Tubes vs Solid State - Imaging, Soundstaging, 3D
Musical Interlude courtesy of the living legend, Neil Diamond. |
Al and Learsfool wanted to know more about the low level detail thing. Here it is. This has to do with the rules of human hearing/perception. Apparently we have the ability to hear into a noise floor and perceive detail which is interesting as normally the human ear has a masking rule wherein louder sounds mask quieter sounds. Apparently hiss is some sort of exception. I personally suspect that this is because wind sounds very much like hiss, and it was important to us as a species (for survival reasons) to be able to hear sounds that are not as loud as the wind. Anyway, hiss seems to be the exception to the masking rule. However not all amplifiers have a noise floor composed of hiss, although this is something that does not show up well on our test instruments. This has to do with the fact that if you apply negative feedback in large quantities to an amplifier, the noise floor becomes that of harmonic and inharmonic low level distortions. The inharmonic distortions (InterModulations) are caused by non-linearlities at the feedback node. The harmonics generated by this practice can go up to the 81st harmonic! (see Norm Crowhurst, who wrote about this phenomena a good 50 years ago). You might look at it another way- that by adding feedback to an amplifier, you don't rid yourself of the energy of the distortion- instead it gets spread out across the spectrum. Of course with many amplifiers those upper harmonics will fall outside the amplifier's passband, but the point here is that the noise floor is not that of hiss. It is that of distortion. The ear can penetrate a noise floor of hiss but if the noise floor is composed of distortion in this manner, the ear will find that to be the lowest level of output from the amp, IOW the ear cannot penetrate that noise floor to retrieve information below it. Hence, amps that apply large amounts of feedback will seem to be less detailed. A further complication comes in when we try to amplify a signal that is in a state of constant change- that is to say does not repeat itself, as in real music. The noise floor of the amplifier is not nearly so low under these circumstances, nor is the over distortion as low as it is with a steady-state signal. Chaos Theory shows that an amplifier with negative feedback is a Chaotic system ('Chaos' being a defined term, not the same as the street meaning of the word). To that effect it may appear to be predictable under certain conditions (steady state signal) but may have other results in other conditions (music). The formula for negative feedback (Crowhurst) is identical to that that we see in classic Chaotic systems (N+1 et.al. if you care to read up on it). What this teaches us BTW is that no application of feedback will solve the problem (Nelson Pass points this out in his excellent article about feedback and distortion on his website). IBM engineers learned this long ago, which led to the invention of the parity bit in digital communications. A different field and application, but the underlying principle is the same. Do I need to elaborate more? |
So hearing hiss is better than not hearing harmonics that can't be heard? I must be missing something? I will say that I find hiss to be more tolerable than other forms of noise that I might hear, but if I don't hear anything, its a stretch to think that noise I do not hear matters more than noise I do. I think I prefer to not hear any noise, hiss or otherwise. If I have to hear low level noise, I suppose I would choose random hiss over anything not randomized. |
Also, I think negative feedback implementations have come a long way over the last 40-50 years, and though the theory may be sound, the significance with good modern implementations is not what it was. My Class D amp is dead silent, and has as good low level detail retrieval as anything I have heard. Of course, the room has to be quiet to hear it, which is a different issue that might often come into play. My tube pre-amp is also mostly quite, though some low level hiss is usually audible in the phono section. AMount varies with tube quality. Not enough to matter for me with the right tube in good working order in play there. |
