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You've saved up for a decent car audio upgrade, and the choice has come down to two four-channel amplifiers with nearly identical power ratings. One is a compact Class D unit. The other is a heavier Class AB design with a slightly lower price and a reputation for smooth, natural sound. Which one actually belongs in your trunk? This is one of the most common debates in car audio. Class AB amplifiers powered aftermarket systems for decades, but Class D has taken over most retail shelves in recent years. The engineering difference is real, and so are the trade-offs. This guide explains what the class label means in practical terms, what bench tests reveal about quality, and how to decide based on your speakers, your electrical system, and your listening habits. What Does "Class AB" Actually Mean in a Car Amplifier? Amplifier classes describe how the output transistors are biased and switched. In a pure Class A circuit, output devices conduct current all the time, which produces extremely linear sound but wastes most of the energy as heat. In Class B, each half of the audio waveform is handled by a separate transistor, which improves efficiency but creates crossover distortion where the two halves meet. Class AB is the middle path that became the car audio standard. A small idle bias keeps both output transistors slightly on around the zero-crossing point, so music at low and moderate levels is reproduced in a nearly Class A fashion. Vocals sound present, cymbals stay clean, and the transition between waveform halves does not add an audible notch. This is the technical reason behind the "warmth" so often attributed to Class AB. Power Supply Signal Input Bias & Driver Class AB Output To Speaker In a typical automotive Class AB amplifier, the signal path runs through an input buffer, a voltage amplifier, and a biased output stage before reaching the speaker. The power supply must keep the rail voltages clean under dynamic load, which is why good Class AB designs use either a generously sized linear transformer or a well-regulated switching supply. Class AB vs. Class D: Reading the Trade-Offs Correctly Class D amplifiers switch their output transistors on and off at frequencies above 100 kHz, then filter the result to recover the audio signal. The switching approach cuts power loss dramatically, which is why Class D amps are smaller, cooler, and easier on the alternator. The catch is that sound quality depends heavily on the quality of the filter, the feedback loop, and the power supply. A cheap Class D amp can sound harsh; a well-designed one can be nearly transparent. The numbers are typical values from manufacturer specifications and independent bench measurements. Actual performance varies with supply voltage and load impedance. Parameter Class AB Class D Typical efficiency 50–65% 75–90% Idle current draw Higher, continuous bias Much lower Heat output High Low Sound character Warm, natural midrange Transparent if well filtered Size and weight Larger, heavier Compact, lightweight Best use Component speakers, full range Subwoofers, tight installs The table is a starting point, not a verdict. A premium Class D amplifier with a clean output filter and robust power supply can measure better than a budget Class AB in distortion and noise. What Class AB gives you is a predictable, musically linear output stage that is hard to get badly wrong. What Bench Tests Reveal About Sound Quality An independent bench comparison published by BestCarAudio showed the efficiency gap in practice. The entry-level Class AB amp in that test drew just over 105 amperes and produced 793 watts during the dynamic power run. The premium Class D amplifier it was paired against produced about 80% more power with the same current draw, because it converted far less input energy into heat. Typical Efficiency at Moderate Output 0 25% 50% 75% 100% Class A ~25% Class AB ~55% Class D ~80% Representative values, not measurements of a particular model. The efficiency difference is not just a laboratory curiosity. The heat generated by a Class AB amp has to go somewhere: into the heatsink, the air behind a dashboard, or the interior of a sealed enclosure. This is what installers mean when they call Class AB "current hungry." Sound quality benefits are real, but they carry an electrical and thermal price. Total harmonic distortion (THD) and signal-to-noise ratio (SNR) also matter when you compare amplifiers. A well-executed Class AB amp commonly measures below 0.05% THD at moderate output. Budget Class D units sometimes show higher