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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For almost every powered speaker project, a well-designed Class AB stage is the safer engineering and commercial choice. Class A only wins when measured linearity at very low listening levels matters more than heat, weight and electricity cost. That single sentence explains why Class AB modules still sit inside most production loudspeakers, even though Class A carries the better reputation on paper. The two classes are not different technologies. They are the same complementary transistor output stage, biased at different operating points. Everything else, from heatsink size to shipping weight, follows from that bias current. The Bias Point Decides Everything Else In a Class A output stage the devices conduct for the full 360 degrees of the signal cycle. Quiescent current is set higher than the peak current the amplifier will ever deliver into its load, so the transistors never switch off. There is no crossover region, no hand-off between devices, and the transfer curve stays linear from a few millivolts upward. Class AB sits between Class B and Class A. A small standing bias keeps both devices conducting for roughly 200 degrees of the cycle, just enough to overlap at the zero crossing and remove the notch distortion that plagues pure Class B. Outside that overlap window each device works only half the time, which is where the efficiency gain comes from. The practical consequence is easy to observe on a bench. A Class A channel draws roughly the same power from the supply whether it is silent or running at full output. A Class AB channel draws little at idle and progressively more as output rises. That is why a Class A chassis is hot ten seconds after switch-on, while an AB chassis stays cool until the programme material arrives. Efficiency and Heat: The Cost of Never Switching Off Typical full-power efficiency (%) Class A 15-30% Class AB 50-70% Class D 80-93% 0% 50% 100% Chart 1: typical full-power efficiency ranges taken from standard linear-amplifier design references The theoretical ceiling for a Class A stage is 50% with a transformer-coupled load, and closer to 25% for the more common single-ended arrangement. Real designs settle in the 15-30% band. Class AB reaches about 78.5% in theory and delivers 50-70% in practice, which is exactly why it became the default for high-power analogue audio. Translate that into a purchasing decision. A 200 W Class AB module might draw 300-350 W from the mains at full output and 20-30 W at idle. An equivalent Class A design needs a supply and heatsink sized for several hundred watts of continuous dissipation, which usually means three to four times the metal, transformer and cabinet volume per delivered watt. Idle power draw, 100 W channel (W) 200 W 150 W 100 W 50 W 0 W ~150 W ~20 W ~10 W Class A Class AB Class D Chart 2: standing dissipation follows bias current, not output level Distortion: A Good Class AB Usually Beats a Cheap Class A Class A's reputation rests on two real advantages: no crossover distortion, and a harmonic profile dominated by low-order, benign second harmonic. Both are genuine. But the comparison that matters in a product is rarely ideal against ideal. Crossover distortion in Class AB shows up as high-order harmonics at low output levels, precisely where the ear is most sensitive. It is controlled by three things: enough bias current, tight thermal tracking of the bias network so the setting does not drift as the heatsink warms, and adequate loop gain. Executed properly, Class AB modules hold total harmonic distortion in the 0.01-0.05% range at rated power, which is inaudible in any normal playback context. Where Class A genuinely pulls ahead is at milliwatt levels into high-sensitivity drivers. If your product is a near-field monitor for critical listening, that margin is worth paying for. If it is a portable PA box, a column speaker or a subwoofer, the difference disappears beneath room noise and programme material. Class A vs Class AB at a Glance The table below condenses the comparison into the parameters that actually drive a bill of materials and a thermal design. Typical values for linear analogue designs; actual figures depend on rail voltage, bias current and load impedance. Parameter Class A Class AB Conduction angle 360 degrees, continuous 180-360 degrees, about 200 typical Full-power efficiency 15-30% 50-70% Idle power draw High, close to full-load draw Low, rises with output Distortion signature Very low, second harmonic dominant Low; crossover artefacts if bias drifts Heatsink and transformer per watt Three to four times larger Baseline Weight and footprint Heavy, often two rack units or more Moderate, easier to fit in a cabinet Typical applications Hi-fi, headphone amps, small monitors PA speakers, line arrays, subwoofers, car audio Cost at equal output High Moderate What This Means for Powered Speaker Module Design Professional powered speakers, line arrays and subwoofers are built around Class AB, and increasingly around Class D and Class H, for one reason: a working product has to survive long duty cycles inside a hot rack. The thermal arithmetic above simply rules out Class A at the power levels those systems demand. Inside a typical AB design the power amplifier module handles voltage gain and current delivery, while tone control, microphone input and limiting live on separate functional modules placed in front of it. That split keeps the analogue signal chain short and lets one amplifier platform be reused across several cabinet models. PL35 Horizontal Analog Front-End Module for Class AB AmplifiersBalanced microphone and line