[LM]

There will be plenty of time to sit perfectly still in "the sweet spot" when I'm old..

Geoff,
I know what you mean but I've been re-evaluating my own version of that stance a bit of late.  My hearing is holding up very well but last week, ironically when using earplugs for protection whilst using a noisy saw bench, I happened to push some wax deep into one ear canal and up against the ear drum.  No problems, it's out now but for a couple of days, I was inadvertently learning just how bad it will probably be for both ears one day.

The loss of high freqs was frustrating enough but what annoyed me most was the complete collapse of a realistic sound stage and the 3D depth the Soraya does so brilliantly for stereo.  No sweet spot at all.  It made me realise what a cranky old coot I'm going to be when my hearing does go.    Normal conversation and background music were not so much of a problem but I do so love to escape into my stereo world with my eyes closed - and that was taken away.    So for the last few days with my hearing fully back again I've been appreciating my system more than ever and using it as much as I can.  I'm just hopeful that Hugh doesn't have much in the way of further upgrades in the pipeline as I would be a very soft touch at present with a strongly reinforced 'use it while you can' philosophy. 

[Geoff-AU]

Hi Lyn,

Those days are already behind me unfortunately, and may account for my change of stance - one or two too many nights in clubs with music too loud has left me with tinnitus so the softest subtleties of sound are lost on me now.  It did make me quite cranky for a few months, causing myself permanent damage like that, but then I realised I'd do it all again, because I was busy chasing after the fairer sex at the time   (and I have worn ear plugs in clubs before - and since - getting the tinnitus.. but even wearing plugs after it started it was still worse the next day so I'm not really sure it's a complete panacea).

Have had a wax blockage myself too, and I know just what a revelation it is to have clear hearing again.  Unfortunately there's no cure for a 14kHz sine wave in the brain.  Win some lose some I guess, probably explains why I like the doof doof now it's all I can hear 

[ginger]

THD and all that, since Hugh made reference above.

Back in 1997 Lynn Olsen wrote an excellent article on amplifier distortion (Glass Audio Vol9 No 4)

In his introduction he wrote:
"....the subjective correlation between the total harmonic distortion (THD) measurement and what you actually hear is close to zero".

He then went on to give an excellent analogy:
"The fault is not with the subjective perception of the listener, but with the measurement itself. There is nothing very new in this; you can measure all you want, but a mass spectrometer is not going to find a lot of difference between lunch at a high school cafeteria and the best dinner at a four-star restaurant. To foolishly assert that the mass-spec. machine is right,.....,is an example of simple ignorance trying to cover its nakedness with a fig leaf of science."

The following summarises, in my own words, what he had to say.

To attempt to make some correlations between harmonic distortion measurements and perceived sound you need to first separate Even harmonics and Odd harmonics and view these as separate sets of data.

Even harmonics are generated by asymmetrical distortion mechanisms and odd harmonics are generated by symmetrical distortion mechanisms.

As an example, look at the differential amplifier on the front end of the AKSA 55, the LIFEFORCE and just about every SS amp from the last 30 years. To reduce the Even Harmonics you must balance the currents in the 2 transistors such that there is little or no asymmetry. Once this is done the residual distortion will be odd harmonics due to the symmetrical nonlinearities in the 2 devices.

This fundamental concept is important, and we get some big clues when we extend are consideration of distortion to include Intermodulation Distortion (IM).

Let us, for example, consider 2 frequencies in the important 1 to 5 kHz region.

For example use 3kHz and 4kHz:
Most of us understand that superimposing these (mixing in a non- linear system) will produce new frequencies and we often refer to these as sidebands. This is too simplistic.

The maths works like this:
Take 2 sinusoidal signal (well we use cosine rather than sine to keep the maths simple)
Signal 1 = a1(cos x)
Signal 2 = a2(cos y)

From output = a1 cos(x) + a2 cos(y)
Exapanding this and substituting in the trigonometric identity cos(x) + cos(y) = 1/2(cos(x+y) + cos(x-y)) and a lot of tedious algebra later we end up with an expression for:

1)  the 2nd order term , For the superposition of 2 signals of x = 3KHz and y = 4kHz then four(4) new frequencies are created, 2 off 2nd harmonic terms and 2 off IM sidebands :
2x = 6kHz , 2nd harmonic of x
2y = 8kHz , 2nd harmonic of y
x + y = 7kHz , IM sideband
x - y = 1kHz , IM sideband

