An experimental 4-th order linear audio power amplifier

[EEVblog]

Not boring, just way beyond me being able to make any valuable input 

Yes, the more obscure the topic gets, the less likely people will be able to give meaningful input.
This is the kinda stuff Doug Ford has wet dreams over (former head audio amp designer at Jands)

[jaxbird]

This is a very interesting project. 

Looking forward to getting a chance to listen to this design.

[Whuffo]

It's a wonderful exercise in design and construction. Without making any judgement on your work, let me offer some observations that might interest you.

First, the best loudspeakers manage 3% distortion; most are 5% to 7%. And the average person can't hear less than 0.5% distortion; it's just not audible. I know that there are many who would argue these figures longly and loudly, but they're the real truth. The limits to audio fidelity aren't the amplifiers (usually) and definitely not the wiring; the transducers (speakers) are where real improvement could be found - but not easily. Remember that speakers are essentially linear motors, and they have back EMF and varying impedance with frequency and the resonant characteristics of the driver(s). Not an easy load to drive precisely.

As long as your amp doesn't clip during normal operation - that's not always an easy one, audio has a lot of dynamic range and transients can push almost any amp into clipping, it'll sound great - as long as its frequency response and phase response are reasonably flat. A good DC coupled class AB amp is more than adequate for this purpose and many of the most highly regarded amps are built that way. A very low impedance power supply will insure that the speaker is tightly controlled.

[madshaman]

I have found that low THD amplifiers tend to be so clinical and accurate that the sound ends up a bit harch/boring and unpleasing, as the human ear kinda expects the distortion of the average amp circuit found in most consumer amplifiers which makes the sound warm and pleasing.

I'll play devil's advocate: when you listen to music live, do you miss the lack of harmonic distortion?

I see the point; much like most home stereo equipment's default eq'ing is the "disco smile" (low and high end are boosted), THD isn't the only thing you want, you want only even order harmonics for that "warm" sound.

Also, imho, other than a guitar/bass amp, an amplifier should *amplify*, other equipment is for colouring the sound.

If you need that "warm" sound, put a class-A vacuum tube biased pre-amp in front of everything and drive it barely into distortion.  I'm sure there are audiophile boxes made just for this.

[GK]

It's a wonderful exercise in design and construction. Without making any judgement on your work, let me offer some observations that might interest you.

A bit presumptuous, don’t you think?

First, the best loudspeakers manage 3% distortion; most are 5% to 7%. And the average person can't hear less than 0.5% distortion; it's just not audible. I know that there are many who would argue these figures longly and loudly, but they're the real truth.

A threshold of audibility of 0.3% for crossover distortion has been demonstrated in blind tests. I don't have the ref at hand right now but it is in Selfs book.
 

 

A good DC coupled class AB amp is more than adequate for this purpose and many of the most highly regarded amps are built that way.

The design presented is class AB, DC coupled. What is your point?

A very low impedance power supply will insure that the speaker is tightly controlled.

Not on its own. An amplifier with good PSR and adequate LF damping factor will ensure that the speaker is "tightly controlled". It's an ass-over-head design approach to neglect these factors and instead concentrate on a stupidly over-engineered PSU; allthough that appears to be the norm in "high end".

[c4757p]

Its an ass-over-head design approach to neglect these factors and instead concentrate on a stupidly over-engineered PSU; allthough that appears to be the norm in "high end".

High end audio generally seems to prefer either "ass over head" or "head in ass" as design theories.

[GK]

No argument there.

[Jebnor]

I love reading about people who do interesting projects even I don't fully comprehend.  I like the challenge of getting up to speed.

Thanks GK. Keep it up.

[jaxbird]

First, the best loudspeakers manage 3% distortion; most are 5% to 7%. And the average person can't hear less than 0.5% distortion; it's just not audible. I know that there are many who would argue these figures longly and loudly, but they're the real truth. The limits to audio fidelity aren't the amplifiers (usually) and definitely not the wiring; the transducers (speakers) are where real improvement could be found - but not easily. Remember that speakers are essentially linear motors, and they have back EMF and varying impedance with frequency and the resonant characteristics of the driver(s). Not an easy load to drive precisely.

It is not unusual for high quality speakers to achieve less than 0.5% distortion at normal levels above a few 100Hz. It's very hard to hear distortion at lower frequencies.

But I agree that speakers are the weakest link in music reproduction, but IMO it's more to do with coupling, dispersion and nonlinearity than actual distortion.

Not saying that all amplifiers sound the same, that is clearly not the case.

[Whuffo]

First, the best loudspeakers manage 3% distortion; most are 5% to 7%. And the average person can't hear less than 0.5% distortion; it's just not audible. I know that there are many who would argue these figures longly and loudly, but they're the real truth. The limits to audio fidelity aren't the amplifiers (usually) and definitely not the wiring; the transducers (speakers) are where real improvement could be found - but not easily. Remember that speakers are essentially linear motors, and they have back EMF and varying impedance with frequency and the resonant characteristics of the driver(s). Not an easy load to drive precisely.

It is not unusual for high quality speakers to achieve less than 0.5% distortion at normal levels above a few 100Hz. It's very hard to hear distortion at lower frequencies.

But I agree that speakers are the weakest link in music reproduction, but IMO it's more to do with coupling, dispersion and nonlinearity than actual distortion.

Not saying that all amplifiers sound the same, that is clearly not the case.

I suspect those speakers that are 0.5% distortion were built by virgins on a mountain and carried to the masses on the backs of unicorns. They probably also require special oxygen-free directional wiring to achieve that impossible level of perfection. Forget those manufacturer marketing specifications - break out the instrumentation and check it out for yourself. I'd be willing to wager that those "not unusual" "high quality" speakers are on the high side of 5%, not 0.5%.

And let's not over complicate this - as far as speakers are concerned, any difference between the signal coming in and the sound coming out is distortion. That's the definition of the word, right?

I'll never claim that all amplifiers sound the same; that's not the case at all. But I will claim that all well-designed and constructed amplifiers sound the same; the differences (if any) are inaudible. The one characteristic that is lacking in many amplifier designs is "headroom" - the peak to average ratio of music is pretty large, and without adequate headroom, clipping results. The amount of output power needed depends on the efficiency of the speakers and the size of the listening room. The average power will be moderate and can be easily handled. The peak power will be at least an order of magnitude greater; I'd recommend 100x the average power being available upon demand.

That whole "tube vs. transistor" debate revolves around how amplifiers using those components behave when they clip. Avoid clipping, and it's an academic debate. You state that people can hear 0.3% distortion; that may be true for some, but the average listener won't hear 0.5%. In any event, any decent amplifier will produce far less distortion.

If we were closer, I'd invite you to come over and hear what audio can sound like. In any event, put the time you'd spend getting that last hundredth of a percent of distortion out of an amplifier into boosting its output power and the result will be much better.

[GK]

You state that people can hear 0.3% distortion; that may be true for some, but the average listener won't hear 0.5%.

Who are you talking to now? I stated nothing of the sort. I simply made reference to a study that demonstrated an audibility threshold of 0.3% THD for crossover distortion in a solidstate amplifier with average test subjects - to counter your "factual" assertion that no one can hear even 0.5% THD.

In any case, speaker distortion is pretty much completely irrelevant when it comes to the potential audibility of distortion generated by amplifiers. Speaker non-linearity is typically benign, low-order and generaly completely negligible at low power levels (where they operate most of the time). Contrastingly a badly designed power amplifier can have the exact opposite characteristic - can generate high levels of high (harmonic) order crossover distortion at low signal levels.

In any event, any decent amplifier will produce far less distortion

Who said otherwise? What is your point?

[Whuffo]

You state that people can hear 0.3% distortion; that may be true for some, but the average listener won't hear 0.5%.

Who are you talking to now? I stated nothing of the sort. I simply made reference to a study that demonstrated an audibility threshold of 0.3% THD for crossover distortion in a solidstate amplifier with average test subjects - to counter your "factual" assertion that no one can hear even 0.5% THD.

