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"Digital" amplification -- is it really analog or does it co

Hi, I am new to this forum, so forgive me for the novice question, but I can't find an answer to my queston about digital amplification, so I'll ask here.

Do class "D" and/or Class "T" amplifiers convert an input from an analog source to digital once it enters and then back to analog on the way out? I've seen praises of a new Onkyo stereo integrated "digital" amp that I see has only rca analog inputs-- that' why it's got me thinking..

http://onkyousa.com/model.cfm?m=A-9555&...plifier&p=i

I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl, but it seems like manufacturers are finally going to start really pusing digital amplification in a big way now that a lot of the problems have been figured out and corrected-- so it might get harder pretty soon to find a solid state amp that isn't digital.

"Digital" amplification -- is it really analog or does it co

Reply #1
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Do class "D" and/or Class "T" amplifiers convert an input from an analog source to digital once it enters and then back to analog on the way out?
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Hi and Happy New Year!
Most integrated amps and receivers nowadays include DSPs - basically sophisticated tone controls. So if you input analog, it gets converted to digital, then back to analog.
This has nothing to do with the amplifier class (A, B, AB, D, T, whatever).
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I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl,

Vinyl has such inherently low quality that any correctly-done digital processing nowadays is unlikely to affect the perceived sound.
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but it seems like manufacturers are finally going to start really pusing digital amplification in a big way now that a lot of the problems have been figured out and corrected-- so it might get harder pretty soon to find a solid state amp that isn't digital.

Actually you can quite easily find a lot of good-quality vintage audio gear for sale, for 1/4 to 1/10 (or less) the price of new gear.
Example:



15 euros on the bidding site. It even has a "Direct" switch that will skip all the tone controls between the MM or MC phone preamps and the power amp stage.

"Digital" amplification -- is it really analog or does it co

Reply #2
Do class "D" and/or Class "T" amplifiers convert an input from an analog source to digital once it enters and then back to analog on the way out? I've seen praises of a new Onkyo stereo integrated "digital" amp that I see has only rca analog inputs-- that' why it's got me thinking..

http://onkyousa.com/model.cfm?m=A-9555&...plifier&p=i

I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl, but it seems like manufacturers are finally going to start really pusing digital amplification in a big way now that a lot of the problems have been figured out and corrected-- so it might get harder pretty soon to find a solid state amp that isn't digital.

Hi audiophil,
I'm just designing a class D amplifier for a university  course.
As far as I know the word “digital” is used only commercially, since they are all class D amplifiers (or theirs variants), maybe because class D amplifiers had a bad name among audiophiles. Anyway  I found the “digital” adjective used only when the source is digital and so the amplifier doesn’t require analog to digital conversion.

Also I found that class D amplifiers that work from an analogical source up till now *work better* than digital ones, because it is still not clear how to make a feedback with a mixed digital/analog system (probably another analog->digital conversion is required).

How much are they good? I don’t know – since I’m not an expert in this field and I’m not an audiophile (because of economical reasons  ). I know they are very popular because of their great efficiency, and their quality is improving very fast in these years.

I suggest you to judge their quality from their electrical specifications and your ears, without worrying about  their internal working. Also the analog to digital conversion that is required is different from the usual one you can find in CDs (PCM), since they often use PWM or sigma-delta modulation.

"Digital" amplification -- is it really analog or does it co

Reply #3
Here's a great article by Rod Elliot about these amplifiers, their advantages and their drawbacks.

I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl,
Why not? Surely you should be interested in the amount of distortion and noise your amp adds to your signal and not what goes on inside it? Don't be biased against a technology just because it says "digital" on the box.

but it seems like manufacturers are finally going to start really pusing digital amplification in a big way now that a lot of the problems have been figured out and corrected-- so it might get harder pretty soon to find a solid state amp that isn't digital.
I doubt that very much. Class AB amp modules are extremely common and cheap to produce and are going to be with us for a long time. PWM amps might take over in the budget and midrange sector, but the high-end will always come with a lot of choice.

"Digital" amplification -- is it really analog or does it co

Reply #4
Hi, I am new to this forum, so forgive me for the novice question, but I can't find an answer to my queston about digital amplification, so I'll ask here.

