What Do the Different Amplifier Classes Mean?

When you look at an amplifier's spec sheet, you will inevitably come across a reference to an amplification class: Class A, Class D, Class G, and so on. These designations are not marketing arguments specific to any one brand -- they are a standardized vocabulary, shared across the entire industry. They describe how an amplifier's transistors or tubes handle the audio signal, with direct consequences for energy efficiency, heat output, distortion, and, to some extent, the sonic character of the unit. Understanding these classes does not require a background in electronics. What matters is grasping the general behaviour of each one and what that means in real-world use.

Before getting into the individual classes, a useful starting point: there are two broad families of amplification -- tube-based and solid-state (transistor-based). Most modern equipment belongs to the second category, but tubes have not disappeared. Understanding why both still coexist also helps explain why amplifier classes exist and what problems they are designed to solve.

Tubes or Solid-State -- Why Both Still Exist

Vacuum tubes preceded transistors by several decades. Before the 1960s, all audio amplification ran through them. Transistors then took over, largely because they are less fragile, less power-hungry, cheaper to manufacture, and capable of measured performance that often surpasses tubes. On paper, transistors won on nearly every technical front and quickly took a dominant share of the market.

And yet, tube amplifiers still hold a genuine place in the high-fidelity market. The reason comes down to how they behave under distortion. When a tube saturates or slightly exceeds its linear operating range, it produces primarily even-order harmonics -- double, quadruple the fundamental frequency. These harmonics are musically consonant; they resemble what an instrument produces naturally. The ear perceives them not as an error, but as a slight warmth or roundness in the sound. This is what is commonly called the "warmth" of tubes, and it is a measurable acoustic reality, not merely a subjective impression.

Solid-state amplifiers, when they distort, tend to produce odd-order harmonics, which are perceived as harsher. In practice, however, well-designed and properly sized transistor amplifiers distort so little that the question barely arises. The best solid-state amplifiers achieve distortion levels so low they are inaudible under any normal listening conditions.

Both approaches coexist because they serve different priorities. Tubes appeal to listeners who put tonal character and sonic warmth first. Solid-state dominates where precision, reliability, and efficiency matter most. Tube amplifiers also require regular maintenance -- the tubes wear out and must be replaced, sometimes after a few thousand hours of use.

Solid-State Amplifier Classes

Class A

In a Class A amplifier, the output transistor conducts continuously throughout the entire audio cycle, even when the signal is at zero. There is never an interruption, never a handoff between two components. Hence the exceptional linearity and zero crossover distortion -- there is simply no crossover to manage. It is no coincidence that Class A remains the reference benchmark in high-fidelity audio.

The downside of this approach is its energy inefficiency. Because the transistor draws current constantly, even at idle, a large portion of the electrical energy is dissipated as heat rather than converted into a usable signal. The efficiency of a Class A amplifier typically falls between 15 and 30%. In practice, this means a heavy unit (requiring a substantial power supply), warm to the touch, with a modest output power relative to its consumption. A 25 W Class A amplifier can draw 150 W or more continuously.

Class A is particularly well suited to stereo music listening in reasonably sized rooms, with speakers of adequate sensitivity. It is less suited to multichannel systems or large installations that require substantial power reserves.

Class B

Class B takes the opposite approach: two transistors share the work. One amplifies the positive half of the signal cycle, the other the negative half. Each transistor conducts for only 180 degrees of the cycle, which significantly improves efficiency (up to 78% in theory).

The problem lies at the transition point between the two transistors, where the signal crosses zero. If this handoff is not perfectly timed, crossover distortion appears. In practice, this distortion is audible and unpleasant. Pure Class B is extremely rare in high-fidelity audio for exactly this reason. It has mainly served as the theoretical foundation for developing Class A/B.

Class A/B

Class A/B is the most widely used amplification class in high-fidelity audio. It combines the advantages of both preceding classes by applying a small bias current to both transistors, causing them to conduct slightly at all times, even when the signal is weak. This overlap eliminates the crossover distortion that undermines pure Class B, without requiring the constant, steady current draw of Class A.

For low-level signals, the section operates almost like Class A. For dynamic peaks that demand more power, one transistor becomes dominant according to the phase of the signal, with a transition smooth enough to remain virtually inaudible. Efficiency typically falls between 50 and 70% -- a genuine compromise: less heat than Class A, far better sonics than pure Class B.

The vast majority of high-end integrated amplifiers and home theatre receivers operate in Class A/B. It is a proven, well-understood topology, and its balance of sonic performance, manufacturing cost, and efficiency remains difficult to beat for general use.