distortion at low levels because of switching noise. Take a familiar music track to your local shop and listen before you buy. Installation Realities: Heat, Fuses, and Wire Gauge Sizing the electrical path is part of any Class AB installation. At full output, a 400W RMS amp draws roughly 53A from a 13.8V system (about 400W divided by 13.8V and the 0.55–0.65 efficiency range). Under heavy bass, that load is sustained, not momentary. Use wiring and fuses rated for continuous operation. Recommendations assume a 13.8V electrical system and premium copper cable. Use the next heavier gauge if the cable run exceeds 5 meters. System RMS Power Class AB Current Draw Fuse Rating Wire Gauge 200W ≈27A 30A 8 AWG 400W ≈53A 60A 4 AWG 800W ≈105A 110A 2 AWG Treat the table as a floor, not a ceiling. In hot climates or enclosed trunks, a 12V cooling fan is an inexpensive upgrade for anything above 400W. The heatsink fin area of a Class AB amp should always be mounted with open airflow; never trap it against carpet or trim. Module-Level Options for Builders, Repair Shops, and OEM Integrators Custom fabrication shops, OEM audio brands, and technicians who rebuild factory sound systems often start with amplifier modules instead of finished chassis amps. A module provides a tested circuit layout, a mechanical footprint, and a documented power rating so you can design an enclosure around it. This shortens development time and reduces component-level debugging. For example, the amp300h-7294 module uses a Bi-Class H output stage and a linear transformer power supply. Its bias behavior resembles a Class AB amplifier, while the switched rail keeps heat production below a conventional AB design. That makes it a practical starting point when you need roughly 300W of low-frequency plus 50W of high-frequency output. When the build demands more headroom, a 400W plus 100W module with a U-shaped aluminum heat sink demonstrates how thermal design is solved at the board level. The radiator on the amp400h-7294 is already sized for sustained operation, so you do not have to guess at heatsink volume or airflow. AMP400H+7294 Class H Power Amplifier Module with U-shaped Aluminum RadiatorThis module combines a 400W Class H bass section with a 100W Class AB high-frequency stage, featuring a U-shaped heatsink and built-in protections for reliable, sustained car audio performance.View Product → Module-level sourcing is common because it shortens the path to production. Car audio builds that emphasize deep bass often combine a full-range module for the mid and treble channels with a dedicated subwoofer amplifier stage, which reduces development work and keeps the final assembly compact. Class AB Car Amplifier FAQ Is a Class AB amplifier good for car audio? Yes, especially for full-range speakers. The linear bias keeps vocals and instruments natural, and the midrange often feels more present than on an equal-cost Class D design. Just budget for the extra current draw and heat dissipation. What is the difference between Class AB and Class D car amplifiers? Class AB uses a biased linear output stage, which sounds smooth but generates heat. Class D switches its output transistors on and off and filters the result, which is more efficient and compact. Sound quality in both depends heavily on build quality and the power supply. Do Class AB amplifiers sound better than Class D? Most listeners find entry-level and mid-priced Class AB amplifiers sound warmer and less fatiguing than budget Class D designs. At the premium end, the gap narrows sharply, and many people cannot consistently tell them apart in blind listening tests. Can you use a Class AB amplifier for a subwoofer? Yes. Class AB bass tends to be tight and well controlled, and many subwoofer amplifiers still use a Class AB output stage. For custom builds, a dedicated subwoofer amplifier module is a good option when space is flexible. How many watts RMS do I need for a Class AB car amplifier? For component speakers, 50 to 100 watts RMS per channel is common. For subwoofers, plan on 300 to 600 watts RMS if your alternator can sustain it. Use RMS power at 4 ohms as the reference, because max power numbers are largely meaningless. Do Class AB amplifiers draw more current at idle? Yes. The bias current runs whenever the amp is on, typically 1 to 3 amperes depending on the design. Class D amplifiers idle at a fraction of that. If you leave the stereo playing with the engine off, battery drain will be noticeable. .article-section table{display:table!important;border-collapse:collapse;margin:0 auto 20px;} .article-section