And another key thing to note ... the NAD system is now at an awkward state of optimisation - something I have experienced over and over again, throughout the years. The quality is now good enough, when everything is correctly organised, for the sound to have that 'magical' quality that so many people chase - but that level of "refinement" is very delicate, fragile; it's not robust in of itself. That is, a single element not in effective alignment means that the conjuring does not happen, and of course that is disappointing, and means one can feel quite negative about the whole exercise - an intense round of examining everything ensues, sometimes with great frustration when no cause pops up immediately, but usually with great relief when the problem is finally tracked down - the diagnosis is the hard bit, the solution is usually quite trivial to implement!
The longer term aim is to move the stability of the system playback quality to the point where the necessary quality "robustness" is well and truly in place, and can withstand degradation of minor aspects without causing the auditory illusion to fail. I have spent large amounts of effort with the many audio experiments over the years trying to establish better control of this, and will strongly emphasis that this is crucial - the frustration of hearing the quality fall back below the necessary standard is very wearing - in my earliest attempts, years ago, it led me to completely give up on trying to achieve high quality sound for a very long time, the disappointments wore me down ...
Sunday 27 December 2015
Friday 25 December 2015
"The essential characteristic of the original signal" ... ?
Referring to a recording, of course. I just saw this phrase commented upon in a thread on an audio forum, its significance or meaning pooh-poohed ...
However, a competent playback system does allow this to emerge, in usually glorious ways! For someone who has never experienced this occurring it might seem almost magical, and in many ways it is remarkable, because it demonstrates the ability of the combination of ear and brain to decipher, unravel a consistent picture of a sound event in a fashion which minimises disturbing or unwanted aural artifacts not relevant to the musical performance. Especially on very poor, ancient recordings, full of detritus - on a typical, ambitious hifi system these will sound absolutely appalling, completely unlistenable to; every defect in the sound will be highly magnified, and will overwhelmingly dominate the listening experience - subjectively !!
Are we listening to the "essential characteristic of the original signal" in that situation? Of course not, our hearing is so distracted by the "noise" and "distortion" that accompanies the "signal", the musical event, that there is almost no chance of relating in an enjoyable way to the latter. So, what to do about that? Some of the options are: never listen to that recording; or, filter out by brute force frequency response manipulation, in the replay chain, what seems to be the worst unpleasantness; or ... raise the standard of the replay mechanism to the point that the human hearing mechanism is fed enough relatively clean information, allowing the ear/brain to do vital, internal, in the brain, filtering, without conscious effort. Even though the last choice may seem fanciful to many, it in fact is done all the time in the "real world" - a good example is listening to a musician busker on a busy street, performing with no sound reinforcing gear, having huge amounts of unrelated racket 'interfering' with the musical sound. The "essential characteristic of the original signal" is very obvious here; some passing by may completely ignore the performance, be oblivious to it, but others will have no trouble "tuning in" to what's important to them at that moment, and enjoy the efforts of the musician.
This should be the aim of quality audio reproduction, and is always achievable. The integrity of the production of the "art" of the music is retained in recordings, and it is only a matter of raising the standard of the replay chain to the point where one's hearing is able to distinguish the "essential characteristics" without conscious effort - this is the essence of the Art of Audio Conjuring!
However, a competent playback system does allow this to emerge, in usually glorious ways! For someone who has never experienced this occurring it might seem almost magical, and in many ways it is remarkable, because it demonstrates the ability of the combination of ear and brain to decipher, unravel a consistent picture of a sound event in a fashion which minimises disturbing or unwanted aural artifacts not relevant to the musical performance. Especially on very poor, ancient recordings, full of detritus - on a typical, ambitious hifi system these will sound absolutely appalling, completely unlistenable to; every defect in the sound will be highly magnified, and will overwhelmingly dominate the listening experience - subjectively !!
Are we listening to the "essential characteristic of the original signal" in that situation? Of course not, our hearing is so distracted by the "noise" and "distortion" that accompanies the "signal", the musical event, that there is almost no chance of relating in an enjoyable way to the latter. So, what to do about that? Some of the options are: never listen to that recording; or, filter out by brute force frequency response manipulation, in the replay chain, what seems to be the worst unpleasantness; or ... raise the standard of the replay mechanism to the point that the human hearing mechanism is fed enough relatively clean information, allowing the ear/brain to do vital, internal, in the brain, filtering, without conscious effort. Even though the last choice may seem fanciful to many, it in fact is done all the time in the "real world" - a good example is listening to a musician busker on a busy street, performing with no sound reinforcing gear, having huge amounts of unrelated racket 'interfering' with the musical sound. The "essential characteristic of the original signal" is very obvious here; some passing by may completely ignore the performance, be oblivious to it, but others will have no trouble "tuning in" to what's important to them at that moment, and enjoy the efforts of the musician.
