Let me end the Premp/Amplifier sound debate ...

I thought "a straight wire with gain"  is unachievable currently but would be the most desirable scenario for ultimately the best possible sound.

It isn't. An amplifier often has to behave as a voltage source, since most speakers are meant to be driven by such. A wire is incapable of this behavior, even if it had gain 😉

The fundamental error of Julian Hirsch, and most 'Alikists' is believing there is a correlation between a static measurement and a complex dynamic process. At best there is a Venn diagram-like overlap. Another poster mentioned that no component can be evaluated 'in exclusis'. With which I agree completely. Also, I encourage people to read the book 'Cork Dork' by Bianca Bosker. Besides a fun read, there is an excellent chapter on the science behind how palates are trained to pick up subtle differences simply lost on others. The parallels to audio are striking and I think valid. No one would state that two wines of equal alcohol content taste the same. 

+1

Amplifiers with insufficient feedback (which is to say: most amplifiers...) can have chaotic reactions to an actual audio signal, which can change depending on the exact volume level. This is due to bifurcations caused by non-linearities at the feedback node.

But today it's almost imposible that 2  different manufacturer designs can sound the same.

That depends on how successful a design is with regards to its application of feedback. If there is enough feedback then issues of coupling caps and power supply design are rejected by the feedback itself. So two very different designs can sound the same if they have sufficient feedback and Gain Bandwidth Product. But it needs to be a lot in both cases, and such is really rare even today (as opposed to 20 or 50 years ago).

those kind of measurements you are asking for just does not exist and in my posts I try to explain about.

Certainly they do! But as I explained earlier, usually such measurements are not published if they are even made. So we have the 'obviously' persistent myth that we can hear something we can't measure.

The question I always had with measurements, are we even measuring the correct things for human hearing parameters?  

We are, if all the measurements are actually made and often they are not.

I think I outlined what is needed to know the sound of any amplifier earlier in this thread. I find it interesting that those interested only in the specs discount the subjective aspect so vehemently. I think this arises out of not understanding that all forms of distortion are audible as tonality. That's a bit of connecting the dots. But once you understand that simple fact, the whole idea of things being 'audible and not measurable' goes away.

But you have to also understand what the measurements are saying.

But I think it is about distortion and and higher order harmonics.
@atmasphere mentioned it in the thread about his new class-D amp… maybe on another forum though?

The “quiet loudness” is something I do not hear often. Mostly because I do not hear nice systems too often.
But I like it when they are that way. Seems that “quiet loudness”  is correlated with it being nice sounding generally.

That is my experience as well. I've said this many times: the mark of a good system is it does not sound loud.

The reason a system sounds 'loud' is distortion; from poor higher ordered harmonics from the amp, poor anti vibration control in the turntable (also poor cartridge condition or setup) or CD transport, early reflections and/or slap echoes in the room and breakups in the loudspeakers.

@holmz +1 My thoughts exactly. that Stereophile nonsense caused me to suspect Mr. Carver so many years ago; the latest debacle with an amplifier that can't make anywhere near full power sealed the deal IMO.

And if you want good S.Q. buy first and to begin with some good gear....It is not so difficult because electronic audio engineering is mature technology for many DECADES... 70 years ? or 60 ? or 50 ? We must ask atmasphere for that, he knows....

Its nice to think audio is a mature technology. But its no-where near as mature as people like to think. If it was, tubes would not still be around; solid state circuits would have replaced them and no looking back (as happens in any field where the new tech replaces the prior art). The problem has been that the semiconductors needed to really supplant tubes (meaning: to make a solid state amp that isn't harsh) didn't exist in the 1970s or 1980s. We had a proper understanding of control theory in that time, but oddly, didn't apply it to audio (probably because if that was actually a goal, no power amplifiers would have been produced 😁). So feedback networks for the most part have been poorly designed and we have several decades worth of solid state amps that come off harsh and bright, especially when you turn it up. IMO this is mostly because the gear was made to make money so the companies making it didn't care that it fell well short of the goal of sounding like real music. Sorry to sound curmudgeonly...

This has been what has kept tube amplifiers in business the last 70 years since they do offer a way around this issue (they make enough lower ordered harmonics to mask the harshness of the higher orders they also make).

