SET vs OTL

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.