thead{display:table-header-group!important;} .article-section tbody{display:table-row-group!important;} .article-section tr{display:table-row!important;} .article-section th{display:table-cell!important;font-weight:bold;border:1px solid #cccccc;padding:8px;} .article-section td{display:table-cell!important;border:1px solid #cccccc;padding:8px;} .article-section table caption{caption-side:bottom;font-size:16px;margin-bottom:12px;font-style:italic;color:#808080;padding-top:8px;} .article-section ol{margin-bottom:12px;list-style-type:decimal;list-style-position:inside;padding-left:0;} .article-section ul{margin-bottom:12px;list-style-type:disc;list-style-position:inside;} .article-section li{list-style:inherit;font-size:16px;margin-bottom:6px;} .article-section h2{font-size:22px;font-weight:bold;text-align:left;margin-bottom:12px!important;} .article-section h3{font-size:16px;font-weight:bold;text-align:left;margin-bottom:12px;} .article-section p{font-size:16px!important;margin-bottom:12px;} .chart-container{width:440px;margin:24px auto;text-align:center;} .chart-container svg{width:440px;height:auto;display:block;margin:0 auto;} @media(max-width:640px){.chart-container{width:100%;}.chart-container svg{width:100%;height:auto;}} .faq-grid{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin:20px 0 12px;} .faq-item{background:#f4f8fd;border-left:4px solid #2b6cb0;border-radius:0 8px 8px 0;padding:14px 18px;} .faq-item h3{color:#1a4a7a;margin-bottom:8px;} .faq-item p{font-size:15px!important;margin-bottom:0;} @media(max-width:640px){.faq-grid{grid-template-columns:1fr;}} .article-section a[data-product-card="true"]{display:inline-block;margin:14px 0;padding:10px 16px;background:#eef4fb;border-left:4px solid #2b6cb0;border-radius:0 6px 6px 0;font-size:15px;font-weight:600;color:#1a4a7a;text-decoration:none;} .article-section a[data-product-card="true"]::after{content:"View module details →";font-size:14px;font-weight:400;} .product-card{display:block;margin:20px 0;border:1px solid #e5e7eb;border-radius:10px;overflow:hidden;font-style:normal;background:#fff} .pc-inner{display:flex;text-decoration:none;color:inherit;align-items:center;min-height:120px} .pc-img{width:160px;min-width:160px;aspect-ratio:4/3;height:auto;min-height:120px;object-fit:cover;flex-shrink:0;display:block;align-self:stretch} .pc-body{padding:12px 16px;flex:1;min-width:0;display:flex;flex-direction:column;align-self:stretch;justify-content:center} .pc-title{display:block;font-size:15px;font-weight:600;color:#111;margin:0 0 6px;line-height:1.4} .pc-desc{display:-webkit-box;font-size:13px;color:#6b7280;margin:0 0 8px;line-height:1.5;overflow:hidden;-webkit-line-clamp:2;line-clamp:2;-webkit-box-orient:vertical} .pc-cta{display:block;font-size:13px;font-weight:600;color:#11700F;margin-top:auto} .pc-inner:hover .pc-title{text-decoration:underline} .article-section a:not(.pc-inner),article a:not(.pc-inner){color:#11700F}.pc-cta{color:#11700F!important}
When a powered speaker manufacturer evaluates amplifier platforms, every option carries a trade-off. Class AB is proven but runs hot, Class D is efficient but introduces switching artifacts, and Class H tends to sit between the two. Its engineering logic is simple: make the power supply follow the audio signal, so the output stage never wastes large amounts of voltage as heat. Class H amplifiers deliver analog sound quality with higher efficiency, and they remain a well-established option in professional audio module catalogs. This guide explains how Class H works, what efficiency you can expect, and how to evaluate Class H amplifier modules for OEM speaker and subwoofer designs. What Is a Class H Amplifier? A Class H amplifier is an analog power amplifier with a modulated power supply rail. In a conventional Class AB amplifier, the supply voltage stays fixed while the audio signal fluctuates, so the output transistors continuously dissipate the excess voltage as heat. In a Class H amplifier, a tracking circuit raises and lowers the supply rail so that it stays just above the level of the audio signal. This technique is commonly called rail tracking. When the music is quiet, the rail voltage drops; when a transient or kick drum hit arrives, the rail voltage rises to give the output stage enough headroom. The result is lower heat generation and better overall efficiency than fixed-rail analog designs. Design discussions among audio engineers often cite current savings of roughly 20-30% at typical music levels compared with a fixed-rail Class AB amplifier. According to the widely cited Wikipedia article on power amplifier classes, Class H differs from