But in more recent times semiconductors have advanced to the point where you can get rid of that pesky brightness/harshness for which solid state is known. IMO we've only just arrived near the top of the R&D sigmoid curve in audio in the last ten years or so.

It's not that the distortion signature is too small, it's that no one measures it. No SPL meter is going to be better than ±0.1db @ 85dB. It likely has a nonlinear response. In absolute terms, it's not even close.

It gets measured alright. But I think I see what the confusion is. An SPL meter is only useful if the speaker is making sound, whereas we've been talking about measured output power. The two are not the same, owing to tube amps having a slightly higher output impedance in most cases- that will cause minor FR errors. My prior statement in this regard is correct:

at any given frequency, for a given amount of power into a given load (8 Ohms for example) the voltage and current will be the same on account on the power formula (1 Watt= 1 Amp times 1 Volt).

Back in the 80's Bob Carver tweaked an amp to sound exactly like another: The Carver Challenge | Stereophile.com

I remember that. He failed too; a 35dB match was all he could get.

FWIW, we gave up on matched level listening tests in the 70's simply because it's about as worthless reading the spec sheet.

If one amp was making more higher ordered harmonics, it likely would have sounded louder. An SPL meter gets to the bottom of that pretty quickly.

Don't conflate the spec sheets with the measurements. Spec sheets are IMO the Emperor's New Clothes. Measurements are not.

The question was about current and voltage. The current and voltage cannot be the same if the harmonics are temporally shifted relative to the fundamental.

Since almost agree that tubes sound different than solid state, then what makes up the ’voltage’ must vary.

If you put a solid state amp and a tube amp on the bench, both might measure quite flat- the H/K Citation 2 is a good example. One amp might sound bright while the other does not. But if you measure the output at the frequencies where the bright amp sounds that way, you'll see that the power can be the same as the amp that is not bright (likely the tube amp).

The difference in sound is higher ordered harmonics which tend to be audible with most solid state amps (audiophiles have been hearing this problem for the last 60 years, which has kept the tube industry alive), and the ear is both keenly sensitive to these harmonics (it uses them to sense sound pressure) and assigns a tonality (as it does for all forms of distortion) of 'bright and harsh'.

Yet at any given frequency, for a given amount of power into a given load (8 Ohms for example) the voltage and current will be the same on account on the power formula (1 Watt= 1 Amp times 1 Volt). This is a simple fact that cannot be undone. The difference you hear isn't on account of power! Its on account of the distortion signature, which is simply too little energy to show up when measuring voltage and current.

The big reveal here is that distortion is always audible! The ear has over a 120dB range and since it uses higher ordered harmonics to know how loud a sound is, its sensitivity to those harmonics is paramount, but poorly understood by audiophiles and engineers alike. Yet that is the main difference you hear between tube and solid state.

This series of frequencies into an 8Ω load resistor gives an idea of the Phase Shift in a tube amplifier output transformer @ 10W RMS. The ear is extremely sensitive to phase, but hardly anyone mentions it.

The ear is only sensitive to phase when it covers a spectrum of frequencies. It cannot tell phase of a single frequency.

FWIW our OTLs have 0 degrees of phase shift at 20Hz to 20KHz. Phase shift at 200KHz isn't a concern. So a tube amp can do much better too 😉

Your answer does not appear to address the question posed.

FWIW, I've never measured a tube amp output voltage into a loudspeaker that was anywhere close to what an SS amp delivered into the same load... going back to the 70's. Admittedly, it's a small sample, and largely irrelevant as measurements mostly don't mean diddly. 😎

Its not that the measurements don't mean anything, its that the measurements have to be performed correctly, and the important measurements have to be made (the latter rarely happens, which has lead to the myth that we can hear things we can't measure...).

Maybe I am simple fellow… but how can the sound (SPL) be the same, if the current (and voltage) are not the same?

 

Its a good question. If a given amp is making 10 watts into a certain speaker at a certain frequency and has an output impedance of 0.01 Ohms, and another amp can make the same power at the same frequency and into the same load, while having an output impedance of 1 Ohm, since both amps are making the same power, the voltage and current will be the same.

The output impedance can affect FR and distortion but it won't affect output power since the example has that aspect being the same. If it really is the same the current and voltage has to be the same too.