In any case, speaker distortion is pretty much completely irrelevant when it comes to the potential audibility of distortion generated by amplifiers. Speaker non-linearity is typically benign, low-order and generaly completely negligible at low power levels (where they operate most of the time). Contrastingly a badly designed power amplifier can have the exact opposite characteristic - can generate high levels of high (harmonic) order crossover distortion at low signal levels.

In any event, any decent amplifier will produce far less distortion

Who said otherwise? What is your point?

Who said what is usually easy to determine from the embedded quotes; I was just having a discussion with someone who drank the audiophool kool aid; nothing more.

That said, I'd still recommend that the effort / cost involved in getting that last hundredth of a percent of distortion out would be much better spent on boosting the output power. To prove amp design excellence, build one that will survive running at full power into a dead short with no component failure - not even a blown fuse.

Low level distortion is almost always due to the output transistors not being biased properly. There's a natural variation in components that means that no amp will be biased correctly unless each one is adjusted manually or elaborate automatic biasing circuitry is included. Matched pairs on the output devices helps, but you'll still need to insure that the bias is correct and that the idling current through the outputs is sufficient to get them into the linear part of their curve. You've got to waste some power supply current here; that idling current is critical.

Low frequency distortion is almost always due to clipping - there's a lot of energy in those low bass notes and an underpowered amp will clip here first.

General "rules" for quality amps: a very low impedance, very well filtered power supply to start; the amp should be able to draw any "legal" amount of current at any time without the rails sagging. In my day, we did this with large power transformers, big rectifiers, and big filter capacitors. That might still be the optimum solution - improve on it if you can. Many references to amp design exist - all I'll add to what they may say is to insure that your amp has an odd number of stages in it. Why? Audio amps have large amounts of gain, and if there's any (even a tiny bit) of coupling from the loud end to the sensitive end, if it's out of phase (due to odd number of stages) it'll reduce the gain. If you have an even number of stages, it'll be in phase and you might just create a high powered oscillator.

And always remember that while the design and math is precise, the components you build with are not. Imagine a world where resistors are 10% and capacitors are 20% or worse. If your design can function properly with any combination of tolerances in the active and passive components, it'll be solid when you use the nicer components you can buy cheaply today. And never forget: simpler is better, the fewer things to go wrong, the fewer things that will go wrong.

PS: Regulation of the main power rails is not too important. What's important is that both rails track each other precisely. A transformer makes this easy; is your approach more cost effective while providing the same functionality?

[GK]

Who said what is usually easy to determine from the embedded quotes;.

Then why do you have so much difficulty replying to things said with any kind of logical coherence? You're just trolling now, right?

**snip**PS: Regulation of the main power rails is not too important. What's important is that both rails track each other precisely. A transformer makes this easy; is your approach more cost effective while providing the same functionality?

Whoooaa! Talk about moving goal posts! Your presumptuous evangelism doesn't give me any confidence that you could design a high performance audio amplifier if your very life depended on it. Just a few post ago you said "A very low impedance power supply will insure that the speaker is tightly controlled" and now you say "Regulation of the main rails is not too important". So which is it? If you knew what you were talking about, you'd be aware that those two things go hand in hand.

My power supply has a transformer (a substantially over sized one). What makes you think otherwise? 

[GK]

Since the issue of power supply design was brought up……………

When it boils down to the aim, in and of itself, of achieving an ever better objective performance from an amplifier design, going overboard on the power supply is the least efficient and the most expensive way of going about it. Big transformers and multiple 10’s of thousands of uF of filter capacitance (especially if the rail voltages are high) can cost the bulk of the money expended.

Our hearing response and perceived loudness does not respond in a linear fashion to power. It is generally accepted that a 4 to 10 times increase in power is required to result in a doubling of perceived loudness. Going overboard on power supply regulation and bulk filter capacitance in order to squeeze out, for a given nominal rail voltage, the last possible volt of dynamic headroom prior to clipping on bass transients isn’t going to have much scope whatsoever for increasing the perceived loudness before clipping becomes objectionably audible.

