solid state vs tubes

Minorl, not to put to fine a point on it but the idea of designing an amplifier to pass a signal without distortion is being heavily glossed over in your arguments, which, other than that, I find myself in agreement with.

The problem lies with the fact that our ears, regardless of the individual, use a set of common rules that govern our perceptions. Now really, that is not a problem except that the bench tests that are commonly used are for the most part not devised with these perceptual rules in mind. So as a result, a common experience is that an amplifier that measures well does not sound good, because **in the attempt to measure well, the human perceptual rules are being violated**.

So it is not true to say that the amplifier that has the 'lowest distortion' will be in fact the amplifier that actually *has* the lowest distortion; the two can be quite different! Often 'higher distortion' amplifiers (as measured on the bench) are lower distortion when subject to the reality of our ears.

In short, the bench measurements come off as an example of the Emperor's New Clothes. Our ears OTOH, are the real thing. Now it is a simple fact that tubes more closely obey these perceptual rules than transistors do, that is why it is so hard to find the transistor amps that really sound like music. However, and I point to Nelsen Pass as an excellent example, when you find such an amplifier it will be because the designer is also looking at how our ears work.

Atmasphere,

Does what you say about loudness cues, our ears and how amps are designed apply mainly in regards to how amps distort?

Loudness cues conveyed by certain harmonics is a natural occurence, correct? What if say a SS amp conveys these cues accurately as they would be if heard live? That may not be as pleasant as them not being conveyed accurately say as a result of tube amplification, but that does not make it wrong, does it? If not clipping/distorting, what if a good amp just conveys what's there accurately, and measurements support that this is what is occurring?

If an amp is not clipping and is fast enough to deliver transient peaks accurately, some might perceive that as a good thing I would think rather than relying on the amp to filter or transform the sound in a manner that makes it more pleasant or digestible. That does not occur when listening to things live. The sound and the harmonics that comprise it are delivered without benefit of amplification or filtering devices to shape it into something more digestible to our ears, for better or for worse.

Mapman- I won't speak for Ralph but I think you are missing something important here. We are not talking about an amp that is filtering or transforming anything. The issue here is that, supposedly, we perceive certain kinds of distortion (can't remember if its odd or even harmonics) as more harsh or objectionable than others. So, (to make up an example) 0.10% distortion of those harmonics sounds "worse/louder" than 1% distortion of those to which our "ears" (actually brains, I think) are less sensitive. Remember the numbers are made up and I also believe that even within the odd vs. even harmonic dichotomy, there are certain specific harmonics to which our ears are more sensitive.

I only say "supposedly" because I don't have any independent knowledge of the science nor am I an auditory physiologist or experimental psychologist. However, this explanation is consistent with what I remember from my long ago undergrad days about differential response to various kinds of stimuli. This also jibes with what I have heard w my own ears; that ss amps that measure 1X% THD do not, in many cases, sound as good as tube amps that measure 10X% THD.

If your hypothesis is that minimizing THD is an important design goal for an audio amplifier, then many would say that, based on their listening tests, your hypothesis cannot be experimentally verified. That statement does not deny that minimizing distortion is not an important design goal, just that minimizing THD (which is measured in a very specific way) cannot be shown to be an important design goal.

Another way to think of this is that it may be a case of "if your measurements don't match up your experience, maybe you're measuring the wrong thing." In this case, measuring THD may not be the right thing. I think Ralph would suggest a weighted measurement that assigns the most "weight" to those harmonics to which we are most sensitive. Call it WHD (Weighted harmonic distortion) or what have you. The relative weight assigned to each harmonic would have to be determined experimentally, in order for this concept to be applied in practice. When you think about it, THD is actually a specific type of WHD, in which the weight is "1" for each harmonic.

I'll stop now before I dig myself in too deep.

Swamp,

It seems much of the argument revolves around how amps distort, in particular how they distort when clipping occurs.

My understanding is that no amp should normally be run in a manner where clipping occurs. If it does occur, then a different and better suited amp for teh application is needed.

What happens when clipping is effectively taken out of the equation with SS? It seems to me that most issues I detect with GOOD SS amps sounding harsh or distorted can be attributed to clipping occurring or some other abnormal operating scenario that perhaps affects transient response, not a normal one.

There is a technique in image processing called contrast stretching. Essentially it means if you clip off the extreme bright and dark extents of the image or picture, you can then stretch out what remains and often show subtle contrast details that are otherwise not apparent. Seems to me soft clipping as found with many tube amps and even some SS, operates similarly. Peaks are clipped in a softer manner that is more digestible to the ear. That allows volume to increase to higher levels than otherwise. Now more subtle variations can be heard in the rest also.

Now take clipping out of the picture by using an amp/speaker combo capable of achieving this at realistic listening volumes. Peaks go louder sooner (though are not cliped or distorted necessarily) and, ouch loud things can hurt now perhaps sooner rather than later because, hey those peaks are loud as they should be. But you now may need a really good ss amp with excellent resolution and detail in order to actually hear the subtle details because, hey things are not louder overall now so subtle differences in teh sound are harder to discern, especially if you do not have good ears and or a very quiet listening environment where what is played can be clearly heard.

Am I off somewhere in describing things this way?