A couple years ago I set out to build the baddest ass audio amplifier that man kind has ever known. Class A designs have always appealed to me, yet they seemed limited in output power due to all of the heat they create. I decided I wanted my cake and the ability to eat it too, so after months of development and hundreds of hours of SPICE simulation, I invented what I call the Extended Class A Output Stage. (XCLA)
It has long been known that Class A biased amplifier designs yield the best Sonics, the lowest distortion (THD specs) and are free from crossover and switching distortions since the output devices never turn off. The drawback to Class A designs – especially in regards to audio output stages – is that they are the least efficient of all the classes and generate tons of heat and wasted power, even when sitting idle. Large heat sinks and big fans are usually required to keep them from melting down under the burn of their own heat. As such, most Class A designs are usually only able to deliver a few tens of watts in class A mode before going into class AB mode. (it should be noted that class AB mode is sonically inferior to optimally biased class B mode due to a phenomenon called GM doubling) So for example, a class A biased amplifier capable of delivering 100 watts RMS into 8 ohms must have its output stage biased at >2.5 amps with supply rails of + / – 45 volts dissipating 225 watts of power – even while sitting idle. If we consider 4 ohm loads, both the bias current and idle dissipation would double to a whopping 450 watts. Burning 450 watts of power per channel is exactly why you don’t see many high power true class A amplifiers out there.
Enter Xtended Class A.
The Xtended Class A output stage design reaps the benefits of class A output stage bias while eliminating much of the wasted power associated with high current biasing. It works by having a lightly biased ( ~ 0.5 amp) Class A output stage (Q1 and Q2 in th e simplified schematic below) that not only drive the load, but also control a current controlled current source (CCCS). The CCCS assist the Class A output stage in driving the load, and Xtend its class A operating region by providing a current gain such that for every 100mA that the class A portion of the output stage (Q1 and Q2) are forced to drive, the CCCS will deliver around 1.2A of current. This enables the output stage to swing several amps of current, but only be biased at a fraction of the current that it would take if it were driving the load unassisted. The Class A stage is always in control of the load, and the CCCS are merely slaved to the Class A portion of the output stage.
This topology yields several benefits. The CCCS’s force the load to appear like a much higher resistance to the Class A stage. This eliminates beta droop effects and improves the linearity of the Class A stage since it “sees” the load as being a higher resistance than it really is. The load presented to the Class A stage is the load resistance (~ 8 ohms) multiplied by the gain of the CCCS (~12X). So rather than driving an 8 ohm load, the Class A stage thinks it’s driving something more like a 100 ohm load. Another benefit of this topology is that it resembles a Complimentary Feedback output stage (CF) which has been demonstrated to be more linear than typical Emitter Follower (EF) output stages. The XCLA 100 amplifier can deliver 200 watts RMS into 4 ohms and never leave class A mode. The idle dissipation is a mere 65 watts- roughly 1/7th of the idle dissipation that would be required for driving that power into that load with a traditional ( un – Xtended) class A output stage.
Armed with the invention of the XCLA output stage, tons of SPICE data suggesting it was going to be super awesome, and a few grand to burn, I set out to incarnate this monster, and breathe life into it. The photos below chronicle the mayhem that ensued.
I also wanted viewing windows to be cut into the top and covered with plexiglass so as to display the gorgeous guts of this beast. As such, I decided to illuminate the interior with blue and white LEDs. Here is what she looks like: