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Topic: Digital Amplifiers (Read 15046 times) previous topic - next topic
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Digital Amplifiers

I have been reading about digital amplifiers for a while.

This is an explanation from one of the internet sites:
Quote
Now a revolutionary new technology called digital amplifier has come to the audio universe. Digital amplifiers make it possible for the audio signal to stay in its pure digital form through the complete audio signal chain to greatly enhance sound quality. In a pure digital amplifier, (one with a digital input ), there is no need for Digital to Analog Converters in the audio signal path. Digital Amplifiers actually synthesize the desired output signal directly on the speaker terminals, thereby creating a high-powered digital-to-analog converter.

Until very recently, the fully digital amplifier was only possible in laboratories, or in extremely expensive high-end audio systems. Now the technology has been reduced to just one or two chips, dramatically lowering manufacturing costs. In many cases this superior new technology can be incorporated into products even more inexpensively than the archaic linear audio amplifiers that will soon be known as "things of the past."


I wonder how digital amplfiers compare to high end amplifiers. Does anyone on this board have some experience? In particular, does anyone have direct experience with Yamaha MX-D1? (more info, Stereophile review, audioreview.com)

Does this direction represent the highest end / future in amplification technology?
The object of mankind lies in its highest individuals.
One must have chaos in oneself to be able to give birth to a dancing star.

Digital Amplifiers

Reply #1 –
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Does this direction represent the highest end / future in amplification technology?
[{POST_SNAPBACK}][/a]
No. Digital amplifiers, in their various forms (pure digital, class D, etc) have several advantages over traditional class AB designs, such as high energy efficiency, low heat and cheap parts.

They do have several problems, however, summed up nicely by Rod Elliot in this quote:
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There are also PWM amps that claim to be truly 'digital', using One-Bit™ technology, or generating the PWM signal directly from the PCM data stream. Although the manufacturers of such amplifiers will naturally proclaim their superiority over all others, such self-praise should generally be ignored. Implementing feedback in a 'pure' digital design is at best difficult, and may be impossible without using a DSP (digital signal processor) or resorting to an outboard analogue feedback system. Including additional ADCs and DACs (analogue to digital converters and vice versa) is unlikely to allow the amplifier to be any 'better' than the direct analogue methods described in this article.

A relative newcomer to the scene is the Sigma-Delta modulator, however at the time of writing this still has problems (challenges in corporate speak). The main issue is that the transition rate is too high, and it must be reduced to accommodate real-world components - particularly the power switching MOSFETs.

The 'pure' digital solutions described above have another shortfall, and that's the fact that the number of different pulse widths is finite, and determined by the clock speed. A digital system can only switch on a clock transition. Based on currently available information, only around 8 x oversampling is possible if a digital noise shaping filter is added to the system. An analogue modulation system has an effectively infinite number of different pulse widths, but this is not possible with any true digital implementation.
[a href="http://sound.westhost.com/articles/pwm.htm]PWM Amplifiers[/url]

Don't get me wrong, you can get excellent sound quality out of a digital (or class D PWM) amplifier, along with great efficiency and a compact size. These amplifiers are extremely attractive for designers of small and battery powered audio equipment. However, claiming that digital amplifiers are in every way superiour to traditional (class A and AB designs) is nothing more than marketing hyperbole.

Digital amplifiers are neither complete crap nor absolute nirvana - the truth lies somewhere in between.

Digital Amplifiers

Reply #2 –
Vynil users won't be pleased.
Acid8000 aka. PhilDEE

Digital Amplifiers

Reply #3 –
Quote
I wonder how digital amplfiers compare to high end amplifiers. Does anyone on this board have some experience? In particular, does anyone have direct experience with
Does this direction represent the highest end / future in amplification technology?
[a href="index.php?act=findpost&pid=357219"][{POST_SNAPBACK}][/a]

I would tend to believe that class D can complete with the best amplifiers but the pure digital route is limited by lack of feedback.

Is the Yamaha amplifier a pure digital amplifier? It has analog inputs doesn't it?

