Tubes vs. solid state.

Alright- here are some basics:

Given an amplifier with a propagation delay (IOW, any amplifier), and a constantly changing waveform (such as a musical signal), the application of loop feedback (negative feedback) creates a 'non-linear dynamic system' which will have chaotic response including signal bifurcation (we audiophiles call bifurcation 'distortion').

In this case, the bifurcation will generate a large amount of harmonics- up to and past the 85th harmonic. There will also be inharmonic distortion- bifurcation that is not exact doubling or tripling due to interaction with intermodulations occurring at the feedback node(s).

The resulting harmonics will be low amplitude and we see this all the time in amplifier specs. The chaotic response cannot be avoided if feedback is applied, and forms an artificial noise floor quite unlike the normal noise floor seen in real life and also in audio circuits that do not employ feedback. The fact that the spectra of noise in a room or a zero feedback audio circuit is nearly identical should not be a surprise as the chaotic behavior of random elements (tubes, transistors, resistors) follow the same rules as is found in a room.

Our ears have the ability to hear about 20 db into natural noise floors, giving us the ability to extract spatial information even if the wind is blowing. The artificial noise floor generated by feedback (chaotic response) cannot be penetrated by our ears in the same manner so any spatial information below that threshold is lost.

This is why amplifier circuits that employ feedback seem to contain less soundstage information. It is also why they seem to sound brighter.

So Chaos Theory is telling us that loop negative feedback cannot be effectively used to eliminate distortion! Other means must be used. To create linearity, we must use the most linear forms of amplification known, otherwise signal bifurcation will be the result.

Now many of you might well be saying 'isn't negative feedack a stabilizing factor in amplifier design?'. While in audio this has been accepted as fact, Chaos Theory teaches something a little different: that the non-linear dynamic system has a stable operating zone (this is the one where we apply a sine wave to the input). In order for the system to have a true stable condition, the feedback has to be **positive** and so will result in the amplifier going into oscillation- when the amplifier oscillates, its condition is now stable; it will not change until the system is shut down. Of course we have no use for that in audio amplification :)

This is the tip of the iceberg, but in this lesson we have seen that Chaos Theory predicts what Norm Crowhurst wrote about in the 1950s- that feedback injects noise into the amplifier. The energy of the distortion is not substantially reduced by feedback- it is chopped up (bifurcated) and spread out over a wide spectrum with a lot of the energy in the ultrasonic range.

Fascinating stuff huh??

It sounds pretty fascinating. Can you reference any papers available online?

Of course, as you well know, the math only takes you so far because our knowledge of the ear/brain system is so limited. For example, who would have thought that upper-order harmonics are so much more objectionable than lower-order? I suppose you could argue that it makes sense that the harmonics closer the fundamental are going to be less objectionable, but I don't believe there's any theory that would have predicted the extent to which the brain objects to high-order HD. Same with stuff like timing of reflections, etc. It's just the way the brain (and to some extent the ear) works. Right?

A little more fuel on the fire......

"A few music listeners still have the illusion that...tubed units are in some mysterious way better than present solid-state units. The overall superior performance of solid-state design has been scientifically proven...Anyone investing in a contemporary vacuum tube product will experience less than the best available performance."
source: McIntosh press release/TAS

"Had I performed a blind A/B listening test, I would have picked the tube amp as being solid-state, and the solid-state unit as being tubed."
source: Sam Tellig/Stereophile

"When it comes to tubes, you will only find solid-state in my personal system....and I manufacture some very good tube gear!"
source: High end audio manufacturer in Minnesota(no names, please)

Paulfolbrecht, yes, General Electric proved that humans use the 5th 7th and 9th harmonics as a means to determine the volume of a sound back in the mid-60s.

So how this relates to the TvsSS debate: The issue centers around feedback- by adding feedback to a tube amp you can make it sound 'solid state' on account of the chaotic harmonic noise floor. I believe the sound of 'solid state' is not so much that of transistors, rather that of a transistor amplifier that has a lot of feedback. Nelson Pass is a good example of someone making transistor amps that don't sound 'solid state'. Many of his designs use no feedback.

For decades, triodes have been known as the most linear form of amplification (at least as far as the specs of triodes appear on paper). The trick it to use the triodes in a way that they will not make distortion **without** also using feedback. IMO/IME this is the primary advantage of tubes- that you can do such a thing in a way that to me seems easier than with transistors.

People such as Nelson Pass are eroding that advantage; I think ultimately though that too few designers are trying to figure out how to crack the nut without feedback. We now know from Norman Crowhurst (55 years ago), General Electric (45 years ago) and the proofs of Chaos Theory (mid 80s to present) that feedback simply does not work- and won't until an amplifier without a propagation delay is somehow devised.

Olesonmd's examples bear this out- the amplifiers used in his examples all use feedback and so have more errors in common with each other and less in common with real music, regardless of being tube or transistor.

Anyone who claims that the "overall superior performance" of solid-state has been "scientifically proven" has no understanding of the real issues.

I guess it was proven in the 70s when we had those wonderful solid-state amplifiers with their amazing THD measurements.