We are a professional audio company integrating R&D, production and sales. We have been focusing on the production of mixers, active power amplifiers, microphones, and related electronic components and equipment for many years.
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Two quotations land on your desk for the same 2 x 500 W powered subwoofer board. The first is built around a Class AB stage with a heatsink roughly the size of a paperback book. The second uses a Class D stage on a board barely larger than a phone. Both are honest about the power rating. Only one of them will fit the cabinet you have already tooled, and only one will pass the thermal test in a closed wooden enclosure at 40 °C ambient. That is the real decision behind the phrase "class amplifiers". The class of an amplifier describes how its output devices conduct current across one signal cycle. That single choice sets how much heat you must remove, what kind of power supply you must buy, how much filtering and shielding the board needs, and how much aluminium goes into the chassis. It does not, on its own, decide whether the product sounds good. Short version: Class AB still wins where linearity and simple supplies matter, Class D wins on efficiency and weight, and Class H exists to keep high output power from turning the chassis into a radiator. Pick the class after you know the cabinet, the load and the supply budget, not before. What the class label actually describes The class name comes from one physical property: how long the output devices stay conductive during a single cycle of the audio signal. Class A keeps them conducting the whole time. Class B splits the cycle between two devices, each handling half. Class AB keeps both devices slightly open at the zero crossing so the handover is smooth. Class D abandons the linear idea entirely and switches the output devices at a few hundred kilohertz, encoding the audio as pulse width. Class H keeps a linear audio stage but makes the supply rail step up and down to follow the signal envelope. Everything else follows from that. A stage that conducts all the time wastes a large share of its supply as heat. A stage that switches loses almost nothing in the devices but pushes the design problem into the output filter, the gate drive and the electromagnetic interference budget. A stage that moves its rails reduces the voltage across the output devices, which is exactly where the heat in a linear amplifier comes from. What the label does not tell you is build quality. A poorly laid out Class D module and a carefully engineered one share the same name and can differ by 15 °C in heatsink temperature at the same output level. Efficiency, heat and distortion side by side Published design references quote wide efficiency bands, because the real number depends on output level, load impedance and supply design. The figures below are the ranges engineers typically use when comparing topologies at or near full rated output. Typical conduction angle, efficiency and distortion character of audio power stages; real values shift with supply design, load and output level. Class Conduction angle Typical efficiency Distortion character Where it shows up A 360° 20-30% Very low distortion, very high steady heat Small-signal stages, audiophile and studio designs B 180° 70-78% Crossover notch at low levels Rarely used alone in audio AB 180-360° 50-70% Low distortion when bias is set correctly Analog PA, powered mixers, budget active speakers C Below 180° 75-85% High distortion with strong harmonics RF transmitters, not linear audio D Switching 80-95% Low, dominated by switching residue and filter quality Powered speakers, subwoofers, portable systems H Rail tracking 65-85% Low, close to a well-biased AB stage High-power line array and subwoofer amplifiers Typical full-power efficiency (%) Class A 20-30% Class AB 50-70% Class B 70-78% Class H 65-85% Class D 80-95% 0 25 50 75 100 Efficiency bands at or near rated output. In real use, a Class D stage idles far below its headline number. Class AB: linearity you pay for in heat Class AB exists to remove the crossover notch of Class B without paying the full cost of Class A. A small bias current keeps both output devices conducting around the zero crossing, so the handover between them stays smooth. Typical quiescent current in a pro-audio module sits in the tens of milliamps per device pair, enough to keep the notch out of the audible range but small enough that the amplifier does not cook at idle. The trade is that heat is continuous. A 400 W Class AB module draws meaningful current even with no signal, and the heatsink has to be sized for the worst case: pink noise at one-eighth power for an hour in a rack with poor airflow. That is why analog modules tend to carry large extruded heatsinks, U-shaped aluminium brackets and sometimes a fan. Buyers who compare