Digital Amplifiers

Reply #4 –
Quote
I have been reading about digital amplifiers for a while.

This is an explanation from one of the internet sites:
Quote
Now a revolutionary new technology called digital amplifier has come to the audio universe. Digital amplifiers make it possible for the audio signal to stay in its pure digital form through the complete audio signal chain to greatly enhance sound quality. In a pure digital amplifier, (one with a digital input ), there is no need for Digital to Analog Converters in the audio signal path. Digital Amplifiers actually synthesize the desired output signal directly on the speaker terminals, thereby creating a high-powered digital-to-analog converter.

Until very recently, the fully digital amplifier was only possible in laboratories, or in extremely expensive high-end audio systems. Now the technology has been reduced to just one or two chips, dramatically lowering manufacturing costs. In many cases this superior new technology can be incorporated into products even more inexpensively than the archaic linear audio amplifiers that will soon be known as "things of the past."


I wonder how digital amplfiers compare to high end amplifiers. Does anyone on this board have some experience? In particular, does anyone have direct experience with Yamaha MX-D1? (more info, Stereophile review, audioreview.com)

Does this direction represent the highest end / future in amplification technology?
[a href="index.php?act=findpost&pid=357219"][{POST_SNAPBACK}][/a]

The idea of PWM class D amps is far from new - they have been around for 20+years. They suffer from various problems, several of which are quite difficult to solve in stereo commercial designs. Their only advantage in the real world is power efficiency and for a high-power Hi-Fi design the benefits aren't enough...yet anyway. I have a design for one if anyone's interested, published years back. It's not high power (5-10W ish IIRC) and doesn't claim to be anything more than "experimental".

A "fully digital amp", where the encoded information is passed to the input and the D/A is in the output is likely to be a major headache. Filtering at high-power is difficult and wasteful, just for a start.

In short, the article is mostly hype and contains little that's new and nothing that's ground-breaking.

Digital Amplifiers

Reply #5 –
1. So difficult to implement feedback?  Difficult or not, it's not impossible correct?

2. Transition rate too high and must be reduced to accomodate real world components?
Uh, well the solution is simple: reduce the transition rates.

3. Only 8x oversampling is possible? Isn't that enough? A better question is, why would you want to oversample anyway? IMO Oversampling DACs are trash in the first place.

Digital Amplifiers

Reply #6 –
(The Rod Elliot quote:)
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/.../

The 'pure' digital solutions described above have another shortfall, and that's the fact that the number of different pulse widths is finite, and determined by the clock speed. A digital system can only switch on a clock transition. Based on currently available information, only around 8 x oversampling is possible if a digital noise shaping filter is added to the system. An analogue modulation system has an effectively infinite number of different pulse widths, but this is not possible with any true digital implementation.
PWM Amplifiers


I don't know what I'm talking about, but the speaker at the end is still analog and if the source is digital, then does it matter that the signal doesn't get the analog infinity with a digital amplifier (edit: in the amplification process itself)?

Digital Amplifiers

Reply #7 –
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1. So difficult to implement feedback?  Difficult or not, it's not impossible correct?
[{POST_SNAPBACK}][/a]
It's impossible to integrate purely digital feedback in a digital design in a way which does not require prohibitively large amounts of processing power. If you go the DAC/ADC approach, you end up with a hybrid design. These hybrid designs have little to recommend them over analog class D.
Quote
2. Transition rate too high and must be reduced to accomodate real world components?
Uh, well the solution is simple: reduce the transition rates.
[a href="index.php?act=findpost&pid=357322"][{POST_SNAPBACK}][/a]
Reducing the transition rate introduces several new problems. A lower rate means more distortion at the high end of the audio spectrum. As soon as you reduce it too far, your THD+N figures start looking pretty bad.

