SETs and Horns

[Al M.]

There has been a lot of enthusiasm expressed on WBF about SETs and horns. Some passionate advocacy in discussions might even cause an impression that there are only advantages to this type of systems, no drawbacks.

So apart from questions of practicality, such as size of horns, WAF and money to be invested, my questions are:

1. In which areas to SET/horn systems excel?
2. In which areas can they not even be matched by other types of systems?
3. Are there any sonic drawbacks to SET/horn systems, in the sense that there are other system types that just can do certain things better?
4. How much money do you have to spend on an excellent SET/horn system? How much money do you have to spend on a good SET/horn system?
5. What about the question of the oft-mentioned horn colorations? Are they still a concern in modern horns? Are they a concern in vintage horns?

 

[bonzo75]

Maybe, just to add to already discussed topics, how do videos of horns compare, and do horns need to be straight facing or inward toed

 

[Atmasphere]

I'd be more concerned about the coloration of the SET than the horns.

Distortion is one of the main reasons we hear differences in amplifiers. For example, the brightness and harshness of solid state is not due to a frequency response error; its caused by higher ordered harmonic distortion (although at what is often called a 'low level'). SETs OTOH tend to make a prominent 2nd order, which results in 'bloom' and 'warmth'. From these two examples we can see that the ear is far more sensitive to higher ordered harmonics; this is because it uses them to gauge sound pressure.

But there is more to it than just that! The presence of the 2nd or 3rd harmonic (both of which are treated by the ear in the same way) can mask the presence of the higher orders if that 2nd or 3rd is present in enough quantity. This is why tube amplifiers sound smoother than solid state, even though they have more higher ordered distortion.

So far this is all easily confirmed both by listening and measurements- they are in agreement.

Now a circuit that generates a 2nd order as its primary distortion component has what is known mathematically as a 'quadratic non-linearity'. If the circuit generates a 3rd as the primary distortion component it has what is called a 'cubic non-linearity'. If the former is present, as the order of the distortion is increased, it falls off at a slower rate than a circuit having the latter.

So we can see that if we can build a circuit with a cubic non-linearity (assuming that real linearity is out of reach) it will be less colored as it will have less distortion, not the least of which is the fact that the primary distortion will be at a lower level than seen if a quadratic non-linearity is the main influence.

So now the question is- how do you do that?? In simplistic terms, these two differences in distortion are seen in single-ended circuits as opposed to fully differential and balanced circuits.

If you mix the two types of circuits, as is common in traditional tube amplifiers like the Dynaco ST70 (single-ended input, push-pull output) you'll get both types of non-linearities. When this happens a more prominent 5th is present, and distortion drops off at a slower rate as the order of teh harmonic is increased.

So far I've kept the effects of negative feedback out of this conversation. But it has a dramatic effect on what we hear. But I think I'll leave that for later, as I only have so much time

Anyway, one advantage of SETs is that as power is decreased, distortion falls off to unmeasurable. This is all about that 'first watt' which has to be clean. SETs are pretty good about this. By comparison, large push-pull amps are not, since below a certain power level (usually around 5% or so), distortion starts to go back up. This is why SET users often talk about that 'magical inner detail'.

But if you have a fully differential circuit (and no feedback) you can also achieve this linearly decreasing distortion curve as power is decreased, with no rise in distortion at lower power levels. And of course you can build small push-pull amps if you want; these are rare in good quality and I would surmise never compared directly watt for watt to SETs.

SETs are traditionally power challenged as it is very difficult to build an output transformer that has bandwidth and power at the same time. This is why the 300b was King in the 1990s, why the 2A3 ascended the throne 20 years ago and why the type 45 power tube (good for about a watt) is now the object of admiration. But if you have a fully differential amplifier you can have the power and bandwidth at the same time, while using the same power triodes (if you want).

So IMO/IME there is no argument for SETs, other than someone simply not having heard something that is both sonically and measurably better.

