The bias current applied to an amplifier’s output stage is a critical factor when it comes to both technical and musical amplifier performance, establishing the optimum operating conditions for the devices chosen. In fact, the bias current is so critical that many high-end amplifiers use heavily regulated bias supplies to ensure a constant current across the output devices. Which is fine in theory, but as so often happens in audio, the theory starts to collapse under the dynamic demands of real-world operation. In particular, the temperature of the output devices has a direct impact on the bias current and thus the operational behaviour of the output stage as a whole. As temperatures rise, the devices draw more bias current, shifting their operating point, moving them outside their optimum operational envelope and increasing distortion levels, with a direct impact on the amplifier’s linearity and ability to accurately track the input signal.
In essence, for optimum performance, the bias current must remain constant, no matter how hard and hot the amplifier is run – something that simple regulation cannot achieve. In order to achieve such an unconditionally stable bias current, CH Precision developed a two-stage electronic system (the ExactBias circuit) that automatically and linearly regulates both ambient (slow shift) changes in temperature and the load related shifts that occur as a result of program demands. To achieve this the temperature inside the output devices is constantly monitored, two extra pins on each transistor allowing us to accurately calculate the internal temperature of the device and regulate the bias current accordingly, taking long and short-term temperature variation into account. The result is unprecedented stability of the output stage bias current, producing more naturally proportioned micro and macro dynamic discrimination, allowing the output stage to respond effortlessly, more naturally and more convincingly to dynamic demands in the program material. This ExactBias circuit is used in all CH amplifiers.