2) The 3rd order term gives six (6) new frequencies. 2 off 3rd harmonics and 4 off IM sidebands
3x= 9kHz , 3rd harmonic of x
3y = 12kHz , 3rd harmonic of y
2x + y = 10kHz , IM Sideband
2x - y = 2kHz , IM Sideband
2y + x = 11kHz , IM sideband
2y - x = 5kHz , IM Sideband

The important thing to note here is that the 2nd harmonic distortion results in new intermodulation product frequencies (sidebands) which are remote from the original frequencies. (1kHz and 7kHz)

The 3rd harmonic results in more intermodulation products, two(2) of which are very close to the original frequencies. (2kHz and 5kHz)

To exaggerate this (but still a valid example) see what happens with 14kHz and 15kHz
2nd order IM terms are 1kHz and 29kHz (very remote from the original 14 and 15kHz)
3rd order products are 13kHz and 16kHz (very close to the original 14 and 15 kHz)

Higher order terms produce even more IM Sidebands
I might be wrong (because I did'nt extend the algebra past the 2nd and 3rd order terms) but I think the 4th order will produce 6 IM sidebands and the 5th will produce 8 IM Sidebands.
 
Then consider other sources of a signal with which we can intermodulate.

One of the most important will be the 100Hz (120Hz in the US etc) residual power supply ripple which depending upon the quality of your power supply may well have 200, 300, 400Hz etc. harmonics as well.

At this point the "stray" IM products can run to thousands.

This is why a no feedback single ended triode amp with 2% of 2nd harmonic distortion but no 3rd, 4th, 5th etc can sound stunning and why an SS amp with 0.001% THD distortion which is primarily odd (and high) order distortion can sound awful.

It is also why some amplifiers which sound lovely with folk or jazz music which have a lower number of simultaneous tones (sparse spectra) can sound seriously rubbish when reproducing large choirs and orchestras (many closely spaced tones).

So what can we do about it?

The answer is simply to follow the established "popular wisdom".
1)  Keep Power Supplies super clean
2) Don?t allow high order harmonic distortion products
3) What harmonic distortion there is should be even harmonics ONLY

So here is my theory / explanation / WAG,
In short, higher order harmonic distortions produce many more Intermodulation Sidebands
Even harmonic distortions create IM Sidebands remote from the original frequencies
Odd harmonic distortion produce IM Sidebands close to the original frequencies.

Any of this make sense to anyone?

Cheers,
Ian (Ginger)
   

[gaetan8888]

Hello ginger

Yes, it's make sense.

Jean Hiraga did a paper about that subject 30 years ago, but there was no math, only experiments, showing that an amplifier with monotonic decrease of the distortions harmonics with a dominance of the even harmonics, do sound much better. In the 70's and early 80's, the Denon PMA-700, Audiotec, Radford SS amps, and Sugden was some of those good amps, especially Audiotec and Radford SS amps.

Nelson Pass did showed in a graph, using AP tests equipments, that a low gnfb amp will have less high order harmonics distortions.

A LTP with balanced currents in the 2 transistors and with a low symmetrical nonlinearities will have low even and odd harmonics with monotonic decrease, and using a phase lead cap will made the gnfb more stable at pole frequency, and all those things give an amp with less high order harmonics distortions and with a better resolution and a better sound stage.

As a side note, the Intermodulation and product frequencies in the preamp and amp folowing a non-os Dac, and cause by the HF noise from the 44khz of the non filtered non-oversampling Dac are why this type of Dac are not very good with large orchestral music and cause fatigues with this type of music.

Bye

Gaetan

[AKSA]

Ian (aka Ginger!) and Gaetan,

Many thanks for your posts.  Makes terrific sense to me.  I strive to do amps with lower odd than even;  difficult with PP Class AB, but it does make a difference.

Recent Clio test with the Soraya at 20W 1KHz into 8R showed these results:

H2 = -78dB
H3 = -87dB
H4 = -89dB
H5 = -89dB
H7 = -94dB
 
THD = 0.014%

These levels are not especially low, as many modern amps will do better than 0.005%, but they do not sound as good as amps with the majority of their distortion even order, such as single ended triodes, as the larger H2 levels do acoustically mask the odds to a degree.  At the risk of sounding a bit self-righteous, this came as no surprise to me, but vindicated my design approach over quite a few amps now.

