fully balanced/differential from input to output. The other input to the differential input voltage amplifier is used for feedback. The amp's distortion signature is expressing a cubic non-linearity, so mostly a 3rd with succeeding orders falling off rapidly. Since the circuit was quite linear without feedback (the feedback is only used to reduce its gain) the distortion signature is relatively undamaged by the feedback. In comparison to a 2A3 based amp I have on hand, this little amp is better in every way- smoother, lower noise, greater detail, overall things sound more 'real'.  Without feedback it has bandwidth past 100KHz...


This is just my experience, but when typically SETs get compared to PP amps, its never PP amps of the same power and quality. Its also never SET amps compared to PP amps using the same kind of power tubes. In past comparisons I've eliminated these variables; IME when you eliminate those variables and also pay attention to the distortion signature, the advantages of SET are eliminated- you can get overall more pure, smoother and more detailed sound with greater bandwidth and power.

@atmasphere, your knowledge on design is at expert level so I wont talk about that.

Whether we call it crossover distortion or "push-pull special" distortion. I have heard the exact same distortion attribute when I compared:

1. Audionote SET to Leben push-pull
2. Ayon Crossfire II SET to Triton II push-pull
3. Trafomatic Experience SET to Traformatic 6550 pushpull
4. Wavac EC-300b SET to Canary audio 300b push pull
5. Border patrol 300b SET to Border patrol 300b push pull
6. Unison research Kt88 SET to CJ Premier KT88 amps
6. The list goes on...
All of them were side by side comparison with my own music.

In every case, the attribute of "oneness" is lost in the push-pull design. It is not a issue of richness of tone. One sounds chopped, the other sounds intact. When the ear catches it, it is unmistakable after that.

Unfortunately error of commission that push-pull introduces is never discussed with the appropriate light in the Hifi community. 

As long as we are talking about bandwidth, power, detail, THD, we are not going to meet anywhere. If there is an instrument which can measure this effect (apart from our ears), I don't know it. But I am surprised it is not measured. 

I have heard the exact same distortion attribute when I compared:

1. Audionote SET to Leben push-pull
2. Ayon Crossfire II SET to Triton II push-pull
3. Trafomatic Experience SET to Traformatic 6550 pushpull
4. Wavac EC-300b SET to Canary audio 300b push pull
5. Border patrol 300b SET to Border patrol 300b push pull
6. Unison research Kt88 SET to CJ Premier KT88 amps
6. The list goes on...
All of them were side by side comparison with my own music.

As long as we are talking about bandwidth, power, detail, THD, we are not going to meet anywhere.

@pani  You may have noticed that I've been talking about *distortion signature*, which isn't the same as THD. You can hear these things very easily- quite literally its the difference you're describing. The reason bandwidth is important is that if the amp is using little or no feedback, bandwidth is needed to preserve phase relationships. Detail is a subjective idea so probably isn't measurable, but we do know that it can be masked by louder sounds as that is one of the human hearing perceptual rules.

Regarding the above list, every. single. one. of them mix single-ended circuits with balanced (PP) outputs. This results in that 5th harmonic prominence I mentioned earlier; IOW  you're making my point.


Its simply erroneous to assume that because all the amps you've heard don't bring home the bacon that *all* PP amps won't. There are detractors of class D amps that make the same mistake- because they heard several class D amps that sounded bad to them, the assumption is that all are bad. In this case I'm making the statement that you need to compare a fully differential circuit against one that is single ended only. You've not done that yet so far as I can see.

@atmasphere

Ralph, are you able to address my comparison between Graaf GM20 and Wavac EC300b in the same way? I am not yet convinced that the issues raised by @pani aren‘t valid in the same way.

For what it‘s worth I have yet to hear a balanced connection that can compete with an optimum single ended setup and I think Ken Shindo, JC Verdier and Keith Aschenbrenner would agree…

This thread is pleasantly devoid of polemics and addressing the core issues: thank you all, this is what the forum should be

Ralph, are you able to address my comparison between Graaf GM20 and Wavac EC300b in the same way? I am not yet convinced that the issues raised by @pani aren‘t valid in the same way.

From what I can see of the Graaf front end circuit, while it is balanced it must have a fairly low CMRR; this based on the lack of a constant current source. This causes the circuit to have higher offsets, lower gain and higher distortion. This in turn muddies the water as far as the distortion product goes (mostly due to the offsets generated, which cause more 2nd harmonic to be created)- you can't say its entirely cubic insofar as its non-linearities are concerned. So I can't really comment about the comparison and certainly can't contest your findings! But of course the Graaf is one of a number of OTLs out there.

