Class A power amplifiers have been thought of as drawing full current at all times including when there is no input signal. In fact, normally a Class A amplifier is biased to the center of its operating curve and draws 50% of current with no signal. Then, input signal swings current up and down from the 50% point. The disadvantage of a traditional Class A power amplifier is at low volume levels where the output stage continues to draw 50% of available current when it may only need 10%.
Responsive bias also known as bias tracking is a method of controlling output tube current to accommodate the tubes demand for current at a particular output level. Essentially the output stage is adjusted up and down its operating curve. At higher output levels the tube draws the full 50% of operating curve current. At lower volume levels the tube is biased down to approximately 10% to 20% of current depending on volume level. A sample of audio signal is used to control a special bias power supply adjusting bias level based on audio signal level. The audio sample is tapped from the amplifier speaker output. Using a sample from the speaker output takes into account varying speaker loads placed on the output stage.
The following conditions are required for responsive bias control. The bias must be able to respond to sudden increases in audio level. To avoid clipping, the audio signal to DC control voltage conversion and bias supply must have a fast rise time. Besides a fast rise time the bias should average itself during continuous signal to avoid a bias riding signal condition. There must be a buffer zone between average signal level and bias controlled current. The buffer zone is required because even with a fast rise time the bias cannot equal instantaneous signals. A buffer zone requires the output stage to draw some current during idle no signal conditions.
The bias system flow diagram shown above has been prototype circuit board built. This particular design is 100% analog and includes a failsafe circuit (not shown) in the event of a bias supply failure. The prototype assembly pictured below has been tested, but requires futher testing in a working amplifier.