This should be the aim of quality audio reproduction, and is always achievable. The integrity of the production of the "art" of the music is retained in recordings, and it is only a matter of raising the standard of the replay chain to the point where one's hearing is able to distinguish the "essential characteristics" without conscious effort - this is the essence of the Art of Audio Conjuring!
Wednesday 23 December 2015
A More Ambitious Upgrade - Part 13
A cautionary tale ...
This one is against me ... it's happened to me many times in this audio game over the years, and us humans being the way we're made, it's so easy to stumble into making the same error, over and over again - a little bit of success encourages a step too far, too fast ... we just want a bit more of that satisfying success, to keep feeding the drip ...
Which is to say that I overdid the dressing of the internal cables I mentioned in the previous episode, in the interim. Let's just make it a bit better, why not, was the thought. Trouble was, I was disturbing the stable physical equilibrium of those cables, established over a long period of time - and the result was a deadening of the sound, I had definitely lost something! Trying a number of recordings to check ... no, I had gone backwards, by a good margin.
Luckily, a good guess and just effectively reversing one of those "improvements" made a substantial difference - I was now back in much better territory in quality terms. So, I will proceed far more carefully with further experiments in refining the cabling layout; the quality of the sound is now such that a single wrong decision in this area can be markedly audible ...
Update: Hooray! Undid a few more of those wrong moves, a bit more fine tuning of alignment of the cables ... and, I can confidently say, we have conjuring!!
An ex-library CD, Vaughan Williams, Fantasia on a Theme by Tallis, 1976, Boult and the LPO, is working its magic - low level strings are special, lovely "bloom" to the sound, deep, rich resonance from the double basses; one can just fall into the sound ... this is what one is after ...
This one is against me ... it's happened to me many times in this audio game over the years, and us humans being the way we're made, it's so easy to stumble into making the same error, over and over again - a little bit of success encourages a step too far, too fast ... we just want a bit more of that satisfying success, to keep feeding the drip ...
Which is to say that I overdid the dressing of the internal cables I mentioned in the previous episode, in the interim. Let's just make it a bit better, why not, was the thought. Trouble was, I was disturbing the stable physical equilibrium of those cables, established over a long period of time - and the result was a deadening of the sound, I had definitely lost something! Trying a number of recordings to check ... no, I had gone backwards, by a good margin.
Luckily, a good guess and just effectively reversing one of those "improvements" made a substantial difference - I was now back in much better territory in quality terms. So, I will proceed far more carefully with further experiments in refining the cabling layout; the quality of the sound is now such that a single wrong decision in this area can be markedly audible ...
Update: Hooray! Undid a few more of those wrong moves, a bit more fine tuning of alignment of the cables ... and, I can confidently say, we have conjuring!!
An ex-library CD, Vaughan Williams, Fantasia on a Theme by Tallis, 1976, Boult and the LPO, is working its magic - low level strings are special, lovely "bloom" to the sound, deep, rich resonance from the double basses; one can just fall into the sound ... this is what one is after ...
The only real problem to solve in Audio, is Attitude
Problem, what problem? Some people might claim that everything is known to made reproduction of recordings as good as one could want - and in one sense they are right. But, if that's the case, why do so many systems, even those which are expensive and lavishly indulged in in terms of time and effort, sound very little like the "real thing", or just downright weird? Of course there are some extremely competent audio systems around, but they're thin on the ground, they stand out as being 'special' when one comes across them!
So, the "problem" is that one can't assemble an audio system from standard, off the shelf components and have it "sound good" - every time. To counter that, my experience over many decades, is that just about every assembly of sound gear, no matter how inexpensive, can be made to come up to, tweaked to an acceptable standard, and with extra effort be evolved into a mechanism that produces highly satisfying audio, and, yes, even conjures ...
