Following the past few day's posts about amplifier efficiency, Class AB biasing, and Class A biasing, let's wrap our little mini-series up with another topology most of us have not heard of, adaptive biasing.
The promise of adaptive biasing is a best of both worlds scenario: the efficiency of a Class AB circuit with the performance of a Class A amp. Sounds too good to be true, right?
The first time I ever heard about an adaptive biasing scheme was way back in the dark ages, the late 1970s. My very dear friend and one of the all-time good guys of audio, Nelson Pass, then of Threshold Corporation, had introduced the idea of what we called a sliding bias scheme, part of what later became known as the Stasis Circuit used in Nakamichi, Threshold and if memory serves correctly, even the Mark Levinson No. 33.
The core of this circuitry is covered in Nelson's patent from 1975 titled Active bias circuit for operating push-pull amplifiers in class A mode.
Simply put, Nelson's design raises the level of Class A bias in cadence with the rising input signal.
Recall in our discussion of Class AB design that a small amount of always-on power keeps small signals always on. In other words, we apply Class A (always-on) bias to the first 10% of the amplifiers output signal level, then switch over to the more efficient Class B for higher signals. Compare that to Class A operation which is always-on for any level of signal—always generating a shit-ton worth of heat (recall Class A amps are at their coolest when at full signal out).
What Nelson cleverly suggested was this. Take what we do in an AB amp where the first 10% of the signal is Class A and actively monitor the signal level. When any given input signal starts to exceed our 10% Class A bias, raise the limit from 10% to, say, 20% (or whatever is greater than the signal level), and continue on the path all the way up to 100%. Then back down again tracking the signal. The heat-producing bias is only enough to accommodate the signal, then goes away when it's not needed.
Thus, we get the benefits of both worlds. Efficient, and sweet-sounding.
Why doesn't every amp use this even today? (Nelson's patents ran their course years ago). Well, as with any design there are problems as well as advantages and this post is long enough already. Ain't nuthin' perfect. (We used this for several models of amps though their model numbers and dates escape me).
In any case, a juicy piece of history I'd thought I would share.