only the wattage on the label often miss this, then discover that the same 400 W in a Class D module needs a fraction of the metal. The upside is well understood: no switching residue, no output filter interacting with the load, simple transformer-based supplies, and behaviour that field technicians already know how to service. Class D: efficiency depends on the supply you pair it with Class D modules switch at a few hundred kilohertz and reconstruct the audio through an LC output filter. The devices spend most of their time either fully on or fully off, so conduction losses are small and the efficiency headline is real. What the headline hides is that the advantage only holds near rated power. At background listening levels, switching and gate-drive losses dominate and efficiency drops sharply, which matters for battery-powered products more than for mains-powered ones. The rest of the design carries equal weight. Dead time that is too short causes shoot-through; too long and distortion rises at low output. The output filter has to stay stable across the actual load, not the dummy resistor on the bench. And the supply has to keep up: a Class D stage paired with a soft, unregulated rail will sag on bass transients and undo the benefit. That is why resonant LLC supplies and power-factor-corrected front ends have become standard in serious modules. EON522SUB Active Subwoofer Power Amplifier ModulePairs an 800 W LLC resonant supply with a 500 W Class D bass stage, addressing rail sag and switching noise in powered subwoofers.View Product → As an example of the pattern, a module such as the EON522D-2092 pairs a 500 W low-pass and 200 W high-pass Class D stage with an LLC resonant supply, which is the combination that keeps rail sag and switching noise under control in a 2.1 enclosure. Class H: rail switching for the top of the power range Class H keeps a linear output stage but gives it more than one supply rail. When the signal envelope crosses a threshold, the amplifier switches to a higher rail; when it falls back, the rail drops again. The output devices therefore only ever see a few volts across them instead of the full rail, and the heat that would have gone into the heatsink simply never gets generated. In practice a Class H module delivers the power of a much larger Class AB design in the same chassis depth. That efficiency is not free. The rail switching circuitry adds parts and cost, the transition can produce artefacts if the comparator thresholds are badly chosen, and the transformer needs multiple taps. It also makes service documentation more important, because a technician who treats the module like a plain AB board will misdiagnose a rail fault as an output device failure. AMP400H+7294 Class H Power Amplifier ModuleCombines a 400 W Class H low/mid stage with a 100 W Class AB high-frequency stage and a U-shaped aluminium radiator for stable output.View Product → Modules in this category, such as the AMP400H-7294 with its U-shaped aluminium radiator, are typically specified for subwoofer and line array duty where continuous high output is expected for hours rather than minutes. What the class choice changes further down the module stack The power stage is one board in a chain, and the class decision ripples through the rest of it. A Class D stage with a fast limiter needs a preamp module that can feed it a clean, well-scaled signal; a Class AB stage with 20 dB of gain headroom will behave differently with the same source. Gain structure, clip behaviour, protection thresholds and even the choice of volume taper all change with the topology. signal path Source module MP3, USB, Bluetooth DSP module EQ presets, limiter Analog function module Mic mix, EQ, crossover Power amplifier module Class AB, D or H The power stage is the last block in the chain: every block above it must match its gain, headroom and protection behaviour. DSP1903 ADAU1701 DSP Module for 2.1 SystemsADAU1701-based DSP module with four preset EQ modes, Bluetooth, and XLR I/O, suited to matching limiter behaviour in 2.1 line array systems.View Product → In practice this is where modular suppliers earn their keep. A DSP module such as the DSP1903, built on the ADAU1701 and carrying four preset EQ modes for a 2.1 line array system, has to be matched to the limiter behaviour of whatever power stage sits below it. If the two are specified independently, the system either clips early or protects late. Our walkthrough of the A, AB, D and H design trade-offs goes deeper into how those thresholds are set. Matching the class to the product you are building Most powered speaker projects fall into a handful of patterns, and each one narrows the class choice quickly: Line array and column systems. Weight and rack density decide. Class D with a resonant supply is the default, with Class H reserved for modules that must run at very high output for long periods. Subwoofers. High continuous power into a low-impedance load. Class D with a robust supply handles this best; the supply, not the amplifier, is usually the limiting part. Portable and battery-powered speakers. Efficiency at part load matters more than the peak number. Class D is the only realistic option, and the idle losses deserve as much attention as the maximum output. Powered mixers and cost-sensitive analog boxes. Class AB with a linear transformer keeps the bill of materials simple and the service story familiar, at the cost of a bigger heatsink. High-power touring subwoofers. Class H or a high-rail Class D design, depending on whether the design team prefers linear behaviour or minimum weight. What to check with a module supplier before you commit The datasheet wattage is the least useful number in the conversation. These are the points that separate a design that survives field use from one that comes back: Efficiency and case temperature at one-eighth power, not just at full rated output, measured in still air at your intended ambient. Distortion at 1 W and at 10 W, where crossover and switching artefacts actually live. Protection behaviour: what happens on a shorted output, a mismatched load, or a supply sag, and whether the module recovers on its own. Supply requirements, including whether power factor correction is needed for the markets you ship to. EMI and idle noise, especially for Class D modules feeding high-efficiency compression drivers. Consistency between prototype and mass production, which is a manufacturing question as much as an engineering one. For buyers who need a non-standard configuration, the sensible sequence is to settle the class and the supply topology first, then take the module through custom development for gain structure, connectors and preset behaviour. Changing the class after the cabinet and power supply are frozen is expensive. Frequently asked questions about class amplifiers Which amplifier class is the most efficient? Class D is the most efficient audio class in normal use, typically 80-95% at rated output. Class C is efficient too but distorts heavily and is used for RF, not audio. What is the difference between Class AB and Class D amplifiers? Class AB keeps its output devices partially conducting and behaves linearly, at the cost of continuous heat. Class D switches devices on and off at high frequency, which is far more efficient but moves the design challenge to filtering and EMI. Is Class D good enough for professional audio? Yes. Modern Class D modules with well-designed output filters and stiff resonant supplies are standard in powered line arrays and subwoofers. The limiting factor is usually the supply and the limiter settings, not the topology. What is a Class H amplifier used for? Class H is used where high output power must fit a compact chassis, such as touring subwoofers and high-power line array modules. Rail switching keeps the voltage across the output devices low, so less heat reaches the heatsink. Do amplifier classes affect sound quality? Less than layout, supply design and gain structure do. A well-engineered Class D module and a well-engineered Class AB module can both sound clean; a poorly biased AB stage or a badly filtered D stage will not. Why do Class A amplifiers run so hot? Because the output devices conduct current during the entire signal cycle, even with no music playing. That constant current produces steady heat, so Class A needs large heatsinks and generous ventilation for modest output power. .article-section{margin-bottom:22px;color:#243b53;line-height:1.7;} .article-section h2{font-size:22px;font-weight:bold;text-align:left;margin-bottom:12px!important;color:#123a5e;} .article-section h3{font-size:16px;font-weight:bold;text-align:left;margin-bottom:12px;color:#123a5e;} .article-section p{font-size:16px!important;margin-bottom:12px;} .article-section ul{margin-bottom:12px;list-style-type:disc;list-style-position:inside;} .article-section ol{margin-bottom:12px;list-style-type:decimal;list-style-position:inside;padding-left:0;} .article-section li{list-style:inherit;font-size:16px;margin-bottom:6px;} .article-section table{display:table!important;width:100%;border-collapse:collapse;font-size:15px;margin-bottom:16px;} .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;background:#eef5fb;color:#123a5e;text-align:left;} .article-section td{display:table-cell!important;border:1px solid #cccccc;padding:8px;vertical-align:top;} .article-section caption{caption-side:bottom;font-size:16px;margin-bottom:12px;font-style:italic;color:#808080;} .article-section .callout{background:#eef5fb;border-left:4px solid #1f7ac0;border-radius:6px;padding:14px 16px;margin:14px 0;} .article-section .callout p{margin-bottom:0;} .article-section .chart-figure{width:440px;margin:20px auto;} .article-section .chart-figure svg{display:block;width:100%;height:auto;} .article-section .chart-caption{font-size:13px;color:#6b7c8f;text-align:center;margin-top:8px;} .article-section .faq-grid{display:grid;grid-template-columns:1fr 1fr;gap:14px;margin-top:16px;} .article-section .faq-item{background:#f2f7fc;border-left:4px solid #1f7ac0;border-radius:6px;padding:14px 16px;} .article-section .faq-item h3{font-size:16px;margin-bottom:8px;color:#123a5e;} .article-section .faq-item p{font-size:16px!important;margin-bottom:0;color:#33475b;} @media (max-width:640px){ .article-section .chart-figure{width:100%;} .