Some argue that SETs are very dynamic and it is often true that they sound that way. But in reality, what is happening is the amp is being driven hard enough that the higher ordered harmonics are showing up as the distortion of the amp is increased (SETs are often 10% THD at full power), and where the power is needed is often transients. Since the ear uses these harmonics to sense sound pressure, and since they are showing up on transients, presto- you have an amp that sounds 'dynamic' but its really distortion masquerading as such. Simply by you're reading this, I may have ruined it for you because of how our brains process music. To avoid this phenom, the speaker should be efficient enough that this does not occur: hence horns.

A mark of any good system is the quality where it does not sound loud even when it is. That takes clean power which SETs cannot provide except at low power. This is often why SET owners will tell you that '90dB is plenty loud enough for me'. If the higher ordered harmonics were not present, it would be natural to turn the volume up higher.

 

[tima]

From @PeterA 's system thread to avoid going off topic:

1. Define what is small and what is large. Your [ @jeffrey_t ] speakers, for example, are not so large and are actually only horn in the mid/high...not that this is a problem it's just not clear where your demarcation lies. Do you mean small, fully horn loaded speakers (i.e. also the bass)? What type of horn, because a TQWT loading is a type of horn that actually delivers a lot of punch in a more or less normal sized cabinet.

Brad, would you offer some examples of a TQWT type of horn, please. I"m totally unfamiliar with both the acronym and implementations.

 

[spiritofmusic]

Ralph, one of the best horns demos ever was Bakoon 13-R, 13W of Class A SS, on Cessaro Liszts.

 

[Al M.]

I'd be more concerned about the coloration of the SET than the horns.
[...]

SETs are traditionally power challenged as it is very difficult to build an output transformer that has bandwidth and power at the same time. This is why the 300b was King in the 1990s, why the 2A3 ascended the throne 20 years ago and why the type 45 power tube (good for about a watt) is now the object of admiration. But if you have a fully differential amplifier you can have the power and bandwidth at the same time, while using the same power triodes (if you want).

Fascinating post, Ralph, thank you for sharing your expertise.

By differential amplifier, do you mean push-pull? How can you have power and bandwidth at the same time with a differential amplifier? Do not the same limitations to output transformers apply as to output transformers for SETs?

Some argue that SETs are very dynamic and it is often true that they sound that way. But in reality, what is happening is the amp is being driven hard enough that the higher ordered harmonics are showing up as the distortion of the amp is increased (SETs are often 10% THD at full power), and where the power is needed is often transients. Since the ear uses these harmonics to sense sound pressure, and since they are showing up on transients, presto- you have an amp that sounds 'dynamic' but its really distortion masquerading as such. Simply by you're reading this, I may have ruined it for you because of how our brains process music. To avoid this phenom, the speaker should be efficient enough that this does not occur: hence horns.

This correlation between perceived dynamics and distortion rings true to me. Recently, with removal in my system/room of some reflections and perhaps some resonances as well (e.g., by moving the main equipment rack further away from the speakers) I noticed that while music sounded not just cleaner -- less distorted --, some music also sounded "less dynamic", due to loss of transient "edge". When I checked on quite a bit of other music it was obvious that the system had not lost any of its true dynamics.

 

[morricab]

From @PeterA 's system thread to avoid going off topic:



Brad, would you offer some examples of a TQWT type of horn, please. I"m totally unfamiliar with both the acronym and implementations.

It stands for tapered quarter wave tube and was invented by Voight (was also called a Voight pipe I believe). It is a mix of horn and reflex and because the mouth is mass loaded it gets reasonablly deep bass in a normal sized cabinet. Commercial examples are the Horning speakers, Tune Audio’s smaller horns, some Bastanis, some Supravox designs (I have one of these), Odeon had some designs (I have one of these too), Cube Audio. There is a good website by a Danish designer Troels Gravessen that goes through some nice TQWT designs using a mid he designed for Jansen Audio.
My Supravox are single 8 inch driver, 99db, and get in-room bass flat to 30hz or so. The main drawback is the single driver doesn’t have a Whizzer so it is not truly full range. So I started mating it with a horn mid/high. It has an effortless and huge sound despite not being overly huge.