This is more of a design tradeoff than anything else. The better the quality you want, the more money you need to spend on FETs, etc. The transistion rate is one of the more important design parameters in a class D/pure digital amplifier.
Quote
3. Only 8x oversampling is possible? Isn't that enough? A better question is, why would you want to oversample anyway? IMO Oversampling DACs are trash in the first place.
[a href="index.php?act=findpost&pid=357322"][{POST_SNAPBACK}][/a]
Oversampling has several advantages. The most important, in my mind, is that it moves the aliases created by the DA process further away from the spectrum of interest. This means that much simpler output filters can be used. What leads you to believe that oversampling DACs are "trash"?

I encourage you to read the rest of Rod Elliot's article on[a href="http://sound.westhost.com/articles/pwm.htm]PWM amplifiers[/url]. PWM amplifiers (class D) are actually analog, not digital but that doesn't stop most marketing departments from trying to convince you that class D means digital.

Digital Amplifiers

Reply #8 –
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I don't know what I'm talking about, but the speaker at the end is still analog and if the source is digital, then does it matter that the signal doesn't get the analog infinity with a digital amplifier?
[a href="index.php?act=findpost&pid=357325"][{POST_SNAPBACK}][/a]
It does matter. Don't confuse this lack of transistion resolution with PCM bit depth. A limitation in PCM bit depth reduces the noise floor, a limitation in switching speed in a digital amplifier both increases noise and introduces harmonic distortion. It's not a huge problem, just something to consider when comparing traditional amplifiers and class D with the pure digital designs.

Digital Amplifiers

Reply #9 –
Quote
Quote
1. So difficult to implement feedback?  Difficult or not, it's not impossible correct?
[{POST_SNAPBACK}][/a]
It's impossible to integrate purely digital feedback in a digital design in a way which does not require prohibitively large amounts of processing power. If you go the DAC/ADC approach, you end up with a hybrid design. These hybrid designs have little to recommend them over analog class D.
Quote
2. Transition rate too high and must be reduced to accomodate real world components?Rou
Uh, well the solution is simple: reduce the transition rates.
[a href="index.php?act=findpost&pid=357322"][{POST_SNAPBACK}][/a]
Reducing the transition rate introduces several new problems. A lower rate means more distortion at the high end of the audio spectrum. As soon as you reduce it too far, your THD+N figures start looking pretty bad.

This is more of a design tradeoff than anything else. The better the quality you want, the more money you need to spend on FETs, etc. The transistion rate is one of the more important design parameters in a class D/pure digital amplifier.
Quote
3. Only 8x oversampling is possible? Isn't that enough? A better question is, why would you want to oversample anyway? IMO Oversampling DACs are trash in the first place.
[a href="index.php?act=findpost&pid=357322"][{POST_SNAPBACK}][/a]
Oversampling has several advantages. The most important, in my mind, is that it moves the aliases created by the DA process further away from the spectrum of interest. This means that much simpler output filters can be used. What leads you to believe that oversampling DACs are "trash"?

I encourage you to read the rest of Rod Elliot's article on[a href="http://sound.westhost.com/articles/pwm.htm]PWM amplifiers[/url]. PWM amplifiers (class D) are actually analog, not digital but that doesn't stop most marketing departments from trying to convince you that class D means digital.
[a href="index.php?act=findpost&pid=357326"][{POST_SNAPBACK}][/a]



1. Curious, just how much is considered a "prohibitively large amounts of processing power?"
2. How far would you actually need to lower the transition rates by to accomodate real world components, and what is the signal to noise ratio at this point?
3. Adding what amounts to little more than "guesswork" into the signal is always bad idea in my book.  If the tradeoff is being able to use simpler output filters then I have to wonder if output filters are that cost/design prohibitive?

Digital Amplifiers

Reply #10 –
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1. Curious, just how much is considered a "prohibitively large amounts of processing power?"
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
For embedded devices (like DAPs), a fairly small amount of extra power can be considered "prohibitive". Processing power required battery power and adds weight. It's not that it can't be done, it's that it can't be done properly while not cancelling out the power advantages of this technology. It might be that it can be done "well enough", but I honestly don't know.
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2. How far would you actually need to lower the transition rates by to accomodate real world components, and what is the signal to noise ratio at this point?
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
Again, I am not trying to say it can't be done, just that this is one of the tradeoffs that might make an alternative technology (like class AB) a better design solution. It really depends on the design requirements.

