What do the different classes of amplification mean? Part 2

A work in progress.

Moderators: Bob Lee, Cameron Shoffner, Thanh Nguyen, Christian Cook, Kirk Fyvie, Martin Barbour, Brad Sambrano, Fred Thomke, Tracey Homan, Gary Evans_QSC

  • In Part 1 we looked at amplifier classes A, B, AB, and D. Here in part 2 we’ll cover class H and—somewhat briefly—class G.

    In QSC amplifiers, classes G and H are class AB (or more correctly, class AB+B; the driver transistors are biased class AB, while the output transistors they drive operate in class B) at their core. The class G or H additions to the output circuitry allow the amplifiers to put out higher power at lower electrical consumption levels, yielding higher electrical efficiencies than possible with class AB+B alone.

    The reason for this is simple: power amps are most efficient—that is, the highest ratio of output power to input AC power—when putting out a continuous signal right at maximum power. However, audio signals are almost never at full power except on occasional peaks. Most of the time, the amp’s output voltage is well below the maximum.

    In a low-powered amp, the inefficiencies of running at well below full power are generally not a significant concern. But in higher-power amp models, improving efficiency can save money on electricity and even air conditioning, and can also allow an amp to have a smaller power supply and cooling system than otherwise possible. Improving efficiency can also give you more amplifier power from the available electrical service.

    Here’s the amplitude envelope (analogous to the signal voltage) of about 25 seconds of a stereo recording of New Blood by Robert Cray. It’s set so that the peaks occasionally hit the maximums, +1 and -1, yet most of the time, the “meat” of the signal doesn’t even reach ±0.5, and is usually down around ±0.25. This is fairly typical of music and other audio signals. If this were the output of a power amp, most of the time in between peaks the output power would be between one-fourth (0.5 squared, because power is proportional to the square of the voltage) and one-sixteenth (0.25 squared) of maximum or “full power.”

    We could improve efficiency if we could somehow use a lower-power amp for the signal most of the time and switch to a high-power amp just for those brief peaks. But can you get more than one power level in a single amp?

    It turns out you can, with a class H output section as found in the higher-power models in the popular QSC RMX, PLX, PowerLight (except the PL380, which is class D), CX, ISA, and DCA series. A class H amplifier uses class AB+B output circuitry with two or more tiers of bipolar supply rails. For example an amp (rated at about 550 watts into 8Ω) may have ±100 volt rails supplying its output section. That will allow its output voltage to reach almost anywhere in between +100 and –100 volts. If it’s a class H amp, it will have another set of lower supply rail voltages at perhaps about ±50 volts. The amp will run on the lower rails except for those brief instances when the signal voltage approaches +50 or –50 volts. Then a comparator circuit will activate a FET that switches the transistors on that side to the high voltage rail so that the circuitry can fully reproduce the audio signal voltage, and then back down to the low voltage when the signal drops back down below 50 volts. Because of this automatic switching between low and high rail voltages, the power that this amp consumes and the heat it generates are a little bit higher than for a lower-power amp that runs on ±50-volt rails, but also significantly less than if the amp ran on ±100-volt rails alone. The downside is the additional and more complex circuitry, which is why class H is used primarily in higher-power models, where the potential for savings outweigh the increased cost.

    QSC and Class H
    Some of the very high-power amp models have not two tiers, but three (RMX4050HD, RMX5050, ISA1350, PowerLight 3.4, PowerLight 4.0) or even four (PowerLight 9.0PFC, PowerLight 6.0PFC, PowerLight 6.0II), for even higher efficiency.

    Here is a conceptual schematic of the output section of a low-power QSC amp. Its driver transistors are biased in class AB by the diodes, and the positive and negative banks of output transistors have grounded collectors. The amplifier circuitry can swing a voltage nearly rail-to-rail (+V to -V).


    This diagram shows the same signal amplitude in a two-tier class H power amp. Its low rail voltages are the same as the rails in the one depicted in the above diagram, but its high rails carry twice the voltage. (Its power rating would be about 4× that of the previous circuit.) When the signal is low enough to be accommodated by the low rails, the amp really behaves almost exactly like the low-power amp depicted above.


    When the signal is large enough that the class H amp has to switch up to the higher rail voltages to reproduce it fully, the amp does this switching automatically, as needed.


    Achieving good class H performance poses challenges for design engineers. The rail commutation must be clean and mustn’t induce glitches into the audio signal. The comparator’s switching points must be optimized so that it does not switch on to the high rail prematurely or off too late, which would waste power, but also does not switch on too late or off too soon, which would cause distortion. QSC is recognized as one of the pioneering companies in the development and refining of class H technology.

    Class G
    Some older QSC amplifier models (MX1500, MX2000, Series 3) used class G to increase efficiency. Like class H, it is based on class AB+B and uses multi-tiered rail voltages, but instead of using an FET to commute each bank of output transistors among different rail voltages, it uses a separate transistor bank for each voltage rail. The thermal and electrical efficiency savings are comparable to class H, but it uses many more output transistors. Therefore, as switching FETs became more suitable for rail commutation, the trend to class H became inevitable.
    Bob Lee
    Technical Communications Developer
    QSC, LLC
    Fellow, Audio Engineering Society
    "If it sounds good, it is good." —Duke Ellington
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    Bob Lee
    QSC Audio Products
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