The Difference Between Analogue and Digital Amplifiers
Understanding the amplification process helps to explain the difference between analogue and digital amplifiers. In an analogue amplifier, the analogue input signal is amplified without any modification. In a digital amplifier, the analogue input signal is converted into a pulse (digital) signal, and then converted back into an analogue signal using a low-pass filter. An analogue signal is constantly changing within a range extending from zero to a maximum value. However, a digital signal is comprised of “pulses”—a series of zeros and ones. The significant difference between analogue and digital amplifiers is the basic principle used for amplification.
In an amplifier, the power supply circuitry (actually, the capacitors) collects electricity. A transistor (valve) opens when an input signal is received, causing some of the collected energy to flow out through the output jacks. This process simply defines how amplification works. Analogue amplifier signals continuously change: the transistor must adjust the size of the “valve” opening to match the constantly changing input signal. On the other hand, with a digital amplifier, the signal consists of either a pulse (1) or no pulse (0)—there are no intermediate values. The “switches” in a digital amplifier are completely open (switch is on) when there is a pulse or completely closed (switch is off) when there is no pulse.
Why the Interest in Digital Amplifiers?
First of all, we should consider an analogue amplifier, where the signal always lies between zero and a maximum value. Therefore, the amplifier elements function as variable resistors that adjust the amount of electricity supplied by the power supply to match the input level. Electricity that does not flow through when the amplifier elements are closed is lost. For this reason, analogue amplifiers can only achieve a maximum power efficiency (relative to the power supply) of about 70%. This large amount of energy loss means that a substantial amount of heat is generated.
In a digital amplifier, the signal level is either 0 or 1, and the amplifier elements function as switches with two states, ON and OFF. The amount of power loss is very small. Consequently, digital amplifiers typically have very high efficiency—90% or so. Very little energy is generated, so heat-dissipating parts such as heat sinks can be smaller and the amplifiers can be more compact.
Possibilities of the Digital Amplifier
At Onkyo, we are not only interested in higher efficiency and a more compact size, we also believe that there is a great opportunity to build a digital amplifier with improved sound. When a digital amplifier’s signal value is 1 (the current is flowing from the power supply to the speakers), the amplification elements in the output stage remain completely open. Broadly speaking, there is little resistance that consumes power between the power supply and the speakers. Consequently, there is no loss of power. In contrast, with analogue amplifiers, there is always some resistance between the power supply and speakers because of the manner in which the amplifier operates.
Furthermore, since the output elements are used as switches in a digital amplifier, properties such as linearity (crucial in an analogue amplifier) are not particularly significant. By reducing the number of parameters that the amplifier must control, it is easier to ensure that the elements will be driven as intended in all circumstances. We believe that the potential of digital amplifiers lies in more accurate signal reproduction.
Another potential attraction is that low-frequency reproduction places little load on the power supply. Analogue recording techniques have limitations when recording low-frequency sounds. However, digital recording, which has become the dominant method for storing and reproducing audio data, has eliminated these limitations. For this reason, more and more of today’s music is based on powerful low-frequency sounds. These recordings contain bass power in all its intensity.
Onkyo’s Approach to Digital Amplifiers
Based on the research of Onkyo’s development team, we believe power supply is essential to achieving quality sound from digital amplifiers, even though their efficiency far exceeds that of an analogue amplifier. If we go back to the basics of amplification, we want to reproduce sound that we can listen to—actually, a sound that we can “feel”. For this purpose, we need a power supply with the lowest possible impedance and superior transient response. Very few manufacturers are building digital amplifiers with power supplies that follow our concept.
A great deal of attention has been given to power supply performance in every Onkyo digital amplifier. In fact, in the A-933 digital amplifier, we have taken this concept even further by including two large-capacity toroidal transformers—quite different from any other amplifier in its class.
Pulse Width Modulation (PWM) and Onkyo’s VL (Vector Linear) Technology
In digital amplifiers, there are two methods of pulse conversion: pulse width modulation (PWM), in which analogue quantity is represented by the width of the pulse, and pulse density modulation (PDM), in which analogue is represented by the number of pulses. Onkyo uses the PWM approach for a number of reasons:
1) PWM produces far less digital noise in the higher frequencies than PDM.
2) PWM is more efficient than PDM in terms of delay relative to the pulse input.
3) PDM is dependent on a large amount of negative feedback (NFB)—approaching 100%. Even in an analogue amplifier, a lot of NFB will negatively affect the sound.
Up to now, Pulse Width Modulation (PWM) has been used as an efficient method of amplifying audio signals. Theoretically, this method should result in accurate analogue-to-digital conversion. In reality, a digital amplifier generates a lot of “noise spikes” from sources external to the modulator circuitry. This spike noise introduces errors into the inversion timing, making accurate conversion into pulse widths impossible. So, to further improve the precision of amplifiers, we’ve had to push even further. Our response is a highly accurate analogue-to-digital conversion circuit—VL Digital—that is unaffected by noise in the analogue signal.
Onkyo’s VL (Vector Linear) Digital technology comprises a vector generator, an integrator (like a charger) and an inversion trigger generator. When the analogue input signal is received, the vector generator outputs a current proportional to the size of the analogue input. This current is sent to the integrator, where it is “charged”. When the charge quantity reaches a specified value, the trigger operates and inverts the output pulse. Circuits charge and invert alternately, performing pulse width modulation proportional to the analogue signal.
The upper and lower portions of the spike noise waveform are symmetrical, so they have the same area. Therefore, if the analogue signal contains spike noise, their charge quantities will cancel each other out. This will ensure accurate pulse width modulation at all times. Onkyo’s third-generation VL Digital technology includes an inverted Darlington circuit that goes beyond earlier versions to accurately produce a current flow based on the input voltage.