My Odeon speakers are two way, 97-98db (depending where you read) and mate a 10inch Mid/bass with a wooden horn round tweeter with 1 inch compression driver. Also effortless dynamics in a normal room with a lot of punch. The bass doesn’t go as deep as the Supravox but the mid bass hits harder.
The speed of the bass is a big part of the horn bass appeal but it often can sound lightweight. TQWT sounds fuller on the bottom than most exponential horns I have heard and have the horn bass speed.

Hornings set up right have awesome bass with a ton of texture that is often missing in normal ported designs.

 

[gestalt]

Hornings set up right have awesome bass with a ton of texture that is often missing in normal ported designs.

Agree. Really lovely.

 

[Atmasphere]

By differential amplifier, do you mean push-pull? How can you have power and bandwidth at the same time with a differential amplifier? Do not the same limitations to output transformers apply as to output transformers for SETs?

Push-pull may or may not be differential, but what we're talking about is cancellation of even ordered harmonics in whatever load the circuit is driving, which will happen in a push-pull output circuit. But if the amp is differential from input to output, then this distortion cancelling property is from one stage of the circuit to the next- and so is not being compounded (with additional harmonics added) in each stage- resulting in a lower distortion amplifier. With lower distortion comes greater neutrality.

I'm sure you're aware of how much more power a push-pull amp can make as opposed to single-ended. This holds true for tubes as well as solid state. A single 2A3 in class A1 can make about 4 watts; if push-pull a pair in class A1 can make 16 watts. Because the current each tube draws through the output transformer is magnetically cancelled in the transformer, the transformer can be made to have much higher power handling and wider bandwidth. Instead of maybe 7 watts to be considered wide enough bandwidth for hifi, you can easily make over 100 watts.

 

[morricab]

I'd be more concerned about the coloration of the SET than the horns.

Distortion is one of the main reasons we hear differences in amplifiers. For example, the brightness and harshness of solid state is not due to a frequency response error; its caused by higher ordered harmonic distortion (although at what is often called a 'low level'). SETs OTOH tend to make a prominent 2nd order, which results in 'bloom' and 'warmth'. From these two examples we can see that the ear is far more sensitive to higher ordered harmonics; this is because it uses them to gauge sound pressure.

But there is more to it than just that! The presence of the 2nd or 3rd harmonic (both of which are treated by the ear in the same way) can mask the presence of the higher orders if that 2nd or 3rd is present in enough quantity. This is why tube amplifiers sound smoother than solid state, even though they have more higher ordered distortion.

So far this is all easily confirmed both by listening and measurements- they are in agreement.

Now a circuit that generates a 2nd order as its primary distortion component has what is known mathematically as a 'quadratic non-linearity'. If the circuit generates a 3rd as the primary distortion component it has what is called a 'cubic non-linearity'. If the former is present, as the order of the distortion is increased, it falls off at a slower rate than a circuit having the latter.

So we can see that if we can build a circuit with a cubic non-linearity (assuming that real linearity is out of reach) it will be less colored as it will have less distortion, not the least of which is the fact that the primary distortion will be at a lower level than seen if a quadratic non-linearity is the main influence.

So now the question is- how do you do that?? In simplistic terms, these two differences in distortion are seen in single-ended circuits as opposed to fully differential and balanced circuits.

If you mix the two types of circuits, as is common in traditional tube amplifiers like the Dynaco ST70 (single-ended input, push-pull output) you'll get both types of non-linearities. When this happens a more prominent 5th is present, and distortion drops off at a slower rate as the order of teh harmonic is increased.

