CLASS D AMPS -
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Class D amplification

Here, in an article originally published in on line forum audioholics Bruno Putzeys explains it all:

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…

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 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

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.

Bruno Putzeys

CLASS D Amplifier makers to consider, with excerpts from each website - trademarks are those of individual companies:

NuForce

1. High bandwidth. Truly faithful music reproduction of music demands bandwidth, and plenty of it. Insufficient bandwidth damages delicate and crucially important spatial information, and strips away tonal complexities.

Nuforce amplifiers deliver high bandwidth, with a ruler-flat response from 20 to 50,000Hz. Only a very few, very expensive high-end linear amplifiers can achieve that.

This high bandwidth allows Nuforce to amplify complex music with faithfulness far beyond the capability of conventional amps, with a vast soundstage that most music lovers have never experienced, because all the spatial information is now completely reproduced.

2. An extremely high damping factor. Damping factor is simply a measure of an amplifier’s ability to drive and control a speaker. The higher the number, the better. An excellent tube amplifier might have a damping factor between 10 and 100. A very high quality Class-A amplifier might reach 200. A Nuforce amplifier has a damping factor greater than 4,000! And unlike conventional amps, Nuforce’s damping factor is totally unaffected by changes in audio frequencies.

In plain English, this means Nuforce will deliver wide, lightning-fast dynamics with any speaker.

3. Ultra-low distortion and noise. In conventional amplifiers, there is no such thing as a fixed specification for distortion or noise. Manufacturers invariably choose to quote the most flattering number, typically measured at 1 watt. But in reality, when power demands increase, so does distortion and noise.

Nuforce employs unique, patent-pending technologies that actively cancel out the distortion at every cycle of operation. As a result, Nuforce amplifiers have the same low distortion characteristics regardless of output level or audio frequency.

4. No phase shift. Many amplifiers use output filters, and these filters cause phase shifts. When there are phase shifts, spatial information cannot be accurate. Most amps have a phase shift over 45 degrees at 20kHz, going quickly towards 90 degrees as frequencies go up.

Nuforce amps are able to take the signal after the output filter, at the speaker terminals, to cancel all these distortions. This unique close-loop design results in zero phase shift across all frequencies.

Repeat, zero.

Audio Research-Technology

We have been designing and manufacturing products under the Analog Research name since 1986. While many of you may not be familiar with us, we have been in existence for nearly 20 years in our present operation, with products sold in over a dozen countries. We have been producing these amplifiers since 2003 for our private customers. The response was such that we have decided to make them available on a worldwide basis.

Not only are we introducing a new product line, but along with that a new look to our products, as typified by the Velluto amplifiers. While we still are called Analog Research, the new products are produced under the brand name of Analog Research-Technology.

Pat DiGiacomo of AR-T comments:

Thanks are in order to the folks at Great Home Theater for providing the opportunity to add comments to this section. The hard part seemed to be what to write about - linear supply vs switch mode; half-bridge vs. full bridge, or any manner of other technical gibberish that would be over the head of the readers. Engineers have a bad habit of doing that. But this is called "Great Home Theater", not "Engineer and Nerds Monthly", so a different approach is needed.

OK, back to square one. Then it became apparent.

Several years ago, a famous design engineer (Jim Williams of Linear Technology) wrote a book called "The Art of Analog Design". He asked 20 or so well-known engineers to submit articles, but gave them no input or clues what to write. They were not even permitted to discuss their submissions with the other contributors. That is the way to approach this. Let the other guys give you the technical stuff, as they most likely will. Hopefully, it will be enlightening without being preachy or boring.

So, how about this:

"Why should I buy a digital amp? I have heard all the technical stuff, but you know what? I know you guys know what you are talking about, and yeah, it sounds impressive, but c'mon! Deliver the goods. Just tell me why. Make it short, make it sweet."

OK, here is your answer:

Because it sounds good. It sounds damn good.

Ok, I know, you are all going "Yeah, right. What else is he going to say? He says digital sounds good because he sells digital."

