Class D - Interesting reading

The Truth About Digital (Class D) Amps
by Bruno Putzeys
Chief Engineer Class D Audio at Philips Digital Systems Labs

The Truth About Digital (Class D) Amplifiers

Firstly I'd like to point out that "digital amps" is a misnomer. There are two categories:

Analog-controlled class D.

Switching amplifiers with an analog input signal and an analog control system. Normally some degree of feedback error correction is present.

Digitally controlled class D.

Amplifiers with a digitally generated control that switches a power stage. No error control is present. Those that do have an error control can be shown to be topologically equivalent to an analog-controlled class D with a DAC in front.

Both use switching power stages and have high power efficiency as their most eye-catching feature.

Why Digital…

Digitally controlled class D initially delivered a success in the form of the Tact Millennium. However, by its mere existence this device (and another one of my own making, the "PPDSD" which performs marginally better) proves that obtaining good performance from such a contraption is largely an analog design exercise - a very complicated and expensive one at that. After all, the distortion phenomena that stand in the way between a perfectly formed digital control signal and a perfect analog replica are inherently analog. Similarly, cheaper digital class D's (such as Sony S-Master and TI's) go on to show that at practical price and complexity levels, performance is quite abysmal (better than 0.1% THD is unusual and be sure it aint just third harmonic!)

One should ask the question: would any D/A converter designer in his right mind build a DAC using power components? Probably not. Then how about the old argument that digital-to-the-end is best? Well, I should think the D/A barrier is best put precisely where it allows the whole signal chain to perform at its best and why should we believe that this is necessarily right at the end? Quite obviously the concept of a digital class D amplifier was dreamt up by DSP folks who presumed that the signal should be kept out of the big bad analog world as long as possible, at the same time expecting the power stage, power supply and filter (all highly analog in nature) to perform flawlessly.

…or Rather, Why Not?

Analog controlled class D (like the so called «Class T») is quite a different story. Although most commercially available implementations are well short of this ideal, proper error control can be used to make analog class D amplifiers with performance figures giving the digital variety a run for their money, at a price well below that of even the cheapest digital class D's. They can have vanishingly low output impedance right across and beyond the audio range (which the digital ones can't!) while frequency-independent distortion (for that "zero-feedback sound") is actually easier to achieve than with digital ones.

To the Ear

So how about sound? The output filter's highish HF output impedance, when uncorrected (in amplifiers without post-filter-feedback, ie. all digital ones and many analog ones too) is responsible for the oft- quoted tube-like warmth and air. At low frequencies the filter impedance is low, resulting in a commanding, dynamic bass. Because switching amplifiers previously had a reputation for sounding harsh (due to people who hadn't heard them but presumed that switching couldn't mean anything else), about every modern entry in the field was heralded as the "first audiophile class D". Read a review about the Bel Canto Evo (which is analog, btw) to see what I mean. Keep in mind that if a device sounds radically different from what you've held in high esteem previously, there's usually something fishy going on.

Unfortunately, while warm and airy is nice, it isn't all you need for real audiophile sound. I like to think audio components should sound neutral and transparent too. A frequency response that wanders 10dB off the line at 20kHz isn't conducive to neutrality. High THD isn't good for transparency, especially when it goes up with frequency. Ergo the Tact Millennium (which has flat and low THD but a nonflat frequency response) sounds transparent but not neutral, and your average transistor amp (which has a low output impedance but sharply increasing THD) sounds neutral but not transparent.

Now, the technology (if you can call a circuit with 16 transistors that) to deliver low output impedance with frequency-independent low distortion in class D exists. Built with audiophile-grade parts it'll scare the pants off any high-end amp (while I'm the designer of that circuit and thus some care reading this statement is warranted, I do have a lot of serious folk to back up my claims). Of course, having these characteristics it sounds more like other high-end amps than that it sounds different, in the same vein as that the best tube gear and the best solid state gear don't differ by miles in sound.

The Upshot

Digitally controlled class D: dead end street.

Analog controlled class D: definitely the future, although you shouldn't expect it to flatten competition from traditional solid- state and tube amps by a tremendous margin.

