Digital / Analog questions

[monkumonku]

Being a total layman, maybe I am asking about apples versus oranges but here goes.

I just got a class D amp which I understand is digital, meaning that it receives an analog signal from a source such as a preamp or processor (like an XMC-1), then converts that signal to digital, and then converts it back to analog to feed the speakers.

Also, it is my understanding that when an analog signal is fed into the XMC-1 and Dirac is engaged (or if not Dirac, then bass management), that analog signal has to get converted to digital for processing, which then is converted back to analog to be output to an amplifier. In fact, for any case except Reference Stereo, there is a conversion that takes place.

So my questions are:

If an analog feed is converted to digital by the XMC-1 then converted back to digital (except in the case of using Reference Stereo mode), then what is the point of using an external DAC to decode the signal to feed the XMC-1 (or any other processor that does the same thing)? Wouldn't the end product be a result of all of the digital to analog and analog to digital conversions?

Same with feeding a digital amp. With all the fuss made about the differences between external DAC's (such as the Yggy versus Western Electric versus whatever), if a digital amp takes an analog signal then converts it to digital and back to analog, why isn't there a lot of fuss made about the DAC inside of a class D amp? That would seem to be the last conversion step in the chain and would influence the result.

Why can't a preamp or processor take a digital signal and instead of converting it just send it out to a digital amp (i.e., have a digital amp with digital inputs) but just use volume control or whatever other signal alterations are made in the digital realm? Why bother converting something to analog then back to digital except for the end step of sending an analog signal to the speakers?

Again, forgive me if this is apples and oranges and my questions don't make technical sense, but they're something I have wondered about.

[geebo]

Good question. If using a class D amp why not just keep the signal in the digital realm from the source all the way to the amp where it would be converted once and only after amplification?

Maybe KeithL could weigh in on this.

[garbulky]

great questions

[AudioHTIT]

I think it is a misnomer to call a Class D amp a 'digital' amp, but I'm not qualified to explain that. However I have heard of power amps (possibly Class D) with digital inputs as you speculate would be beneficial, but I haven't explored them further.

[Percussionista]

Nov 5, 2015 13:45:09 GMT -5 geebo said:

Good question. If using a class D amp why not just keep the signal in the digital realm from the source all the way to the amp where it would be converted once and only after amplification?

I seem to remember this has been done, maybe by Meridian (well, not sure about the amp class)? And connect up all components with ethernet cable.

[geebo]

Nov 5, 2015 14:14:30 GMT -5 Percussionista said:

Nov 5, 2015 13:45:09 GMT -5 geebo said:

Good question. If using a class D amp why not just keep the signal in the digital realm from the source all the way to the amp where it would be converted once and only after amplification?

I seem to remember this has been done, maybe by Meridian (well, not sure about the amp class)? And connect up all components with ethernet cable.

I checked a little on wiki and see that a digital amp may use Pulse Width Modulation (PWM) vs Pulse Code Modulation (PCM). So maybe that has something to do with it?

[geebo]

PCM




PWM

[yves]

Class D amps are not digital because digital amps don't exist.

www.eetimes.com/document.asp?doc_id=1274757

[monkumonku]

Nov 5, 2015 14:29:57 GMT -5 yves said:

Class D amps are not digital because digital amps don't exist.

www.eetimes.com/document.asp?doc_id=1274757

Thanks for posting that, although most of it was over my head. So then why do people refer to these as digital amps? Is it because of this:

In this type of amplifier, the switches are either fully on or fully off

So that's like things being either a 0 or a 1 which likens them to digital signals?

[Boomzilla]

Hi monkumonku

As I understand it, no - "Class D" amplifiers do NOT convert back & forth from analog to digital. Not in the sense that DAC and ADC converters do. My understanding may be wrong, so take the following as speculation based on reading.

The analog input signal is "digitized" at the amp's inputs, but not in the usual way. The "sampling" of the analog input signal is amplified (in the digital domain), and then filtered to remove the high frequency digital noise and to somewhat smooth the output signal. This does not mean that the output is "converted" back to analog. Instead, the smoothed digital signal is sent directly to the speakers.

Now I'm confident that I've botched this explanation sufficiently that Mr. Levkof will need to wear out his fingers correcting my misconceptions, but that's OK - I'll expect to learn something too!

As to your concerns about the analog signal being re-digitized by the XMC-1 for processing, you are EXACTLY correct, as I understand it. Therefore, using a digital input to the XMC-1 (and letting the processor use its own DACs) is the right way to go (theoretically). The fewer AD / DA conversions in the signal path, the better the sound quality.

The only exceptions to the above, to my understanding, are the NAD series - which keeps the signal digital throughout without needing ANY conversion back to analog.

And, as always, I may be totally wrong!

Boomzilla

I did find this online, but it isn't too technical: Class D – This class of amplifier is a switching or PWM amplifier as mentioned above. This class of amplifier is the main focus of this appli- cation note. In this type of amplifier, the switches are either fully on or fully off, significantly re- ducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier sig- nal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

[monkumonku]

Nov 5, 2015 14:42:10 GMT -5 Boomzilla said:

Hi monkumonku

As I understand it, no - "Class D" amplifiers do NOT convert back & forth from analog to digital. Not in the sense that DAC and ADC converters do. My understanding may be wrong, so take the following as speculation based on reading.