So far I've kept the effects of negative feedback out of this conversation. But it has a dramatic effect on what we hear. But I think I'll leave that for later, as I only have so much time

Anyway, one advantage of SETs is that as power is decreased, distortion falls off to unmeasurable. This is all about that 'first watt' which has to be clean. SETs are pretty good about this. By comparison, large push-pull amps are not, since below a certain power level (usually around 5% or so), distortion starts to go back up. This is why SET users often talk about that 'magical inner detail'.

But if you have a fully differential circuit (and no feedback) you can also achieve this linearly decreasing distortion curve as power is decreased, with no rise in distortion at lower power levels. And of course you can build small push-pull amps if you want; these are rare in good quality and I would surmise never compared directly watt for watt to SETs.

SETs are traditionally power challenged as it is very difficult to build an output transformer that has bandwidth and power at the same time. This is why the 300b was King in the 1990s, why the 2A3 ascended the throne 20 years ago and why the type 45 power tube (good for about a watt) is now the object of admiration. But if you have a fully differential amplifier you can have the power and bandwidth at the same time, while using the same power triodes (if you want).

So IMO/IME there is no argument for SETs, other than someone simply not having heard something that is both sonically and measurably better.

Some argue that SETs are very dynamic and it is often true that they sound that way. But in reality, what is happening is the amp is being driven hard enough that the higher ordered harmonics are showing up as the distortion of the amp is increased (SETs are often 10% THD at full power), and where the power is needed is often transients. Since the ear uses these harmonics to sense sound pressure, and since they are showing up on transients, presto- you have an amp that sounds 'dynamic' but its really distortion masquerading as such. Simply by you're reading this, I may have ruined it for you because of how our brains process music. To avoid this phenom, the speaker should be efficient enough that this does not occur: hence horns.

A mark of any good system is the quality where it does not sound loud even when it is. That takes clean power which SETs cannot provide except at low power. This is often why SET owners will tell you that '90dB is plenty loud enough for me'. If the higher ordered harmonics were not present, it would be natural to turn the volume up higher.

Differential circuits tend to produce predominantly odd and higher order harmonics, unless you pump up the feedback. See Boyk and Sussman’s modelling of basic circuit elements. Single ended produces an exponential decay of harmonics from low to high order that is consonant with our ear/brain...when done right of course. This “ideal” harmonic pattern was described by Jean Hiraga and seems his ears were pretty good.

 

[Gregadd]

When Ralph writes I lead. Ihis 30 watt and 60 watt amp sound excellent with the Classic(TM) horns.
I am going to go out on a limb and say Loudness is not is not the the issue. Dynamics is. When a low powered SET drives horns the anp is already in the upper limit. When it comes time to "expand" it can run out of gas.

 

[Atmasphere]

Differential circuits tend to produce predominantly odd and higher order harmonics, unless you pump up the feedback. See Boyk and Sussman’s modelling of basic circuit elements. Single ended produces an exponential decay of harmonics from low to high order that is consonant with our ear/brain...when done right of course. This “ideal” harmonic pattern was described by Jean Hiraga and seems his ears were pretty good.

No doubt single ended circuits are euphonic. But its important to understand what's happening in a differential circuit!

Both halves are single-ended circuits, but they communicate and become differential because their emitter circuits are made to be common, and then tied to either through a constant current source or minus voltage of enough magnitude that the circuit is caused to be differential.

So that means both sides make a 2nd harmonic and have a quadratic non-linearity. This in turn means that the most significant harmonics it makes will be the 2nd and 3rd, and the 3rd will be at the level seen in a single-ended circuit. But the even orders are cancelled at the load of the differential circuit, leaving the 3rd as the primary and most significant distortion component.

At this point (just as if the 2nd were present) the ear's masking principle comes in. The 3rd harmonic masks the presence of the higher ordered harmonics. If the differential circuit is not perfect (and none are) there will be some even ordered content too, along with a 2nd to mask their presence. But over the decades I've been making differential circuits, over and over again its been the 3rd that is predominant.

The ear treats the 2nd and 3rd the same way. So we're dealing with a harmonic that adds warmth and bloom, but at a lower level than if there were a 2nd harmonic. But because of its relationship to the higher orders (its got more amplitude) its masks those higher orders.