To date, I've yet to see a tube amp measurement that does not look similar to the Black Line into the Stereophile simulated speaker load. Many speakers' impedance curves make the Simulated load look like a resistor.

Have you looked? As long as the speaker impedance does not go well below 4 Ohms, 15dB should be sufficient feedback to allow most tube amps to act as a voltage source within limits.

If the 15 dB of negative feedback was meant for discussion of the Melton MKT-88 tube amp

@toddalin I did not have the Melton in mind.

If a tube amplifier is able to act as a voltage source, how it does it is by cutting its power in half as impedance is doubled and not the other way 'round. This means that to get proper voltage response with some speakers and some speaker loads (aka Stereophile) you might have to use the 4 Ohm tap. You can see that this is one of the ways that tube power is more expensive than solid state power.

15dB is enough feedback that the output impedance of most transformer coupled amplifiers will be less than 0.1Ohm. Since most speakers do not need a damping factor of more than 10:1 (some need 20:1) you can see that this will work out just fine.

The problem IMO is that 15dB isn't enough feedback- it will add distortion of its own (mostly higher ordered, causing brightness and harshness) as a result. This is why feedback has a bad rap. But if you can get over the hump with enough feedback, then the circuit will be able to clean up those higher orders that feedback otherwise produces.

Speaking from my personal experience, I believe the measurements we use as standard do not tell the whole story. Not at all.

 The only reason this happens is that all the measurements are either not published or simply were never made. One of the more important measurements you rarely see, but has an enormous impact on what you hear is distortion vs frequency. Many amplifiers have rising distortion with frequency- always a bad sign.

Another measurement is that of the distortion spectra. That can show the difference between harsh and smooth.

Its usually not enough for the distortion to be low- we've all heard amps that 'measure well' but don't sound good; the distortion spectra isn't good on an amplifier like that, and very likely the distortion was only measured at 1KHz which tells you almost nothing.

Because these measurements are rarely made,  the idea has arisen that we can hear things we can't measure- its false, we simply don't see the measurements we need to see!

Voltage from a tube amplifier OPT will vary considerably depending on the speaker impedance at the test frequency.

@ieales This statement is incorrect. This amp uses about 15 dB of feedback, allowing it to behave as a voltage source with most speakers. The variance at the output transformer is thus minimized. If it was zero feedback then it would behave as you describe.

Most everything has a signature sound characteristic. Every resistor and capacitor has their own sound.  That also goes for transformers and chokes.  Even the chassis material has a sound. 

While this is correct, it is incorrect to say that in all cases the 'sound' of the various parts actually influences the output. If the amplifier has enough Gain Bandwidth Product to support a large amount of feedback (+35dB) over the entire audio band then the various parts 'sound' will go away. Most amplifier designs cannot meet this criteria and so the parts influence the sound as you describe.

He said 'an amplifier is a straight wire with gain', defining his belief that 'all well designed amplifiers sound the same'.

@clearthinker When he said that, apparently no 'well designed amplifiers' existed- they all sounded different! On top of that, the 'straight wire with gain' thing is false!

Starting in the late 1950s with Mac and EV, there has been a move to drive loudspeakers with a voltage source. This is the idea that an amplifier that can make constant voltage regardless of the load impedance. This was done to improve 'plug and play' since prior to that speakers had level controls to adjust the speaker to meet the amplifier's voltage response (those controls were not there to adapt the speaker to the room).

A straight wire (regardless of gain) cannot do this. So 'thanks' Peter Walker, for creating one of the longer standing myths in audio.

These alleged sonic difference between power amplifiers, are they musically significant?

They can be! I think the mistake that Julian Hersch and a lot of the folks over at ASR were/are making is arbitrarily deciding how low distortion has to be before its inaudible. IMO/IME it needs to be considerably lower than previously thought. I think this is because the ear is so keenly tuned to the presence of higher ordered harmonics, since it uses them to tell how loud a sound is. IME when the amp is at 6dB below full power those higher ordered harmonics have to be below -100dB in order to be masked, otherwise the amp will sound harsh at volume.

Harshness is unpleasant and IMO, not worth paying for.

Alternatively the harmonics can be masked by a lower ordered harmonics- the 2nd or 3rd. In a nutshell, the differences you hear between amps is mostly their distortion signature.