Is there actually a difference?

When I asked about the viability of adapting the F5 to power JFETs he pointed me to an example he'd built using the Sony devices

Sure- but that's not the same as saying they are the same. Google is your friend.

SITs are not in production- power JFETs are.

Just for those unaware, SITs and VFETs are just power JFETs.

SIT stands for Static Induction Transistor. VFETs are SIT devices. Power JFETs are a different beast:https://en.wikipedia.org/wiki/JFETThere's a lot less info on SITs:https://en.wikipedia.org/wiki/Static_induction_transistor
But there's a pretty good thread about Nelson Pass's rework of the Sony VFET circuit in DIYAudio.com. The kit that he came up with was excellent- all sold out now... It is one of the best solid state amps I've heard.

Why doesn’t someone else use this approach??

No-one makes SITs. They can be a bit fragile too!

I'm not sure input capacitance is an insurmountable issue.

I'm not one of those who automatically cringes at feedback.

I'm not either, but one thing should be abundantly clear at this point- even though tubes have their weaknesses, they are a lot easier to use to build a circuit that makes less of the distortions that the ear finds particularly offensive. Funny that no-one has stepped up to the plate to fix what's wrong with transistors!

If the industry had a weighting system on the harmonics produced by an amplifier, we'd probably see some change. We've known since the 1930s how much more sensitive the ear is to higher ordered harmonics than it is to lower orders, but little has been done about that in terms of circuit design- we've actually made it worse! That is why we are having this conversation right now: in a nutshell, the industry doesn't **want** to do anything about it, because it *costs too much money*.  So it puts its head in the sand, hoping somehow our ears will change despite millions of years of development :)  For example, if there was a range of SIT devices; signal, driver and various output devices, tubes would be gone. All done- history. But that isn't going to happen, so in all likelihood, if you have grandchildren that care about audio, they will be having this conversation decades from now.

Yes I can.

The short explanation is that tubes generally are more linear. Semiconductors have a significant non-linear input capacitance which as any solid state designer knows can be tricky to deal with (in fact radios are tuned by the use of varactor diodes, which take advantage of this capacitance), and often requires the application of loop negative feedback to correct the non-linearities that result.
In a nutshell, the more linear the circuit, the better it is at low level detail. It is true that

The same phenomenon that give rise to various orders of harmonic distortion exist with both tubes and transistors.

But it is also true that tubes have less of that phenomena than transistors, which is why the odd ordered harmonics to which you refer are at a much lower level in all tube circuits. That is the basis of the tubes/transistor debate, since that lack of higher ordered harmonics is why tubes sound smoother. It really is too bad that Sony sat on the SITs, if you will pardon the expression :)  They really did offer the solid state replacement for a tube!

Usually tube has good presence but loss of fine detail. Solid state sounds more tight but very detailed.

I find that tubes generally have more detail than solid state, in particular when it comes to low level detail. Its the low level detail that is the argument for tubes. Solid state by contrast is brighter (due to higher ordered harmonic distortion), and often people mistake that for detail.

Indeed. And you and I both know why. Well matched parts. When we talk about measurements here I'd like to think we're talking about more than the most basic facts on the side of the box. To really understand what a distortion figure means it needs to be related to an FFT. I don't think .005% is very impressive or desirable if it's 5th order.

+1

Yes- you aren't going to get much in the way of parts matching when the amp is built to a 3-figure price target.

The ability to drive difficult loads. How robust is the power supply? etc. You can make an amp that sounds acceptable at low levels driving easy loads. But, when you really need power (not volume), it is like trying to get large flows of water through a small diameter pipe. not going to happen. That is why power supply design to date is still extremely important.

The ability to drive difficult loads doesn't seem to be helpful to any amplifier, because its easy to see on the spec sheet and by listening to the amp on a hard load that its making more distortion. The problem is, such distortion is audible as brightness, harshness and a loss of detail. You can take any amplifier and just by putting it on a higher impedance/easier-to-drive load, it will immediately sound smoother with more detail. That is true whether its tube, transistor or class D. You can both hear it and see it in the specs- this is one of the few areas where the ear and the specs agree.
I do agree though that robust power supply design is one of the things that set high end amps apart from the mid fi.

Looking at basic specs like damping factor and THD+N doesn't tell you much of anything about an amp. I'd rather have an amp with .01% THD+N with a beautiful negative 2nd harmonic than one that's .001% and all high odd order distortion.

The problem is that spec sheets generally don't tell you information like this- so it can easily appear that two amps are measuring the same when in fact they are not. I regard that as the bigger problem- the spec sheets are an attempt to make the gear look better on paper than it really is- a good example of the Emperor's New Clothes.

Actually an amplifier that exhibits a 3rd harmonic as its primary distortion component will have overall inherently lower open loop (no feedback) distortion than one that makes a 2nd.

^^The specs on the amp appear to be misleading. If it were really capable of 150Amps at any time, at the very least it would be making 22,000 watts or more!
Constant current mode involves using current feedback rather that voltage feedback, and will cause the output impedance of the amplifier to much higher than most loudspeaker loads.

“Longtime readers of The Audio Critic know the drill that comes at this point: I repeat, for the nth time, that all amplifiers having high input impedance, low output impedance, flat frequency response, low distortion, and low noise floor sound exactly the same when operated at matched levels and not clipped. (Those who are unable to stomach this simple truth, proved over and over again in double-blind listening tests, should stick with Stereophile.)”

--

Tube amplifiers with their typical output impedance of >1 ohm will act as tone controls as they interact with the typical varying impedance of most speakers. Ohm’s Law applies here! Go look it up!

These two statements purvey the idea that its all about frequency response and low distortion. This promotes a falsehood, since frequency response is not the most important issue (meaning that low output impedance isn’t either).


Its all about distortion.
Less expensive amplifiers tend to be designed to look good on paper- i.e to have ’good specs’. More expensive amplifiers are often designed by engineers that understand that the audio industry hasn’t progressed much in the last 50 years as far as spec sheets go, and so look to advances in our understanding of human physiology that have been made in the last 50 years instead.


For example, the human ear/brain system uses higher ordered harmonics in order to sense sound pressure. This is easy to prove with very simple test equipment and this fact has been known for decades, but the spec sheets and what we think of as ’good specs’ still ignore this simple fact!
But many high end designers, including Nelson Pass and John Curl (both giants in the high end designer pantheon) understand that designing the equipment to not make certain types of distortion to which the ear is keenly sensitive will result in a better sounding amplifier, even if other types of distortion (to which the ear isn’t very sensitive at all) are present in higher quantities. It is this fact that is why tubes are still commonplace in high end audio.

A further fact about how the ear/brain system perceives sound is that all forms of distortion are converted into some form of tonality. The 2nd harmonic contributes to ’warmth’; the 7th harmonic (in much lower quantities!!) contributes to a harsh metallic quality. Both are colorations, and the ear has tipping points where these colorations predominate over frequency response; IOW you can have flat frequency response yet the system won’t sound flat at all due to the kind of distortion the amp makes.

The above quoted statements don’t take this into account.


What is needed is a weighting system, so that the distortions that the ear cares about are given more weight than those it does not. Spec sheets would look a lot different if that were the case!
Anyway, a well-designed amp that sounds good to the ears probably won’t have "good" distortion specs as it will likely have more lower ordered harmonics, but it will **sound** better and **more neutral**, if the designer was careful in the design. You won’t see that in cheaper gear- its all designed to look good on paper.


So yes, there can be an appreciable difference between high end and mid-fi gear.