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In Reply to: RE: Corrections posted by Ralph on March 16, 2017 at 15:37:51
These units don't work with analog signals, so your forced statement IT'S ANALOG doesn't apply.
For those who don't think this is different, I invite them to read TAS review of Technics R1. Here, Robert Harley gives a good explanation of this approach.
From the link below (some images are missing- see the link):
Often erroneously referred to as "digital amplification", Class D amplifiers represent the zenith of amplifier efficiency, with rates in excess of 90% being achieved in the real world. First things first: why is it referred to Class D if "digital amplification" is a misnomer? It was simply the next letter in the alphabet, with Class C being utilized in non-audio applications. More importantly, how is 90%+ efficiency possible? While all the amplifier classes previously mentioned have one or more output devices active all the time, even when the amplifier is effectively idle, Class D amplifiers rapidly switch the output devices between the off and on state; as an example, Class T designs, which are an implementation of Class D designed by Tripath as opposed to a formal class, utilize switching rates on the order of 50MHz. The output devices are typically controlled by pulse width modulation: square waves of varying widths are produced by a modulator, which represents the analog signal to be reproduced. By tightly controlling the output devices in this way, efficiency of 100% is theoretically possible (although obviously not achievable in the real world).
Class D TopologyIQ M300 Amplifier
Example of a full bridge Class D circuit (left; sourced from sound.westhost.com), and the IQ M300 Class D amplifier, a pint sized 300W wonder (right).
Delving deeper into the world of Class D you'll also find mention of analog and digital controlled amplifiers. Analog-controlled Class D amplifiers have an analog input signal and an analog control system, usually with some degree of feedback error correction present. On the other hand, digitally controlled Class D amplifiers utilize digitally generated control that switches a power stage with no error control (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). Overall, it's worth noting that analog controlled Class D tends to have a performance advantage over its digital counterpart, as they generally offer lower output impedance and an improved distortion profile.
Next, there is the (not so) small matter of the output filter: this is generally an L-C circuit (inductor & capacitor) placed between the amplifier and the speakers in order to mitigate the noise associated with Class D operation. The filter is of considerable importance: shoddy design can compromise efficiency, reliability, and audio quality. In addition, feedback after the output filter has benefits. While designs that do not utilize feedback at this stage can have their response tuned to a particular impedance, when such amplifiers are presented with a complex load (i.e. a real world loudspeaker as opposed to a resistor), frequency response can vary considerably depending on the loudspeaker load it sees. Feedback stabilizes this issue, ensuring a smooth response into complex loads.
Ultimately, the complexity of Class D has its rewards: efficiency, and as a good consequence, less weight. As relatively little energy is wasted as heat, much less heat sinking is required. Ratcheting that up a notch, many Class D amplifiers are used in conjunction with switch mode power supplies (SMPS). Like the output stage, the power supply itself can be rapidly switched on and off to regulate voltage, leading to further gains in efficiency and the ability to shed weight relatively to traditional analog / linear power supplies. Taken together, it's possible for even high powered Class D amplifiers to weigh only a few pounds. The disadvantage of SMPS power supplies over traditional linear supplies is the former typically don't have much dynamic headroom. Our limited testing of Class D amps with linear supplies vs SMPS supplies have shown this to be true where two comparably rated power amps both delivered rated power, but the one with the linear supply was able to produce higher dynamic power levels. Still SMPS designs are becoming more commonplace now, and you can expect to see more high powered, next generation Class D amplifiers employing them.
This piece offers more proof that there are 'digitally-controlled amplifiers', which the link states.
IOW, not a digital amp, but a digitally controlled amp. No worries there.
But when referring to digital-controlled, it doesn't say 'analog'.
But as any technician can tell you, there's a huge difference between being 'all digital' and 'digitally-controlled'!
I play in a band and have a variety of synthesizers. Most are entirely analog but a few are digitally-controlled. However despite the digital control they are still considered analog synths because the oscillators and filters and such are still analog despite the digital control.
Much the same is going on with class D amps. As I've mentioned several times, the switching means that class D uses lends itself quite well to use with a digital input (with no analog input). But to get the output transistors to switch properly from the bit stream of the digital source there is an analog conversion system employed. Further, the usual considerations of power supply voltage, the risetime of the output devices and how long it takes for them to shut off, plus the stripping of the switching frequency from the signal at the output of the amp are all concerns in the analog domain.
Its easy to understand how a person might think that such an amplifier is a 'digital amp' but that moniker is erroneous as that is not how the amp works.
I think your confusion here comes from the fact that analog processes cover a lot more ground than just a simple audio signal! For example if there is a reflection on a transmission line you have to use analog techniques to tame it- if you don't, the digital system in which the transmission line (an analog device in itself BTW) resides will generate errors.
There is no analog stage - as John Atkinson, Robert Harley and the schematics show.
Its a lot more than just an audio signal...
Are you aware that the process of extracting data from a CD or magnetic disk drive is also an analog process?