Class D and pure digital amplifiers implemented with fairly mainstream components can achieved excellent performance.
Quote
3. Adding what amounts to little more than "guesswork" into the signal is always bad idea in my book.  If the tradeoff is being able to use simpler output filters then I have to wonder if output filters are that cost/design prohibitive?
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
Oversampling is in no way "guesswork". Depending on the implementation, the noise shaping that oversampling allows can bring better, cheaper DACs. Saying "oversampling is always bad" certainly isn't an informed perspective.

My point in this debate is not "digital is always good". I am merely trying to show that the OP, who's quote suggested that digital amplifiers were perfect in every way didn't reflect the reality of the situation. Digital (and PWM) amplifiers certainly have their place and are likely to improve even further in future, but they aren't a panacea.

Digital Amplifiers

Reply #11 –
Thanks. Very helpful posts so far.

How can I learn about how different classes of amplifiers work to better understand the bottleneck with Class D amplifiers?

If the input to some of these digital amplifiers are actually analog, why are they advertised as "digital"? (Yamaha Amp is said to have a pair of XLR and RCA inputs. Is XLR digital? If so what soundcards sport this interconnect?)

Does there exist competitively priced, good performing, pure digital amplifiers in retail?
The object of mankind lies in its highest individuals.
One must have chaos in oneself to be able to give birth to a dancing star.

Digital Amplifiers

Reply #12 –
Quote
Quote
1. Curious, just how much is considered a "prohibitively large amounts of processing power?"
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
For embedded devices (like DAPs), a fairly small amount of extra power can be considered "prohibitive". Processing power required battery power and adds weight. It's not that it can't be done, it's that it can't be done properly while not cancelling out the power advantages of this technology. It might be that it can be done "well enough", but I honestly don't know.
Quote
2. How far would you actually need to lower the transition rates by to accomodate real world components, and what is the signal to noise ratio at this point?
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
Again, I am not trying to say it can't be done, just that this is one of the tradeoffs that might make an alternative technology (like class AB) a better design solution. It really depends on the design requirements.

Class D and pure digital amplifiers implemented with fairly mainstream components can achieved excellent performance.
Quote
3. Adding what amounts to little more than "guesswork" into the signal is always bad idea in my book.  If the tradeoff is being able to use simpler output filters then I have to wonder if output filters are that cost/design prohibitive?
[a href="index.php?act=findpost&pid=357335"][{POST_SNAPBACK}][/a]
Oversampling is in no way "guesswork". Depending on the implementation, the noise shaping that oversampling allows can bring better, cheaper DACs. Saying "oversampling is always bad" certainly isn't an informed perspective.

My point in this debate is not "digital is always good". I am merely trying to show that the OP, who's quote suggested that digital amplifiers were perfect in every way didn't reflect the reality of the situation. Digital (and PWM) amplifiers certainly have their place and are likely to improve even further in future, but they aren't a panacea.
[a href="index.php?act=findpost&pid=357349"][{POST_SNAPBACK}][/a]



How is oversampling not guesswork?  Sure it might be an informed guess based on strong algorithms but when you are adding information that wasn't there in the first place, the only thing you can do is "guess."
I agree that digital is not perfect.  I have a T-Amp which is a class D "digital" amplifier when in actuality the "digital" is naught but a buzzword.  However it is a good performer considering its price of 30 USD, and I am pretty happy with what I got for my money.
The OPs post alludes to a "pure" digital amplifier.  If this is just another way to say Class D amplifier then whatever, but if it's something different, we'll just have to wait and see how it pans out.

Digital Amplifiers

Reply #13 –
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How can I learn about how different classes of amplifiers work to better understand the bottleneck with Class D amplifiers?
[{POST_SNAPBACK}][/a]
I hope the debate between CookieFactory and myself has provided some enlightenment for you, one way or the other. 