What is clear, something recently even more strongly confirmed, is that there is very little desire to think beyond the standard cliches of audiophile think: "it's all about the frequency response"; "loudspeakers are what cripples the sound no matter what"; "huge effort must be put into creating a perfect acoustic environment for the audio equipment"; etc ... but a little thought puts these ideas into better perspective: an actual musical instrument or voice of a person in the room or any natural, non-reinforced sound doesn't require any of these sort of shenanigans, it will still have the impact of the "real thing", every time, irrespective of how casually the sound is produced, and how non attentively one listens to it. And the latter is my experience of competent playback: when it works well there is the same impact as hearing 'natural' sounds, no, absolutely zero, excuses or allowances have to be made for the fact that it is "only a recording" ...
It's easy to see that the attitude of the majority in audio land doesn't echo this thinking at all; firstly by reading what they have to say about things, and secondly by listening to what the vast majority of systems produce, the acoustic end result. Sometimes spectacular in a circus-like manner, often disappointing, irritating, fatiguing, uninteresting to listen to - nearly always sounding like a "hifi system", very rarely they are convincing.
But there is a solution ... just get a few people thinking that there may be a smarter way, which in essence is to view the business of reproducing sound from a recording as a process which has to have minimal flaws in every aspect, every area, to get a worthwhile result. Yes, people worry now about "flaws", but they're usually the 'wrong' flaws - they are of low importance, they matter little in the quest to produce an illusion that the ear/brain combination can believe in ... it's only a shift in attitude that's actually required, to focus on the key flaws that make the crucial difference in the quality of the sound that's heard.
So, the "problem" is that one can't assemble an audio system from standard, off the shelf components and have it "sound good" - every time. To counter that, my experience over many decades, is that just about every assembly of sound gear, no matter how inexpensive, can be made to come up to, tweaked to an acceptable standard, and with extra effort be evolved into a mechanism that produces highly satisfying audio, and, yes, even conjures ...
What is clear, something recently even more strongly confirmed, is that there is very little desire to think beyond the standard cliches of audiophile think: "it's all about the frequency response"; "loudspeakers are what cripples the sound no matter what"; "huge effort must be put into creating a perfect acoustic environment for the audio equipment"; etc ... but a little thought puts these ideas into better perspective: an actual musical instrument or voice of a person in the room or any natural, non-reinforced sound doesn't require any of these sort of shenanigans, it will still have the impact of the "real thing", every time, irrespective of how casually the sound is produced, and how non attentively one listens to it. And the latter is my experience of competent playback: when it works well there is the same impact as hearing 'natural' sounds, no, absolutely zero, excuses or allowances have to be made for the fact that it is "only a recording" ...
It's easy to see that the attitude of the majority in audio land doesn't echo this thinking at all; firstly by reading what they have to say about things, and secondly by listening to what the vast majority of systems produce, the acoustic end result. Sometimes spectacular in a circus-like manner, often disappointing, irritating, fatiguing, uninteresting to listen to - nearly always sounding like a "hifi system", very rarely they are convincing.
But there is a solution ... just get a few people thinking that there may be a smarter way, which in essence is to view the business of reproducing sound from a recording as a process which has to have minimal flaws in every aspect, every area, to get a worthwhile result. Yes, people worry now about "flaws", but they're usually the 'wrong' flaws - they are of low importance, they matter little in the quest to produce an illusion that the ear/brain combination can believe in ... it's only a shift in attitude that's actually required, to focus on the key flaws that make the crucial difference in the quality of the sound that's heard.
Saturday 19 December 2015
A More Ambitious Upgrade - Part 12
Are we there yet ... well, if not it's now mighty, mighty close ... last round I mentioned bypassing the amplifier's speaker relays, and in between I did so in a quick and dirty way - the sound appeared to improve, but I was still not 100% comfortable with the overall result; I didn't feel that it was worthwhile yet to do some tests, removing the bypass and comparing that with the kludge included ... something more needed to be done ...
One thing that was always on the list was the typical rat's nest of wiring inside the amp, tying all the modules and bigger lumps of hardware together electrically. The NAD was far, far better than many in this regard - nearly all the circuitry resides on a single, large circuit board. But, the remaining discrete wiring was the usual mess one finds in audio gear, roughly and loosely tied in bundles in a couple of places - I hadn't done anything yet to improve this because the unit had been so impressive so far ... was the remaining lack of "conviction" in the sound due to this ?? In particular, low level orchestral music was not there yet ...
So, I did a round of stabilising and separating the strands of connecting wire - in simple terms, making sure that any low amplitude vibrating of wires was minimised in level and impact. Only part way through this exercise, but for now let's see what that's got us ...
Ah-hah!! Significant lift in quality, now essentially have 'invisible' speakers, the system generates a deeply immersive sound field, and conveys a convincing sense of the performers doing their thing to the far end of the house, on an aggressive, messy recording. The sense of conjuring is very strong, and perhaps only a little more tweaking will be required now for a rock solid illusion to be always on tap.