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;box-shadow:0 0 5px rgba(31,41,55,.08)} .pc-inner{display:flex;text-decoration:none;color:inherit;align-items:center;min-height:148px} .product-card .pc-img{width:auto!important;min-width:0!important;max-width:min(36%,240px)!important;height:auto!important;min-height:0!important;max-height:148px!important;aspect-ratio:auto!important;object-fit:contain!important;flex:0 0 auto;display:block;align-self:center} .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}
A 100 W Class B amp sitting at idle draws almost nothing and barely warms its heatsink. Feed the same circuit into a 4 ohm speaker and run it at one watt, and a notch appears right where the waveform crosses zero. Efficiency is the easy part of this topology; linearity around the zero crossing is what decides whether it belongs in a finished audio product. Here is the conclusion first: for audio equipment, pure Class B is rarely the right final answer. It is the reference point every other amplifier class is measured against, and it is the reason Class AB exists at all. If you are specifying an amplifier module for a powered speaker, a subwoofer or a portable system, understanding Class B tells you exactly which trade-offs you are paying for — and which specifications to demand from a supplier. What a Class B Amp Really Is Industry amplifier references define Class B by its conduction angle. Two output devices share the work, and each one conducts for exactly half of the signal cycle — 180 degrees. One device pushes current into the load on the positive half, the other pulls current out on the negative half, and together they rebuild the full waveform at the output. Because both devices sit at cutoff when no signal is present, there is almost no quiescent current and almost no idle heat. That single property is the appeal. The theoretical efficiency ceiling of an ideal Class B push-pull stage is pi/4, or roughly 78.5%, compared with about 25% for a Class A stage whose devices conduct through all 360 degrees. The catch is that the two halves must meet precisely at zero volts. In real silicon they do not, and that mismatch defines the reputation of the class. +VCC -VCC NPN positive half PNP negative half Input Load Push-pull output stage: the NPN device drives the positive half cycle and the PNP device the negative half, meeting at the zero crossing where crossover distortion is generated. Class B Against Class A, Class AB and Class D Efficiency alone does not decide which amplifier class belongs in a product. Idle current, distortion behaviour and thermal design matter just as much — and the table below shows where Class B actually sits. Figures are classical benchmarks for each topology; Class AB and Class D practical values reflect the ranges commonly quoted in audio engineering literature, while 78.5% is the ideal Class B ceiling. Class Conduction angle Efficiency Idle current Distortion behaviour Common use Class A 360 degrees About 25% Very high Lowest and smoothest Low-power hi-fi, preamp stages Class B 180 degrees Up to about 78.5% Near zero Crossover notch at low levels RF transmitters, battery gear, legacy designs Class AB Just over 180 degrees 50% to 70% in practice Low Low; crossover largely removed Powered speakers, pro audio, subwoofers Class D Switching Above 90% typical Very low Low with a well-designed filter Portable speakers, PA, subwoofer modules Efficiency ceiling by amplifier class Class A 25% Class B 78.5% Class AB 50-70% Class D 90%+ 0% 25% 50% 75% 100% Class B shares the top of the linear efficiency range; the Class AB bar shows where real products land once bias current and supply losses are included. Two patterns stand out. Class B wins on idle current, because unbiased output devices draw almost nothing until a signal arrives — which is why the topology survives in battery-powered products and RF transmitters. It loses on linearity exactly where small signals live, and that is the quiet listening range that dominates real-world use. Crossover Distortion: The Real Cost of That Efficiency At the zero crossing, both output devices are close to cutoff at the same instant. Gain collapses briefly, the transfer curve flattens, and the output waveform picks up a notch. On a spectrum analyser that notch appears