Sound Quality Vs. Measurements

[dvv]

Originally Posted by ThorstenL

Hi,



It was stated several times that the compensation is a placeholder and may need additions. HOWEVER, Tina-Ti does suggest the result is stable.

Maybe it WILL live up to it potential precisely BECAUSE of these choices.

I know what Amplifiers sound like that are designed around your tenets (maximise global looped feedback at the expense of individual stage linearity, transient performance, harmonic distribution etc.), and I have little use for this kind of Amp.

I also rather doubt Dejan has any more use than I for this kind of Amp but that is between you and him.

Ciao T

Damn right, Thorsten. I have heard only a few amplifiers with little or no local degeneration which sounded good, but a whole truckful of those which sounded from bad to worse.

Oddly enough, one of the "good guys" was a German design from circa 1978 (or 1979?), from a company called Linear Audio Systems (LAS). I'll see if I can dig up the priginal schematic, I used to stare at it admiring the simplicity and outrageous thuinking behind it. I don't know who actually designed it, but I'd love to buy him a beer; if that man satyed in the business, I would love to know how far he has gone by now.

I nicked their protection scheme, experimented with it for years, and still use it to this day, you saw it in my schematics. One of very few circuits I can honestly say I know everything of importance about it.

[dvv]

Originally Posted by ThorstenL

Dejan,

You must understand, as soon as Wahab sees any degeneration or attempts to reduce local non-linearities while subtracting global loop gain he sees an amplifier that "won't live up to it's potential" as it will not have as low a measured THD (actually, it seems more simulated THD) as could be done using the same circuit un-degenerated.

And he has expressed many times that he does not believe that an amplifier can be improved by applying degeneration (Wahab, feel free to correct me if I have misunderstood your points despite having read them at least dozens of times across a wide variety of threads - I would not want to misrepresent your position - what I am writing is simply what I get from reading your posts).

Ciao T

Wahab's concept is his own, and it's pointless trying to dissuade him from it, nor is there any reason to do so.

The problem is, I own two integrated amps with low global feedback, where "low" means 17 dB for H/K 6550 (and it's a SEPP design!) and 12 dB for H/K 680. Thus, I have personal hands-on experience with such designs, as well as quite a few others as well - by now, it should be obvious to one and all that I hold Harman/Kardon in very high regard, and low global NFB is one of their cornerstone philosophies. I'm not saying they are the best ever, but I am saying that they are, design wise, a very consistent and serious company. Whether you like or hate their sound, that's a matter of personal preference, and ultimately, not all of their products sound exactly the same.

But I become very cautious each and every time someone has a different view which serves to criticize others, but offer no practical proof of their own ideas. You think you can do better? Fine, let's see what you've done, perhaps we can all learn something from it.

Perhaps I'm too inquisitive, but I am not concenrned with theory only. Theory without practical confirmation is meaningless, it should not exist for its own sake, but as a guide towards new and presumably better solutions.

As an example, I admit to feeling a bit challenged by your amp shown here. Because that's so, I will take the time and trouble to actually make it, with only one minor change, I will add a fourth pair of output devices. I can do that easily, since unlike you, I have no existing basis, I am starting from scratch, and I have a box full of Fischer heat sinks, 300x40x100 mm, about 50+ kilos of them. It can serve as another take on your work, but the key point is, I want to hear it play music. My faith in its musical qualities is strong.

And anyway, The Force is strong in my family.

(Sorry about that, a few days ago I purchased a Blue Ray version of Star Wars, The Complete Saga, and boy oh boy, am I enjoying it, or what! Such fine memories it conjurs up!)

[wahab]

Originally Posted by ThorstenL

It was stated several times that the compensation is a placeholder and may need additions. HOWEVER, Tina-Ti does suggest the result is stable.



Maybe it WILL live up to it potential precisely BECAUSE of these choices.

I know what Amplifiers sound like that are designed around your tenets (maximise global looped feedback at the expense of individual stage linearity, transient performance, harmonic distribution etc.), and I have little use for this kind of Amp.

I also rather doubt Dejan has any more use than I for this kind of Amp but that is between you and him.

