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un compresseur mono/stéréo en kit DIY basé sur de la modulation de largeur d’impulsion (PWM)

Crane Song’s single-channel Trakker ($2,550) and dual-channel STC-8 ($4,450) compressors use a unique Pulse Width Modulator (PWM) FET design to solve the distortion problem. Unlike traditional FET designs, a PWM FET does not modulate the gain reduction, so there is less distortion at high gain. In technical terms, the PWM is essentially a switch that turns the audio signal on and off at a 1.1 MHz frequency, letting only a percentage of its energy through at any given time to control the gain. A filter rids the signal of switching artifacts. Because the signal is either on or off, it doesn’t modulate the gain-control element’s resistance or resulting gain reduction. The result is that the distortion content is linear as gain reduction increases. In addition, attack time can be as fast as a few microseconds.

Digital control through PWM is not new, and neither is the use of gain-changing FETs: The '60s vintage EMT 156 Stereo Compressor/Limiter is an example of a PWM-controlled processor and the revered UREI 1176LN is a good example of an FET-based peak limiter. The technique of converting an analog waveform into a digital pulse train (A/D conversion) for use as a control voltage has two distinct advantages: Once digital, the control signal can be easily manipulated, and in ways not always possible with analog circuitry. Second, the quality of the analog signal path remains pristine from input to output. In the case of the Crane Song Trakker, a pair of parallel FETs, responding to the PWM sidechain control signal, control the audio output by shunting the output audio to ground. This is the same way a resistive passive volume control works--the most neutral-sounding gain reduction circuit available.