Class G in that the supply rail varies continuously rather than in discrete steps in response to the input. For a manufacturer, this same architecture is available as a building block: an active power amplifier module integrates the preamp, tracking controller, and output stage on one board, so you can adopt Class H performance without designing a tracking supply from scratch. A module such as the Amp300H-7294, for example, delivers 300 W low-frequency plus 50 W high-frequency output from a linear transformer supply for bi-amped speaker systems. Efficiency at a Glance: How Class H Compares Efficiency figures depend on load, signal content, and circuit design. Audio engineering references typically cite the following ranges at continuous high-level output: Class A ~25% Class AB ~55% Class H ~75% Class D ~88% Typical efficiency (percent) Class A dissipates the most energy because its output devices always conduct fully. Class AB improves on that but still burns significant heat at idle and at partial output. Class H reduces this dissipation by following the signal envelope, while Class D reaches the highest efficiency by switching its output stage at high frequency. General characteristics of the main amplifier classes relevant to professional audio design. Class Typical efficiency Supply rail Output stage Heat management Class A 20-30% Fixed Linear, always conducting Large heat sinks required Class AB 50-65% Fixed Linear Moderate heat sinks Class H 70-80% Tracking Linear Small-to-moderate heat sinks Class D 85-90% Switched Switching Minimal, but output filtering required How the Tracking Supply Rail Works The diagram below illustrates the key behavior. The central signal is a sine wave; the dashed lines above and below are the positive and negative supply rails. The positive rail rises only during the positive half of the signal, and the negative rail deepens only during the negative half. headroom Output signal Tracking +V rail Tracking -V rail Class H rails stay close to the signal, so the output stage dissipates little excess heat. 0V The tracking controller performs this modulation continuously. It monitors the input signal, predicts the required headroom, and drives an active regulation stage that feeds the output transistors. In practical module designs with a linear transformer power supply, the controller adjusts the rail so that the output stage always sees the minimum sufficient supply voltage. The input preamp conditions the audio signal and sends it to the tracking controller. The controller compares the instantaneous output envelope with the current rail voltage. The supply regulation stage raises or lowers the rail within microseconds, keeping a small margin above the signal peak. The output transistors amplify the signal using a rail that is just high enough, converting far less excess power into heat. Strengths and Trade-Offs of Class H What Class H does well Higher efficiency than Class AB at music duty cycles, with less heat sink burden Clean analog output with no switching noise or high-frequency artifacts Lower idle power consumption benefits thermally constrained or battery-powered products Peak headroom remains available for transients because the rail can rise quickly Where Class H makes you pay More complex power supply stage increases module cost relative to Class AB Tracking controller must be stabilized carefully to avoid distortion on fast transients Efficiency still falls short of Class D at sustained full power Linear transformer supplies add weight compared with compact switching supplies Design discussions among audio engineers note that Class H never became the dominant topology in consumer hi-fi, largely because the added cost and complexity of rail tracking were hard to justify for low-power, single-voltage applications. In professional speaker products, however, the efficiency and heat benefits matter much more, and module-based Class H designs sidestep much of the complexity concern. What to Check When Choosing a Class H Power Amplifier Module Not every Class H module behaves the same way. These points separate a rugged professional module from a board that will create problems in the field: Rated power and impedance: confirm the LP and HP power ratings and the nominal load impedance, such as 400 W and 100 W into 8 ohms. Supply topology: linear transformer supplies provide robust transient current and low noise, while switching supplies reduce weight and cost. Heat sink design: a large U-shaped aluminum radiator improves long-term reliability at high output levels. Protection circuits: look for overload, short-circuit, and over-temperature protection built into the module. Customization: if you are an OEM, check whether the manufacturer will adjust EQ presets, input sensitivity, or connector layouts. Manufacturing quality: SMT assembly, automated testing, and consistent component sourcing are essential for repeatable module performance. Linear transformer U-shaped aluminum heat sink Rail tracking control board Base PCB with output stage Isometric view: major building blocks of a compact Class H amplifier module. An example in this category is the Amp400H-7294, which delivers 400 W low-frequency plus 100 W high-frequency output with a U-shaped aluminum radiator and a linear transformer power supply. It is designed for OEMs who need dependable headroom in a compact module footprint. AMP400H+7294 Class H Amplifier Module with 400W LF and 100W HF OutputThis compact Class H module offers 400W low-frequency and 100W high-frequency power with a linear transformer supply and U-shaped aluminum radiator, making it a reliable choice for powered speakers requiring headroom.View Product → Where Class H Fits in Professional Audio Class H modules are most useful in powered speakers that play material with a high crest factor: short peaks above a relatively modest average level. This includes PA monitors, column speakers, line-array cabinets, and especially subwoofers. Subwoofers: bass transients demand sudden current, and the tracking rail supplies exactly the headroom needed without idling at the full supply voltage. Line arrays: amplifier modules are installed inside sealed cabinets where heat is difficult to remove; Class H reduces the thermal load compared with Class AB. Column speakers: compact form factors make efficient heat sink use essential, and the tracking supply helps keep the enclosure temperature manageable. For subwoofer duty, the Amp600S-FP delivers 600 W through a professional linear transformer power supply, making it a direct example of how Class H suits high-power low-frequency output. The same reasoning that makes it useful in subwoofer designs applies broadly when you evaluate how audio amplifier modules enhance sound quality in speaker systems: the amplifier must deliver clean power without overheating the cabinet. Frequently Asked Questions Is a Class H amplifier better than Class AB? Class H is more efficient than Class AB because the supply rail tracks the signal, which reduces heat at typical music levels while keeping an analog output character. Class AB remains simpler and less expensive for low-power, fixed-level applications. What is the efficiency of a Class H amplifier? Typical efficiency is around 70-80% at high output levels, depending on design and load. That is higher than Class AB at 50-65% but lower than Class D at 85-90%. Is a Class H amplifier good for speakers? Yes. Class H works well in powered speakers, column speakers, and subwoofers because it reduces heat and idle power loss while producing clean analog output without switching artifacts. What is the difference between Class D and Class H amplifier? Class D switches the output stage on and off for very high efficiency and needs output filtering. Class H keeps a linear output stage but modulates its supply voltage; it uses more current than Class D but is easier to integrate for clean analog performance. Does a Class H amplifier sound good? Class H can sound very good in practice. The output stage is linear, and the tracking supply preserves transient headroom, so the sonic character is close to a well-designed Class AB amp without the heat burden. Why use a Class H amplifier in a subwoofer? Bass content has a high crest factor: short peaks above a modest average level. Class H delivers the headroom when the peaks arrive and lowers the rail in between, which minimizes heat and power loss exactly where it matters most. .article-section{line-height:1.6!important;} .article-section table{display:table!important;border-collapse:collapse;} .article-section thead{display:table-header-group!important;} .article-section tbody{display:table-row-group!important;} .article-section tr{display:table-row!important;} .article-section th{display:table-cell!important;} .article-section td{display:table-cell!important;} .article-section table{caption-side:bottom;font-size:16px;margin-bottom:12px;font-style:italic;color:#808080;} .article-section th{font-weight:bold;border:1px solid #cccccc;padding:8px;} .article-section td{border:1px solid #cccccc;padding:8px;} .article-section ol{margin-bottom:12px;list-style-type:decimal;list-style-position:inside;padding-left:0;} .article-section