It's fairly hard to find a good article on amplifiers that is not too technical and doesn't attempt to make crazy statements about sound quality in favour of one kind of design. Oddly you'll find about an equal number of pages punting class A, class AB, class D, pure digital and even odd things like class C (which are generally considered as only good for RF). Here are some balanced views:
[a href="http://forums.afterdawn.com/thread_view.cfm/79383]Amplifier Classes[/url]
Rod Elliot's Articles Page

Quote
If the input to some of these digital amplifiers are actually analog, why are they advertised as "digital"?
[a href="index.php?act=findpost&pid=357358"][{POST_SNAPBACK}][/a]
Digital sounds good as a marketing term. People know their CDs are digital and assume analog amps went out with LPs. By some stretch of the imagination, you could call analog PWM (class D) amps digital, but it's certainly not accurate.

Quote
How is oversampling not guesswork?  Sure it might be an informed guess based on strong algorithms but when you are adding information that wasn't there in the first place, the only thing you can do is "guess."
[a href="index.php?act=findpost&pid=357359"][{POST_SNAPBACK}][/a]
Digital noise shaping is not guesswork. Oversampling is not about guessing some extra samples (because it's pointless - you've lost the bandwidth already) but either about reducing costs or improving quality. Quality can be improved through a number of noise shaping methods, which move much of the quantisation noise energy out of the audible spectrum. Cost can be reduced by reducing the complexity of output filters.

Oversampling is not without it's problems, however. Like so many things in engineering, there are no clear cut answers.
Quote
I agree that digital is not perfect.  I have a T-Amp which is a class D "digital" amplifier when in actuality the "digital" is naught but a buzzword.  However it is a good performer considering its price of 30 USD, and I am pretty happy with what I got for my money.
[a href="index.php?act=findpost&pid=357359"][{POST_SNAPBACK}][/a]
Low component cost is one of the major benefits of class D. There's really no excuse for a good amplifier of moderate power to be expensive, these days.
Quote
The OPs post alludes to a "pure" digital amplifier.  If this is just another way to say Class D amplifier then whatever, but if it's something different, we'll just have to wait and see how it pans out.
[a href="index.php?act=findpost&pid=357359"][{POST_SNAPBACK}][/a]
I have tried fairly hard to find out more about these pure digital amplifiers with little success. Their designs are kept fairly secret and there's little in the DIY space because designing and constructing high frequency digital circuits isn't really fun DIY.

As far as I can see, there are two popular types - digital PWM and Sigma-Delta power DACs. Digital PWM is just like class D, but with digital rather than analog PWM control. I know nothing about the specifics of how the sigma-delta power designs work. Like pretty much everything, they are unlikely to live up to the hype, especially considering that the quality differences between decent amps is so small already, if it exists at all.

Digital Amplifiers

Reply #14 –
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Thanks. Very helpful posts so far.

If the input to some of these digital amplifiers are actually analog, why are they advertised as "digital"? (Yamaha Amp is said to have a pair of XLR and RCA inputs. Is XLR digital? If so what soundcards sport this interconnect?)


[a href="index.php?act=findpost&pid=357358"][{POST_SNAPBACK}][/a]

XLR, also known as a "cannon" (or "canon", I'm not sure), is a balanced line connector commonly used with more professional equipment rather than the unbalanced RCA ("phono") type more often used on domestic equipment. It isn't necessarily digital but it could be, although I can't think of any advantage of an XLR over RCA for digital. I've never seen one on a sound-card, though I admit to being no expert on sound-cards! XLR is a more robust connector and usually has a locking ring (which people often don't bother to lock!).

Digital Amplifiers

Reply #15 –
Also, Class D amplifiers are NOT digital amplifiers!

As briefly mentioned (by cabbagerat, if I remember correctly, and maybe Cookiefactory) Class D amplifier are Pulse-width modulation amplifiers. The marketing people, however, thinks that "Digital" sounds "modern".

Big advantages for Class D is very high efficiency, meaning the amplifiers can be much smaller and lighter and doesn't create a lot of heat. They can also be made with very little distorsion.

