SET vs OTL

@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