Why do we want to distortions?

Sorry have issues with postings. Very sorry.  I get page error when posting. I will try to sort it out

Thanks atmasphere for clarification about IM.

I agree with you that either very much feedback to clean up high order distortions or less than perhaps 10dB or so. There are different types of feedback and I’m referring to traditional "loop" feedback. Degeneration is a bit different. But I think error correction is much better if that is possible in the design. What is your opinion about that?

A very interesting thread about distortion. It's really a subject that is hard to understand as there are so many aspects also related to our hearing and perception of sound.

To me it seems like if we in a playback amplifier add some low order harmonic distortion with a distribution similar to our aural harmonic distortion, this may be an interesting way forward. In a paper from Cheever, 1989 there is an interesting discussion about aural distortion and high order distortion. Both Cheever, Pass and several others agree on that to much negative feedback create high order harmonics that we are very sensitive to. Cheever also discuss aural masking and the effect by adding such distortion to the playback system. But does this masking effect also apply to to some extent to intermodulation distortion? I have not a clue.

Is it so that we should focus on how much an amplifier deviates from our hearing aural harmonics? I had the opportunity to borrow a Benchmark AHB2 for a weekend. To listen to it and to measure its performance using my audio measurement system. From a pure "standard" measurement point of view its really hard to beat its performance. Super low distortion for example. But to me it was not playing musical. Transparent and with high resolution of course. Not engaging but very neutral. This amplifier is a technical masterpiece. Using forward error correction that from a measurement point of view more or less eliminate distortion. At least to very, very low levels. By the way, error correction, is to my knowledge a superior method to manage different kind of distortions compared to for example global negative feedback. But that's another story.

To me, there is more going on than masking effects by adding a falling level of low order harmonics. Maybe its about the improved dynamics?

Here is my idea... Instruments may be more clearly defined in all of its natural harmonic spectra by adding well defined low order harmonics. We get the masking effect and we also "enhance" the natural sound of voice and instruments if we add some harmonics that are inline with our hearing harmonics. We are very sensitive to unnatural and high order harmonic distortion and this would just make use of the way our hearing and perception of sound works. As long as we stay close to the aural harmonics we just get a more musical presentation. Without changing the timbre (I think). I think this might have a good effect on the holographic image as well. Hope these ideas may bring something to the subject.

Interesting. A few years ago I made a prototype mosfet amp with adjustable output impedance (or damping factor if you will). By using adjustable current feedback. Damping factor of 20 was preferred with my speakers. I tested with DF 30 down to negative output impedance. Just to see how the sound was affected by this. But it comes down to what the speaker was design together with. I also did a spice simulation model of my speakers to be able to simulate the behaviour with respect to source impedance. Used this for some simulations of a current amplifier together with the speaker simulation model. However, my experiments using the MOSFET (class A voltage gain and push-pull output stage and some global negative feedback) did not show a strong correlation with tube sound. There where some to changes in the base as the base driver gets a bit more freedom. But this was not a scientific analysis, and it was only tested on my setup. I could not increase the DF over 30 in the design.

A very interesting topic. I recently read a paper from Cheever (1989) that's quite interesting. Our aural distortion seem to play a role here. Both from a masking perspective, but also from a musicality and holographic soundstage perspective.

I got the opportunity to borrow a Benchmark AHB2 over a weekend some weeks ago. For listening and measurements. I have an audio measurement system that I use for development so it would be interesting to measure some on the very high performance AHB2 amp. And indeed, it has a very very low distortion. Its a real masterpiece in that sense. And by using forward error correction instead of traditional negative feedback they have reached some incredibly good numbers regarding "traditional" amplifier measurements. That's one part of the coin. The other task was to listen to it. And to me it was not musical or engaging to listen to. But of course transparent, good dynamics and with high resolution.

Papers I have read, own experiments with harmonic distortion and discussions have got me into an idea around distortion. If we add low order harmonics inline with our hearing harmonics we get a masking effect but we also, for us as humans, a natural way to increase harmonics from instruments and voices. Without affecting timbre (I think). This could maybe also explain improved holographic imaging. A least to how I think.

A very interesting topic. I recently read a paper from Cheever (1989) that's quite interesting. Our aural distortion seem to play a role here. Both from a masking perspective, but also from a musicality and holographic soundstage perspective.

I got the opportunity to borrow a Benchmark AHB2 over a weekend some weeks ago. For listening and measurements. I have an audio measurement system that I use for development so it would be interesting to measure some on the very high performance AHB2 amp. And indeed, it has a very very low distortion. Its a real masterpiece in that sense. And by using forward error correction instead of traditional negative feedback they have reached some incredibly good numbers regarding "traditional" amplifier measurements. That's one part of the coin. The other task was to listen to it. And to me it was not musical or engaging to listen to. But of course transparent, good dynamics and with high resolution.

Papers I have read, own experiments with harmonic distortion and discussions have got me into an idea around distortion. If we add low order harmonics inline with our hearing harmonics we get a masking effect but we also, for us as humans, a natural way to increase harmonics from instruments and voices. Without affecting timbre (I think). This could maybe also explain improved holographic imaging. A least to how I think.

Some months ago I listened on wonderful tube amp sound from Engstrom Arne (Swedish highend) as maybe one of my most memorable listening experiences with tube amps. The new Marten Parker speakers where used. But I still think you are right. Tube amps in general should work much better with speakers at my age... or older as speaker impedance can be very low these days.

I have owned a hypex based class D amplifier from Nord Acoustics in UK. I think Nord’s amplifiers gives very good performance for the money. They now have versions using the newer Purify modules. You can specify OP-amp for the input stage. An example is the Nord Three SE 1ET400A MKII Dual Mono Stereo Amp for £1649 including Sparkos OP-amps.

"Nord Acoustics based in Cheltenham UK, design and hand-build world-class Class D Amplifiers that are changing Audiophile’s perceptions about Class D Amps. We take the very best designs from around the world and integrate them into our finished products."

Thanks @bruce19 for opening the door to the acoustic aspects of encountering pleasure in music. I'm not an expert in this subject but I can at least give my view. I see the room is an extension of the instrument and one reason is room reflections.

We don't like listening to music in sound-wise dead rooms. We usually say, and I agree on that its much harder, if even possible, to have a good musical experience in such room. The reflections in a room increase the time a harmonic or any tone live - reverbing. Short pulses of tones of <100ms are less audible for us than longer pulses. Room reflections will thus have an effect on how loud we hear frequencies with short duration and maybe this is one part of musicality. I have good experiences by using sound diffusers to make the decay times a bit smoother over the frequency range of room reflections. 

Another interesting part of our hearing ability is localization of sound sources. This very developed ability may be a part of our evolution as both hunting and dangerous animals require very precise localization - sometimes in darkness or in high grass for example.

Our sound localization ability is made of a mix of detecting sound level differences between our ears together with the timing difference when the sound hit our ears. I did an experiment on myself using headphones that showed that it was possible to detect a time difference between the ears of as little as ~6-8 microseconds. In a stereophonic playback system, maybe this can be relevant when thinking about the holographic image?