In the very high-end segment the three are bound to coexist for a very long time. In mainstream gear, class D is certain to take over the scene completely, although one serious problem remains: building a good class D amp is an order of magnitude tougher than a linear amp, and the knowledge required is much more diverse. It may take long before each large company has at least one knowledgeable designer. It won't stop them from putting class D based products on the market, but until then and unless they buy completed amplifier modules from specialist vendors (which eastern companies rarely do, they'd rather commit harakiri than having to swallow their pride), they will be putting out seriously substandard products for years to come

 

Thanks Antonio. Very interesting indeed. I shall read it, re-read it and commit it to memory (hopefully with some interesting responses below).

 

I agree with what the guy is saying it's slightly confusing to refer to analog controlled Class D amps as "so called Class T" as it's only Tripath that uses that term which is purely a marketing excercise as their amps are just PWM Class D amps like everyone elses

Antonio, if you posted that text on AudioAsylum you'd get a very heated debate going on...especially from all those people who have just fallen in love with the Panasonic SAXR45 (which is digital input). I'd love to see their reaction to having their amps' technology decsribed as "dead end street" :JOEL:

Michael.

 

maybe Tact sussed this, and are now using the 'D2' style of analogue<>digital<>analogue amp.
Prehaps using ones own ears will yeild yet more clues trawling through acres of white papers is great fun for students & internet bikerless bikers.
Imagine senerio, Proud owner of piece of Kit 'a', has guests round they love it, some are curious, go away, switch on the information super highway, and find some great intelliectual mind with seventeen letters after their name who then proceeds to write a misnomer on said piece of kit, listener then shows owner copy of this, owner is now in a quandry, his pride and joy is being diss'd by Mr grey matter, so now he unhappy, time for a new product
Mr Putzeys, assumption of digital v's analogue switching, is (imho) pretty close to the mark, of the 'True digital' amps I've heard 3 of, Tact 2150/Spectron musician II/Ps audio Hca 2 all do the accuracy thing with poise and grace, lots of the air and spacial detail good dynamics too, but devoid of life, drive & groove, that the analogue d/t derivetives give.
I suspect the inital designers concept of the signal in the digital domain until the L/pass filter before exiting to the speakers, was intended to keep the 'signal' as untainted as possible, not really failed as failed to motivate the buying public by sales
They also respond very well to the odd 'massage of internals'
I haven't heard a another solid state I'd own, this side of Spectral 360 mono's, but thats me I'm just odd

 

Yes.

 

Point of order m'lud

The PS Audio HCA-2 is of the 'analogue' type as described above. I thought the Spectron was too, but wouldn't swear to it.

reg

 

As for your first point, you cannot win that one...

Regarding the second, I already did, of course... :JPS:

 

Reg, the Spectron, has a digital feed ability directly straight from a dac, the PS audio, you may be right, I'll have a look tomorrow, I've had it for a week or 2 and haven't had the lid off yet too busy, you may be right), however, it's devoid of the interesting stuff, but at the price, I felt, be worth a look and fart about with.

 

I always love "the truth"

The author conveniently fails to mention that the frequency response issues with many digital amps are related to the loudspeaker load, that a speaker whose impedence is around 6ohms above say 5khz will result in a ruler flat frequency response.

Of course the real issue with units like the Tact S2150 is the prohibitive cost of the power supply itself, therefore it is fair to suggest that this solution will remain a high cost option.

The problem is that live music sounds radically different to just about any hifi system, going to a live gig will show that all too painfully. Speaking subjectively, I would agree that the Tact amps simply don't do groove in it's accepted sense. Having said that, they do transparency and detail like no others, so it's acase of horses for courses. Where they are good, they are earth shatteringly good. Stick an analogue signal into them and people gush. This suggests the amps themselves are excellent, it's just that we as listeners prefer some kind of embellishment.

Tact are launching an SDA, I hope to have one to play with in the near future. The idea is that this technology is used to juice up the bass, so it should be interesting. It also retails for just £1200 and puts out 275wpc with masses of current. This is not a replacement for the full digital amps, merely an addition to the range.

