SET vs OTL

@antigrunge2,
It is interesting how people hear audio products and how it formulates opinion and decision making. Some OTLs may sound faster than some SET amplifiers. Some do not. Your experience with the Graaf and Wavac is similar to mine.Ive heard SET with as much "speed" clarity and transparency as fine quality OTLs. Some may experience "sweetened" or "lush" coloration with certain SETs but they all are not the same.

I would caution against painting with the proverbial broad brush. My SET is organic and full of tone/body, very natural and has an  emotionally engaging presentation. There is no sacrificing of nuance or natural detail. There are many variables that determine what works best for a given listener and their respective audio system. Many roads do in fact lead to Rome.
Charles

Well actually no! True the quality of the iron as well as the circuit and its design do contribute and can make significant improvement in any amp design, the absence of a transformer contributes hugely to the intrinsic difference in sound. Provided that the amplifier requirements are met, I think an OTL is a superior design. 

Antigrunge,
What speaker were you using for this comparison? 

Phantom a/v is correct in that impedance is crucial with OTLs which can be a serious impediment. Your experience mirrors mine. What amazed me was how quickly the SETs we used in the comparisons went into soft clipping. They got more muscular but confused sounding. It took hours of listening and direct comparison to figure this out. 

I have used a Graaf Gm20 for about 10 years before upgrading to a Wavac Ec300b some 10 years ago. In short, there is no going back: while the Graaf had twice the stated output power, making that power useful was challenging: using autoformers to ideally match speaker impedance as well as pretty regular rebiasing, neither of which is required by the Wavac. If anything, the Wavac is faster than the Graaf despite its transformers and the bass is substantially more solid. Botj amps are classics in their line of design

What speaker were you using for this comparison? 

The speaker can be an enormous influence when doing comparisons. There are other variables as well- for example most OTLs employ feedback (ours are some of the very few that do not). Some OTLs use pentodes as opposed to triodes. So they sound different; you can't categorically state that because you've heard one OTL you've heard them all and actually be telling the truth.

The distortion signature of the amplifier also plays a big role in the results. In a nutshell there are two types of non-linearities that show up. In single-ended circuits you get a quadratic non-linearity. In a differential circuit you get a cubic non-linearity. The cubic is preferred because its lower distortion; this is because even orders are cancelled and distortion does not compound as much from stage to stage as the signal progresses through the circuit. The 3rd harmonic is the primary distortion product and will mask the presence of the higher orders, allowing for a very smooth sound, but more detailed that that of a single-ended circuit because there is less distortion to mask it.

In a single-ended circuit the primary distortion product is the 2nd and there is usually a prominent 3rd. These two harmonics mask the higher orders so this circuit sounds very smooth as well. The problem is that as the order of the harmonic is increased, its amplitude falls off at a slower rate, causing more low level detail to be masked. But it has a rich sound due to the prodigious 2nd harmonic created.

When you combine the two (such as a single-ended input with a push-pull output), as a good number of OTLs (and other push-pull amps) do, algebraic summing occurs and there usually results more of the 5th harmonic (this has been known a good long time as Norman Crowhurst was writing about this 65 years ago...), which is the main reason SET guys object to the 'sound' of push-pull. But if the PP amp is fully differential you don't have this problem.


Since the feedback used in any tube amplifier is insufficient for it to really do its job, these distortion artifacts remain in the distortion signature.


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. But all OTLs are **faster** than any SET (simply out of the fact that transformers slow down risetimes based on their bandwidth limits; this is simple physics); if it sounds 'slower' its likely because the amp is not matching well with the speaker in the high frequencies.

"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.