My opinion, and it is just that, is that THD correlates to the subjective listening experience about as well as the color of the paint on an automobile correlates to its performance.  This is heresy, of course, and many will disagree, that's cool, but I have tailored my amp distortion characteristic to accord with my beliefs and the general consensus seems to be that they sound good.  I feel that the time is rapidly coming where the low THD dictum will no longer appeal to the the purist, engineering set, as many are now aware of the strange pscyhoacoustic issues which lie at the foundation of human aural perception.  I do feel that low THD amplifiers are fundamental in servo and instrumentation applications.

Cheers,

Hugh

[Geoff-AU]

An interesting read, thanks guys!

[andyr]

I feel that the time is rapidly coming where the low THD dictum will no longer appeal to the the purist, engineering set, as many are now aware of the strange pscyhoacoustic issues which lie at the foundation of human aural perception.

Cheers,

Hugh

Mmmm, not sure I agree with you, Hugh! 

I know where I stand on this matter but there seems still to be a lot of argument about tube distortion vs. ss distortion ... which means that the participants (or should I say the "fundamantalists"?  ) will never accept that any distortion is a good thing. 

No problem, IMO .. let them listen to their crappy-sounding amps!  We know we're backing the right horse! 

Regards,

Andy

[LM]

Hi Hugh et al,

I think this thread may have to be re=titled 'QUIET IN HERE!!'  I'm sure there is much a buzz at Aspen HQ but the sound insulation seems to be working very well.  Come on Hugh, there must be something up your sleeve you can tell us about.

I've got little to say personally as all the minor changes to my system of late have been of non-Aspen origin though still amazed at the underlying refinement instilled by my current Soraya.  So what is happening with everybody else in the Aspen community?

Not so quiet at my place a few minutes ago though.  Lots of 2cm or 3/4 inch hailstones crashing down on the roof for a few minutes.  What a din but no damage I can detect.  Perhaps the hail also made it noisy at Aspen HQ which is only about a 10 min drive away.  How's your roof Hugh? 

[AKSA]

Hi Lyn,

Yes, too quiet by halves.....

I'm working away on the next model, a higher power Soraya, 150W per channel, to top the range.

In this endeavour I'm working with two others, Colin Brown, and Jon Pippard.  Soon there will be something to announce, and it will be really quite something.

I was driving west of Melbourne when the hailstorm struck.  A couple of thumps on the windscreen threatened to break it.  The freeway was covered in water, in some low spots to a depth several inches.  Traffic slowed to a crawl for a time.  Fortunately it passed in about half an hour, but it was traumatic on the roads and many drivers cars stopped beneath bridges to avoid denting their panels.  As far as I can see my car is undamaged, but it was certainly a surreal experience.

Be assured that though the forum is quiet, Aspen is busier than ever......

Hugh

[LM]

Hi Hugh,
You were lucky.  An acquaintance was on the road and had every panel of his near new Subaru dented and the windscreen smashed.

Anyway, really interesting news about the amp so keep the noise up.

[Seano]

Fortunately.....we missed Melbourne's hail.  But we missed most of the rain event too.  Damnit.

I too have been quiet on the AKSA front.  My GK-1 remains in pieces...awiaitng its upgrade caps and what have you.....plus a re-wire and and a re-case.

But real life gets in the way....but in a good way.

My thinking is that I may end up sending the tube board and extraneous parts to Hugh and he can put them together for me whilst also doing an idiot check (as in what idiot soldered it like this!).  That way I know the thing is done right if not at least finally done!

Might also have to talk to Hugh about a power supply for the old 100N+ too....

[AKSA]

Seano,

I was caught driving westward when it struck, hail the size of large marbles, fortunately nothing broken or dented, but harrowing indeed, lots of cars under bridges, waiting it out...

Never mind, the flood from QLD moves ever southward and will hopefully fill the Darling River.

It's amusing, SE QLD hit by crippling drought, now outback towns completely inundated, incredible.  Climate change anyone?  Charleville's river peak easily exceeded the 1890 flood, so this is more than a 100 year flood.....  still, it has happened before.

Go easy on sending me anything.  I'm snowed under with work, of the worst, most difficult kind.  Amps are a delight, preamps are a nightmare, so speak first with me to find how the land lies.

Power supplies are not so bad.

Some time ago one of your posts on DIYaudio was removed, why was that?  I'm sure you were misread......

Cheers,

Hugh

[Seano]

Yep....they didn't know how to handle my bespoke kind of sarcasm and shit stirring. Fear was enhanced by the fact I was a 'new' poster.

I got a please explain email that amused me so much that I couldn't respond without even more shit stirring.....which kind of negated the entire effort of contrition - so I let it slide.

No bruises, no bark lost.