as a spray of high-order harmonics — the gritty edge listeners describe as harsh or transistorised. It is worst at low output levels, where the notch is a large fraction of the signal, and it fades into the background once the amplifier is driven hard. Distortion across the output range (illustrative) Class B Class AB Distortion Output level Illustrative trend rather than measured data: an unbiased Class B stage distorts most at low output, while a well-biased Class AB stage stays linear until the onset of clipping. Three fixes exist. Bias the output devices slightly into conduction so they never fully switch off, which is precisely what turns a Class B stage into Class AB. Apply enough negative feedback to linearise the crossover region. Or leave the linear topology behind and switch to Class D with a well-designed output filter. The first option shaped modern audio hardware: most speaker amplifiers sold as Class B push-pull audio stages are in fact running a small bias current. Once the output stage is biased correctly, the sound character of the finished product is decided by the analogue front end in front of it — tone controls, graphic EQ, microphone mixing and limiting — which is where a design analysed in this Class AB amplifier design analysis usually earns or loses its reputation. PL1 Horizontal Treble and Bass EQ Module for Class AB AmplifiersHorizontal analog module adds treble/bass EQ, mic and line inputs, volume controls, and clip indication to Class AB amplifier front ends.View Product → Where Class B Still Makes Sense Pure Class B is not obsolete, only specialised. It remains common in RF power amplification, where transmitters and constant-envelope signals tolerate the nonlinearity, and in battery-powered equipment where near-zero quiescent current translates directly into runtime. It also survives in cost-sensitive designs where a modest amount of crossover distortion is an acceptable price for a smaller, cooler output stage. RF and constant-envelope transmission, where the class B and class C stages in a transmitter are standard practice. Battery-powered devices, where standby and idle consumption decide how long the product runs. Legacy or low-cost platforms where the alternative is a physically larger and hotter Class A stage. For anything that reproduces music at low levels — desktop speakers, studio monitors, home audio — the distortion penalty outweighs the efficiency gain, and Class AB remains the mainstream answer. High-power professional systems take a different route entirely: Class H modules that modulate the supply rail to follow the signal, keeping the voltage drop across the output devices small while preserving linear operation. AMP400H+7294 Class H Power Amplifier Module with U-Shaped RadiatorClass H module delivers 400 W into 4 Ohm, 100 W Class AB into 8 Ohm, with U-shaped aluminum radiator and protection.View Product → Choosing an Amplifier Module: Specs That Decide the Purchase For anyone specifying a module rather than designing a discrete stage, the checklist below is what separates a smooth integration from a redesign six weeks later. Rated power at a stated impedance. A 500 W figure means nothing without the load, because 500 W into 4 ohm draws twice the current of 500 W into 8 ohm and changes both heatsink and protection requirements. Supply topology. Linear transformer supplies are simple and robust; switching supplies with PFC and LLC resonance are lighter and tolerate a wider mains range. Continuous versus peak rating. Check the thermal rating rather than the marketing peak, and confirm what the heatsink is doing — a U-shaped aluminium radiator carries real load in a high-power module. Protection circuits. Over-current, overload, DC offset and thermal shutdown protect both the module and the loudspeaker, while a limit circuit stops the front end driving the output stage into hard clipping. Front-end functions. Graphic EQ, microphone priority, echo and output-mode switching determine how much extra board area and wiring you need around the amplifier. Manufacturing evidence. An SMT line and an acoustic test room tell you whether a supplier can hold tolerances across production batches, not just on the first sample. Because these choices shift from product to product, most module projects need some level of adaptation — supply voltage, connector placement, EQ presets or output mode. A supplier with an established module customisation process will usually shorten that loop considerably. PL21 / PL21E Vertical 5-Band Graphic Equalizer and Limit Protection ModuleVertical audio module offers 5-band graphic EQ, limit protection, balanced mic/line inputs, XLR output, volume controls, and PL21E microphone echo.View Product → Frequently Asked Questions What is a Class B amplifier used for? Pure Class B stages appear mainly in RF power amplifiers, battery-powered equipment