inputs plus a five-band graphic equaliser make this horizontal module a compact front end for Class AB amplifier platforms.View Product → PL21 / PL21E Vertical Functional Module with Limit ProtectionA vertical front-end option for narrow enclosures, combining balanced microphone and line inputs, a five-band equaliser and limit protection; PL21E adds microphone echo control.View Product → The two front-end modules above follow that pattern: balanced microphone and line inputs with a five-band graphic equaliser in a horizontal format, or the same equaliser with limit protection in a vertical format when the enclosure is narrow. Both are designed for Class AB amplifier platforms, so the bias behaviour of the output stage stays the dominant variable. Heatsink Class AB stage Front-end board EQ, mic pre, limiter Figure 3: how an amplifier plate stack is assembled in an analogue powered speaker If the thermal budget of an AB design stops scaling, usually above a few hundred watts per channel inside a compact cabinet, the next step in professional audio is not back to Class A but forward to Class H, which switches the supply rail in step with the signal. A module such as the AMP400H keeps a linear-transformer supply and a U-shaped aluminium heatsink, retaining the analogue output stage while cutting dissipation at low and mid levels. AMP400H+7294 Class H Power Amplifier ModuleA Class H module with 400 W mid/low and 100 W high-frequency outputs, a linear-transformer supply and U-shaped heatsink, plus multiple protection circuits.View Product → For teams mapping the whole family before committing, our guide to amplifier classes A, AB, D and H for powered speaker module design walks through the same trade-offs for each topology. You can also browse the current Class AB loudspeaker amplifier modules to compare output levels and supply options. Choosing Between Them for a New Product Four questions usually settle the decision faster than any listening test: What is the duty cycle? A speaker that runs eight hours at high output cannot afford a stage that dissipates its full bias power the entire time. Where does the heat go? In a sealed plastic enclosure, an AB design already needs a thermal path to the plate; Class A would need a far larger one. At what level is distortion actually audible? Measure at the levels the product will really play at, not at 1 W into a dummy load. What does the finished unit weigh? Transformer and heatsink mass is a recurring freight and installation cost, not a one-off. Answer those honestly and the bias point, along with the module platform, tends to choose itself. Frequently Asked Questions Q1. What is the main difference between a Class A and a Class AB amplifier? Only the bias point. A Class A output stage conducts through the whole signal cycle; a Class AB stage conducts for roughly 200 degrees per device, which removes crossover distortion while using far less standing current. Q2. Is Class A really better than Class AB for sound quality? Not automatically. A well-biased Class AB module will usually measure and sound better than a poorly built Class A amplifier. Class A only has a clear edge at very low levels into high-sensitivity speakers. Q3. Why do Class A amplifiers run so hot? Because the bias current flows continuously, whether or not there is a signal. Idle dissipation is close to full-load dissipation, so the heatsink must be sized for the worst case at all times. Q4. Which is more efficient, Class A or Class AB? Class AB by a wide margin. Typical figures are 15-30% for Class A against 50-70% for Class AB at full power, and the gap at idle is even larger. Q5. Can a Class AB power amplifier module drive a subwoofer? Yes, especially in bridge-tied load configuration. Check the module's rated load impedance and confirm the supply and heatsink can sustain continuous low-frequency output rather than short bursts. Q6. How do I size the power supply and heatsink for a Class AB module? Start from the rated output power, divide by realistic efficiency to get input power, then subtract the audio output to find dissipation. Size the transformer for roughly 1.5 to 2 times the rated output and keep heatsink thermal resistance below the calculated limit. .article-section{margin-bottom:28px;color:#243342;font-size:16px;line-height:1.7} .article-section p{font-size:16px!important;margin-bottom:12px} .article-section h2{font-size:22px;font-weight:bold;text-align:left;margin-bottom:12px!important;color:#0f4c81;border-left:4px solid #e8a33d;padding-left:10px} .article-section h3{font-size:16px;font-weight:bold;text-align:left;margin-bottom:12px;color:#1b5e8a} .article-section ul{margin-bottom:12px;list-style-type:disc;list-style-position:inside;padding-left:0} .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:16px;font-size:15px} .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:#eef4f9;text-align:left;color:#0f4c81} .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 a{color:#0f4c81;text-decoration:underline} .chart-figure{width:440px;margin:0 auto 18px;text-align:center} .chart-figure svg{display:block;width:100%;height:auto} .chart-caption{font-size:13px;color:#6b7a88;margin-top:6px;font-style:italic} .faq-grid{display:grid;grid-template-columns:repeat(2,1fr);gap:12px} .faq-card{background:#eef4f9;border-left:4px solid #0f4c81;border-radius:6px;padding:12px 14px} .faq-card h3{color:#0f4c81!important} .faq-card p{margin-bottom:0!important;font-size:15px!important;color:#33475a} @media (max-width:640px){.chart-figure{width:100%}.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}
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. 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