The result is sound as smooth as any single-ended circuit, but considerably more neutral owing to less distortion (and so there is more detail too). When you get smoother sound and more detail, that's a plus in my book; its not brightness masquerading as detail, its the real thing.

I've not read Boyk and Sussman's works (if you have a link I'm happy to look into it) but if your interpretation of their work is represented here correctly and is in absolute form, I'm going to say that they missed something or are outright incorrect. What that 'missing' thing is has to do with real music that does not contain extra harmonic information other than those imbued to the instruments themselves. If the higher orders made by the reproduction are rendered inaudible as I described, then what is left is music, plus the 3rd. At that point our ear/brain system is not going to be worried about exponential (quadratic) decay of ascending orders of added harmonic content.

To be clear I'm not saying that SETs sound bad! But I am saying they are not and cannot be neutral. Specifically with regards to the OP I am responding to these bits of that post:

3. Are there any sonic drawbacks to SET/horn systems, in the sense that there are other system types that just can do certain things better?

5. What about the question of the oft-mentioned horn colorations? Are they still a concern in modern horns? Are they a concern in vintage horns?

3.: Yes
5.: I hear more colorations in SETs than I do in modern horns. I own horns (Classic Audio Loudspeakers T-3.3 with field coil drivers) and I do that because they are some of the most neutral and revealing speakers I've heard. They operate at the same level as ESLs, but without the need for more power, with less sensitivity to humidity and with greater overall dynamic range.

 

[Al M.]

I'm sure you're aware of how much more power a push-pull amp can make as opposed to single-ended. This holds true for tubes as well as solid state. A single 2A3 in class A1 can make about 4 watts; if push-pull a pair in class A1 can make 16 watts. Because the current each tube draws through the output transformer is magnetically cancelled in the transformer, the transformer can be made to have much higher power handling and wider bandwidth. Instead of maybe 7 watts to be considered wide enough bandwidth for hifi, you can easily make over 100 watts.

(Emphasis added.)

Unfortunately I know too little of output transformer theory. Could you please elaborate on what you mean by the bolded part? What does magnetic cancellation mean? Is there no cancellation in SET configuration, and is that a reason why the transformer cannot have high power handling and wide bandwidth at the same time? Thanks.

 

[jeff1225]

It stands for tapered quarter wave tube and was invented by Voight (was also called a Voight pipe I believe). It is a mix of horn and reflex and because the mouth is mass loaded it gets reasonablly deep bass in a normal sized cabinet. Commercial examples are the Horning speakers, Tune Audio’s smaller horns, some Bastanis, some Supravox designs (I have one of these), Odeon had some designs (I have one of these too), Cube Audio. There is a good website by a Danish designer Troels Gravessen that goes through some nice TQWT designs using a mid he designed for Jansen Audio.
My Supravox are single 8 inch driver, 99db, and get in-room bass flat to 30hz or so. The main drawback is the single driver doesn’t have a Whizzer so it is not truly full range. So I started mating it with a horn mid/high. It has an effortless and huge sound despite not being overly huge.

My Odeon speakers are two way, 97-98db (depending where you read) and mate a 10inch Mid/bass with a wooden horn round tweeter with 1 inch compression driver. Also effortless dynamics in a normal room with a lot of punch. The bass doesn’t go as deep as the Supravox but the mid bass hits harder.
The speed of the bass is a big part of the horn bass appeal but it often can sound lightweight. TQWT sounds fuller on the bottom than most exponential horns I have heard and have the horn bass speed.

Hornings set up right have awesome bass with a ton of texture that is often missing in normal ported designs.

Brad, I don't actually consider what I own horns. The bass is just normal ported bass and the highs come through a wave guide.