and cost-driven designs where idle current matters more than a small amount of crossover distortion. In audio products, the same push-pull idea is almost always implemented as Class AB. What is the difference between Class A, Class B and Class AB amplifiers? The difference is conduction angle. Class A devices conduct through all 360 degrees and idle hot, Class B devices conduct 180 degrees each and idle cold, and Class AB conducts slightly more than 180 degrees with a small bias current that removes most crossover distortion. Why does a Class B amplifier produce crossover distortion? Around zero volts both output devices are near cutoff, so gain momentarily drops and the waveform flattens into a notch. That notch generates high-order harmonics and is most audible at low output levels. What is the maximum efficiency of a Class B amplifier? The theoretical maximum is pi/4, roughly 78.5%, for an ideal push-pull stage. Real amplifiers fall below that because of supply losses, device saturation voltage and the bias current used in practical designs. Can a Class B amplifier module drive a subwoofer? It can deliver the current, but the crossover notch and thermal stress make it a poor match for subwoofer duty. Most powered subwoofer modules use Class D or Class H output stages with a DSP front end for filtering and limiting. Is Class AB better than Class D for powered speakers? Neither is universally better. Class AB offers linear behaviour and simple EMC performance, while Class D runs cooler and lighter at high output. The decision usually comes down to thermal budget, weight targets and how much processing the design already includes. Class B is best treated as a boundary rather than a destination. It defines the efficiency limit that linear audio amplifiers can approach — about 78.5% in theory — and it marks the point where crossover distortion becomes the dominant design problem. For speaker and powered-module products, the practical answer is almost always a Class AB front end built on the same push-pull idea, or a move to Class D where efficiency and size matter more than the last fraction of linearity. Knowing which side of that line your product sits on is the whole specification exercise. .article-section{color:#22303f;line-height:1.7;margin-bottom:24px;} .article-section h2{font-size:22px;font-weight:bold;text-align:left;margin-bottom:12px!important;color:#0d3b66;padding-left:12px;border-left:4px solid #f4a259;} .article-section h3{font-size:16px;font-weight:bold;text-align:left;margin-bottom:12px;color:#0d3b66;} .article-section p{font-size:16px!important;margin-bottom:12px;} .article-section ul{margin-bottom:12px;list-style-type:disc;list-style-position:inside;} .article-section ol{margin-bottom:12px;list-style-type:decimal;list-style-position:inside;padding-left:0;} .article-section li{list-style:inherit;font-size:16px;margin-bottom:6px;} .article-section table{display:table!important;width:100%;border-collapse:collapse;margin-bottom:6px;} .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;background:#e9f1fa;color:#0d3b66;text-align:left;font-size:15px;} .article-section td{display:table-cell!important;border:1px solid #cccccc;padding:8px;font-size:15px;vertical-align:top;} .article-section caption{caption-side:bottom;font-size:16px;margin-bottom:12px;font-style:italic;color:#808080;text-align:left;} .article-section.intro{background:#f5f8fc;border-left:5px solid #1c7ed6;border-radius:6px;padding:16px 18px;} .article-section.compare{background:#f5f8fc;border-radius:6px;padding:16px 18px;} .article-section.faq-section{background:#eef4fb;border-radius:8px;padding:18px;} .article-section.callout{background:#0d3b66;border-radius:8px;padding:18px;} .article-section.callout p{color:#e6eef8;} .chart-figure{margin:0 auto 16px;width:440px;text-align:center;} .chart-figure svg{display:block;margin:0 auto;} .article-section figcaption{font-size:13px;color:#5a6b7d;text-align:center;margin-bottom:8px;line-height:1.5;} .faq-grid{display:grid;grid-template-columns:1fr 1fr;gap:14px;} .faq-item{background:#ffffff;border:1px solid #d3e0ee;border-left:4px solid #1c7ed6;border-radius:6px;padding:12px 14px;} .faq-item h3{margin-bottom:6px;color:#0d3b66;} .faq-item p{margin-bottom:0;} .faq-item:nth-child(2n){border-left-color:#f4a259;} @media (max-width:640px){.chart-figure{width:100%;}.chart-figure svg{width:100%;height:auto;}.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;box-shadow:0 0 5px rgba(31,41,55,.08)} .pc-inner{display:flex;text-decoration:none;color:inherit;align-items:center;min-height:148px} .product-card .pc-img{width:auto!important;min-width:0!important;max-width:min(36%,240px)!important;height:auto!important;min-height:0!important;max-height:148px!important;aspect-ratio:auto!important;object-fit:contain!important;flex:0 0 auto;display:block;align-self:center} .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}