Soon I will make some further recordings, done in a more careful way, of the system's reproduction that hopefully will convey a better sense of what I'm mentioning ...
One thing that was always on the list was the typical rat's nest of wiring inside the amp, tying all the modules and bigger lumps of hardware together electrically. The NAD was far, far better than many in this regard - nearly all the circuitry resides on a single, large circuit board. But, the remaining discrete wiring was the usual mess one finds in audio gear, roughly and loosely tied in bundles in a couple of places - I hadn't done anything yet to improve this because the unit had been so impressive so far ... was the remaining lack of "conviction" in the sound due to this ?? In particular, low level orchestral music was not there yet ...
So, I did a round of stabilising and separating the strands of connecting wire - in simple terms, making sure that any low amplitude vibrating of wires was minimised in level and impact. Only part way through this exercise, but for now let's see what that's got us ...
Ah-hah!! Significant lift in quality, now essentially have 'invisible' speakers, the system generates a deeply immersive sound field, and conveys a convincing sense of the performers doing their thing to the far end of the house, on an aggressive, messy recording. The sense of conjuring is very strong, and perhaps only a little more tweaking will be required now for a rock solid illusion to be always on tap.
Soon I will make some further recordings, done in a more careful way, of the system's reproduction that hopefully will convey a better sense of what I'm mentioning ...
Monday 14 December 2015
Yes, Virginia, it is like cleaning a window ...
It has become a cliche that improving playback sound is like having a clearer 'window' onto the recording, but, that remains a very apt analogy - sorry!! Achieving convincing, realistic recreation of a recorded musical event requires that the listener focuses only on the scene beyond the window - the latter representing the mechanism of the playback system, and the former being the actual 'data' encoded on the recording. A window that is dirty to some degree, or that distorts the image of what is on the other side can be largely ignored depending upon one's mood, and especially if the landscape beyond is already well known ... but removing the glass from that window, suddenly, will be quite noticeable, or dramatic, or even an epiphany ...
If an individual is content to always observe the world through that lesser window, oblivious to its effect, then that's fine - but if one were to be exposed to the "reality" beyond the imperfect glass barricade, then a burning desire to never be aware of that window lessening the impact of what one is seeing will form in many people. That it is impossible to actually eliminate the window may be part of the deal - you always require a playback system to experience recorded music - but your awareness of any filtering effect should be as close to zero as possible.
Thus, the exercise is to make the window as "perfect" as possible. And there are many ads of window cleaners where the punchline is that a person walks into the glass, not picking there is an "illusion" of nothing being in his way ... this is what one is after in audio - and, fortunately, it's very achievable ...
A large part of the "battle" in getting the glass 'transparent' enough is not understanding what audible artifacts are due to the glass, not realising that supposedly bad recordings, say, come across that way because the mind is struggling to make sense of the fine detail beyond the imperfect pane - many times it's easier to give up, mentally, and simply decide that the music is so flawed in the recording that nothing can be done about it ... but, nothing could be further from the truth ...
And so a key first step is learning to recognise what the imperfect window is doing to what you're perceiving: luckily, in the visual area everyone can easily adjust the focus of their eyes so that they "look" at the dirt on the window, the scene beyond is blurred out. People can use various ways, intuitive most of the time, to register what's wrong with the glass, being able to completely ignore what is actually seen through the pane - and then clean up and correct the window. Unfortunately, in the aural area, most people are not so well attuned to what's going on, and haven't learnt to switch, shift their focus on the sounds in what they hear.
One needs to be able to "see" the dirt in the playback, and know it's not part of the recording, the scene beyond, itself. This can be learnt, and once acquired, like riding a bike, never goes away. Then, you will motivated to fix the problem, and ,be far better equipped to make the right moves in sorting things out ...
If an individual is content to always observe the world through that lesser window, oblivious to its effect, then that's fine - but if one were to be exposed to the "reality" beyond the imperfect glass barricade, then a burning desire to never be aware of that window lessening the impact of what one is seeing will form in many people. That it is impossible to actually eliminate the window may be part of the deal - you always require a playback system to experience recorded music - but your awareness of any filtering effect should be as close to zero as possible.
Thus, the exercise is to make the window as "perfect" as possible. And there are many ads of window cleaners where the punchline is that a person walks into the glass, not picking there is an "illusion" of nothing being in his way ... this is what one is after in audio - and, fortunately, it's very achievable ...