We're not talking about those things. This is all-new technology, invented by the Danish (TacT Millennium -1998). Robert Greene of TAS reviewed it -twice.
You really don't have any idea do you!
I don't need an idea. These products are real-world, accept or not...
You don't to be confused with the facts huh?
Alright- I'm not going to argue with you. You don't want to know and apparently that can't be helped by your own admission.
These units don't work with analog signals, so your forced statement IT'S ANALOG doesn't apply.
This statement is outright false.
In order for a digital bit-stream to drive an amplifier, it gets converted into a series of pulses, which works out nicely for a class D amp, since that's what the converters in a normal class D amp do as well. So its a simple matter to marry a DAC and a class D amp together.
But that process is entirely analog. If it were not, there would not be a DAC in the circuit. There is no such thing as a pure digital amplifier.
Any hardware technician can tell you that the process that allows a computer or any digital product to work is also an analog process. The analog signals are merely interpreted as being digital ('on' or 'off'). But anyone that works with digital chips knows that they have a margin, for example a 5Volt digital chip is designed to interpret anything from about 4.2 volts to 5 volts and an 'on' signal. So its even worse than you imagined!
The reason there are things like 'errors' and the need for parity bits and the like is the simple fact that a digital signal can be eroded by the analog process in which it resides; degradation of magnetic or optial media, noise on transmission lines between ICs and so on. Its **all** analog. That's the world in which we live.
The situation here is that you like your gear and I'm not attacking it. What I am attacking is your argument which is heavily flawed, as others here are confirming.
You don't work with digital circuits, so your comments are ignored.
As to the unit in question, there is NO DAC in the circuit.
These type of products amplify in the digital domain, not analog. "Digital amp" would be the correct term.
And the output filter is one part, not a stage.
Under 'digital amplifiers':
This from your linked review:
"however, they accept digital rather than analog input signals. These "digital" amplifiers take in the pulse-code modulation (PCM) signal from a music server or other source and convert those audio data to a pulse-width modulated signal. This PWM signal then drives the output transistors, just as in a Class D amplifier . "
While its input may be digital, its output is necessarily the same as an analog switcher.
It might be -but the point is that the amp removes all analog stages. Inc. D to A converter, current to voltage, DAC output-ramp, etc.
-but the point is that the amp removes all analog stages.
This statement is false.
PWM (pulse width modulation) and PCM (pulse code modulation) are both analog processes. There is no such thing as a true digital amplifier.
I've never heard anyone say PCM is analog.
As for the unit, even Robert Harley says it: *digital* amp.
PCM is for the encoding of analog signals to digital.
But we're not encoding. We're only dealing with digital signals.
You won't be doing PCM. Instead, you'll be using PWM, which is an analog process. It can be adapted very nicely for use with digital systems.
The specific forms of PWM used in the amps to which you referred is either Delta or Delta Sigma.
Read the Wiki article at the link.
Notice things like 'waveform', 'integration' and so on- all analog thingys.
The fact is the world is analog and true digital does not exist in it. That is why IBM developed the parity bit.
I looked it up in a few different dictionaries. The consensus is that in the context of signals and data, "digital" means representing a quantity as a series of numbers, presumably finite, usually but not necessarily binary numbers.
So in the case of pulse width modulation, if the pulse width is constrained to be one of a finite number of possible values, then it would be fair to call it a digital signal.
Once you low pass filter it after amplification by a high power switching output stage, the output at the binding posts is analog. Kind of like a R2R ladder DAC, which is why some people call this type of Class D amp a "power DAC".
J.Phelan has a point in that the amplifiers he mentioned are different from other Class D amplifiers having digital inputs (e.g. Tripath) which have a more conventional DAC followed by PWM formed in the analog domain.
Clearly the line between what is an analog process is what is considered 'not' is a bit dubious. Your point is well taken, and not germane to where this thread started, although once that got debunked it drifted pretty wildly afterwards...
I look at things from an engineering standpoint as that is my training. And having built class D amps, know that they are an entirely analog process. As I mentioned earlier on in this train wreck, class D lends itself nicely to hybridization with a DAC for a more direct-conversion process, but its wholly inaccurate to say that such an amplifier it truly digital when an obviously analog process is employed. The guy that started this was insisting that class D was digital all the way to the output, which really isn't true at all- otherwise dead time circuits and other aspects of class D operation simply would not be needed.
You never built a class D amp, therefore your comments (should be) ignored.
Look at the Harley piece. A sample-rate converter is used -that is NOT analog. These are digital all-the-way, as far as the amp is concerned.
Clearly you've not read my prior posts...
I have in fact built class D amps. We've been developing our own for some time and have a patent in the works. I also linked a simple class D amp for you to look at, easy to build, and had you read the simple article (with lots of photos) at that link, you would also know that all class D amps are an analog process.
