Direct Digital Amplifiers

The tact s2150 amplifier is all digital with built in dac. Just plug in your transport via aes into the amp and you're ready to go.

Bombaywalla, Class D amp doesn't convert anything to digital. It converts analog voltage to analog duty cycle of square wave and back to analog voltage thru filtering....

you are right, Kijanki. I was trying to be simplistic in the interest of explaining the concept.
OTOH, Kijanki, a 2-level signal which is going from logic0 to logic1 could be considered a (very simple) digital (or binary) signal. Such a signal is both analog & digital. It has infinite resolution like an analog signal & at the same time it is discrete since it transitions from logic0<-->logic1 with no other state in between.
So, I do not think that I confused anything here. Both your comments & mine are correct.
But, yes, it's not a tradition digital signal like the one that is quantized with finite # of bits.

Kijanki, I know of a least one amplifier brand (tube) that only has one stage of gain. That's pretty simple- far more simple than any class D amp.

Atmasphere, that's true but output power is very limited while in class D it depends only on power supply and size of output Mosfets.

Bombaywalla, Adjusted analog quantity is duty cycle. For instance, when you adjust oscillator's frequency by turning knob it doesn't matter if output signal is square, triangle or sinewave - your adjustment is still analog. The fact that voltage jumps between two level's doesn't make it digital if adjusted quantity is duty cycle. Also, amplifier doesn't make any use of these levels other than converting to output voltage by taking average value (filtering).
Distorted electric guitar (square wave) dosn't make it digital. FM radio is not digital in spite of voltage moving between two levels.

Taken from the tact s2150 owners manual:

The digital input is taken to the central processor where it is reformatted into apulse width modulated signal of extreme precision. The pulse rate is measured atprecisely 384,000 pulses per second. Each pulse can have 256 different widths,with the narrowest pulse being a mere 10 nanoseconds wide. The clock frequencytherefore is 98 MHz. The central processor uses proprietary patented algorithms(Equibit) to arrive at exactly the right combination of pulse widths produce a highlyaccurate waveform. This is the most fundamental departure from conventional am-plifiers. TacT defines the waveform mathematically - we are not trying to follow oremulate a waveform by using feedback or feed-forward.Once the decision of the duration of the pulse is made the central processor con-trols FET-switches at the output with extreme precision. Voltage and current aredrawn from the power supply and fed to the speakers.The level of playback is controlled by adjusting the voltage of the power supply. Asthis voltage is switched directly to the speakers, it is of paramount importance thatthe power supply be totally free of ripple and noise. For TacT digital amplifiers, aswitch mode power supply of extreme precision with ripple rejection of more than135 dB has been developed. At full volume (voltage) the TacT M/S2150 delivers 58volts, equivalent to 150 Watts into 8 ohms. To reduce the volume the voltage of thepower supply is reduced. This means that the volume control is no longer part of anactive circuit.