Put your hand on the back of a 500 W powered speaker after an hour of playback. A Class A or Class AB design is usually too hot to touch; a Class D amplifier is often just warm. That one difference explains most of what matters. A Class D amplifier is a switching amplifier. Its output transistors are driven fully on or fully off at a high frequency, and the audio information is carried by the width of those pulses rather than by the linear operating point of the devices. An LC low-pass filter at the output strips the switching carrier and leaves the amplified audio. Because the transistors almost never sit in the linear region, very little energy turns into heat: typical efficiency lands between 80% and 95%, against roughly 25-30% for Class A and 50-70% for Class AB. What Is a Class D Amplifier, Exactly? The letter D does not stand for "digital." It is simply the next letter in the sequence of amplifier classes, and the audio path inside a Class D stage is analog: a pulse-width modulation (PWM) process, not a stream of ones and zeros. What is genuinely digital in a modern powered speaker is usually the DSP feeding the amplifier, not the amplifier itself. A typical Class D power amplifier module contains five functional blocks: Pulse-width modulator - compares the incoming audio with a high-frequency triangle carrier and produces a square wave whose duty cycle follows the instantaneous signal level. Gate driver and MOSFET output stage - switches the supply rails at several hundred kHz, arranged as a half bridge or, at higher power, as a full bridge (BTL) configuration. LC output filter - averages the pulse train back into an audio waveform and keeps switching residue away from the driver. Feedback network - samples the output and corrects errors, which is what separates a clean Class D design from a harsh one. Switch-mode power supply - often a PFC front end plus an LLC resonant stage in professional modules, holding the rail steady across a wide AC input range. A B C D A - PFC and LLC resonant power supply; B - PWM modulator and gate driver; C - MOSFET output stage; D - LC output filter with feedback. How a Class D Amplifier Works, Step by Step Input conditioning. The line or balanced input is buffered and, in DSP-equipped modules, passes through EQ, crossover, limiting and delay before the power stage. Modulation. A comparator mixes the audio with a triangle carrier. Loud passages produce wide pulses; silence produces a 50% duty cycle that the filter averages to zero. Switching. The gate driver turns the MOSFETs hard on and hard off, with dead time inserted so both devices in a half bridge never conduct at once. Reconstruction. The LC filter passes audio and blocks the carrier, with its cutoff placed well above 20 kHz but well below the switching frequency. Error correction. Feedback returns a scaled copy of the output to the modulator, lowering distortion and improving damping factor at the terminals. Two details decide whether the result sounds clean or coarse: switching frequency and loop design. Raising switching frequency shrinks the filter and improves high-frequency behavior, but increases switching loss and EMI. Pre-filter feedback is simple but load-sensitive; post-filter feedback corrects the filter's own errors and is standard in modules that must stay flat into 4 ohm or 8 ohm loads. Class D vs Class A, AB and H: What Actually Changes Class is not a quality rating. It describes how the output devices conduct, and that single choice drives efficiency, heat, cabinet size and power supply cost. Efficiency ranges reflect typical textbook figures and manufacturer datasheet values for each class at full rated output. Class How the output devices conduct Typical efficiency Heat and heatsink Where you still meet it A Conducting 100% of the time 25-30% Large and heavy Small-signal and high-end hi-fi AB Conducting 50-100% of the time 50-70% Moderate Analog powered mixers, value speakers H Rail voltage tracks the signal 65-80% Moderate, U-shaped aluminium radiators High-power subwoofers, touring racks D Switching on and off 80-95% Small, often the chassis itself Powered speakers, subwoofers, line array Typical full-power efficiency by amplifier class Class A25-30% Class AB50-70% Class H65-80% Class D80-95% The gap compounds quickly. At 90% efficiency, a 1000 W module turns roughly 110 W into heat; at 55% the same output means more than 800 W of heat. That is the difference between a fanless moulded cabinet and a rack unit with forced cooling. Class D was once dismissed for EMI, load-dependent frequency response and higher distortion near the top of the band. Those complaints came from low switching frequencies and pre-filter feedback, and they have largely been engineered out. What matters more than the class label today is the implementation: loop topology, switching frequency, power