There sure are some high-class class D amplifiers, you have a top performer here: http://www.nuforce.com/ - I've heard their amps and they sound really great for that price.

Digital Amplifiers

Reply #16 –
Quote
Stereophile review, [

Anathema! Anathema!
Sounds like quite the product, in all seriousness.

Digital Amplifiers

Reply #18 –
Hypex amps are highly regarded

Digital Amplifiers

Reply #19 –
Quote
I was thinking of using them in a speaker project I have in mind. Perhaps, somebody has some experience?
[a href="index.php?act=findpost&pid=357762"][{POST_SNAPBACK}][/a]
If the figures in the hardwareanalysis article are accurate, then the amp seems to be pretty good. The increase in output impedence with frequency might interact badly with some speakers, but shouldn't be a problem with most speakers. Frequency response seems excellent and IMD is also good.

However, at $150 per module, these are pretty costly if you don't need the full 400 watts.

Digital Amplifiers

Reply #20 –
A friend of mine, Alfsch is offering his poweramp diy-design in a german forum. It is called cheap d-amp and is in a kind of experimental ongoing developement.
If someone is interested and understands german can have a look here:
DIY cheap d-amp mk1
The thread is pretty long meanwhile and shows the evolution so far and shows how efficient and easy they can be made.

This is no digital wonder machine but may be interesting for some hobbyists of you. At least you could understand the shematics shown there.
Is troll-adiposity coming from feederism?
With 24bit music you can listen to silence much louder!

Digital Amplifiers

Reply #21 –
I actually went ahead and purchased the Yamaha MX-D1 amp on eBay for $2250 a little while ago (also got the YPC-1 unit separately). Total paid is about $3k.

Thank you for all the feedback on this thread esp. about the shortcomings of digital amps. Nevertheless I think it's a superb unit for the size.

I wanted a small / light + high quality amplifier that could drive 500W @ 4ohms / channel (Infinity Prelude MTS Tower/Woofer need lots of current to drive). All traditional amps at this range looked huge and bulky. I was thinking of getting a Krell KAV-400xi but decided otherwise b/c it is rated 400W. My only other option seemed to be PS Audio GCC250 (another digital amp) which was around the same price range but with worse specs.
The object of mankind lies in its highest individuals.
One must have chaos in oneself to be able to give birth to a dancing star.

Digital Amplifiers

Reply #22 –
Quote from: tommypeters on
Also, Class D amplifiers are NOT digital amplifiers!

As briefly mentioned (by cabbagerat, if I remember correctly, and maybe Cookiefactory) Class D amplifier are Pulse-width modulation amplifiers. The marketing people, however, thinks that "Digital" sounds "modern".


The actual amp itself is analogue, the switching can allow for analog variations on the pulse width, the only diference between analogue and digital class D amps (the former was originally Class S by the way) is that way that the PWM is generated, not the actual amplifier stage, in analogue applications the sound signal is combined with a triangular wave in a comparator to create the signal, therefore creating it in the analogue domain, while in the same chassis as "digital" amplifiers, the PWM signal is generated in the digital domain using a DSP chip. Other than that their operating principles are the same. One could design such an amp that could operate as either analogue or digital, by having both analogue inputs (for formats such as record and tape, as well as live microphone feed*) and digital inputs for digitally recorded media and digital radio. So, this means that the amplifier is essentially analogue if the source media is analogue and digital if the source is all digital.

*While nearly all microphones directly output a baseband analogue of what they hear, one type, the RF condenser microphone, produces an FM analogue of what it hears (these microphones have very wide and flat frequency response and theoretically operate down to DC, making them good microphones for base frequencies, while FM detectors generally convert the FM to AM so that it can be envelope detected (as in the Foster-Seeley and ratio detectors), one method (Quarature detection) can be desgnied to convert the FM signal to PWM, which could easily be Class D amplified, how is that for an idea?

