Should Class D amps be fed analogue or digital?

[dshreter]

With my small brain understanding of Class D amplification, they can operate with a digital input that goes straight into the amplification circuit without first requiring digital to analogue conversion. And that when provided with an analogue input, they do what could be described as analogue to digital conversion first before amplification. In other words, Class D natively works with a digital input.

Is this an accurate understanding? And if so, should we be seeking out amps that accept a digital input instead of analogue? It used to be easy where the basics were source (which could be a DAC), pre-amp, amp, passive speaker. Now with class D amps, powered speakers that can accept digital inputs, and all digital sources, I’m having trouble understanding the merits of when d/a conversion should take place and when volume control should take place.

 

[Doodski]

A Class D amplifier uses a analogue input. If there is a digital input then there is digital signal processing circuitry operating before the Class D amplifier stage.

 

[dshreter]

I understood that a digital input could directly feed the pulse width modulation, and was my takeaway from the way it is described here:
https://www.audioholics.com/audio-amplifier/class-d-digital-amplifier

Perhaps there are not associated performance benefits, or that I just have this wrong. Regardless I’d like to have a better understanding. The amp I’m using (Sonos Amp) I believe is described as taking digital input, and does ADC on the line input for this reason.

 

[Doodski]

I don't see where it says a digital signal can be processed in the Class D amplifier. The web link says, "...a digital amplifier that accepts only a digital input must rely on the incoming digital signal to lower distortion." It's a pretty general statement and I think they mean using a digital signal input is a better method but they generalized so much that they left out the part about a DAC being required before the Class D amplifier stage. Is that the part that got your attention?

 

[Doodski]

This link explains the Class D amplifier better than the Audioholics web link you gave.
https://en.wikipedia.org/wiki/Class-D_amplifier

 

[dshreter]

It was only a convenient source I found. More generally, I understood that the switching amplifier was fed via a pulse width input which is digital in nature. Which ties back to the original thought that you either have an ADC or can run fully digital until the gain stage.

 

[Doodski]

The amp is a Class D. It's not a digital amplifier as some are labeling it. It's pulse width modulation and that is not digital.

 

[0bs3rv3r]

I understood that the switching amplifier was fed via a pulse width input which is digital in nature.

No it isn't. You need to convert the digital numbers into the analogue pulse width signal?

 

[Doodski]

No it isn't. You need to convert the digital numbers into the analogue pulse width signal?

The digital signal needs to be put through a DAC before the Class D amp and then that analogue signal can be converted and amplified.

 

[0bs3rv3r]

The digital signal needs to be put through a DAC before the Class D amp and then that analogue signal can be converted and amplified.

Yeah, in my editing, I left in a ? at the end. I was indeed trying to say, you can't directly feed a digital signal into an analogue class D amp.

 

[Doodski]

Yeah, in my editing, I left in a ? at the end. I was indeed trying to say, you can't directly feed a digital signal into an analogue class D amp.

That's right. The Class D amp is a analogue circuit. The quasi square wave in the pulse width modulation stage might confuse people but it is a analogue signal amplifier.

 

[dshreter]

If it is fed a square wave signal, why would it require a true analogue input? I understand the topology of the popular amplifier modules is based on analogue input, but is that fundamental? I would think a digital input would be able to directly produce the square wave input without any conversion to the full analogue wave form.

 

[0bs3rv3r]

I would think a digital input would be able to directly produce the square wave input without any conversion to the full analogue wave form.

The "square wave" is a pulse width modulated ANALOGUE signal, not digital.

 

[0bs3rv3r]

Actually there is a way to generate the pulse width modulated signal from a digital input. You can use a computer to read the digital input, and generate the PWM signal using software - but I did not think that was being done by anyone. May be wrong. Still ends up as analogue PWM though.

 

[pma]

A Class D amplifier uses a analogue input. If there is a digital input then there is digital signal processing circuitry operating before the Class D amplifier stage.

Of course! It is a popular misunderstanding that class D is a "digital" amplifier. Oh no, it is an analog amplifier with impulse type modulation!

 

[dshreter]

Actually there is a way to generate the pulse width modulated signal from a digital input. You can use a computer to read the digital input, and generate the PWM signal using software - but I did not think that was being done by anyone. May be wrong. Still ends up as analogue PWM though.

Can you tell by looking at this brief if this operates in that manner?

https://www.qualcomm.com/media/documents/files/csra6620-ddfa-controller-product-brief.pdf

I really appreciate all of the responses here, so much to learn.

 

[Doodski]

Can you tell by looking at this brief if this operates in that manner?

https://www.qualcomm.com/media/documents/files/csra6620-ddfa-controller-product-brief.pdf

I really appreciate all of the responses here, so much to learn.

Yes, it appears this new Qualcomm IC has a digital audio input, processes the signal and outputs PWM.

 

[0bs3rv3r]

Can you tell by looking at this brief if this operates in that manner?

https://www.qualcomm.com/media/documents/files/csra6620-ddfa-controller-product-brief.pdf

I really appreciate all of the responses here, so much to learn.

That is just the controller chip, but yes, there appears to be a fully digital PWM modulator as part of the block diagram.

 

[pma]

There might have been one interesting exception - if the input was in DSD, then, in theory, only a LPF (low pass filter) operating at necessary power level would be enough to reconstruct the analog signal. The drawback would be quite high switching frequency, however, as a principle - maybe we shall see it sometimes in future.

 

[JohnYang1997]

The D in class D doesn't mean digital. End of story.

Also
The ones that accept digital inputs are generally having digital processing so they will convert analogue to digital then back to analogue if the input is fed analogue signal.