A much smarter approach is to start with a design having a decent power supply rejection; one that doesn’t pass on, to an excessive degree, variations of, and crud on, the rail voltages to the speaker.

The n-th order feedback method and ultra-low open-loop bandwidth / maximal feedback design “approach” that I have proposed in this thread makes it theoretically and practically possible to construct amplifier modules having absurdly good and far more than adequate power supply rejection - particularly at LF; literally hundreds of dB. The challenge then becomes, particularly for the ultimate minimization of main hum, in adequately isolating earth loops, arranging hierarchical grounding and designing the chassis layout and PSU component placements to minimize the measurable effects of electromagnetic induction.

[ElectroIrradiator]

Note that if we are talking simple passive power supplies, then there always exist a limit on filter capacitance beyond which it no longer makes sense to add more. The loss resistances in transformer, rectifiers and ultimately the mains wiring puts a hard limit on the peak charging current. So you will always have a certain minimum of ripple and/or average DC voltage drop, even for an infinitely large filter cap.

It very quickly becomes the case that a simple CRC or CLC filter has very much better ripple suppression, compared to the single cap filter, though those solutions have other inherent problems.

An excessively large filter cap also hugely increases the apparent VA load seen by the transformer, potentially making a mockery of any attempt at choosing an oversize unit. Which frequently explains why DIY people sometimes runs into problems with their mains transformer overheating, despite the massive overkill factor employed for all PSU components.

[Whuffo]

Who said what is usually easy to determine from the embedded quotes;.

Then why do you have so much difficulty replying to things said with any kind of logical coherence? You're just trolling now, right?

**snip**PS: Regulation of the main power rails is not too important. What's important is that both rails track each other precisely. A transformer makes this easy; is your approach more cost effective while providing the same functionality?

Whoooaa! Talk about moving goal posts! Your presumptuous evangelism doesn't give me any confidence that you could design a high performance audio amplifier if your very life depended on it. Just a few post ago you said "A very low impedance power supply will insure that the speaker is tightly controlled" and now you say "Regulation of the main rails is not too important". So which is it? If you knew what you were talking about, you'd be aware that those two things go hand in hand.

My power supply has a transformer (a substantially over sized one). What makes you think otherwise?

I'm not trolling, I'm just trying to pass some useful knowledge on to you. And regardless of what you may think, I've designed and built audio amplifiers before. The last one would do full power over 10-30 KHz, and with less than 0.01% distortion at any frequency or output power. Measured with real instruments, and it'd survive full power output into a dead short, too. That was before you were a gleam in your father's eye, and it was much, much simpler than what you propose.

One opamp and 4 transistors per channel, basic transformer / rectifier / big filters power supply. If you can do better, then trot it out. Otherwise, get a clue from those who know much more than you do. Your nonsense about the rails doesn't matter - as long as the two rails are the same and opposite voltages, their exact voltage is irrelevant as long as it doesn't exceed the safe voltage of the output devices. Consider that with a center tapped supply, the ripple on the rails cancels out.

This is important: as long as the rails are opposite and equal voltages, the amp will perform well no matter how much you load it. Sags in the + voltage are matched by sags in the - voltage and the DC center point remains stable. I won't offer suggested component or filter cap values, because you're so smart and you can figure it out all by yourself. If the power transformer is sized to provide peak load, it won't be a problem.

The more complex you make it, the worse the ultimate performance will be. And for those talking about diode and capacitor loading - you're thinking about this wrong. The power supply isn't only needed to supply the average demand, it also has to supply the peak demand which is at least a magnitude higher. If the components are sized to handle the peaks, normal operation won't stress them at all.

Quality audio amplification is a solved problem. Until better technology comes along, there's no opportunity for big improvements. Check out the schematics for amplifiers by your favorite brand; they're almost always available online. Check several of them - you'll find that they're more identical than you imagined. Most of the differences quoted in their spec sheets are due to marketing, not electronics.

