One of the cleverest solutions to amplifier designs came to our attention through our old friend Nelson Pass, then of Threshold. We always referred to it as the “sliding class A bias circuit” and what was clever about it is it took less than $0.25 in parts to work.
All traditionally designed power amps are class A to some extent. Class A simply means that the output transistors are fully on and drawing power from the wall, even if there’s no audio signal. Most amps are only class A for the first 30 watts of power and then they switch to what we call Class B where the transistors only draw enough power to make your speakers work. This type of circuit is called Class A/B.
A fully Class A amplifier constantly generates tons of heat – most of it wasted. In fact, class A amplifiers create most of their heat when they are just sitting there without a signal – and less heat at full output power. They are veryinefficientdevices but great sounding. Nelson figured out how to have both Class A performance and efficient operation – and he did it for less than a quarter’s worth of parts.
Using a simple resistor and a diode in series with each other, he connected one end to the amplifiers output and the other end of the two parts to an internal circuit (used by all amplifiers) that adjusts the bias level. What this did is made the bias go up and down in synch with the output music signal. So, remember the 30 watts of class A every amplifier has? This amount of power tracked up and down with the music so for any given signal, there was always class A performance. But when there was no signal, there was no heat being generated.
I think Nelson patented the circuit and came up with a clever name.
But to this day it’s the best twenty five cent improvement I’ve ever seen.