ul{margin-bottom:12px;list-style-type:disc;list-style-position:inside;} .article-section li{list-style:inherit;font-size:16px;margin-bottom:6px;} .article-section h2{font-size:22px;font-weight:bold;text-align:left;margin-bottom:12px!important;} .article-section h3{font-size:16px;font-weight:bold;text-align:left;margin-bottom:12px;} .article-section p{font-size:16px!important;margin-bottom:12px;} .article-section svg{width:440px;height:auto;display:block;margin:16px auto;} .article-section .chart-caption{text-align:center;font-size:14px!important;color:#888;font-style:italic;margin-top:-8px;} .article-section .intro-block{background:#f7fafd;border-left:5px solid #2980b9;padding:22px 24px;border-radius:6px;} .article-section .efficiency-block{background:#fafbfc;padding:22px;border-radius:8px;border:1px solid #eee;} .article-section .sc-columns{display:flex;gap:20px;margin:14px 0;} .article-section .sc-col{flex:1;background:#f4faf4;border:1px solid #cfe8cf;border-radius:8px;padding:14px 16px;} .article-section .sc-col:last-child{background:#fdf3f3;border-color:#eec9c9;} .article-section .faq-grid{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin:16px 0;} .article-section .faq-card{background:#f0f7ff;border:1px solid #cfe2f3;border-left:5px solid #2e86c1;border-radius:10px;padding:14px 16px;} .article-section .faq-card h3{color:#1a5276;} .article-section .faq-card p{margin-bottom:0;color:#2c3e50;} @media(max-width:640px){ .article-section svg{width:100%;} .article-section .sc-columns{flex-direction:column;} .article-section .faq-grid{grid-template-columns:1fr;} } .product-card{display:block;margin:20px 0;border:1px solid #e5e7eb;border-radius:10px;overflow:hidden;font-style:normal;background:#fff} .pc-inner{display:flex;text-decoration:none;color:inherit;align-items:center;min-height:120px} .pc-img{width:160px;min-width:160px;aspect-ratio:4/3;height:auto;min-height:120px;object-fit:cover;flex-shrink:0;display:block;align-self:stretch} .pc-body{padding:12px 16px;flex:1;min-width:0;display:flex;flex-direction:column;align-self:stretch;justify-content:center} .pc-title{display:block;font-size:15px;font-weight:600;color:#111;margin:0 0 6px;line-height:1.4} .pc-desc{display:-webkit-box;font-size:13px;color:#6b7280;margin:0 0 8px;line-height:1.5;overflow:hidden;-webkit-line-clamp:2;line-clamp:2;-webkit-box-orient:vertical} .pc-cta{display:block;font-size:13px;font-weight:600;color:#11700F;margin-top:auto} .pc-inner:hover .pc-title{text-decoration:underline} .article-section a:not(.pc-inner),article a:not(.pc-inner){color:#11700F}.pc-cta{color:#11700F!important}
Specify a 500 W Class AB stage for a sealed powered speaker and you take on roughly 400 W of waste heat inside a cabinet the size of a carry-on bag. Specify a Class D module for the same 500 W and the heat load drops to around 55 W. The acoustic output on the spec sheet can look almost identical; the thermal design, transformer rating, cabinet volume and shipping weight will not. The short answer: for new powered speakers, line arrays and portable systems, Class D is the default. Class H still earns its place in high-output subwoofers and installed sound systems where a linear output stage and a simple transformer supply are preferred. Class AB remains widespread in legacy designs, instrument amplifiers and powered mixers. The sections below explain what the class letter actually controls, where each class wins, and which checks keep a module from failing in production. What an Amplifier Class Actually Describes An amplifier class describes the bias condition of the output stage, and specifically how much of the input signal cycle each output device conducts. That single parameter sets the theoretical efficiency ceiling, the distortion mechanism and the heat the stage has to shed. Class A — the device conducts through the full 360 degrees of the cycle. Excellent linearity, lowest efficiency of any linear class. Class B — two devices each conduct 180 degrees and hand over at the zero crossing. Efficient in theory, but the handover creates crossover distortion. Class AB — each device conducts somewhere between 180 and 360 degrees, pushing crossover distortion below audibility at the cost of some efficiency. Class D — the output devices switch on and off at high frequency instead of tracking the waveform, so efficiency comes from switching rather than from linear amplification. Class G and H — linear stages whose supply rail steps or tracks the signal, so the output devices only ever see the voltage they need. The class letter is not a quality rating. A well-implemented switching