Do class "D" and/or Class "T" amplifiers convert an input from an analog source to digital once it enters and then back to analog on the way out? I've seen praises of a new Onkyo stereo integrated "digital" amp that I see has only rca analog inputs-- that' why it's got me thinking..

http://onkyousa.com/model.cfm?m=A-9555&...plifier&p=i

I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl, but it seems like manufacturers are finally going to start really pusing digital amplification in a big way now that a lot of the problems have been figured out and corrected-- so it might get harder pretty soon to find a solid state amp that isn't digital.

If it doesn't have digital inputs there is probably no conversion to digital. Some class d amplifiers (panasonic and sony) convert to digital but most are analogue.

"Digital" amplification -- is it really analog or does it co

Reply #5
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If it doesn't have digital inputs there is probably no conversion to digital.

This Onkyo in particular does A/D conversion to drive its switching output stage, even though it has no S/PDIF input (check the link provided by the OP). They (Onkyo) probably saved a few bucks in connectors, etc, and it looks more "audiophile-ready" like that...
As we all know an optical link has non-audiophile characteristics and S/PDIF will degrade sound... or so they claim.

"Digital" amplification -- is it really analog or does it co

Reply #6
Class D does NOT equal "digital."  Some marketing nitwit created that confusion at some point in time.  It's just a different mode of amplifying a signal that uses PCM, which is often associated with "digital."  Pretty much every AB receiver on the market converts analogue signals to digital on the way in and vica-versa on the way out.  That's what DACs and ADCs are for.  If you don't want the conversion, then pick an amp with a "pure direct" mode.  Such an amp can be any class.

"Digital" amplification -- is it really analog or does it co

Reply #7
Do class "D" and/or Class "T" amplifiers convert an input from an analog source to digital once it enters and then back to analog on the way out? I've seen praises of a new Onkyo stereo integrated "digital" amp that I see has only rca analog inputs-- that' why it's got me thinking.


Class D/T amplification has nothing to do with digital audio. The whole confusion stems from stupid marketeers trying to brainwash people into replacing all their existing components with ones with a "digital" sticker on them. See

http://en.wikipedia.org/wiki/Electronic_amplifier#Class_D

These amplifiers use Pulse Width Modulation(PWM) to represent the input signal, the are "digital" in the sense that the components are either on or off at a particular instant in the time and the amplitude of the signal is represented by the average value of the resulting square wave. This is the same mechanism which is used to control continuously variable RPM fans on newer CPUs.

They are not digital in the way a CD player is digital. CDs use Pulse Code Modulation, in this the wave form is sampled at a particular frequency (44.1KHz for CDs) the amplitude of the waveform at each point is represented by a number.

The advantage of class D/T amplifiers is power efficiency and as such most amplifiers in portable devices are class D/T, there is very little reason to use one for a mains powered amplifier. Most low end HiFi amplifiers are class AB, higher end ones are class A and most high power professional amplifiers are class H.

Refer to

http://en.wikipedia.org/wiki/Electronic_amplifier

for information on what each class means.

However as has been said elsewhere in the thread most home cinema receivers and amplifiers will convert everything to PCM digital as it comes in no matter if it is from an analogue input a S/PDIF input or a compressed stream such as AC3 or DTS. It then processes it and coverts it back into an analogue signal in order to amplify it and pass it to the speakers. The "direct" buttons on some such amplifiers will sometimes bypass the ADA conversion. On more conventional HiFi amplifiers they generally just bypass the tone controls.

"Digital" amplification -- is it really analog or does it co

Reply #8
These amplifiers use Pulse Width Modulation(PWM) to represent the input signal, the are "digital" in the sense that the components are either on or off at a particular instant in the time and the amplitude of the signal is represented by the average value of the resulting square wave. This is the same mechanism which is used to control continuously variable RPM fans on newer CPUs.

They are not digital in the way a CD player is digital. CDs use Pulse Code Modulation, in this the wave form is sampled at a particular frequency (44.1KHz for CDs) the amplitude of the waveform at each point is represented by a number.
Some PWM amplifiers are truly digital. They control the PWM digitally and use digital feedback (through an ADC). This approach has several disadvantages, partially related to having an ADC in the feedback loop. The more common designs control the PWM in analog, using analog feedback. This includes most so-called "digital" amplifiers, and most of what calls itself "class D". There are other non-PWM approaches, such as PDM and PAM which also exist.