The analog input signal is "digitized" at the amp's inputs, but not in the usual way. The "sampling" of the analog input signal is amplified (in the digital domain), and then filtered to remove the high frequency digital noise and to somewhat smooth the output signal. This does not mean that the output is "converted" back to analog. Instead, the smoothed digital signal is sent directly to the speakers.

Now I'm confident that I've botched this explanation sufficiently that Mr. Levkof will need to wear out his fingers correcting my misconceptions, but that's OK - I'll expect to learn something too!

As to your concerns about the analog signal being re-digitized by the XMC-1 for processing, you are EXACTLY correct, as I understand it. Therefore, using a digital input to the XMC-1 (and letting the processor use its own DACs) is the right way to go (theoretically). The fewer AD / DA conversions in the signal path, the better the sound quality.

The only exceptions to the above, to my understanding, are the NAD series - which keeps the signal digital throughout without needing ANY conversion back to analog.

And, as always, I may be totally wrong!

Boomzilla

I did find this online, but it isn't too technical: Class D – This class of amplifier is a switching or PWM amplifier as mentioned above. This class of amplifier is the main focus of this appli- cation note. In this type of amplifier, the switches are either fully on or fully off, significantly re- ducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier sig- nal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency.

Thanks. You must have been typing when I posted right before you. Yves said the same thing about the amps not really being digital, and that paper he referenced says basically what you said in your reply.

So that answers my question on the amp but as far as the other one - if the XMC-1 converts an analog signal to digital for processing (except in Reference Stereo), then what would be the point to using an external DAC with it? Is there any point, or does the processing within the XMC-1 make it moot?

[yves]

The article that I linked in my first reply can be a little confusing, I agree. Perhaps a more easy to understand explanation can be found under "D Does NOT Equal Digital" in this article: www.extron.com/company/article.aspx?id=ts122001

[jmilton]

However, some class D can take digital and/or analog inputs. The Lyngdorf SDA2400 is an example.

www.lyngdorf.com/products/sda-2400

[monkumonku]

Nov 5, 2015 15:07:39 GMT -5 yves said:

The article that I linked in my first reply can be a little confusing, I agree. Perhaps a more easy to understand explanation can be found under "D Does NOT Equal Digital" in this article: www.extron.com/company/article.aspx?id=ts122001

Thanks, that one was easier to understand!

[monkumonku]

Nov 5, 2015 15:18:55 GMT -5 jmilton said:

However, some class D can take digital and/or analog inputs. The Lyngdorf SDA2400 is an example.

www.lyngdorf.com/products/sda-2400

So, a question about this $2,400 amp. Since it has no volume control, what happens if you plug a digital source that likewise has no volume control such as a CD player into it? Does the signal go out at 100% volume?

[DYohn]

As has been said, calling Class D "digital" is marketing BS. There is nothing digital about a class D amplifier except for the fact that the output transistors simply switch on and off very quickly in response to the changing analog waveform, varying the output voltage. It is an analog amplifier as all audio amplifiers must be) but it is more efficient than class A or A/B because the transistors switch off, using less power over time. Amplifiers that accept digital inputs contain DAC circuitry on board to covert to analog for the output stage. They are simply DAC + Amp in one package. They require a digital preamp to control volume, etc.

[jmilton]

Yes, it is recommended that the "source" have a volume control. I plan on testing it via the Oppo 103's volume. Certainly a DAC like the DC-1 or XDA-1/2 would also meet that criteria.

[geebo]

But if the analog signal is converted to a series of on or off pulses, can it be correct to call it analog?

[yves]

If neither the digital source nor the power amp has a volume control, then you need to either grab an Emotiva Control Freak or, alternatively, add a preamp that has a volume control. Neither my separate DAC nor my XPA-2 have one, so I am using the volume control of my UMC-200 for this exact reason.

[monkumonku]

Nov 5, 2015 15:43:20 GMT -5 jmilton said:

Yes, it is recommended that the "source" have a volume control. I plan on testing it via the Oppo 103's volume. Certainly a DAC like the DC-1 or XDA-1/2 would also meet that criteria.

On a tangential note, is Lyngdorf stuff supposed to be good? I don't know anything about them but looking around their web site reminds me of Bose. All talk and no real specs or measurements to back anything up. Lots of nice pictures and claims of excellence, though.

Like this blurb from the product page, for example:

Lyngdorf is the pioneer in digital amplifiers, and the SDA-2400 power amplifier is not your typical Class D digital power amplifier. It’s better. It has very low and linear distortion and an impressive signal-to-noise ratio, making it the perfect choice for those seeking the purest sounding high-power amplifier on the market.

The output stage uses Pulse Width Modulation with a patented switch speed optimization technology. Together with a fixed switching speed of 390 kHz and minimal feedback control, this ensures low and linear distortion compared to typical Class D amplifiers.

Yet they offer nothing in the way of practically demonstrating what they are claiming. Maybe they're truly a Rolls Royce of audio but their web site makes me skeptical.