From the Wikipedia page on Class D amps:
The term "class D" is sometimes misunderstood as meaning a "digital" amplifier. While some class-D amps may indeed be controlled by digital circuits or include digital signal processing devices, the power stage deals with voltage and current as a function of non-quantized time. The smallest amount of noise, timing uncertainty, voltage ripple or any other non-ideality immediately results in an irreversible change of the output signal. The same errors in a digital system will only lead to incorrect results when they become so large that a signal representing a digit is distorted beyond recognition. Up to that point, non-idealities have no impact on the transmitted signal. Generally, digital signals are quantized in both amplitude and wavelength, while analog signals are quantized in one (e.g. PWM) or (usually) neither quantity.
emphasis added; after that follows why class D is in fact not digital.
Your company never sold a class D amp.
And you keep linking *outdated* information.
Besides Harley, here's another. Note the statements "digital input fed directly to the output stage", "keeps everything in the digital domain".
So how it works for you is that facts go out of date. Would it help if that Wiki page got refreshed recently??
So Norman Crowhurst (was writing 60 years ago) can be ignored? How about Shannon- I suppose he's pretty out of date too huh? and that Nyquist dude- he's so old he's dead? Sheesh!
The Wiki page is factual. If class D had somehow become true digital in the meantime, it would have been an article worth the front cover of every audio magazine in the world.
But apparently you only believe people who listen to stuff for magazines as the only possible experts in the world.
How about people that actually have an engineering degree or work on class D amps or anything like that?
Do you think that because we've only sold tube amps so far, that tube amps are somehow the only thing I know??
Is there any possibility that you simply don't me as a person? Any rational person would know the answer right away- you don't know me, don't know my qualifications, don't know what my job entails, etc. You just have made-up stories.
You've been taken in by advertising; hype, hook, line and sinker.
You have no proof you built a class D amp. I could say *I* built one !!
Now, it's not "marketing", it's "magazines". John Atkinson and Robert Harley were "taken" by advertising. I don't think so.
Digital amps are the future !!
Don't confuse the situation with facts huh?
I'm sure it does not matter how many awards I've gotten (about 35 or so) or how long I've been in business (over 40 years) or anything like that. You'll go with someone that writes instead of someone in the industry that actually makes amps for a living. Because you have to be right.
It'll be interesting when someone actually makes a digital amp.
From the link below:
Footnote 1: The D in class-D does not stand for digital, as some commentators have suggested. Rather, D was just the next available letter in the alphabet when amplifier circuit topologies were being classified. A class-D amplifier can be either digital or analog in operating principle.
The author, John Aitkinson, happens to be misleading in this comment, as there is no such thing as a digital amplifier. If you change the last sentence above to read " A Class-D amplifier is analog in operating principle" then the entire paragraph would be correct. How about I see if we can get JA to weigh in on this?
Mmmm...John Atkinson 'misleading', I hope he sees this.
You're the only person who says "no such thing as a digital amp". There is.
Now a schematic, which eschews analog stages. No wait, this is phony too !!
I've spent some time on that site in the past as it often comes up in Google searches. I would direct you to other portions of the site which refute your claims but since you've not read a lot of that sort of thing in the past there is no expectation that you would do so now.
JA would be welcome in the discussion. I'm sure he can clarify.
Can't refute the page I linked.
Atkinson and Harley have enough credibility, they don't have to 'clarify'.
By causing this thread to 'spiral', you pulled more attention to it. Thanks to you, they'll now be a buzz on (true) digital amps...
If you're claiming that the amp at that thread is a digital amp, well it uses analog process to do its job. Quite literally there is no way it can't.
This is true of any amplifier that can drive a loudspeaker.
If a reviewer that has never built an amplifier before wants to contest that, all that happens is he's wrong. In JA's case, I'm pretty sure he was using shorthand with his use of 'digital amplifier' and I'm also sure he would be happy to clarify that for you.
Did John tell you he would clarify ? These are digital amps, as the schematic shows...
And they're not easy to make. Noise-shaping is required, but I wonder if the Technics R1 system has a clock frequency high enough to avoid it.
The output stage has 1-part -analog. But this too looks different -it's now high-speed. Compared to the typical slow, losing-its-shape analog waveform.
The Technics is one of the few that has a clock speed approaching what is needed and so far looks like one of the better class D amps out there.
Your links don't prove anything, at least in this discussion.
Harley said digital amp and the signals are amplified in the digital domain.
Digital is everywhere inc cable-TV systems and satellites. For audio, DSP crossovers could be placed before a digital amp. (1) part in the output stage sends the signal to the speakers.
This technically is analog, but it's far different than the slow-speed circuits and gain stages we're all accustomed to.
So, faced with the rather obvious facts, you prefer to go with one person, who, if you were to ask him, would confirm everything I said?
Point him at this thread and see what he says!
Don't know what 'point him' means.
And it's more than one person -it's any co. making this type of product -inc. Classe. Plus AES papers explaining all this (which can't be linked).
That's because said papers don't exist :)
t might be -but the point is that the amp removes all analog stages.
Except of course for the output stage driving the speakers with an analog signal. :)
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