 

[cjfrbw]

I have been blessed with these large, hi efficiency midrange ribbons that love simple 45 amp (active crossover, range from 300Hz to 7Khz). I was listening to them today with globe 183 DHT (an uncommon 45 with a 5v cathode) driving the 45 tube amp. Talk about being writ large with a fine brush. The 45 already has one of the loveliest upper midrange extant, but 'stacking' the DHT makes exotic seem like an understatement.

Still, I think every committed audiophile should try to hear a 45 type driving large efficient horns. It's a bit of a revelation and a mind bender.

 

[Ron Resnick]

Ralph,

I did not understand all of the elements of your explanations, bit I think I understand most of them. I appreciate deeply your expertise and experience, and I, for one, am very grateful that you participate here on WBF.

Thank you.

 

[Ron Resnick]

There has been a lot of enthusiasm expressed on WBF about SETs and horns. Some passionate advocacy in discussions might even cause an impression that there are only advantages to this type of systems, no drawbacks.

So apart from questions of practicality, such as size of horns, WAF and money to be invested, my questions are:

1. In which areas to SET/horn systems excel?
2. In which areas can they not even be matched by other types of systems?
3. Are there any sonic drawbacks to SET/horn systems, in the sense that there are other system types that just can do certain things better?
4. How much money do you have to spend on an excellent SET/horn system? How much money do you have to spend on a good SET/horn system?
5. What about the question of the oft-mentioned horn colorations? Are they still a concern in modern horns? Are they a concern in vintage horns?

I would say "horn-loaded" as the driver feeding the horn could be any of several different types, including conventional cones. Separately, a driver can be back horn-loaded, as in the Tannoy Westminster Royal GR.

I am much more inclined than most to use higher than typical power on high sensitivity loudspeakers. So I am answering your questions more from the point of view of horn loudspeakers than from the point of view of SET amplifiers.

I am answering these questions having in mind the very best and most ambitious speakers of each topology I have heard.


1. In which areas to SET/horn systems excel?

midrange realism, naturalness and presence

reproduction of the sounds of brass instruments

dynamics and "jump factor"

wider listening zone (no narrow "sweet spot")

Horn-loaded bass can sound very "fast."

An overall sonic presentation which is different from any other speaker topology in that horn speakers project the sound towards you in an energetic way that contributes to "aliveness," presence and realism.

2. In which areas can they not even be matched by other types of systems?

the reproduction of the sounds of brass instruments

I think there is something about the way horn-loaded loudspeakers reproduce the sounds of brass instruments which is consonant with the way those instruments themselves produce sounds.

If my primary musical genre interest were jazz, I definitely would have a horn speaker system. If my primary musical genre interests were jazz and classical, I probably would have a horn speaker system (unless I could have a Wilson XVX + Subsonics in which case it would be very difficult to make a decision).

3. Are there any sonic drawbacks to SET/horn systems, in the sense that there are other system types that just can do certain things better?

I find almost all contemporary design horn speakers to be a bit aggressive-sounding and fatiguing. The Tune Audio Avaton and the Cessaro Zeta and the Pnoe are my only three exceptions to this personal subjective experience. For example, I have yet to hear any Avantgarde or Acapella speaker system which I did not find a bit fatiguing.

I continue to feel that, for me, there is something slightly more transparent and in-the-room real about the way planar speakers reproduce vocals than any other speaker topology I have heard.

I think big cone speakers systems with a lot of driver surface area in the midrange and from the midrange on down can reproduce piano and big classical symphony orchestra at least as well as big horn speaker systems.

4. How much money do you have to spend on an excellent SET/horn system? How much money do you have to spend on a good SET/horn system?

Small horn speakers don't do the trick for me. I feel the same way about planars.

Unfortunately, with contemporary designs the horn loudspeaker system has to be big (and this inevitably means expensive) to achieve the sonic attributes I am attributing to them. (I have never heard from a 30" tall, two-way JBL or similar what I have heard from big horn systems.)

5. What about the question of the oft-mentioned horn colorations? Are they still a concern in modern horns? Are they a concern in vintage horns?