Ciao T

I m not talking of reducing degeneration and maximize the OLG
but to find the good balance between all theses parameters
to yield unconditionnal stability even with 2uF load , lower
possible THD and IMD as well as excellent harmonic distribution,
not counting better PSRR and DF..

As a clue , using your last schematic with almost no modification:

[benb]

Originally Posted by dvv

I couldn't agree more. And it works both ways.

When I was working on the solid state versions of my headamps, I allowed space for the compensation as on the design. The simulator model was sort of iffy, maybe I'd need it, and maybe I wouldn't, clear as mud.

In the end, it turned out that I don't actually need it.

Think of it this way, Thorsten - that simulator has to ASSUME something, for example, that yopu will be using PC boards with say 30...35 microns of copper, as an industry standard. Thus it also assumes things like parasitic capacitances and so forth.

I used 70 micron PC boards (my personal standard) as I always do, and the board came out very nicely compact, but not too compact (meaning that we didn't push for still more compact at the cost of copper trace width, I like nice, wide and thick lines). Appearently, the actual parasitic capacitance was on a lower level than the simulator assumed, and the actual measured results were at worst 10% better than the simulator told me - for example, it put the higher -3 dB point at 1.2 MHz, and it actually measured 1.6 MHz. The compensation simply was not needed in real life.

However, mostly it's the other way around. The sim says say 2 pF, and you discover you actually need 3.3 pF, etc. No matter what the simulation looks like on the screen, until you have the actual, live model on your test bench, it's just a more or less educated guess.

Which sim is this??? Is is some form of SPICE? SPICE is an acronym for Simulation Program with Integrated Circuit Emphasis, which makes me think that, if it adds anything at all (and I'm doubtful it adds anything not already in the models, though they might include lead capacitance and inductance), it's unlikely that it would add some "typical" capacitance between all nodes (!) based on what printed circuit boards might have.

You're of course free to add any capacitor between any two nodes, and this might even be useful depending on your layout, frequencies and impedances involved, whether it's on a PCB or IC. I could imagine some back-annotation where a layout (with netlist tied back to the simulation schematic) is scanned with board thickness and material specified, and the resulting capacitances are added to the schematic. But does anything actually do this for either a PCB or an IC layout? I recall hearing of software that calculates impedance of a trace at microwave frequencies, but that's specific to RF design.

But the idea that a simulator would add such things automatically "just because all circuits have 'em" is a little scary. It seems it would too often be a bad guess on the simulator's part. It sure seems easier to add capacitors than to go through menus and help screens to take out invisible ones.

[magnoman]

On the subject of stability...
Why dont people doing DIY for personal use take advantage being able to measure the speaker load (over frequency) to be driven?
My designs quickly get over-constrained and being able to slightly relax one constraint for the sake of improving another aspect is always welcome.
I'm not suggesting having the amp break into oscillation if not loaded or when driving "typical" speakers.

Wahab, the transient response plot you present looks good to me especially if the 1uF case is supposed to show stable but not necessarily optimal response. What criterion do you for phase margin when subjected to the extreme load capacitances?

Thanks
-Antonio

[dvv]

Originally Posted by benb

Which sim is this??? Is is some form of SPICE? SPICE is an acronym for Simulation Program with Integrated Circuit Emphasis, which makes me think that, if it adds anything at all (and I'm doubtful it adds anything not already in the models, though they might include lead capacitance and inductance), it's unlikely that it would add some "typical" capacitance between all nodes (!) based on what printed circuit boards might have.

You're of course free to add any capacitor between any two nodes, and this might even be useful depending on your layout, frequencies and impedances involved, whether it's on a PCB or IC. I could imagine some back-annotation where a layout (with netlist tied back to the simulation schematic) is scanned with board thickness and material specified, and the resulting capacitances are added to the schematic. But does anything actually do this for either a PCB or an IC layout? I recall hearing of software that calculates impedance of a trace at microwave frequencies, but that's specific to RF design.

But the idea that a simulator would add such things automatically "just because all circuits have 'em" is a little scary. It seems it would too often be a bad guess on the simulator's part. It sure seems easier to add capacitors than to go through menus and help screens to take out invisible ones.