Besides, the distinction between digital and analog in PWM is arbitrary and meaningless. Getting excited about whether quantized PWM (where there are a countable number of pulse widths) is digital or analog is meaningless and adds nothing to the debate.

"Digital" amplification -- is it really analog or does it co

Reply #9
Besides, the distinction between digital and analog in PWM is arbitrary and meaningless. Getting excited about whether quantized PWM (where there are a countable number of pulse widths) is digital or analog is meaningless and adds nothing to the debate.


I quote cabbagerat, but I thought  that a true "digital" PWM need to be time-quantized. I mean "digital" as a signal that can be represented only with numbers with a finite precision. Class D doesn't require this "digital" PWM, if this can give some safety to audiophilosophy 

btw as cabbaregat said all this is meaningless, the main problems of the Class D amplifiers are in the power stage, after the modulation.

Using or not the word "digital" doesn't add or subtract anything to the internal working of the amplifier, neither to its quality. 

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Most low end HiFi amplifiers are class AB, higher end ones are class A and most high power professional amplifiers are class H.


I saw something about class H - their principle of working seems almost more problematic than class D. Are they used when it is needed high power more than sound quality? I think they will probably be replaced by class D in the future.

"Digital" amplification -- is it really analog or does it co

Reply #10

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If it doesn't have digital inputs there is probably no conversion to digital.

This Onkyo in particular does A/D conversion to drive its switching output stage, even though it has no S/PDIF input (check the link provided by the OP). They (Onkyo) probably saved a few bucks in connectors, etc, and it looks more "audiophile-ready" like that...
As we all know an optical link has non-audiophile characteristics and S/PDIF will degrade sound... or so they claim.

The link bandies about the word "digital" but it still could be analog-controlled class D. They may think that the on-or-off PWM signal gives them the right to call it digital.

"Digital" amplification -- is it really analog or does it co

Reply #11
I saw something about class H - their principle of working seems almost more problematic than class D. Are they used when it is needed high power more than sound quality? I think they will probably be replaced by class D in the future.


Class H amps are mostly used where you need power, you can't make a 1KW class A amp unless you have some kind of exotic cooling. Almost all pro power amplifiers, what you hear when you go to the cinema, a theatre, a club etc. are class H. Some of these designs can put 7KW into 4 Ohms in bridged mono mode.

Recently companies such as QSC, Crest and Crown have been moving over to switch mode PSUs reduce the weight of their amps.

I don't think conventional class D amps will replace class H amps soon simply due to the difficulty of designing them, the incumbent companies know how to design a class H amp and a well designed class H amp is going to sound better than a badly designed class D amp.

There are some companies with products in the market for example Yamaha's EEEengine which is some kind of improvement on a class D amp however I've never heard one so I can't comment on the sound quality.

"Digital" amplification -- is it really analog or does it co

Reply #12
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I would prefer to have an amp that did not convert an analog signal to digital if I were using it to listen to vinyl,

Vinyl has such inherently low quality that any correctly-done digital processing nowadays is unlikely to affect the perceived sound.


Maybe not if in mint condition and played on a properly set up high end tunrtable, anyway, would you conversing prefer one that can generate the pulse train using DSP if you were listening to a digital source.

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The link bandies about the word "digital" but it still could be analog-controlled class D. They may think that the on-or-off PWM signal gives them the right to call it digital.


My response is: False advertising. On-off PWM gives them the right to call it that if it is generated by actual DSP, and thus every pulse spans between two clock transitions each, which in turn means that the number of possible pulse widths depends on the bitrate (detrimined by the master clock (MCK) rate) to switching frequency ratio. So, on the topic, what is the formula for calculating which MCK to switching frequency ratio is equivalent to PCM at a given resolution? A PWM amplifier with a switching frequency of 705.6 or 768khz and an MCK ratio equivalent to 14 bit (remember that averaging 4 consecutive samples (without noise shaping) adds one extra bit of resolution, another bit is added every time we multiply this by another four), would be a good choice for 16 bit at 44.1khz (with the former switching frequency) or 48khs (with the latter switching frequency).

 
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