Quote from: tommypeters on
Big advantages for Class D is very high efficiency, meaning the amplifiers can be much smaller and lighter and doesn't create a lot of heat. They can also be made with very little distortion.

There sure are some high-class class D amplifiers, you have a top performer here: http://www.nuforce.com/ - I've heard their amps and they sound really great for that price.


Have you listened yourself, and do you have a sharp hearing acuity for sound quality (this may be if you periodically listen to live, unamplified instruments (and especially if that's the sound you grew up with) and/or your hearing is well trained, if you only listen to recorded music and lack this training you most likely wouldn't), and if so, do you trust your own ears?

Digital Amplifiers

Reply #23 –
Folks, if we want to get technical about power amplification, we need some fundamental knowledge in power electronics. So here is a good source;

Amplifier Classes

If you have no time to read that, then understand this; Class D Amplifier

Traditionally, Class A have the cleanest sound. Not because it is first in class but because it is operated in the most linear range and there is no crossover distortion. But it is terribly inefficient. Class B is more efficient but suffers from crossover distortion. A good compromise is Class AB that has the transistor or valve biased so it doesn't switch off. But if it is biased too low, it too will suffer from crossover distortion. Class C is even more efficient than Class B but distortion is high and linearity is not good. On the other hand Class D operates like a digital device and is highly efficient. Traditionally, it is only used in subwoofers because of its limited bandwidth and relatively high distortion. But advances in electronics has made it possible for this class of amplifiers to be used as full range audio amplifiers with distortion levels similar to class AB. So, this is a relatively new development in Class D amplifiers for high fidelity audio. But it is definitely not the ultimate design for the purest sound. In pre-amplifiers and headphone amplifiers, Class A is still almost universal.
Life is short, delaying enjoyment is pointless.

Digital Amplifiers

Reply #24 –
Quote from: beyondipod on
Traditionally, Class A have the cleanest sound. Not because it is first in class but because it is operated in the most linear range and there is no crossover distortion. But it is terribly inefficient.


Ever heard of sustained plateau bias? Developed by Krell as a way of inproving the efficiancy of Class A, where the bias is adjusted according to the amplitude of the sound, as to allow full Class A operation but cut power consumption at lower levels, I wonder how this is done, do they use a regulating transformer of some sort? I wonder how the history of electronic amplifers would have tunred out if this had been developed in the days of valves (which Class A was almost universal).

Quote from: beyondipod on
A good compromise is Class AB that has the transistor or valve biased so it doesn't switch off. But if it is biased too low, it too will suffer from crossover distortion.


As for disotrtion, I have nothing much to say but, some Class AB amplifiers (known as Class G) also improve efficianly using rail swithing (I wonder how, do they also use veriable transformers or a sort?), some go even further by modulating the power supply (Class H).

Quote from: beyondipod on
Class C is even more efficient than Class B but distortion is high and linearity is not good.


Class C (as far as I know) is never used as either an audio or vision amplifier, its main market is IF amplifcation (like in radio trasmitters).

Quote from: beyondipod on
On the other hand Class D operates like a digital device and is highly efficient. Traditionally, it is only used in subwoofers because of its limited bandwidth and relatively high distortion. But advances in electronics has made it possible for this class of amplifiers to be used as full range audio amplifiers with distortion levels similar to class AB.


What is yet to be exaplained to me is how such and amplifer can have in-band THD+N, if the switching frequency of the amplifer is greater than twice the signal bandwidth, as required by the Nyquist theorum, a bandlimited signal represented by a series of points my be reproduced unambigously provded the sampling rate is greater than twice the signal bandwidth (not simply an octave above the upper limit of the bandwidth). Could it be that in reality, the reconstructed signal never(!) exactly matches the orginal and is always superimposed by some kind of noise (some of it in band), even when the theorum is satisfied.

Quote from: beyondipod on
But it is definitely not the ultimate design for the purest sound.
Nor is Class A, even they aren't perfectly linear.
Quote from: beyondipod on
In pre-amplifiers and headphone amplifiers, Class A is still almost universal.
Maybe then Krell should license their technology out.