I don't know what you mean here by "colorations." (Is ruler-flat frequency response which sounds unnatural and unrealistic a "coloration"?)

I think some people refer to horn "colorations" as the horn "shouty-ness" phenomenon or the "cupped hands" phenomenon. I don't hear either of these on the big horn loaded speakers I have heard.

I will define "coloration" here as frequency anomalies. Other than the Shindo system Keith took me to hear in Hollywood, which sounded to me like bourbon dripping down a warm brownie, I do not think horn-loaded speakers have any ubiquitous infirmity regarding colorations.

 

[morricab]

Brad, I don't actually consider what I own horns. The bass is just normal ported bass and the highs come through a wave guide.

The JBL waveguide doesnt provide any gain for the driver? I agree it is not reallya horn speaker although some would say it is because the horn covers a substantial range.

 

[morricab]

I have been blessed with these large, hi efficiency midrange ribbons that love simple 45 amp (active crossover, range from 300Hz to 7Khz). I was listening to them today with globe 183 DHT (an uncommon 45 with a 5v cathode) driving the 45 tube amp. Talk about being writ large with a fine brush. The 45 already has one of the loveliest upper midrange extant, but 'stacking' the DHT makes exotic seem like an understatement.

Still, I think every committed audiophile should try to hear a 45 type driving large efficient horns. It's a bit of a revelation and a mind bender.

imagine that driver at 96db (with Nd magnets) and maybe a waveguide...horn loaded ribbons or AMT drivers can be awesome.

 

[acg]

I'd be more concerned about the coloration of the SET than the horns.

I'm new here, but I think you have pretty much nailed it...at least on the face of things. I use SET amps and horns.

Most SET systems I have heard have sounded compromised in at least one or more ways. Being driven too hard, unsuitable loudspeaker impedance loads, feedback from the bass driver affecting the other channels to name a few. The Single Ended Triode is near the simplest and oldest electrical amplifying circuit we humans have devised and its use comes with some serious caveats which most users tend to either be unaware of or ignore.

For any given SET amp the following things must be overcome:
(a) Output impedance is generally, but not always high. This means that the frequency response follows the speaker impedance plot. Unless the speaker is designed to be agnostic (flat impedance curve) or specifically designed to be used with the output impedance in question, the frequency response is going to be problematic.
(b) Distortion increases with power output. Run them light and they are fantastic. Run them hard and you will hear it.
(c) Electromotive force from bass and larger drivers can feed back via the output transformer and alter the frequency response in other loudspeaker channels, eg. tweeters or mids.
(d) There is not much power on tap to start with so your speaker will need to be big.
(e) Output transformers for bass need huge cores, thick wire and as much inductance as you can jam in there. Output transformers for mids and highs need much smaller cores, thinner wire, less inductance for optimum sound. The two never shall meet in a single output transformer which is why the great OPTs are only really good, and the good ones and not so good...if you want great sound.


All of these caveats can be beaten.

Anyway, one advantage of SETs is that as power is decreased, distortion falls off to unmeasurable. This is all about that 'first watt' which has to be clean. SETs are pretty good about this. By comparison, large push-pull amps are not, since below a certain power level (usually around 5% or so), distortion starts to go back up. This is why SET users often talk about that 'magical inner detail'.

Yes. If you have 100dB/w/m sensitivity loudspeakers the smallest SET I would recommend is 15w. Absolute minimum so that the amp retains plenty of headroom. My personal opinion is that any loudspeaker less than 100dB/w/m sensitivity is unsuitable for SET amps. If the loudspeaker does not have a benign impedance load (i.e. is a normal commercial loudspeaker, not one design to be agnostic toward higher output impedance amplifiers) then the SET amp needs to have low output impedance. This can be done, but it limits the power tubes that can be used. Low plate resistance tubes must be found and used here. My zero feedback SET amp for my bass channel has 0.19ohm output impedance, which is pretty decent.