A large part of the "battle" in getting the glass 'transparent' enough is not understanding what audible artifacts are due to the glass, not realising that supposedly bad recordings, say, come across that way because the mind is struggling to make sense of the fine detail beyond the imperfect pane - many times it's easier to give up, mentally, and simply decide that the music is so flawed in the recording that nothing can be done about it ... but, nothing could be further from the truth ...
And so a key first step is learning to recognise what the imperfect window is doing to what you're perceiving: luckily, in the visual area everyone can easily adjust the focus of their eyes so that they "look" at the dirt on the window, the scene beyond is blurred out. People can use various ways, intuitive most of the time, to register what's wrong with the glass, being able to completely ignore what is actually seen through the pane - and then clean up and correct the window. Unfortunately, in the aural area, most people are not so well attuned to what's going on, and haven't learnt to switch, shift their focus on the sounds in what they hear.
One needs to be able to "see" the dirt in the playback, and know it's not part of the recording, the scene beyond, itself. This can be learnt, and once acquired, like riding a bike, never goes away. Then, you will motivated to fix the problem, and ,be far better equipped to make the right moves in sorting things out ...
Saturday 12 December 2015
A More Ambitious Upgrade - Part 11
Some parameters relevant to the posted recordings of the NAD system playing:
As a reference for comparisons, other YouTube clips with the actual source material are The Faces - That's All You Need - YouTube and Jimmy McCracklin, EVERYBODY ROCK - YouTube.
The NAD amp uses relays driven by protection circuitry to connect to speakers, delayed closing of contacts on switch on, etc - and I was hoping that these wouldn't prove to be a quality bottleneck. Unfortunately, even though earlier listening didn't show any major issues this is ever more becoming noticeable, as improvements elsewhere make it easier to pick this happening - especially on pre-WWII recordings, which so easily descend into sounding like AM radio broadcasts, the harsh edge that typically vocals acquire when poor connections are a factor was clearly evident. This was easily tested by switching off the amp while music was playing, then switching back on again some seconds later, cleaning the contacts temporarily - the natural sweetness of the voices returns, only to slowly degrade with continuing playing of the track.
Hard wiring, bypassing the switch contacts is a permanent solution, from previous experience; but I lose the protection mechanism, plus have possible power on thumps through the speakers. Anyway, I will use this method for now to confirm that what I'm hearing is directly caused by the particular contacts - and then decide what to do about it, long term ...
- Room is approx. 7 x 4 metres, with sloping ceiling whose height varies from 2.5 to 3.5 metres lengthways
- Speakers facing shorter end of room, about 2.5 metres from that end, LH speaker about 0.5 metre from side wall
- As already mentioned, single microphone 2 metres behind RH speaker
- The room, a work room, is extremely cluttered, full of hard and soft surfaces, carpet on floor
- Deliberately started part way through track so that the the sound of the room and noise levels could be picked up during the gap between the the two tracks captured
As a reference for comparisons, other YouTube clips with the actual source material are The Faces - That's All You Need - YouTube and Jimmy McCracklin, EVERYBODY ROCK - YouTube.
The NAD amp uses relays driven by protection circuitry to connect to speakers, delayed closing of contacts on switch on, etc - and I was hoping that these wouldn't prove to be a quality bottleneck. Unfortunately, even though earlier listening didn't show any major issues this is ever more becoming noticeable, as improvements elsewhere make it easier to pick this happening - especially on pre-WWII recordings, which so easily descend into sounding like AM radio broadcasts, the harsh edge that typically vocals acquire when poor connections are a factor was clearly evident. This was easily tested by switching off the amp while music was playing, then switching back on again some seconds later, cleaning the contacts temporarily - the natural sweetness of the voices returns, only to slowly degrade with continuing playing of the track.
Hard wiring, bypassing the switch contacts is a permanent solution, from previous experience; but I lose the protection mechanism, plus have possible power on thumps through the speakers. Anyway, I will use this method for now to confirm that what I'm hearing is directly caused by the particular contacts - and then decide what to do about it, long term ...
Saturday 5 December 2015
To conjure, first learn to be a detective ...
I have an audio friend down the road who isn't afraid to try the weird and wonderful to get answers, and possible solutions - and I've been visiting over many years now. He had a new toy he wanted me to appraise, so last night I toddled over for a listen ... for many years he has been using competent, pocket media players as his digital source, and he had just acquired a well recommended, lowish cost unit which he was pleased with. And I had been impressed with the previous units he had used - usually significantly better than the tweaked CD player that had served him for many years.
