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Some Class D amps are purely analog and some are (almost) purely digital

Posted by Dave_K on May 30, 2018 at 05:15:47

In Reply to: RE: No semantic aspect to it. Class D incorporates digital technigues... posted by AbeCollins on May 26, 2018 at 23:33:45:

Class D means PWM-based amplification, nothing more. If the quantity represented by the pulse width is continuously variable, it's analog. If the quantity represented by the pulse width is constrained to discrete value (quantized), it's digital.

The earliest Class D amplifiers were all analog, e.g. John Ulrick's Infinity and Spectron amps and the original Sonys. They took an analog signal in, used it to modulate a pulse width - using an analog modulator, and linearized the output using analog feedback.

At the other end of the spectrum lies Toccata's Equibit which first came to market in the TACT Millennium. Toccata was bought by TI and renamed PurePath digital. Equibit amps have no DAC. The PWM waveform used to drive the output stage are digital because the pulse widths are discrete. The limited number of discrete pulse-widths leads to quantization noise. Noise shaping is used to push the quantization noise out of band where it gets filtered out by the low pass filter on the output stage. There is also no feedback. Non-linearities of the output stage are pre-corrected in DSP.

Tripath was similar in having a PCM input and eschewing feedback in favor of DSP pre-correction, but the main reason Tripath eliminated the feedback loop was low cost. Tripath amps have a DAC preceding the pulse-width modulation, so unlike the Equibit/PurePath amps it shouldn't really be called a digital amplifier.

A more common approach today seems to combine an analog modulator with a digital control loop. The feedback loop in this kind of amplifier consists of A/D, DSP, and D/A. NAD Direct Digital is a good example. I haven't yet bothered to read the Hypex Ncore patent, but the whitepapers suggest it uses a self-oscillating analog PWM modulator (which avoids the need for a highly time-stable carrier) and a digitally implemented sliding mode control, so I think it falls into the category of analog modulation with digital control.

And then there are the Sharp digital amplifiers, which are not really Class D because they use 1-bit modulation (PDM rather than PWM). They're basically a high power delta-sigma DAC, so you could presumably call them digital amplifiers.

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