I wonder if someone could explain the difference between converting an analogue signal into PWM, and converting PCM to PWM? I for one cannot grasp the advantage of the former, save for the massive reduction in build costs, and the potential for adding some attractive distortion.

As for the full Tact system, I am constantly amazed at the results acheivable for relatively little money. Expect to see some further observations very soon

 

I won't claim to know all the technical details but the advantage of an amp that takes an analog input (which then gets converted to PWM) is that you can use it with sources which don't have PCM outputs, which is all analog tuners, all TTs, all SACD and DVD-A players, all home cinema pre/pros (assuming you don't just want a stereo downmix) - so, quite a lot of sources then An amp with PCM inputs is only useful for CD players, and not even Naim ones at that

One advantage of an analog input PWM amp is that you can use negative feedback. With a straight PCM -> PWM amp that's not possible so you just have to live with the distortion the varying speaker load causes :MILD:

Michael.

 

Forgive me Michael, but isn't that something the analogue amp manufacturers are trying to avoid?

At the end of the day, it's the musical enjoyment that counts and that is a personal thing. For me, the phase coherence of a fully digital chain enables stereo to sound as it should. Along with room correction, you really are able to get the "you are there" sensation, something unfamiliar to most. The typical top end hi-fi set up gives you "they are here", which whilst somewhat different, is no less enjoyable.

Of course if you use a full Tact setup, you have the facility to convert analogue systems to PCM, sampled at 24bit/192khz, so can't be too bad. Still have to say, there's something to be said for colouration and even order harmonic distortion

 

Yes, they are trying to avoid it (negative feedback) but when used in an analog controlled PWM amp it's effect is not damaging as it is with conventional analog amps.

To quote from the original article: "They [analog PWM amps] can have vanishingly low output impedance right across and beyond the audio range (which the digital ones can't!) while frequency-independent distortion (for that "zero-feedback sound") is actually easier to achieve than with digital ones".

All in all, I haven't ever heard a full digital PWM amp so I can't comment on the sound. Putzey's conviction that full digital amps are a "dead end street" I'm not so sure of (even he doesn't explain why he thinks that) but I do think they are bound to be a small niche compared to analog controlled PWM amps which are about to (well, already started to) explode onto the scene of amplification from walkmans to megabucks audiophile systems.

Being a technology that's only just becoming mainstream, set to be big bucks, and with quite a few players with an axe to grind there's a significant amount of bullshit flying around from all quarters unfortunately

Michael.

 

Looks like powerful class A amps will rule the roost for a while yet then

 

Agreed

The only way to decide is to listen, in a controlled enviroment of course

 

An 'analogue' Class D allows you to use a feedback loop, possibly containing the output filter. Digital Class D moves this responsibility out to the PSU. And probably also has to use the PSU to range the volume control.

So you trade relatively simple high performance for very complex possibly high performance. Your amp becomes totally dependent on the quality of the PSU regulation (which inevitably involves an analogue feedback loop...)

Paul

 

Thanks for that Paul.

So basically one relies on the quality of the analogue stages of the source equipment, the other on the quality of the amp's PSU.

I can't for the life of me see why one would have a future and the other not, especially if you are talking high end. Still I guess the cost involved in the Tact PSU does put it at a disadvantage when it comes to pricing, so maybe long term, they are going to have to try other technology to meet price points.

It's a shame, because once you've heard music amplified without hundreds of little components screwing up the signal, you kind of miss it

 

Joel, the immortal words "get the funk out of my face - get the funk out of my face" spring to mind.

Besides which, what's that got to do with amplification?

 

I think Joel was referring you to the several thousand components the music goes through before it even gets to a master tape so a couple extra in your amp isn't going to make much difference

Whether any component is necessarily "screwing up" the music is also up for debate. You seem to be one of the people that Putzeys was referring to with:

"Quite obviously the concept of a digital class D amplifier was dreamt up by DSP folks who presumed that the signal should be kept out of the big bad analog world as long as possible, at the same time expecting the power stage, power supply and filter (all highly analog in nature) to perform flawlessly. "



Michael.

 

Wot you mean like an ear trumpet or those old gramophone thingies?