If you want great sound from flea-powered SET amps then you are going to need much more sensitive speakers than 100dB/w/m...think 110dB/w/m. Horns. Big horns.

So IMO/IME there is no argument for SETs, other than someone simply not having heard something that is both sonically and measurably better.

In most situations I agree with you. However, if you have a loudspeaker that is not adversely impacted by the shortfalls of the SET topology, then I will guarantee that there is nothing sonically better. I'll explain below.

Some argue that SETs are very dynamic and it is often true that they sound that way. But in reality, what is happening is the amp is being driven hard enough that the higher ordered harmonics are showing up as the distortion of the amp is increased (SETs are often 10% THD at full power), and where the power is needed is often transients. Since the ear uses these harmonics to sense sound pressure, and since they are showing up on transients, presto- you have an amp that sounds 'dynamic' but its really distortion masquerading as such. Simply by you're reading this, I may have ruined it for you because of how our brains process music. To avoid this phenom, the speaker should be efficient enough that this does not occur: hence horns.

A mark of any good system is the quality where it does not sound loud even when it is. That takes clean power which SETs cannot provide except at low power. This is often why SET owners will tell you that '90dB is plenty loud enough for me'. If the higher ordered harmonics were not present, it would be natural to turn the volume up higher.

SET's are ridiculously dynamic when paired with the correct speaker, but it is not the type of flawed dynamics that you correctly describe when overdriven, they are no-compromise dynamics.

A mark of any good system is the quality where it does not sound loud even when it is. That takes clean power which SETs cannot provide except at low power. This is often why SET owners will tell you that '90dB is plenty loud enough for me'. If the higher ordered harmonics were not present, it would be natural to turn the volume up higher.

This can be done with SET, but it takes a big effort. My listening is usually at concert levels, 105dB peaks, and it does not sound loud or even with a hint of discomfort in the ears. The opposite actually happens to what you desrribe...the louder my system gets the softer it sounds. I attribute this to in-room response flat to 18dB and loads of headroom in the amplifiers. The Fletcher Munson curves of equal loudness show that as SPL increases we able able to hear a higher proportion of the bass frequencies. If the speaker output is clean and capable of producing these big SPLs at low distortion then the sound should get softer as volume increases, but most playback systems I have heard are unable to achieve this.


So, how do you side-step the topological flaws of SET amps?
(a) Match amplifier output impedance to the speaker load.
(b) Run the amp at a low percentage of its potential power output. This means very high sensitivity loadspeakers.
(c) Multi-amping. There are a number of ways this is possible: SET for mids/highs and SS for bass drivers; SET for mids/highs and another SET suitable for bass drivers.
(d) Great big speakers that need a fraction of watt to be really loud. Minimum 100dB/w/m for high powered SETS, 110dB/w/m or better for low powered SETs.
(e) Multi-amping so you can use the best output transformer (and amplifier circuit) for the job. Big core, high inductance OPT for bass duties. Smaller core, lower inductance OPT for mids/highs.



By my judgement, a "good" SET system uses at least two amplifiers (bass can perhaps be SS) to separate the influence of the bass drivers from the mids/highs and to optimise the OPT's, has a high sensitivity loudspeaker preferably with a benign/flat impedance curve.

A great SET system goes further. One SET amp for each driver with transducers directly driven (no passive crossover between amp and driver). Each amplifier circuit is optimised for each driver in terms of output power and output transformer (among other things). Higher frequencies need much less power than lower frequencies so requirements change with frequency.

Do smaller output SET amps sound better? Usually, within limits and the other compromises of the playback system. My system is dual mono, six channels per side, mixed two stage and single stage SET amps with output varying from 17w to perhaps 1w, output impedance from 0.19r for the bass channels to much higher for the highs, direct coupled, passive line level crossovers at the input to the amplifiers, horns circa 110dB/w/m with bass channel 100dB/w/m. The speakers are 2.4m tall.

To make a "great" SET based system that side-steps the amplifier shortfalls is a lot of work...but it is worth it.