A Class H amplifier is an audio power amplifier topology that dynamically tracks the input signal to adjust its supply voltage, minimizing wasted heat in the output stage. In a traditional linear amplifier, the output transistors act like variable resistors, burning off the difference between the fixed power supply rails and the instantaneous audio signal as heat. A Class H design fundamentally changes this by modulating the supply rails—either continuously or via high-frequency discrete steps—so they sit just a few volts above the output signal at any given microsecond. This drastically reduces the voltage drop across the output devices, shrinking the required heatsink mass and pushing efficiency from a sluggish 50% up to 80% or higher.
The Core Concept: Tracking the Signal to Save Watts
To understand why Class H exists, you have to look at the Safe Operating Area (SOA) and thermal limits of bipolar junction transistors (BJTs) or MOSFETs. In a Class AB amplifier running off fixed ±80V DC rails, if the output signal is sitting at 10V, the remaining 70V is dropped across the output transistors. Multiply that 70V by the load current, and you get massive instantaneous power dissipation.
A Class H amplifier solves this by inserting a tracking modulator between the main power supply and the output stage. This modulator reads the input signal (or the output signal) and adjusts the rail voltage. When the music is quiet, the rails might drop to ±15V. When a massive transient hits, the rails shoot up to ±80V in microseconds. Because the voltage across the output transistors (V_ce or V_ds) is kept to a minimum, the heat generated is a fraction of what a fixed-rail design produces.
Class H vs. Class G vs. Class D: Clearing Up the Confusion
When discussing amplifier topologies, Class H is routinely confused with its close cousin, Class G, and its completely different counterpart, Class D. Here is how to tell them apart on the bench:
| Topology | Output Stage | Power Supply Behavior | Typical Efficiency | Primary Trade-off |
|---|---|---|---|---|
| Class AB | Linear | Fixed rails | 50% - 65% | Massive heatsinks, heavy iron transformers |
| Class G | Linear | Switches between 2 or 3 fixed rails | 65% - 75% | Rail-switching distortion if not carefully compensated |
| Class H | Linear | Continuous tracking or high-res stepped rails | 75% - 85% | Complex tracking modulator design, higher EMI |
| Class D | Switching (PWM) | Fixed rails (usually SMPS) | 85% - 95% | Requires output LC filters, potential high-frequency EMI |
The most common mistake is calling a Class D amp a 'Class H' just because it's highly efficient and uses a switching power supply. Class D uses pulse-width modulation at the output stage to create the audio waveform. Class H uses a strictly linear, analog output stage; it is only the power supply feeding that stage that switches or tracks.
The Math: Heat Dissipation at 1/3 Power
Let's run the numbers on a workbench scenario. Assume we are designing a 1000W RMS amplifier into a 4Ω load. To achieve 1000W, we need an RMS voltage of 63.2V, which means a peak voltage of 89.4V. We will use ±90V fixed rails for the Class AB baseline.
Audio test standards (like FTC and IEC) often evaluate thermal performance at 1/3 of maximum continuous power, as this represents a typical steady-state music load with a standard crest factor. At 1/3 power, the amplifier is delivering 333W to the load. The peak output voltage at this level is 51.6V.
- Class AB (Fixed ±90V Rails): The DC power drawn from the supply to deliver a 51.6V peak sine wave into 4Ω is calculated as P_dc = (2 * V_cc * V_peak) / (π * R). That gives us (2 * 90 * 51.6) / (3.1415 * 4) = 739W drawn from the wall. Subtract the 333W delivered to the speaker, and the output transistors must dissipate 406W of heat.
- Class H (Tracking Rails): The tracking modulator keeps the rails at roughly 5V above the peak signal. So, the rails sit at ±56.6V. Running the same formula: (2 * 56.6 * 51.6) / (3.1415 * 4) = 465W drawn from the supply. Subtract the 333W output, and the transistors dissipate only 132W of heat.
