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Digital amps: what's the deal?

vert

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#1
I figured that would be the right section to post this, as these new amps are called digital amps.

Recently while browsing the Audiophonics site (French distributor and manufacturer of audio gear) I noticed a model by SMSL called the AD18, a digital amp that has apparently made a small splash in audiophile circles (at least in France). There are 80 reviews for the amp which is an unusually high number for the site, from people who have tried it sometimes with really good speakers and compared it to serious amplifiers. Assessments range from the SMSL being called revolutionary and ground-breaking, to people saying it is competition for mid-range analog amps at most and not truly audiophile. But most seem to think it's a pretty incredible deal for just over €100.

I'm used to the analog vs. digital debate as regards guitar amps, which can easily veer into fanaticism. I'm all for innovation. I think Leo Fender and Christopher Kemper have a lot in common. In terms of convenience, at the minimum, I think digital guitar amps are a fantastic invention. I wasn't aware until now the technology was emerging too in audio land. Is anyone familiar with that trend? I think it's quite interesting, and given the reviews the SMSL gets, I can't help but wonder where the technology will be just a few years from now.
 

dc655321

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#2
I think class-D audio amps have been around for quite a while. But, you may be correct that this "new" tech is catching on at an accelerating rate.
 

DonH56

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#4
For the record, the "D" in class-D does not stand for "digital". It is just another is the series of amplifier classes -- A, B, C, D, etc. The concept has been around a very long time (I do not recall off-hand, many decades).

Was the 1968 or so Infinity Servo-Statik bass amp class D or just a normal class-AB using a servo loop? I know it was a servo amp, not sure on the class...

Class D amplifiers have really improved in the last decade or so and now are very competitive. Advances in technology (faster transistors and such) and circuit design (stable feedback using feed-forward compensation and such) have really helped.
 

dc655321

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#5
For the record, the "D" in class-D does not stand for "digital". It is just another is the series of amplifier classes -- A, B, C, D, etc
SMSL advertise the amp as "full digital". Not sure what that means exactly (has a whiff of marketing to it).
I took it to mean a "switching" amp.
 

sergeauckland

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#6
A Class D, i.e switching, amplifier can be entirely analogue if the switching is PWM, where the pulse width
corresponds to the analogue envelope. In those cases, 'digital' is entirely marketing. PWM amplifiers go back to the 1960s albeit they weren't very good due to limited output devices.

S
 

amirm

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#7
FYI I have the SMSL AD18. Will review it soon. :)

For now, the "advancement" has come from highly integrated implementations of class-D which make the designs much simpler and cheaper. I think the original motivation was for car audio as saving weight and size was important there. Then a second massive market has developed which is Bluetooth speakers. Efficiency. low power consumption, and size/cost are very important.
 

DonH56

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#8
SMSL advertise the amp as "full digital". Not sure what that means exactly (has a whiff of marketing to it). I took it to mean a "switching" amp.
Me neither. More than a whiff to me. Digital to me would imply digital IO and all gain and control is digital. And presumably the output amp is class D since Marketing tends to think it is "digital".
 
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#10
Me neither. More than a whiff to me. Digital to me would imply digital IO and all gain and control is digital. And presumably the output amp is class D since Marketing tends to think it is "digital".
Full digital refers to the TI amplifier chip; they come in either digital or analog input configurations with PWM out. The AD18 uses TAS5342A which is digital input from the TAS5508C processor and eliminates the D/A conversion.

I have an AD18 and it's a really great amp. I feed it via USB and WASAPI but the Apt-X Bluetooth works well too. It also has two optical inputs and a summed subwoofer line out. Oh and a remote too.

As for sound quality I'm anxiously waiting for Amir's test. Subjectively sometimes the little amp sounds great and sometimes it doesn't depending on source, volume, and content. I lay the blame on their DSP. Before I commit to digital amplification I'd like to go through only one DSP processing preferably within the player software (I use Foobar) then have wireless transmission of the I2S directly to one of these digital input chips. Until then for my system I've gone back to the Dac, analog pre-amp, analog amp chain.
 

Soniclife

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#11
The AD18 uses TAS5342A which is digital input from the TAS5508C processor and eliminates the D/A conversion.
That's the bit I'm confused by, it is doing D/A conversion, digital goes in, amplified analogue signal comes out to go to the speakers, so what does it mean?
 

dc655321

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#13
Interesting topology with these chips: I2C data --> PWM --> class D amp --> noise-maker
Thanks for the chip info, @Dilliw.
 

amirm

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#14
That's the bit I'm confused by, it is doing D/A conversion, digital goes in, amplified analogue signal comes out to go to the speakers, so what does it mean?
It is a two-chip solution. The first chip is in charge of digital to analog conversion to a PWM signal rather than analog waveform. That PWM "digital" signal is then sent to the second chip for amplification and filtering which converts it to analog waveform.

Here is the first chip diagram:

1529514798585.png


And second:

1529514822658.png


As you see the input to the second one is a PWM signal which is a switching waveform and hence the use of the word "digital."
 

Soniclife

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#15
As you see the input to the second one is a PWM signal which is a switching waveform and hence the use of the word "digital."
So it really is a digital amp, and it does not use a conventional DAC. So it's not DS or R2R?

It must be the same as used by Eve Audio, they say.
And as the PWM-amplifiers are directly connected to the DSP section, no additional conversion is necessary. This guarantees an extreme reliability.
 

sergeauckland

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#16
How is a PWM switching amplifier 'digital'? If the width of the pulses is an analogue of the 'analogue' output signal, then the amplifier isn't digital at all. A digital amplifier should add bits to the output, each bit adding to the total.

My understanding is that's how the Harris Digital AM transmitter works, each bit pumps energy into the output, so there's no DAC function, just an ADC for analogue inputs.

S.
 

amirm

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#18
How is a PWM switching amplifier 'digital'?
It is "digital" in the sense that it has pulses that swing to power supply rails. It is not digital in the sense of conveying digital data.

In this case they are distinguishing between an amplifier that accepts an analog waveform and one that accepts PWM.
 

DonH56

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#20
Marketing and engineering often define "digital" differently, and perhaps there is some difference in terminology among engineers. In this case, they mean to imply no conventional analog signal anywhere in the signal chain until the amplifier's output. So the output is not truly "digital" but the signal is all digital to that point, and then marketing takes over to say that a class D amplifier is also digital. Whatever they call it, at least I think I understand the signal path now, thanks.
 
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