Ben, to be perfectly honest, I have no idea what the simulator I am using (NI Multisim Pro 10.3) does or does not assume. I assume that it assumes simply because its results are so incredibly close to what I later get from a living model, and if there is an error, it errs on the ideal side, being very conservative. This suits me just fine.

But the proof is in the pudding - it's a simple fact that over the last 6 years, and around 30 models made in it, it never once failed me in its predictions.

[dvv]

Originally Posted by wahab

I m not talking of reducing degeneration and maximize the OLG
but to find the good balance between all theses parameters
to yield unconditionnal stability even with 2uF load , lower
possible THD and IMD as well as excellent harmonic distribution,
not counting better PSRR and DF..

As a clue , using your last schematic with almost no modification:

Wahab, if I read it correctly, in your graph of say 1 kHz distortion, with a peak output of 40 V, the distortion is around 2 mV, or 86 dB down, or at 0.005%. This is "unrealized potential"?

I would also like to remind you that in his explanation, Thorsten DID say in several places that something needed more work, thus clearly indicating that this was still a rough prototype, and not the final form. Knowing him for many a year, I am aware that this means he is simply occupied with other issues, but being very pedantic (sometimes to a fault), he WILL get back to it and do what needs to be done.

In the end, you or I may not agree with what he's done, but I feel certain it will perform better than it does at this time. I have never known Thorsten to leave anything he starts unfinished. This is why I will make a version of his amp, if he provides the complete schematic once he feels it's ready.

If however you post a schematic of one of your complete works, rest assured I will give it proper attention as well. I believe I am not alone in saying that the proof is in the pudding, the actual living model, not merely in simulations, although we all start there.

[dvv]

Originally Posted by magnoman

On the subject of stability...
Why dont people doing DIY for personal use take advantage being able to measure the speaker load (over frequency) to be driven?
My designs quickly get over-constrained and being able to slightly relax one constraint for the sake of improving another aspect is always welcome.
I'm not suggesting having the amp break into oscillation if not loaded or when driving "typical" speakers.

Wahab, the transient response plot you present looks good to me especially if the 1uF case is supposed to show stable but not necessarily optimal response. What criterion do you for phase margin when subjected to the extreme load capacitances?

Thanks
-Antonio

I can't speak for others, but my default design and test load is 4 Ohms, not 8. If it can stand its ground with 2 Ohms clean, or 4 Ohms in parallel with 1 uF, I will then consider it ready to be made as a model. I expect its distortion to rise as the load deteriorates, but at no point can they rise over 0.1% peak value.

This may not appear to be impressive, but then the very idea of somebody sitting and listening to full power sine waves is hardly realistic, isn't it? If you want no clipping, you would need to lower the volume by at least 3 dB, and 6 dB would be more realistic.

Also, I use load models of my own loudspeakers, the actual circuits. This is of limited value only, as it overlooks what happens after the speaker heats up and changes its properties, but it's still more realistic than a plain dummy resistor load.

You mention the DIY people. I find that that too many times, it's a matter of personal promotion and preceived glory to put out whatever as soon as possible, without REALLY testing it.

My first model is doomed from the outset, because its final fate will be to be burned out - I need to know its actual, real life absolute limits. So, throw out the overvoltage/overcurrent protection circuits and pump him stupid until he gives up the ghost. But then, there's no more guessing, no more arguing, there are simply FACTS you adjust your protection circuitry for. As a result of this perhaps fatalistic approach, beside the material loss, is that over the years, NOT ONE of my amps has ever burnt down or gone dead on its operator, a statistic I am rather proud of (not that I've made an awful lot of amps, something like 12 or 13).

Thus, in full agreement with you, I would like to enlarge your argument - it's not only testing for pure 8 Ohms as a workload, but also it seems very important to LEAVE OUT any form of common sense protection circuitry, which can be executed in many different ways. Some (insert your favorite swear work here) idiot launched the thesis that protection circuits deteriorate the sound, so it's best to leave them out altogether - same as with tone controls on preamps. As a result, I have seen at least 30 amps blown to bits with their "protection" fuses intact.

