A Class H amplifier is an audio power amplifier topology that dynamically switches or modulates its power supply voltage rails to closely track the audio signal, drastically reducing wasted heat compared to traditional linear designs. By keeping the voltage drop across the output transistors as low as possible at any given moment, this architecture solves the massive thermal penalty of Class AB designs without introducing the high-frequency switching noise inherent to Class D amplifiers. In a real circuit, implementing a Class H topology changes the power supply from a single fixed-voltage secondary into a multi-tap transformer or a tracking switch-mode pre-regulator, fundamentally altering the thermal management and physical footprint of the amplifier.
The Core Concept: Dynamic Rail Tracking
In a standard Class AB amplifier, the power supply rails are fixed at a high voltage to accommodate the maximum possible peak signal. When the amplifier is playing a quiet passage or low-frequency signal that doesn't reach those peaks, the output transistors must absorb the difference between the high rail voltage and the actual output voltage. This voltage drop, multiplied by the load current, is dissipated entirely as heat.
Class H solves this by introducing a secondary, lower-voltage rail (or a continuously variable rail). For low-level signals, the output stage is powered by the low rail. As the audio signal approaches the low rail's limit, a high-speed comparator detects the impending clip and instantly switches the output stage to the high rail.
What people commonly confuse with Class H is Class G and Class D. Class G is closely related but typically relies on a fixed number of discrete, stepped voltage rails (e.g., two or three fixed taps). Class H often implies a more complex, continuously modulated tracking supply or a higher number of switching thresholds. Class D, on the other hand, is entirely different: it uses pulse-width modulation (PWM) at the output stage itself, switching at hundreds of kilohertz, whereas Class H keeps the output stage strictly linear and analog, doing all its switching at the power supply rails.
The Math: Class AB vs. Class H Power Dissipation
To see why Class H is favored in high-power professional audio, we need to look at the instantaneous thermal dissipation. Let's run a worked numeric example using a 500W amplifier driving an 8-ohm load.
- The Signal: A continuous 20V peak sine wave (which delivers 50W RMS into 8 ohms).
- The Current: At 20V peak into 8 ohms, the instantaneous peak current is 2.5 Amps.
Scenario A: Traditional Class AB
The amplifier uses fixed ±80V rails to ensure it can deliver its full 500W peak capacity. When outputting our 20V peak signal, the voltage drop across the output transistors is:
V_drop = 80V - 20V = 60V
The instantaneous power dissipated as heat per device is:
P_diss = 60V × 2.5A = 150 Watts
Scenario B: Class H Topology
The amplifier uses a low rail of ±25V and a high rail of ±80V. Because our 20V peak signal is below the 25V threshold, the amplifier stays on the low rail. The voltage drop is now:
V_drop = 25V - 20V = 5V
The instantaneous power dissipated as heat per device is:
P_diss = 5V × 2.5A = 12.5 Watts
By simply tracking the signal to the lower rail, we achieved a 91.6% reduction in instantaneous thermal dissipation for that signal level. Over a full music program—which spends 90% of its time at low-to-moderate levels—this translates to an amplifier that runs cool enough to use a fraction of the heatsink mass, or fit into a 1U rack chassis without thermal throttling.
| Topology | Typical Efficiency | Output Stage | Primary Heat Source | EMI / RFI Risk |
|---|---|---|---|---|
| Class AB | 50% - 65% | Linear | High V-drop across output BJTs/MOSFETs | Very Low |
| Class G | 65% - 75% | Linear | Moderate V-drop, stepped rail switching | Low |
| Class H | 75% - 85% | Linear | Low V-drop, tracking supply losses | Low to Moderate |
| Class D | 85% - 95% | Switching (PWM) | Output filter and MOSFET Rds(on) | High (Requires LC filtering) |
For deeper theoretical breakdowns of linear versus switching topologies, the electronics-tutorials.ws amplifier class guide provides excellent foundational schematics, while Texas Instruments' audio amplifier portfolio offers modern datasheets showing how these topologies are implemented in silicon today.
Where You Meet Class H in Practice
You won't typically find Class H in cheap consumer Bluetooth speakers or budget home theater receivers. The complexity of the power supply and the precision required to avoid switching glitches pushes it into specific, demanding professional and audiophile niches:
- Touring Pro Audio Racks: Sound reinforcement requires massive power (2000W+ per channel) in lightweight, 1U or 2U rack enclosures. Class H allows amps like the legendary Crest Audio Pro series or modern equivalents to deliver Class AB sound quality without the 40kg heatsinks.
- Active Studio Monitors: Inside a sealed speaker cabinet, heat buildup is a major enemy of driver longevity and crossover components. Class H provides the low-noise floor of a linear amp without cooking the internal electronics, avoiding the high-frequency EMI of Class D that can interfere with nearby analog mixing consoles.
- High-End Subwoofer Plate Amplifiers: Bass frequencies require high current but move relatively slowly. Class H tracking supplies can easily follow low-frequency envelopes, delivering massive dynamic current for kick drums and synth bass without the thermal compression seen in Class AB plate amps.
Frequently Asked Questions
Is a Class H amp better than Class D for audiophile listening?
It depends on your definition of 'better.' Class H maintains a strictly linear, analog output stage, meaning it avoids the high-frequency PWM switching noise, dead-time distortion, and load-dependent frequency response variations that plague poorly designed Class D amps. For critical listening environments with highly sensitive tweeters or analog signal chains, a well-engineered Class H amp offers the pristine noise floor of Class AB with much better thermal stability. However, modern premium Class D modules (like those using Hypex Ncore or Purifi Eigentakt technology) have largely closed this gap, offering audiophile performance at even higher efficiencies.
What is the exact difference between Class G and Class H amplifiers?
While marketing departments often use the terms interchangeably, audio engineering convention draws a line at the power supply behavior. Class G uses a finite number of discrete, fixed voltage rails (usually two or three) and switches between them. Class H implies either a much higher number of switching thresholds or, more accurately, a continuously variable tracking supply where a switching pre-regulator dynamically adjusts the rail voltage to sit just a few volts above the audio signal at all times. Class H is essentially the continuous, optimized evolution of Class G.
Do Class H amplifiers require special power supplies?
Yes. A traditional Class AB amp only needs a single secondary winding on its toroidal transformer, rectified into a single high-voltage DC bus. A discrete-rail Class H amp requires a transformer with multiple center-tapped secondary windings to create the low, medium, and high voltage rails. If the amp uses a continuous tracking Class H topology, it requires a high-frequency switch-mode power supply (SMPS) acting as a pre-regulator ahead of the linear output stage, which adds significant design complexity and requires careful EMI shielding.
Why don't portable battery-powered speakers use Class H amps?
Class H is designed to optimize heat dissipation from high-voltage AC-derived DC rails. Portable Bluetooth speakers run off low-voltage DC battery packs (typically 3.7V to 14.8V) and prioritize absolute maximum battery life over thermal management. In low-voltage, battery-constrained environments, Class D is the undisputed king. Class D's 90%+ efficiency directly translates to longer battery life, and at low voltages, the EMI and filtering challenges of Class D are much easier and cheaper to manage than building a multi-rail tracking supply for Class H.