Regarding the balanced connection thing, I always encourage people to look at whether the gear doing the balanced connection supports the balanced standard or not. The reason is simple: if you don't support the standard you don't accrue the benefits either! FWIW most high end audio manufacturers don't seem to support the standard. One of the more important aspects is that in balanced mode, neither the inverted or non-inverted signal references ground- they reference each other and ground is ignored. Its this bit that makes supporting the balanced standard a bit tricky- most of the time you need an output transformer to do the job. To do that right is expensive.We patented a means to get around that problem but that simply means we're the only one doing it our way, which is direct-coupled.

@atmasphere I have not heard your amps yet and also my exposure to differentially balanced amps is very limited hence I would eagerly wait to hear them out. I would be very happy if I can find the cohesion of single ended design in a powerful balanced push pull design

My exposure to OTL was only with the Tenor OTL75 which I owned for about 6 months. Lovely amplifier but surprisingly even with 75 watts it could not control my Tannoys' bottom end like a 10 watt Wavac 300b (which I owned just prior to the Tenor). The bass was loose. In my relatively small room it was an issue. The rest of the spectrum was deliciously to my taste. It still missed the cohesion of a Single ended amp though.

@pani Which Tannoys did/do you have?

I have the Tannoy Turnberry SE

That speaker seems a bit on the low side of efficiency (93dB) for an amp of only 7-10 watts! Nice speaker though. I'd be putting at least 10x more power on that unless the speaker was in a small room.

This is what I know about the Tenor (we had a few in here for service some years back): It is a differential design but using a circuit similar to what is in the BAT VK-60, which is to say two cascaded gain stages driving the output section. That is how our very early amps worked but back in the early 1980s I realized that there were too many problems with the approach (in particular DC Offsets which would result in unequal drive to whatever stage was succeeding, resulting in distortion, especially the 2nd harmonic).

The 2nd harmonic suggests a quadratic non-linearity is present. The differential design suggests a cubic will be present also. Further that amp employs feedback (although not nearly as much as is needed) while the WAVAC does not. I would expect the Tenor to be fast and transparent but a bit dry, perhaps a bit bright and less engaging compared to the SET. Do I have this right?

You are correct @atmasphere that Tenor had the better PRAT. But it did not control my speakers as well as the 10 watt Wavac. That was wierd. It also did not sound as illuminated from within as the Wavac. I have again noted this difference between typical SET vs Push-Pull amps. SETs illuminate the entire spectrum evenly (DHTs more, non DHT) but push-pull amps have a darkness to the presentation. Can't explain it well. That is also one reason SETs sound more vibrant & energetic compared to Push-Pulls.  

That is also one reason SETs sound more vibrant & energetic compared to Push-Pulls.  

Actually what is happening is how the SET makes distortion. The energetic quality is due to higher ordered harmonics showing up on the transients when you ask the amp to make any significant power. Those higher orders are interpreted by the ear as 'louder' although if you were to use a sound level pressure meter, you'd find that its an illusion. They are a sign that the speaker isn't efficient enough to take advantage of the better properties of the amp. Any time you ask an SET to make more than about 20-25% of full power these higher ordered harmonics are in the mix.

What Atmasphere is exactly what we experienced in our comparisons. Takes a bit of time to figure this out, but all 5 of the participants eventually came to the same conclusion. 2 of us heard this within moments. 

There is another quality that I have clearly noticed in all SETs vs Push-pull. And this is even in systems which are super efficient and made for SETs (single drivers, Audio note speakers etc). SETs sound like "they are here" whereas push-pull sound like "we are there". At any volume and with any speaker I have heard, this is another phenomenon that was consistent

Sorry Charles you probably have more experience then i do but  being an audiophile for over 25 years and the budget to spend as much as i like on this hobby.

Years of buying and selling has tought me many lessons, i would say an evolution in trainning the ears.

I have owned Solid States and SET Amplifiers, Horns and Full Range Speakers, bookshelf and studio speakers to active speakers from studio and audiophile markets, your conventional speakers to even PA Speakers like Danley Sound-labs too. DIY builds on Speakers and Amplifiers from different designs some well known and others not so much.

So far to my Ears with my current experience OTL are superior over any SET Amplifiers in almost any way and so far over any Solid State Amplifiers too.

If you want super lush sounding or slightly colored music then SET would be your weapon of choice but that is more to do with preference and their are many wonderful Sounding SET Amplifiers i personally still would never Sell i like the SET sound too but over each other you obvious know which i would pick. That being said in my main Room for musical playback OTL will always be a permanent keeper. So to my ears and my experience and my preference and conclusion. 