Class D

Class D operates on a fundamentally different principle from the classes above. Rather than having a transistor conduct continuously or partially, it uses transistors as switches that open and close at very high frequency -- often between 300 kHz and 1 MHz. The audio signal modulates the duration of these switching pulses, a technique known as pulse-width modulation, or PWM. A low-pass filter at the output then reconstructs the continuous analogue signal.

Because the transistors are never in an intermediate state (they are either fully open or fully closed), almost all of the consumed energy is converted into a useful signal. Class D efficiency commonly exceeds 85 to 95%, resulting in compact, lightweight units that run cool.

Class D long carried a poor reputation on the sonic front, partly justified in the early generations of circuits. Output filters introduced artifacts in the upper frequency range, and some inexpensive Class D amplifiers did indeed sound less natural than their A/B counterparts. That reality has changed over the past decade. Modern Class D modules achieve levels of distortion and signal-to-noise ratio comparable to the best Class A/B amplifiers, and are now found in high-end high-fidelity components. Class D is also used today in active subwoofers, soundbars, home theatre receivers, and a growing number of integrated amplifiers. The historical skepticism toward this class is increasingly difficult to justify as the technology continues to improve.

Class G (and Class H)

Class G starts from a simple observation: in a Class A/B amplifier, the power supply delivers a fixed voltage continuously, even when the signal does not need it. The excess energy is dissipated as heat. Class G corrects this waste by using multiple power supply rails at different voltages. When the signal is low, the amplifier runs on the low-voltage rail, just like an ordinary Class A/B. When the signal demands more power, a high-voltage rail takes over.

The result is an amplifier that sounds essentially like Class A/B, but consumes significantly less energy and runs cooler. Class G efficiency typically falls between 60 and 80%, depending on the signal content. This is why it appears in some high-end home theatre receivers, where driving seven, nine, or more channels simultaneously creates a significant thermal load. Class G allows the amplifier to sustain output power without overheating.

Class H operates on the same principle, but with a continuously variable supply voltage rather than discrete steps. It follows the signal in a quasi-analogue manner, improving efficiency further in very demanding configurations. In practice, the distinction between G and H is most relevant to engineers; for the listener, both offer a sonic character comparable to Class A/B with better thermal management.

Other Notable Approaches

Some manufacturers have developed their own proprietary variants. Cambridge Audio uses Class XD, which shifts the crossover point slightly to prevent crossover distortion from occurring at the zero crossing. NAD has long worked with Class D amplifiers based on Hypex modules. Marantz and other manufacturers integrate HDAM (Hyper Dynamic Amplifier Modules) into their Class A/B output stages as a replacement for generic op-amps. These proprietary designations all refer to refinements of an existing class rather than entirely new categories.

Class Summary

How to Use This Information When Choosing an Amplifier

Amplification class is not the only factor that matters, but it meaningfully shapes the decision once you know your priorities. Here are a few questions worth asking.

• What is the sensitivity of your speakers? Low-sensitivity speakers (below 87 dB/W/m) require more power to reach satisfying listening levels. In that context, a 20 W Class A amplifier may run out of headroom. A 100 W or more Class A/B will be better suited. High-sensitivity speakers (90 dB and above) reveal more of an amplifier's character at low power, making them ideal partners for Class A or tubes.
• Are heat and power consumption a concern? A Class A amplifier runs warm continuously. Placed inside a closed cabinet or in a poorly ventilated room, it can cause problems. Class D is the most appropriate solution when space is limited or when power draw is a priority.
• Are you prepared for tube maintenance? A tube amplifier requires periodic tube replacement, a bias adjustment depending on the model, and more care in day-to-day use. Some find this part of the enjoyment of ownership; others see it as an inconvenience.
• How large is your listening room? In a small room, a low-power Class A amplifier may be more than sufficient. In a large home theatre, a multichannel receiver in Class A/B or G provides the dynamic headroom needed for complex, high-level passages.
• What is your total budget? Class A is more expensive to design and build correctly. Tubes also represent a higher investment, both at purchase and over time. Class D has made quality amplification accessible at lower price points. Class A/B remains the best value proposition for the majority of configurations.

In practice, most listeners building their first quality system will be very well served by a good integrated amplifier in Class A/B or Class D. Those looking for a particular sonic character, with suitable speakers and an appetite for more subjective territory, will find something different in Class A or tubes. And those building a complete home theatre with many channels will naturally look toward receivers in Class A/B, G, or D, depending on their thermal and energy priorities.

Frequently Asked Questions