No, you have it backwards: We build "digital" amps because they sound good.

(BTW: they are not "digital". Digital amps would have a digital input, such as a PCM signal. PCM is the form the data is for CDs. Nope, they are analog amps, because they have an input stage that accepts an analog signal. And that input stage modulates a power supply, just like in the amp you already have. The difference being the type of power supply used, and the way it modulates it. Call them "self-oscillating Class D amps". Class D for short. You knew I could not make it through an entire page without at least one time reverting to "nerd mode".)

I hear your skepticism, already having experienced it a few years back. Long story short, a friend wanted to show off his new "digital" amp.

"It sounds really good. It does. You will just have to believe me. You know me, we have been buddies for over 30 years. Would I BS you?"

Well, he was a stereo salesman at one time. After that, he opened his own high-end store. Exaggerate?, be prone to be overly enthusiastic about anything new?, say anything to "close the sale?"

No, he would never do that, would he? Of course not!

Well, folks, it was stunning. The thing that impressed both of us the most was that it sounded the same at what ever level it was played. (There are technical reasons why that is so, but, no, no lecture why. You really are not in the mood for one, anyway.)

Anyway, the next day, it was time to call one of my amp designer buddies.

"Hey, ever hear of something called ICEpower?"

"ICEpower? What about it?"

"Well, I heard an amp that was made with them last night. Sounded pretty damn good."

"It did?"

"Yep. Hard to believe that some little PCB could put everything that we have done in the last 25 years or so sound sick by comparison."

"Think so?"

"Yep. Sick. Just plain sick. I am fixing to dump all the amps around that are half-finished and switch over to that ICEpower stuff."

"Uh................yeah...........uh...................."

Silence.

"HEY! Are you still awake?"

"..........Uh.............yeah..........uh....................... YOU DIDN'T TELL ANYONE ELSE ABOUT THEM YET, HAVE YOU? I HOPE NOT!!!!!! They are too damn good for them."

"So, what do you know about them?"

"Well, our first production run is finished and they are hitting the market right now."

"And you didn't tell me?"

"Oh, well, I knew you would figure it out sooner or later. The rest of those guys, they are so wrapped up in their ego that they will never be able to let go of what they have been building. Let 'em build it while they can, because they will all be gone if they don't get with the program. But they won't."

You know, the thing is, the idea is to make a living selling great audio gear. When we were younger, it was one thing to try to outdo everyone else by coming up with something different before everyone else did. But now, we are all too old for that. And besides, you are right. This is better than anything any of us has ever done. I am man enough to admit that someone has me beat by a mile. They make money by selling us raw modules. We make money by making them sound good. Yeah, there are challenges. You can't just stick one in a box and expect it to sound great. But it does make our life easier."

"So, when I call ICEpower, I can tell them that you highly recommend them, right?"

"Sure. Do you need their e-mail address?"

That's the truth folks. My company has built SS amps for retail sale for 20 years. Add to that over 15 years of amps for personal use and sale.

Yes, I like the fact that they run cool, are light, and make designing a 5-channel amp easier. All of that is nice.

Good sound is mandatory. We would not put our name on it if it didn't sound good. No matter how lucrative the market is.

You can trust me. I'm an engineer, not a salesman!

H2O

Henry Ho is a gentleman dedicated to the pursuit of excellence in audio. His amps have converted several serious tube amp fanatics I know into the new digital world. Versus the lightweight NuFroce, for example (7 pounds, 350watts at 4 ohms), his Signature models weigh nearly 60 pounds, due to his implementation of a heavy transformer in the power supply.

Channel Islands

D•200 receives Positive Feedback "Writers Choice 2005" Award!

