Tuning with current

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When it comes to traditional class AB amplifiers there are two schools of thought about bias. Engineers can add enough class A current to satisfy their distortion analyzers or their ears. Most keep the analyzers happy without worrying about sound quality and it’s pretty easy to see why. Class A bias produces unwanted heat that is costly to dissipate.

Some degree of class A bias is required in a low distortion amplifier. It is bias that keeps the output devices from switching off and creating distortion. The minimum current needed to eliminate distortion isn’t much, but not all of us place economy above performance.

How an amplifier sounds—particularily in the areas of detail and low-level music—has a lot to do with its bias level. Typically more is better, but only to a point. Successful high-performance designs balance higher output current with available power supply and design goals so as not to cross the line of excess. Too much bias can actually weaken the slam and bass grunt of an amplifier if its power supply is stressed.

As in most things audio related it is a matter of trade-offs that wins the race. When we design class AB amplifiers—wheteher BHK, myself, Bob Stadtherr or Darren Myers—we start by assigning sufficient output current to eliminate notch distortion, then drag the amp into the listening room for the final bias settings.

The design of high-performance audio equipment is a mix of both science and art. If you want to learn more of my thoughts on class A biasing, you can go here to watch the video.