[GK]

I'm not trolling, I'm just trying to pass some useful knowledge on to you. And regardless of what you may think, I've designed and built audio amplifiers before. The last one would do full power over 10-30 KHz, and with less than 0.01% distortion at any frequency or output power. Measured with real instruments, and it'd survive full power output into a dead short, too. That was before you were a gleam in your father's eye, and it was much, much simpler than what you propose.

One opamp and 4 transistors per channel, basic transformer / rectifier / big filters power supply. If you can do better, then trot it out. Otherwise, get a clue from those who know much more than you do. Your nonsense about the rails doesn't matter - as long as the two rails are the same and opposite voltages, their exact voltage is irrelevant as long as it doesn't exceed the safe voltage of the output devices. Consider that with a center tapped supply, the ripple on the rails cancels out.

This is important: as long as the rails are opposite and equal voltages, the amp will perform well no matter how much you load it. Sags in the + voltage are matched by sags in the - voltage and the DC center point remains stable. I won't offer suggested component or filter cap values, because you're so smart and you can figure it out all by yourself. If the power transformer is sized to provide peak load, it won't be a problem.

The more complex you make it, the worse the ultimate performance will be. And for those talking about diode and capacitor loading - you're thinking about this wrong. The power supply isn't only needed to supply the average demand, it also has to supply the peak demand which is at least a magnitude higher. If the components are sized to handle the peaks, normal operation won't stress them at all.

You certainly are trolling here. You never properly address comments made or back up assertions when challenged. You just rant away and pontificate on multiple tangents and different directions instead. What I wrote about the power supply design definitely isn't nonsense, and the points I made aren't refuted or even addressed by the additional (again tangential) "useful knowledge" that you have once again so kindly provided.

I've made it quite clear right from the beginning of this thread that the design presented here is an over-the-top, esoteric exercise/study in the practical implementation of a complex, high order control loop. Your continual banging on about the objective levels of performance and minimal complexity needed to make a subjectively transparent amplifier are irrelevant and besides the point. If it bothers you so much that I should embark on such a project and share some of the design theory I suggest that you share your angst with your shrink instead - I'm not particular interested. Or perhaps if you really feel that you have so much important wisdom to impart and are hankering for attention and acknowledgement, why don't you start your own thread: "Whuffo's quide to the ultimate power amplifier" and stop derailing this one?

And thanks for the further presumption, but I really don't need advice of how to work out an adequate value of power supply filter capacitance. As detailed already, I have built, tested and measured (on equipment designed and built by myself) the design (not my first). It measures great deal better than 0.01%. For a  large part I'm struggling to see harmonic components out of a <0.0001% THD+N measurement floor. Some might even regard that to be by some degree an engineering achievement.
 

[GK]

Note that if we are talking simple passive power supplies, then there always exist a limit on filter capacitance beyond which it no longer makes sense to add more.

Tell that to the audiophiles and tweakers! I've seen reports/boasts of 200,000uF per rail for an LM3886 "chip amp"!
 

[Whuffo]

I'm not trolling, I'm just trying to pass some useful knowledge on to you. And regardless of what you may think, I've designed and built audio amplifiers before. The last one would do full power over 10-30 KHz, and with less than 0.01% distortion at any frequency or output power. Measured with real instruments, and it'd survive full power output into a dead short, too. That was before you were a gleam in your father's eye, and it was much, much simpler than what you propose.

One opamp and 4 transistors per channel, basic transformer / rectifier / big filters power supply. If you can do better, then trot it out. Otherwise, get a clue from those who know much more than you do. Your nonsense about the rails doesn't matter - as long as the two rails are the same and opposite voltages, their exact voltage is irrelevant as long as it doesn't exceed the safe voltage of the output devices. Consider that with a center tapped supply, the ripple on the rails cancels out.

This is important: as long as the rails are opposite and equal voltages, the amp will perform well no matter how much you load it. Sags in the + voltage are matched by sags in the - voltage and the DC center point remains stable. I won't offer suggested component or filter cap values, because you're so smart and you can figure it out all by yourself. If the power transformer is sized to provide peak load, it won't be a problem.