First impressions ... hmmm, okay, but there was something amiss - he is usually capable of getting very satisfying sound, sometimes after some judicious fiddling on the fly ... but it wasn't happening at the moment. There was a flatness, a lack of sparkle - the sound wasn't as good as as I had heard many times previously, with other configurations he had optimised.
Long story short, after many, many experiments, including swapping amplifiers, there was no major movement forward ... the dullness, lack of 'musicality' wasn't going away. Then, there was a period of inspired combining of our thinking, and intuition, which caused us to focus on the improvised damping of the top cover of the amplifier ... over time, it had been worked out that the typical, thin metal top plate had an audible impact, and adding some damping and weighting materials helped. But now, in a seemingly crazy way, removing that and just using some scraps of fluffy viscoelastic material on top was giving us the best sound! What's going on, ... ??!!
Then, the "Ah-hah!!" moment occurred: he had replaced the smoothing capacitors in the amplifiers - the manufacturer's parts were under-rated for the job, and failed early in the amplifier's life; my friend's replacements were better suited, but were significantly taller; he couldn't get higher voltage rated units that better matched the original profile. And now some experiments confirmed that adding the mild damping precisely over where those capacitors were situated under the cover plate was the key!
It appears that the top of those capacitors were now too close to the sheet metal of the covers, and there was a sound modulating interaction between those two parts. Okay, rip off the cover, and insert two circles of damping material between the caps and plate - so when the latter was screwed back on the material was under compression, slight but unnoticeable bulge to the cover now ...
And, the good sound was back, with a vengence! Full, sweet, satisfying music on every track - the system was now well and truly in the "zone" ...
An excellent example of how important it is, firstly, to acknowledge that an audio system is not up to scratch; and secondly, to start investigating all the possible causes, reasonable and unreasonable, to try and track down where the flaws in the reproduction are originating - dedication to this approach is so important to being able to conjure up convincing sound.
Addendum: for anyone who wonders why the amplifier swap didn't impact the sound quality issue- the two units were in fact from the same manufacturer, Naim: Nait models which were different versions, but had the same casing and essential circuitry within; the power supply capacitors in both had been replaced by my friend, so the same problem manifested in both units.
First impressions ... hmmm, okay, but there was something amiss - he is usually capable of getting very satisfying sound, sometimes after some judicious fiddling on the fly ... but it wasn't happening at the moment. There was a flatness, a lack of sparkle - the sound wasn't as good as as I had heard many times previously, with other configurations he had optimised.
Long story short, after many, many experiments, including swapping amplifiers, there was no major movement forward ... the dullness, lack of 'musicality' wasn't going away. Then, there was a period of inspired combining of our thinking, and intuition, which caused us to focus on the improvised damping of the top cover of the amplifier ... over time, it had been worked out that the typical, thin metal top plate had an audible impact, and adding some damping and weighting materials helped. But now, in a seemingly crazy way, removing that and just using some scraps of fluffy viscoelastic material on top was giving us the best sound! What's going on, ... ??!!
Then, the "Ah-hah!!" moment occurred: he had replaced the smoothing capacitors in the amplifiers - the manufacturer's parts were under-rated for the job, and failed early in the amplifier's life; my friend's replacements were better suited, but were significantly taller; he couldn't get higher voltage rated units that better matched the original profile. And now some experiments confirmed that adding the mild damping precisely over where those capacitors were situated under the cover plate was the key!
It appears that the top of those capacitors were now too close to the sheet metal of the covers, and there was a sound modulating interaction between those two parts. Okay, rip off the cover, and insert two circles of damping material between the caps and plate - so when the latter was screwed back on the material was under compression, slight but unnoticeable bulge to the cover now ...
And, the good sound was back, with a vengence! Full, sweet, satisfying music on every track - the system was now well and truly in the "zone" ...
An excellent example of how important it is, firstly, to acknowledge that an audio system is not up to scratch; and secondly, to start investigating all the possible causes, reasonable and unreasonable, to try and track down where the flaws in the reproduction are originating - dedication to this approach is so important to being able to conjure up convincing sound.
Addendum: for anyone who wonders why the amplifier swap didn't impact the sound quality issue- the two units were in fact from the same manufacturer, Naim: Nait models which were different versions, but had the same casing and essential circuitry within; the power supply capacitors in both had been replaced by my friend, so the same problem manifested in both units.
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