OTL Is Superior Amplifier ( I dont intend to tick anyone off)   This is based on my life journey as a Audiophile.

I currently use the Classic Audio T1.5 and T1,3 Both Field Coils In my home in diffrent rooms both are 16 ohms

I also own Western Electric Field Coil Speakers which is a clone made by Line Magnetic these are also 16ohms

I have the Cube Audio nenuphar Speakers Mated with SET Amplifiers in my bedroom.

I have other rooms with Active Studio Speakers with Tube Pre-amps. Various rooms have different setups.

Overall the combination of Classic Audio Field Coils with OTL is by far the most superior of all my audio systems and i would extend this to saying i have never wanted to change this combo even when i have visited various shows with uber priced audio gears.

And this is even in systems which are super efficient and made for SETs (single drivers, Audio note speakers etc). SETs sound like "they are here" whereas push-pull sound like "we are there". At any volume and with any speaker I have heard, this is another phenomenon that was consistent

People occasionally ask me how a stereo is supposed to work, and I explain that the best model I've come up with is that the stereo can act like a time/space machine that can graft your room onto a musical event.


Some recordings are very intimate and sound like they are in the room. Others are larger events, like a full orchestra, in which case you feel more like 'you are there' rather than 'they are here'. IOW IME the way that presents really depends more on the recording if the stereo's ducks are in a row.

’Sorry Charles you probably have more experience then i do but being an audiophile for over 25 years and the budget to spend as much as i like on this hobby.’

@phantom_av , not a problem at all. You definitely have  very  broad experience with various types of amplifiers and OTL is your preference based on much listening . I’ve heard nearly every type of amplifier topology you can name but for me SET when all is considered moves me the most and is the best for listening to music.

Your findings parallel @audition_audio. Mine parallel @pani. I wrote earlier in this thread that both can be excellent choices and people have certainly move (In both directions) from one to the other. Depends on what you’re sonically seeking and hold most dear. I have no doubt that you are as happy with your OTL amplifier as I with my SET amplifier. As far as I’m concerned we both made the right choice. It turns out that SET just does more of what I like.
Charles

The problem I have with a previous post is with the use of the word "superior."

This presupposes that:

1. The poster is the arbiter and ultimate authority of what’s superior

2. Superiority is a given or an absolute

It would be infinitely more accurate to say you, or I, or we "prefer"

...which surprisingly (or is that unsurprisingly) is used to describe the state of those who "prefer" SET amps.

My point is that the words we use are important...

Let’s use them with greater care and strive for comity in this amazing hobby of ours.

[On "superior"]

So far to my Ears with my current experience OTL are superior over any SET Amplifiers in almost any way and so far over any Solid State Amplifiers too.

...OTL Is Superior Amplifier ( I dont intend to tick anyone off) This is based on my life journey as a Audiophile.

[On "preference"]

If you want super lush sounding or slightly colored music then SET would be your weapon of choice but that is more to do with preference

Couldn’t agree more. Words are important. Let us strive then to use them with accuracy and precision.

Author means from or of itself- autonomous, automatic, etc. Everything then is everyone’s opinion. Yes even "facts", as the choice whether or not to consider something a fact is itself a decision someone made. There is no getting around it. Never has been, never will be, any Divine providential entity posting from on high.

This being the case then it is redundant to be qualifying everything with "in my opinion" or anything to that effect. To be redundant is just plain sloppy writing. Only thing worse would be to apologize for doing nothing wrong.

2. Superiority is a given or an absolute

This is correct.


IMO/IME the OP question is a bit too broad. Some OTLs sound more like SETs (like ours, due to no feedback and otherwise masked higher ordered distortion content); some sound more like traditional PP tube amps (due to feedback, combined single-ended and PP circuitry, and the use of pentode output tubes).


All these things make and enormous difference in an OTL because there's no transformer to hide what's going on.


On the SET side, you have traditional SETs, parafeed, Loftin-White circuits; Jack Elliano developed a class A3 output system (which he patented; it seems that Western Electric is using his system) and believe it or not some SETs employ feedback. They all sound different, and they also all sound different depending on whether you have a type 45 output (arguably the ’best’ since they have the widest bandwidth although they are not the best in that they have almost no power), 2A3, 300b, 211 and so on, and whether there are paralleled output tubes and of course how well or poorly the output transformer is designed.


Its also important that if you are doing a comparison, that both amps be comfortable on the loudspeaker chosen for such a comparison. Smaller OTLs need higher impedance speakers, but if you want the best out of any SET, higher impedance (16 ohms) works out well in their favor as well! I think @pani did a good comparison between his amp and the Tenor on this latter account. But while the Tenor used many of the concepts we pioneered (fully differential voltage amplifier, Circlotron output stage) it was a *very* different sounding amp from our stuff.


