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Old 10th October 2013, 11:47 AM   #1
bimo is offline bimo  Indonesia
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Default Why do we need analog line until now?

I wonder why i can not find yet, amplifier with digital input only that D/A produce very high voltage then we need only current amplifier (buffer) to drive speaker?
Is this topology amplifier impractical?

Usually, now music is recorded digitally. Then why do we need analog line? I know there are digital class D amplifier (according Bob Cordell). If this amplifier's performance can not achieve the performance of class A or A/B amplifier, then we need very high voltage D/A.

I think, line level must be digital (volume control, balance control , mixer, equalizer).

What do you think?
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Old 10th October 2013, 01:00 PM   #2
DF96 is offline DF96  England
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The music starts as an analogue signal and ends up as an analogue signal. A better question might be: why do we want to put some digital in the middle of this analogue chain? The reason is recording and playback, which digital can in principle do better (given a restricted dynamic range, although not as restricted as analogue recording requires). Having recorded and played back, staying in analogue is better.

Direct digital right through to just before the final buffer could have at least three problems:
1. High voltage DAC could require compromises not needed for low voltage DAC e.g. thermal management, dielectric non-linearity.
2. Digital volume controls require more bits in the DAC in order to maintain low-level signal integrity.
3. Combining DAC and power amp mixes two challenging technologies together - an analogue link between them means they can be done by different people with different skills.

Don't confuse Class D with digital. Class D is not digital, but just switching - it is pure analogue.
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Old 11th October 2013, 03:04 AM   #3
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Quote:
Originally Posted by DF96 View Post

Don't confuse Class D with digital. Class D is not digital, but just switching - it is pure analogue.
Bob Cordell write this in his book: " Finally, there is direct digital class D. In this case a PCM
digital input signal is converted in the digital domain to the string of pulses required to
drive the output MOSFET switches."

A class D amplifier with digital input. Is Bob Cordell wrong?
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Old 11th October 2013, 03:35 AM   #4
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There would be still the BW problem, as a lowpass filter is needed at the very end of the chain. Coils are so analogue! So maybe it is a problem of 'rising BW' you have to fight.
So the power devices, I don't know about class D (same for any class ,either..! ) but I suppose that several devices are used to 'reconstruct the wave' and they must be matched. Indeed, we see lots of multi-legged micro ICs with few power ( and very efficient)
for portable equipment. An amplifier for home use -about 30 W- I haven't seen any, not full-D

Last edited by picowallspeaker; 11th October 2013 at 03:45 AM.
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Old 11th October 2013, 09:55 AM   #5
DF96 is offline DF96  England
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I guess direct digital Class D might be possible, but people often think ordinary Class D is digital when it isn't. Ordinary Class D is analogue, except that the signal is carried by pulse width or pulse repetition rate (both analogue representations) rather than voltage level. Direct digital Class D would require very fast switching rates, or an astonishingly complicated output filter, in order to suppress images.
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Old 11th October 2013, 11:00 AM   #6
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I think what Bob is talking about is commonly referred to as a "Power DAC". Digital in, Analog out - with enough power to drive a speaker. I tired to build one circa 1990, and made a complete mess of it.

I do have a couple of amp eval boards here with digital only inputs. Not sure it's digital all the way thru, tho.
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Old 18th October 2013, 01:46 PM   #7
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by Pano View Post
I think what Bob is talking about is commonly referred to as a "Power DAC". Digital in, Analog out - with enough power to drive a speaker. I tired to build one circa 1990, and made a complete mess of it.

I do have a couple of amp eval boards here with digital only inputs. Not sure it's digital all the way thru, tho.
If your design is not for commercial, can you share to me?
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Old 18th October 2013, 02:04 PM   #8
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Quote:
Originally Posted by bimo View Post
I wonder why i can not find yet, amplifier with digital input only that D/A produce very high voltage then we need only current amplifier (buffer) to drive speaker?
Is this topology amplifier impractical?
I'm of the view that this topology might well have some merit. I'm planning to experiment with the following signal chain which seems quite similar to your own suggestion :

NOS current-out DAC -> common base transistor -> passive I/V from B+ supply -> LC anti imaging filter -> darlington buffer -> speaker

So far I've not decided where the best place for the volume control might be. The DAC has a reference input so some of the volume control range can be implemented there, but lowering the reference current too much lowers the dynamic range. So possibly a hybrid (two part) volume control will be called for. Another option is to employ some variation on a Gilbert cell in place of the CB transistor.
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Old 20th October 2013, 02:40 AM   #9
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Quote:
Originally Posted by abraxalito View Post
I'm of the view that this topology might well have some merit. I'm planning to experiment with the following signal chain which seems quite similar to your own suggestion :

NOS current-out DAC -> common base transistor -> passive I/V from B+ supply -> LC anti imaging filter -> darlington buffer -> speaker

So far I've not decided where the best place for the volume control might be. The DAC has a reference input so some of the volume control range can be implemented there, but lowering the reference current too much lowers the dynamic range. So possibly a hybrid (two part) volume control will be called for. Another option is to employ some variation on a Gilbert cell in place of the CB transistor.
I want to know about your progress. I glad that someone has almost same idea.

I think digital pre-amp must contain DSP (digital signal processor). It has volume control, user defined parametric equalizer, at least 3 independent digital output with user defined active crossover. RTA (real time analyzer) is great feature to measure room frequency response. It is like computer for manipulate audio signal with simple user interface.
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Old 20th October 2013, 03:09 AM   #10
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Quote:
Originally Posted by bimo View Post
I want to know about your progress. I glad that someone has almost same idea.

I think digital pre-amp must contain DSP (digital signal processor). It has volume control, user defined parametric equalizer, at least 3 independent digital output with user defined active crossover. RTA (real time analyzer) is great feature to measure room frequency response. It is like computer for manipulate audio signal with simple user interface.
And all signal connections can use wireless. Remote control is using smartphone. Maybe just my dream
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