stage can measure better than a mediocre linear one, and a poorly filtered Class D module can produce artifacts a Class AB design would never show. What the letter predicts reliably is wasted power, heatsink and supply size, and the cost of building and shipping the finished product. Typical full-power efficiency by amplifier class Class A25–30% Class B50–70% Class AB50–65% Class H65–80% Class D80–92% Practical ranges for audio output stages. Class H efficiency depends on how often its rails step or track. Amplifier Class Comparison at a Glance The table below summarises the classes you are most likely to see in a module catalogue, together with the design decision each one forces. Typical published efficiency ranges rather than guaranteed figures for a specific module; real numbers depend on supply design, load impedance and programme material. Class Conduction or mode Typical efficiency Where it appears Design consequence A 360 degrees 25–30% Studio monitors, hi-fi Large heatsink, high idle current B 180 degrees per device 50–70% Rare in audio, common in textbooks Crossover distortion at handover AB 180–360 degrees 50–65% PA amps, instrument amps, powered mixers Simple supply, heatsink sized for continuous output C Under 180 degrees 60–80% RF transmitters, not audio Distortion too high for audio use D Switching on and off 80–92% Powered speakers, line arrays, subwoofers EMI layout, output filter, dead-time control G Stepped supply rails 60–75% High-power hi-fi, installed systems Rail switching adds cost and complexity H Continuously tracking rails 65–80% Subwoofers, large installed sound Rail modulator plus linear transformer supply Classes E and F belong to radio frequency design and rarely appear in audio hardware, which is why they are left out of the table above. Why Efficiency Turns Into Cabinet Size, Weight and Cost Efficiency percentages become useful once you convert them into watts. For a 500 W output stage, a Class A design at roughly 30% efficiency draws about 1667 W from the mains, a Class AB stage at 55% draws about 909 W, a Class H stage at 75% draws about 667 W, and a Class D module at 90% draws about 556 W. Every one of those watts has to be supplied by the transformer and removed from the enclosure. That drives heatsink mass, fan noise, ventilation openings and the weight of every unit you ship. Temperature also affects reliability: a widely used rule of thumb in power electronics is that every 10 °C drop in operating temperature roughly doubles the life of an electrolytic capacitor. Mains power drawn per 500 W of audio output 1667 WClass A 909 WClass AB 667 WClass H 556 WClass D Illustrative calculation using typical efficiencies of 30%, 55%, 75% and 90% respectively. Class D Amplifier Modules: The Default for New Powered Speakers Class D output devices switch rather than amplify linearly, so losses are dominated by conduction and switching instead of by standing bias current. Practical modules reach 80–92% efficiency, which is why nearly every new portable speaker, line array element and compact subwoofer is built around a switching module. The trade-offs move into the supply and the board layout. A resonant LLC supply with power factor correction keeps the bus stable across a wide mains range and holds switching noise in a predictable band, which makes EMI filtering easier. Output filter inductors, dead time and gate-drive routing decide whether the module passes emissions limits and stays stable into a 4 ohm load. A representative example is the eon520-2092 module, rated LP500W/HP200W with a PFC and LLC resonant front end and wide-voltage input. Class H Modules: Still the Right Answer for High-Output Subwoofers Class H is a linear class with supply rails that step or track the audio signal. Because the output devices never drop the full bus voltage across themselves at low output levels, efficiency lands in the 65–80% band — well below Class D, but with far less high-frequency switching content. That is one reason Class H designs remain common in large subwoofer amplifiers and installed sound systems. Modules in this category typically use a linear transformer supply and rely on substantial heatsinking. The amp400h-7294, for instance, is rated LP400W/HP100W and uses a U-shaped aluminium radiator to handle continuous output, while the amp600s-fp is aimed specifically at subwoofer duty. AMP400H+7294: LP 400W+HP 100W Poweful Output with U-shaped Aluminum Radiator Class H Power● Linear transformer power: 500W ● Mid and low-frequency power amplifier: Class H 400W-4Ohm ● High-frequency power amplifier: Class AB 100W-8 Ohm ● U-shaped aluminum r...View Product → Matching the Amplifier Class to the Rest of the Module Set The amplifier class only makes sense in the context of the whole module set. A powered speaker usually stacks four functions: input and volume control, a DSP core, a crossover or EQ stage, and the power stage that drives the transducers. Typical four-layer module stack in an active speaker Input and volume DSP core ADAU1701 Crossover and EQ Class D power stage signal flow Functional layers of an active speaker module set, from input to loudspeaker output. Matching matters at the boundaries. The DSP module sets the limiter threshold and the crossover point, so it needs to know the real output voltage swing of the power stage, and the power stage needs a load it can drive safely at the driver's minimum impedance. Modules built on a single DSP platform keep the control interface and preset structure consistent across models — the dsp1903, for example, stores four customised preset EQ modes for a 2.1-channel line array system. A supplier that designs both halves in house, such as Ningbo Huage Electronics, can adjust those boundaries together rather than negotiating them between two vendors. DSP1903: 4 Customized Preset EQ Modes 2.1CH Linear Array Speaker ADAU1701 Based DSP Functi● Designed based on ADAU1701 sound audio system. Use sigma studio DSP software to configure signal processing parameters. ● Selectable four customized preset modes whi...View Product → Practical Checks Before You Commit to a Class Whichever class you shortlist, these five checks catch most problems before tooling starts. Work from minimum impedance, not nominal. A driver rated at 8 ohm nominal can dip well below that at some frequencies, and amplifier module and speaker impedance matching has to be verified against that minimum. Convert efficiency into heat at the worst case, not the average. Programme material has a crest factor, but a limiter will hold long-term power close to the rating during a show. Check the supply against the mains. Wide-voltage switching modules tolerate brownouts and generator supplies better than fixed linear rails. Confirm the protection scheme in writing. Over-current, over-temperature and DC-offset behaviour should be specified, not assumed. Verify the thermal path. Heatsink mounting torque, thermal pad specification and airflow all influence field failure rates. Class AB, D and H compared across five design criteria Efficiency Thermal headroom Compact size Low distortion Cost per watt Class DClass HClass AB Qualitative comparison on a five-point scale for typical module-level designs. Frequently Asked Questions About Amplifier Classes Q1. What are the main amplifier classes used in audio? Class A, Class AB, Class D and Class H cover almost all audio equipment. Class A conducts through the full signal cycle, Class AB through more than half of it, Class D switches the output devices at high frequency, and Class H is a linear stage whose supply rails track the signal. Q2. Which amplifier class is best for a powered speaker or subwoofer? For powered speakers, line arrays and portable systems, Class D is usually the starting point because of its efficiency and small thermal footprint. For very high output subwoofers and installed systems, Class H often gives a better balance of cost, linear behaviour and simple transformer supply design. Q3. Is a Class D power amplifier module as good as Class AB? When it is engineered properly, yes. Class D modules now drive line arrays and subwoofers routinely. The deciding factors are the output filter, gate-drive layout, EMI performance and the protection design rather than the class letter itself. Q4. How efficient is a Class D amplifier compared with Class A and Class AB? Practical Class D audio modules run at roughly 80–92% at full power, against about 50–65% for Class AB and 25–30% for Class A. A 500 W Class D module therefore dissipates in the region of 45–125 W of heat instead of the 400 W or more a Class AB stage would produce. Q5. What does Class H mean in a power amplifier module? Class H is a linear output stage whose supply rail steps up or tracks the audio signal, so the output devices only see the voltage they need. Efficiency typically lands between 65% and 80%, higher than Class AB but below Class D, with less high-frequency switching content than a switching module. Q6. Does amplifier class affect sound quality, or only efficiency and heat? Class changes the distortion mechanism and the thermal behaviour rather than the sound quality by itself. 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