Argyro is completely right. There are tube preamps that go down to 1 Hz and can pass a 10Hz squarewave without measurable tilt. IOW, as good as any solid state. Hum has nothing to do with either tubes or transistors- that points to a setup problem, a malfunction or a cheesy design. |
Anyone here interested in what Chaos Theory has to say about amplification? |
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?? |
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. |
Minorl, sounds to me as if you are saying something that I've been harping on a lot- that the playback system has to take into account the rules of human hearing/perception and obey those rules. Designing to specs on a piece of paper doesn't do it (unless those specs take the ear into account, which, these days, they don't). |
Minorl, of course there are electric cars too, and energy storage systems that are a lot better than the internal combustion guys would have you believe.
There have been improvements in amps in the last 50 years. There are new topologies that did not exist in the 1950s or 60s. So even if the parts had not gotten better (which they have) there would still be progress.
A lot of that progress comes out of the understanding of how the human perceptual system works. IOW we are not going to make progress if we design something to look good on paper, but the paper rules fail to take in how we hear sounds. Dr Herbert Melcher (famous in the world of neurochemistry) has done some recent studies that show that the more an audio playback system violates human perceptual rules, the more the processing moves from the limbic system to the cerebral cortex. |
Pinkus, the only issue with transistors is that without feedback, they will make more distortion than tubes will. However, if the design is competent, the higher orders will not manifest nearly so much as they will if feedback is applied. Its my opinion that in time we (as designers) will sort out what must be done to make them work properly.
One of the best amplifiers I have heard is transistor, made by Ridley Audio. It was zero feedback and class A and fully differential. It employed a heater to heat the output devices to a fairly high temperature (and then regulated the heat) and made as much heat as any tube amp of the same power. The amp made 100 watts and cost about $100,000 for a pair (they could drive a 1 ohm load); a bit pricey for my blood but proof that the extra distortions of transistors can be tamed if handled right.
For the time being as I mentioned earlier, it is a lot easier to use tubes to keep the distortion down. It might be cheaper too. If one looks at the whole amp/speaker thing from the point of view of obeying human perceptual rules, the issue of higher output impedance of zero feedback amps becomes moot- that is 'what is' so you deal with it in the design. |
Guidocorona, feedback is very definately a factor in preamp design. A very famous Ampex recorder (model 351-2), used by RCA and Mercury as well as many others, featured a zero feedback record circuit.
EQ of phono preamps is often handled in the feedback loop of the preamp. The Dyna PAS-3 is a good example as is the ARC SP-3. If not, you will often see the term 'passive equalization' used by the manufacturer. Harmon Kardon used passive phono EQ in their famous Citation 1 preamp decades ago. IMO/IME passive EQ is harder to execute, but allows the phono preamp to sound better- smoother, with less ticks and pops. Refer to my prior posts as to why that is.
Chatta, there is of course an intellectual preference that each audiophile carries. Despite this conscious preference, the human brain apparently knows when it is being fooled, whether we consciously acknowledge it or not. Dr Herbert Melcher, a famed neuro-chemical scientist, has proved this recently in tests using audio playback- its fascinating stuff: as the brain detects violations of how reality actually sounds (false noise floor, slow waveform delivery, etc.), the processing of music moves from the limbic centers to the frontal lobes of the human brain. When we are comparing cables or listening to MP-3s, the processing is entirely in the frontal lobes. |
Hi Sam, IME the most transparent systems I have heard are also some of the more relaxed, but of course I have heard many systems wherein 'transparent' equates to 'analytical'.
This has to do with the types of distortion the system makes. If the higher ordered odd harmonics are not being enhanced (distorted), **and** if the system is also lacking on lower ordered distortions, then it will be both transparent and easy going. Believe it or not there really are not very many ways to do that: somehow you have to get rid of distortion without using feedback. It can be done, but those that have accomplished it are few.
I think you are right in the emotionally engaging systems also having an 'organic' quality. IMO music itself has an organic quality- the fullness of reality- and if a stereo is violating basic human perceptual rules there seems to me no way the system will be 'organic', emotional and transparent all at once. |
Tube dampers work but they have limited effect. You are best off avoiding microphonic tubes if you can. |