So I agree you can’t make blanket statements, but IMO/IME you can indeed make very specific statements! For example I’d put up our S-30 against any of the more expensive Audio Note amps out of Japan and as long as both amps were comfortable with the speaker load, expect the S-30 to be better in every way that can be described. But there are some speakers that the Audio Note amps can drive that the S-30 cannot (to be specific: 4 ohms) and in those comparisons the S-30 would fall flat on its face. So I imagine that its important to pay attention to what the designer of the amp had in mind when it comes to loudspeakers.


IMO/IME the distortion signature of any amp is far more important than what kind of amp it is. If the amp has, as its primary distortion product, enough of the lower harmonics to mask the presence of the higher ordered harmonics, you’ll get that organic musical sound that so many SET users crave and it won’t matter if the amp is SET, OTL, solid state or class D.

I owned an OTL amp for 8 years and then switched to SET. No one has mentioned that OTL amps require many output tubes. For example, my OTL amp had 8 output tubes. The obvious issue is that OTL generates a lot of heat, but also with that many tubes, there is more potential for tube hiss and other tube related noise. I was always tracking down a gremlin. My OTL amp always had tube hiss through the speakers, but the more annoying thing was dealing with tracking down the cause of humming noise that would arise. When my amp was quiet (albeit with the hiss still there), it was glorious. I’m not saying that other OTL amps such as Tenor or Atmasphere have noise related issues, as I have no experience with those brands, but mine did.

My current SET amp has been 100% reliable since I got it over 5 years ago and is the most quiet tube amp that I have ever owned. I have gone through a lot of amps over the years. One thing I really enjoy about my SET amp is that I can more easily tell the difference between tubes, cables, and source components. With my OTL amp, the differences were much more subtle. For me, I would not say that OTL is "superior" as that has not been my experience. I would like to re-visit OTL one day, but I am thoroughly enjoying my SET amp so there is no rush.

@thaluza  I think you're overgeneralizing.

https://www.lineartubeaudio.com/products/zotl-40-reference-amplifier

@twoleftears,

No, I am just relating my extensive experience with one brand. I made it clear I have no experience with Tenor and Atmasphere. Nor do I have experience with other brands. The link you provided does not say anything about user experiences with Linear Tube Audio, so it means nothing to me. I take manufacturer's hype with a grain of salt. I would be very interested in reading about what you and others think about LTA, though. As I said, I would like to re-visit OTL one day. Seems like LTA is more like a Berning amp?

Again, I do not know how to measure this or if there is any way to measure this. SETs simply sound more direct. Instruments and voices sounds realistic, as in small, focused, dense and direct. Just the way they sound in real world. This doesn't happen on any of the Push-Pulls. In fact this is the primary sonic difference I was trying to nail between the 2 topologies. A vast majority of the amps therefore do not sound like real thing. Even a large instrument like Cello, when bowed has a very direct and focused sound, like it is speaking to you. Only SETs get this directness. Is it because it just amplifies without any further "processing" ? May be. Whether it is measurable? I don't think so, I have never seen any amps having a measurement which talks about these characteristics.

I have heard SETs with negative feedback (Unison Research) and without (Audio note, wavac, trafomatic, Allnic). The directness is there regardless of feedback and output type. Feedback does make the sound more controlled, sometimes too controlled, affecting the flow. I prefer zero feedback every time.

@pani,
’Even a large instrument like Cello, when bowed has a very direct and focused sound, like it is speaking to you. Only SETs get this directness. Is it because it just amplifies without any further "processing" ? May be.’

Yep! Precisely my same reaction with initial listening exposure to a SET amplifier. In a nutshell,  more real, more natural.   Different people hear and are drawn to various things when listening to music via home audio. There is not a single topology that gets everything right, no perfection. So people resort to choosing what comes closest to sounding ’more’ right than other available alternatives. So it may be SET, OTL, class A solid state, class D or whatever. SET for me and something else for somebody else.
Charles

How about an OTL SET ?

I don’t think it is possible.  The tubes in a SET amp are constantly outputting dc that is the modulated up and down in output by the audio signal.  That changing output fed to the primary windings of the output transformer then induces an alternating current in the secondary.  Absent a transformer, a SET amp would be sending a very large dc signal to your speakers, particularly when there is no music playing, and dc is very bad for speakers.

Could a DC offset protection circuit be used to make a OTL SET?

No, it would constantly be in protection mode, meaning no output.