The more complex you make it, the worse the ultimate performance will be. And for those talking about diode and capacitor loading - you're thinking about this wrong. The power supply isn't only needed to supply the average demand, it also has to supply the peak demand which is at least a magnitude higher. If the components are sized to handle the peaks, normal operation won't stress them at all.

You certainly are trolling here. You never properly address comments made or back up assertions when challenged. You just rant away and pontificate of on multiple tangents and different directions instead. What I wrote about the power supply design definitely isn't nonsense, and the points I made aren't refuted or even addressed by the additional (again tangential) "useful knowledge" that you have once again so kindly provided.

I've made it quite clear right from the beginning of this thread that the design presented here is an over-the-top, esoteric exercise/study in the practical implementation of a complex, high order control loop. Your continual banging on about the objective levels of performance and minimal complexity needed to make a subjectively transparent amplifier are irrelevant and besides the point. If it bothers you so much that I should embark on such a project and share some of the design theory I suggest that you share your angst with your shrink instead - I'm not particular interested. Or perhaps if you really feel that you have so much important wisdom to impart and are hankering for attention and acknowledgement, why don't you start your own thread: "Whuffo's quide to the ultimate power amplifier" and stop derailing this one?

And, thanks for the presumption, but I really don't need advice of how to work out an adequate value of power supply filter capacitance. As detailed already, I have already built, tested and measured (on equipment designed and built by myself) the design (not my first). It measures great deal better than 0.01%. For a  large part I'm struggling to see harmonic components out of a <0.0001% THD+N measurement floor. Some might even regard that to be by some degree an engineering achievement.

Fine. I'll just ignore your insults and name calling and leave you to your moment of glory.

[ssassen]

@GK,

I've been reading through your 4th order power amplifier topic with great interest. You mention that this design has been featured in Linear Audio magazine, however I can't seem to find in what edition? Now it so happens I own every edition, and have looked through a number of them, but can't seem to find your article, could you perhaps point me in the right direction?

Thanks and best regards,

Sander.

[med6753]

I was reading this very interesting and informative old thread. I curious as to what the final results were. But I was also wondering when it was going to "happen". And true to form, it did.

Why do discussions about high end audio always, ALWAYS, turn into a monkey poo flinging contest? 

[jsmn4vu]

> Why do discussions about high end audio always, ALWAYS, turn into a monkey poo flinging contest? 

Pretty sure it's because in the field of high-end audio, there are so many passionate opinions that are factually unsupportable. I could list a couple of notorious ones, but it'd really amount to no more than trolling.

[dom0]

Doesn't have to be about high end audio. Try convincing someone (most people anyway) working pro audio that differential signaling isn't about levels or using three wires, but about impedances.

I curious as to what the final results were.

Me too. Thanks to ssassen for bumping the thread and of course a big thanks to GK for his patience

[fivefish]

>that differential signaling isn't about levels or using three wires, but about impedances.

I think you meant "balanced lines"   -- I agree, balanced lines is all about impedances.

To OP:
I'd like to see an actual board stuffed with components! What's the latest on this?
And ignore the naysayers. 

[T3sl4co1l]

Hmm, I didn't see this. Fascinating as usual, GK

Only problem I have, which is rather fundamental in my opinion, and I'm sure you're aware of the drawback: why screw it up by introducing explicit discontinuities?! 

The analog switches can probably be replaced by OTAs, or even just clamp diodes (back to back zeners from OUT to -IN) on the integrators really, so that a more modest nonlinearity can be used to transition from integrator to inverter behavior.

The system is still nonlinear either way, so I wonder if it could be abused to exhibit chaotic behavior.  (I'm kind of suspecting not, since it will tend to be more damped, rather than less, as it goes around.)  An exploration of that regime (if any) might be informative, in regards to minimizing it (maybe turn "conditional" into "unconditional" stability -- in a wider, nonlinear whole-system sense, rather than the linearized LTI sense being used here).

Tim