supply quality and thermal design. Where Class D Belongs in a Powered Speaker or Subwoofer For an OEM building active loudspeakers, the cabinet usually makes the decision. A column speaker or a moulded two-way box has no room for a linear supply and a large heatsink, so a single switching board delivering, for example, 400 W to the low-frequency driver and 100 W to the high-frequency driver is the practical answer. The same logic applies to BTL subwoofer channels, where one module can push four figures of wattage into an 8 ohm load without a fan. Three module families cover most of that ground: EON520: LP400W+HP100W PFC+LLC Resonance Wide Voltage Class D Power Amplifier Module● High Efficiency Switching Power Supply with PFC+LLC Resonance: 800W ● Self-adaptive Wide Voltage Input: 100V-240V ● Mid and Low-Frequency Power Amplifier: CLASS D 40...View Product → EON522SUB: LP 500W/4ohm LLC Resonant Active Subwoofer Power Amplifier Module●LLC Resonant High Efficiency and High Reliability Switching Power Supply: 800W ●Bass Amplifier: CLASS D 500W-4 Ohms ●U-shaped Aluminum Radiator, Stable Power Output, ...View Product → EON521+1500:LP 1500W/8ohm Pro Audio BTL Subwoofer Power Amplifier Module●LLC Resonant High Efficiency and High Reliability Switching Power Supply: 1800W ●BTL Power Amplifier: CLASS D 1500W-8 Ohm ●U-shaped Aluminum Radiator, Stable Power Ou...View Product → Matching the module to the transducer matters as much as matching wattage. A two-way module pairs a low-pass channel with a high-pass channel and expects a crossover point that suits your HF driver; a subwoofer module expects a band-limited input and the load impedance it was designed around. Mismatched impedance and crossover points are the most common reason a Class D module measures well on the bench and sounds wrong inside the cabinet. What to Check Before You Specify a Class D Module Efficiency at the load you actually use Datasheets quote peak efficiency at one operating point. Ask for the curve across 10% to 100% of rated power, and at 4 ohm, 8 ohm and bridged loads, because efficiency falls as load impedance drops. THD+N and residual noise 0.05% THD+N or lower is a normal target for a modern module. Residual noise matters more than distortion in a quiet listening room, so check the unweighted noise floor rather than only the headline figure. For a wider comparison of topologies, the amplifier class guide covers A, AB, D and H side by side. Switching frequency, EMI and dead time Higher switching frequency means a smaller filter and better high-frequency performance, but tighter dead-time control and more careful PCB layout. If the module will sit near a radio tuner or a wireless microphone receiver, ask for conducted and radiated EMI data. Power supply topology and protection PFC plus LLC resonance is the usual answer for wide-voltage operation and stable rails. Confirm that overcurrent, overvoltage, DC offset and thermal protection are included, and check what happens at brownout rather than only at nominal mains. When a standard part does not fit the cabinet, the load or the DSP feature list, the practical route is to adapt an existing platform: custom amplifier module development usually starts from a proven power stage and adjusts output channels, EQ modes and interface rather than designing a new amplifier from scratch. Class D Amplifier FAQ Q1. What is a Class D amplifier used for? Powered speakers, active subwoofers, line array modules, portable battery speakers and car audio - anywhere high output power must fit a small enclosure with limited cooling. Q2. Is a Class D amplifier better than a Class AB amplifier? For high-power powered systems, usually yes: better efficiency, less heat and a smaller footprint. Class AB remains a reasonable choice for low-power analog designs and for engineers who value its simpler EMI profile. Q3. Do Class D amplifiers sound harsh or distorted? Early models did. Modern modules using post-filter feedback and switching frequencies in the hundreds of kHz hold THD+N near 0.05% across the audio band, which is comparable to the analog amplifiers they replace. Q4. What is the efficiency of a Class D power amplifier module? Typically 80-95% at rated output. A 1000 W module therefore dissipates roughly 50-250 W as heat, while a 50-70% Class AB stage at the same output dissipates 430-1000 W. Q5. Can a Class D amplifier module drive a 4 ohm subwoofer? Only if it is specified for that load. A BTL subwoofer module optimized for 8 ohm is not interchangeable with a 4 ohm design; the wrong load can trigger overcurrent protection or overheat the output stage. Q6. What is the difference between Class D and Class H amplifiers? Class H keeps a linear output stage but varies the supply rail to follow the signal, cutting heat while preserving linear behavior. 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