With the proliferation of digital sources for audio, opportunities exist to amplify the signal entirely in the digital domain, right up to the loudspeaker. The performance of an open loop digital amplifier can be significantly improved by applying feedback. In this article the Direct Digital Feedback Amplifier (DDFA) is compared with other methods of achieving this goal.
So what actually is a digital amplifier?
The output of any Class D amplifier is generated by a Pulse Width Modulated (PWM) switching signal so discussion of this subject starts with an immediate opportunity for confusion. For some time, there has been significant debate on the definition of the digital amplifier and even more on the merits or otherwise of such a product in comparison with its analog equivalents. The majority opinion is that any Class D amplifier's pre-filter output is either high or low, so it must be digital!
While this explanation is simple and convenient and serves as a rough explanation of Class D, it hides major differences between the different methods of implementation. In actual fact, a large proportion of Class D amplifier products are actually analog in every stage from the original input signal to the infinitely variable pulse width modulation at the output of the power stage. The conversion of the logic level pulses to high output voltages remains a high frequency analog power electronics challenge.
A better definition
A digital amplifier actually takes a digital input, which is typically in a Pulse Code Modulation (PCM) format, and the implementation of the PCM to PWM conversion is performed digitally. The PWM output stream shows digital characteristics, in that the pulse widths are quantized. This differs from the infinitely variable pulse widths exhibited in analog systems and requires the use of noise shaping techniques to minimize quantization noise.
(Click here for digital amplifier overview)
Looking at the function of the digital amplifier, it's clear that it performs a greater task than its analog cousins. The major difference is that the Digital to Analog Conversion (DAC) is incorporated into the digital amplifier. A quick examination of digital modulators on the market shows that many offer many other features ranging from volume control through to complex Digital Signal Processing (DSP) capabilities. In the minimum form, the digital amplifier is equivalent to the DAC, pre-amplifier and power-amplifier combined. It is very much more than the simple power-amplifier and this distinction is very important when we investigate the subject further.