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Switched-On Amplification

As with may aspects of digital technology, the mathematical constructs have been in place for many decades.

This month’s review of the QSC GX7 offers an example of just how far the concept of Pulse Width Modulation-based amplifiers has come in a few short years. Using a PWM approach uses the amplitude of analog audio to determine the width of the individual pulses in a high-frequency stream of pulses. The pulses then are used to achieve a higher voltage version of the source signal (amplified) by turning a high voltage on and off very rapidly in proportion to the input amplitude. As with may aspects of digital technology, the mathematical constructs have been in place for many decades, early implementations in use for far less time, with an exponential leap in use of the concepts in recent times. Switching power supplies and light dimmers are examples of non-audio PWM devices, adjusting the pulse widths as necessary to maintain a certain available output voltage.

The first PWM audio application I ran across was in an AM radio broadcast transmitter. It was a reasonably proven design at the time, PWM transmitter control being pioneered in the 1970s. Instead of the typical brute-force approach of amplifying the audio input to a level that could be fed, essentially as a control voltage, to the transmitter output tube thus varying the RF output at an audio rate, this Continental Electronics transmitter converted the audio to a PWM signal (at 70 kHz sampling, if memory serves) and used a fiber-optic connection to couple the signal to the modulator stage (my first experience with fiber optics as well).

The modulator stage was powered by 1,500 VDC. The fiber optics allowed the tube’s input to be fed by conventional power transistors for drivers, working within the transistor’s operating voltage range of around 150 VDC by offsetting the supply voltages down from 1,500 VDC, not up from ground. The modulator tube in turn, based on the PWM input, controlled the supply voltage on the 5 kW output tube stage.

Now that our ability to implement the technology has caught up with the theory, PWM topologies (Class D, H, I are being aggressively employed, as these amplifiers are amazingly efficient, require far less cooling and offer high output power in a small package. The sonics are such that even high-end studio loudspeaker manufacturers are implementing digitally controlled amplification in their powered monitors.

The digital revolution marches ever forward.