2. amps-preamps
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Digital input amp

I see a few coming on market.  Previous threads were quite old.

They take a USB or other input. No DAC on the front end. Just the DSP engine to do volume, eq, etc, then whatever conversion to PCM to feed the class D output. So the only "DAC" is the output stage.   This makes sense as it further reduces the functions in the chain.  I have not seen objective testing or any reliable subjective testing. The ones I have seen are Infineon processer based. No idea if the output filtering or feedback implementation is up to the Purify quality.   I guess the next innovation is the GaNFET output.  One has a DAC to feed a sub out. Easy as any old $5 DAC will wo there. 

I was browsing and came across the Sajab A30a. ( quite inexpensive)  Peachtree has the old Gan-1 but coax PCM only.  That would be fine if it managed buffering and clocks internally. Unfortunately my all-in-one only has USB out. 

I suspect there is a lot to be learned here but it makes sense to me for the future. 

tvrgeek
atmasphere's avatar
atmasphere
12,138 posts
 

Not these designs.  They take a bitstream , do DSP, do the digital conversion to PWM and feed the output. So the actual integrator and filter is the output.  There is no DAC on their input. 

@tvrgeek What you are describing above means that the DAC is at the input of the class D amp portion of the product. Put another way, the PWM portion of the circuit is at the input of a class D amp, not its output.

Digital at its fastest might be 192KHz sampling frequency. Class D amps often switch at 500KHz or higher. You can design a DAC that can produce a PWM output, but it really does have to be at the input of the class D amp in order to work. I know a lot of companies say they have a 'digital amp' but if they also say its class D then the DAC is always at the input to the class D circuitry.

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tvrgeek OP
299 posts
 

Maybe you should look at the block diagram of these products.  They take digital in. Process within a DCP chip which produces the PWM signal to feed the output.  

cleeds
6,226 posts
 
atmasphere

I know a lot of companies say they have a 'digital amp' but if they also say its class D then the DAC is always at the input to the class D circuitry.

That's what I always thought. Thanks for confirming, Ralph.

tvrgeek OP
299 posts
 

This technology HAS NO DAC. Please go look at the products. Look at the Peachtree GAN1 as well as the Sabaj.   I think NAD has one with built in streamer. 

PCM in. Convert to PWM. Switch outputs.  Integrate.   NO DAC. 

I don;t know what other companies are advertising. The copy writers may not have a clue either. 

erik_squires
16,795 posts
 

OP:

Exactly how are you defining a DAC? From your description, the amp IS A DAC.

Simply put, a DAC stands for digital to analog converter. You don’t listen to PWM. You listen to, must listen to, music as an analog event.

So that PWM signal must go somewhere, and the moment it becomes an analog signal you have a DAC.

I should also point out that Class D is an analog, but switched system. A purely PWM driven amplifier has complications of it’s own, especially how the lack of feedback is a challenge.

To borrow a term from marketters, this is a powerDAC.

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