Only incompenently designed and adjusted protection circuits deteriorate the sound, same for tone controls, and the bad rep they garner comes from flashy on the outside, junk on the inside Japanese mostly products from the 70ies. They used highly strung up amps, made so for commercial reasons only, which were pushed to their limits with even 6 Ohms, let alone 4 and less Ohms; that was the Lab Era of audio, when products were made or failed in the lab stage. On the opposing side, speaker X was thought of as a very difficult load, only partly because it may really have been difficult, but mostly because amps were not made to drive actual, real world loads.

At least, not in Japan. In Germany, it as a different story altogether, simply because their amps were made to DIN (Germany Industry standards, 45500 for audio) standards, which assumed a 4 Ohm load as default. Thus, in terms of watt delivery capability on paper, they were way behind the Japanese, but in real life, at least 70% of the German serious production of the day sounded better than 70% of the Japanese production in your room. Even your average Grundig (considered Low Fi forever, despite some stunning work they produced here and there) sounded WAY better than your average Pioneer, or some such.

Thus, the DIY arena seems to have adopted as standard the really worst of it out there, although there have been really remarkable projects here and there (eg. Hafler in its time with their DIY kits, Erno Borbelly, Nelson Pass, etc).

And here's where Nelson Pass' philosophy backfires (although through no fault of Nelson). His single-transistor-do-it-all approach has created many bastards in his name, but without his knowledge in the background, and the otherwise sound KISS principle has been vulgarized to the extreme. Yes, keep it simple, but no more than you can without stressing its operating parameters. Too simple is just as bad as too complex. Simple often creates real world drive problems.

[jan.didden]

Originally Posted by dvv

Ben, to be perfectly honest, I have no idea what the simulator I am using (NI Multisim Pro 10.3) does or does not assume. I assume that it assumes simply because its results are so incredibly close to what I later get from a living model, and if there is an error, it errs on the ideal side, being very conservative. This suits me just fine.

But the proof is in the pudding - it's a simple fact that over the last 6 years, and around 30 models made in it, it never once failed me in its predictions.

The simulator itself doesn't assume anything. It's just a bunch of mathematical equations that are solved.
What you CAN do is add parasitics to models. For instance, you can define a cap as a capacitor of say 1uF, but a more sophisticated model could include a 10meg parallel resistance to model the losses, and a 4nH series L to model the connection wires. And you can really get carried away here, with ESR, temperature effects etc.
The simulator wouldn't care less, it just churns through the equations.
So it's not the sim that assumes anything, its the model designer.
Many opamp models for instance include 2pF cap on the input terminals to model parasitics, but that's just an assumption. If your PCB causes more or less than that 2pF obviously your results will deviate from the real world results.

jan didden

[wahab]

Originally Posted by magnoman

Wahab, the transient response plot you present looks good to me especially if the 1uF case is supposed to show stable but not necessarily optimal response. What criterion do you for phase margin when subjected to the extreme load capacitances?

Thanks
-Antonio

First things i look for is sufficient phase/gain margin to drive highly
capacitive loads , at least 2uF or even more, unconditionnal stability
whatever the load being a plus.

Generaly used numbers are 26db and 90° , although 120° is better,
one never knows.
Of course this will translate in not so impressive perfs on the THD/IMD
as well as in the slew rate fronts , still , good enough numbers are
easy to obtain with very few components.

This will mandate either heavy degenerations or compensation or boths,
depending on the designs preferences.

Originally Posted by dvv

Wahab, if I read it correctly, in your graph of say 1 kHz distortion, with a peak output of 40 V, the distortion is around 2 mV, or 86 dB down, or at 0.005%. This is "unrealized potential"?

This is the modded version simulation.
The schematic he published , if ever he manage to stabilize it
enough , will produce 20dB higher THD , not counting an erratic
behaviour on capacitive loads.


The main mistake is that a cascoded VAS cant be compensated
the way he wants to , as the VAS local loop ,comprising the common emitter
and common base VAS transistors, is so fast that the input stage is unable to transmit the signal derivative back to the VAS input , as its slowness will integrate the said signal and render any phase lead compensation unfonctionnal.