Our new D•200 Class D Monoblock builds on the technology of our highly acclaimed D•100. The new higher voltage D•200 version incorporates independant power supplies for input and output sections, dual discrete diode bridges, and oversized filter capacitors to produce the power to drive the highly efficient amplifier section. Don't let the compact size fool you, the D•200 produces a muscular 200 watts into 8 ohms and 325 watts into 4 ohms. High current capability and low output impedance allow the D•200 to drive difficult loads with excellent control. The mute function puts the amplifier in a standby mode, allowing critical circuitry to be kept powered and eliminate any warmup period. Being over 90% efficient means the amplifier runs cool and extends the life of all internal components, and built-in safety features protect it from shorted speaker cables and components. Built to last a lifetime, our compact amplifier is housed in a heavy gauge steel chassis with 3/16" brushed and anodized aluminum front and rear panels, secured by stainless steel hardware, with highest quality gold plated speaker binding posts. Available with Neutrik single-ended RCA or balanced XLR input, and your choice of gain (26dB/32dB).

Many manufacturers have introduced Class D designs but all have had their shortcomings, whether it be sound quality, safety, excessive EMI/RFI, or reliability. Due to new technology and careful design, we are now able to offer a product which realizes the full potential of switching amplifiers.

In our new D Series, we have addressed and eliminated these shortcomings and improved the performance to better any other design we've tested...their sound quality is on par with the best amplifiers in the world. From a solid and mighty bottom end, through smooth and textured midrange, to sweet extended highs with a great sense of air and space, these amplifiers produce sound matched only by the most expensive.

Jeff Rowland Design Group

Jeff Rowland produces several digital amps - fromtheir website, here is a look at the Model 201:

An extraordinary journey into the undiscovered country of audio excellence, the Model 201 announces a new order in amplifier design. This amazing amplifier offers an unprecedented alliance between power and efficiency in a small yet elegantly engineered package. Don’t be fooled by its surprisingly compact size - the Model 201 Mono Amplifier from Jeff Rowland Design Group is an amplifier capable of prodigious displays of power and performance. Featuring state-of-the-art power conversion, the Model 201 brings a quiet revolution to music and film score and marks the first true expression of amplifier technology free of the limitations of traditional amplifier design.

The Model 201 offers an innovative solution to one of the most stubborn conflicts in audio design: to build an amplifier with the power and presence to recreate all of the subtlety of the world’s great recordings without the troubling by-products of conventional design such as excessive power consumption, massive size, questionable reliability and objectionable heat. With its patented ICEPOWER® power conversion technology, the Model 201 solves each of these dilemmas. The Model 201 combines incredible power and composure with a cool, quiet efficiency never before realized in the art of amplification.

Ideally sized, light-weight, and perfect for multi-channel use, the Model 201 offers maximum system flexibility for use in audio-video applications. A striking solid billet aluminum chassis provides the Model 201 a stable, non-resonant platform that is completely EMI and RF shielded. Balanced inputs and custom wheel lock binding posts complete an exterior that practices what the Model 201 preaches: no compromise in appearance or performance.

TACT

Innovation includes room correction processors.

See review at Stereo Times: http://www.stereotimes.com/MW2005.shtm

Audio Research Corporation

Audio Research, long known for it's tubed product, ventures into digital, based on the Tripath chip - thus Class "T". From their website:

150.2 Class-T Stereo Power Amplifier

After extensive development, we are happy to announce the availability of a new solid-state amplifier from Audio Research that combines high efficiency, musical sound and ample power - all at a very affordable price. The new 150.2 combines the groundbreaking Class T digital switching technology first used in the 150M multichannel amplifier with a fixed (non-modular) two-channel design in a package sized like the older 100.2 amplifier.

The 150.2 produces a strong 150 WPC into 8 ohms, and 300 WPC into 4 ohms. BAL and SE inputs are provided, along with a 12V trigger and detachable A.C. power cord. Like the 150M, the 150.2 produces very little heat because of its high efficiency, so it is ideal for in-cabinet installations.

Bel Canto

From their site:

Bel Canto’s tradition of innovation and performance in modern audio technologies began with the broad acceptance of the eVo analog switching amplifier technology. The e.One series continues the evolution of performance audio technology by bringing even higher levels of performance, efficiency and flexibility in a compact and cost effective package.

The foundation of an e.One audio system are the power amplifiers which provide the power to energize your life with music. These amplifiers use the best switching amplification and power supply technology to provide audio realism that has been largely missing from the experience of most music lovers. Bel Canto has based these new amplifier developments on the later generation of technology from the ICEpower group. We have evaluated and implemented this technology using our years of experience in developing and producing the eVo amplifiers. This experience has insured that our design of the e.One products allows full expression of the remarkable performance potential promised by this technology.

All of the e.One amplifiers utilize a common technical base, starting with the power supply topology. The foundation of any electronics product is the power supply and the e.One amplifiers utilize the latest switching power technology to provide a solid, fully regulated power source to build on. When coupled with the compact and efficient fully-balanced analog switching amplifier the e.One amplifiers deliver unprecedented sonic performance through the audio band.

The music emerges with great integrity and subtlety from a silent background, untouched by the 50-60Hz related residual inherent in traditional, massive, low frequency transformer-based amplifier power supplies. Removing this low frequency residue reveals a purity through the mid frequencies that simply sounds like music and the regulated supply liberates the low frequencies, seemingly adding an octave or more to your loudspeaker’s performance.

B&O

B&O are inventors of this breed of amp module, and as such sell modules and license the technology to others, as well as build it into their own powered speakers and amplification systems.

ICEpower Benefits

Compactness The efficiency of ICEpower results in ultra compact amplifier designs, which doesn’t need a bulky heatsink to dissipate heat. This allows ultra compact product designs without compromising audio performance.

Cost Competitive ICEpower’s combination of high sound quality, compactness, a flexible business setup and cost savings due to reduced R&D effort, power supply size, chassis size & heat sinking offers a cost competitive solution for the complete end product.

Reliability ICEpower ensures continuous performance and stable protection features under adverse conditions such as high temperatures & power fluctuations. This is the result of ICEpower’s cool operation and invariance to power supply variations.

Energy Savings ICEpower provides significant energy savings. In normal use, the energy consumption is reduced by a factor of 10 compared with conventional technology. This is extraordinary beneficial in battery-based applications.

Hypex

http://www.hypex.nl/

From their site:

Concept

“UcD” stands for “Universal class D amplifier”. This is a reflection of the requirements put forward when it was developed, and of the extent in which it embodies them.

Requirements

Universality meant that the new amplifier be able to replace linear audio amplifiers in all fields of use. Following are the chief considerations made: The amplifier should be as easy to use, if not easier, as a linear amplifier. As a module, its application should not require special EMC knowledge. Its audio performance should not depend on special thought from the user either. This dictated good PSRR and the use of differential signal inputs. It should have excellent EMC performance. Many products use it in one box with a radio tuner. The reception quality should not be appreciably affected even if the antenna was a piece of wire dangling next to the speaker wires. It should be simple circuit-wise. This would reflect itself in cost (the company’s main desire) and in its usefulness in audiophile applications (my main desire). Self-oscillation becomes the automatic choice. Less common was the decision to construct the active electronics with discrete parts only. It should be completely load-invariant. The loudspeaker-dependent frequency response deviations that other class D amplifiers exhibited, while usually being of a euphonic nature, were to me an impediment to their use in true high-end audio. Euphonic colouration is still colouration and therefore not acceptable. THD should be low enough such that it would not produce any sound colouration. My experience with tube amps and low-feedback amps of various sorts was that 0.05% THD in itself does not manifest itself as colouration, as long as it is independent of frequency and as long as 2nd and 3rd harmonics dominate. The spec was pinned at a maximum of 0.03% up to half rated power and loop gain had to be constant across the audio range. Reducing THD at lower frequencies is not hard to do (class D amplifiers delivering 0.005% at 1kHz were already on the market) but sound quality would actually be worse off.

We have auditioned 4 different class D amplifiers in our listening room and find the sound of each was very, very good, yet each was different.

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