By tracking the rails, we just eliminated 274W of thermal dissipation. In physical terms, this means replacing a massive, 15-pound extruded aluminum finned heatsink and a forced-air fan with a small, stamped metal bracket and natural convection.
Where You Meet This in Practice
You won't typically find true Class H topology in cheap consumer electronics or basic hobbyist audio kits due to the complexity of the tracking modulator. Instead, you will encounter it in applications where weight, thermal density, and acoustic output are critical:
- Touring Pro Audio: Line array amplifiers from manufacturers like L-Acoustics, d&b audiotechnik, and Powersoft rely heavily on advanced Class H (and Class I, a variant) to pack 10,000W of output into a 2U rack chassis that a single roadie can lift.
- High-End Active Subwoofers: Brands like SVS and REL use Class H tracking rails in their flagship 16-inch and 18-inch sealed subwoofers to deliver massive transient current without the amplifier's internal thermal protection shutting down during movie explosions.
- Car Audio SPL Competitions: When competitors wire dozens of 15-inch woofers to 1-ohm loads, Class H architectures allow the alternator to keep up with the current demands without melting the vehicle's wiring harness.
Bench Scenario: The Generator Sags and the Amp Shuts Down
Theory is clean; the jobsite is messy. Here is a real-world scenario that highlights a specific vulnerability of Class H designs when paired with weak AC sources.
The Numbers: Electronic music with a 12dB crest factor means the average continuous draw is only about 250W per sub. However, the transient bass drops demand 1500W instantly. The generator is rated for the average load, and the 4500W peak rating seems sufficient for the transients.
The Outcome: During the heaviest bass drop of the night, both subs abruptly mute. The generator engine bogs down, recovers, and the amps remain in protect mode until manually reset.
What Went Wrong: A Class H amplifier's tracking modulator is usually fed by an internal Switch-Mode Power Supply (SMPS) that generates the high-voltage DC rails from the AC wall power. When the massive transient hit, the SMPS demanded a huge burst of AC current. The portable generator's mechanical throttle could not spool up the engine fast enough to meet this instantaneous demand. The AC line voltage sagged from 120V down to 95V. Because the SMPS lost its AC headroom, it could no longer maintain the high-voltage tracking rail required for the peak signal. The amplifier's Under-Voltage Lockout (UVLO) circuit detected the rail collapse and muted the output to prevent severe clipping, which could have sent a lethal DC offset voltage straight into the subwoofer voice coils. The fix? Upgrading to a generator with a larger mechanical flywheel and faster AVR (Automatic Voltage Regulator) response, or adding a heavy-duty power conditioner with a capacitor bank to buffer the AC transient.
Frequently Asked Questions
Can I convert my existing Class AB amplifier to Class H?
No. Converting a fixed-rail Class AB amp to Class H requires completely redesigning the power supply to include a high-speed tracking modulator, adding secondary windings to the transformer, and implementing complex compensation networks to prevent rail-switching distortion. It is not a bench mod; it is a ground-up engineering project.
Does the tracking supply introduce switching noise into the audio?
In poorly designed or budget implementations, yes. If the tracking modulator switches at a frequency close to the audio band, or if the power supply rejection ratio (PSRR) of the output stage is inadequate, you can hear a faint 'hash' or high-frequency distortion on quiet passages. Premium pro-audio designs push the tracking frequency well above 500kHz and use heavy localized decoupling capacitors to mask the switching noise.
Is Class H more reliable than Class D?
Reliability depends entirely on thermal and electrical engineering, not just the topology. However, because Class H operates the output transistors at a much lower V_ce, the devices run significantly cooler. Lower junction temperatures exponentially increase the lifespan of silicon junctions, often giving well-designed Class H amps a legendary reputation for road-worthiness and longevity compared to early, poorly cooled Class D designs.
For deeper technical specifications on audio amplifier topologies and thermal design, refer to the Texas Instruments Audio Amplifier Design Hub and the application notes provided by Analog Devices.
