In this post, I’m going to show you exactly how to choose between Class AB and Class D for multichannel amplifiers and what measurements actually matter when you install them.
I’ve seen installers pick the wrong class because they focused on specs that don’t matter in the field. You’ll get: clear topology differences, the SPEC metrics to compare (efficiency, THD/IMD, damping, switching freq), thermal and EMI trade-offs, and installer-safe mitigation steps. Let’s dive right in.
How Class AB and Class D Work (quick primer)
Class AB and Class D use fundamentally different output strategies one is linear, the other is switching.
Why? Because they handle the output stage differently, which drives efficiency, heat, and noise behavior.
Class AB uses complementary push‑pull devices operating in their linear region with a small bias to reduce crossover distortion. That means continuous conduction around the crossover point and predictable analog behavior across the band. The trade-off is heat: linear conduction produces steady dissipation in the output stage.
Class D converts the audio into a high‑frequency pulse train (PWM or variants) and uses fast switching MOSFETs. The output is reconstructed with an LC filter. “D” does NOT mean digital it’s a switching topology that can reach very high efficiency.
How these scale to multichannel: AB multiplies heatsink and PSU needs per channel; D multiplies switching/power‑rail complexity and EMI control per channel. For multi‑channel racks, that difference drives enclosure size, ventilation, and grounding strategy.
Key Takeaway: Class AB = linear, heat-heavy; Class D = switching, efficient, but needs EMI control.
Which brings us to the SPECs you should actually compare when choosing a multichannel amp.
Key Specifications to Compare for Multichannel Use
Don’t pick an amp by name pick it by the metrics that predict real-world performance.
Why? Because two amps that advertise the same watts can behave very differently under sustained multi‑channel load.
Efficiency This is the obvious one. Expect typical modern multichannel Class AB efficiency in the neighborhood of ~60% for real‑world operating points. Expect modern Class D designs to run ~85-90%+ in many cases. Put another way: at 100 W output a Class AB at 60% needs ~167 W input and dumps ~67 W as heat; a Class D at 90% needs ~111 W input and dumps ~11 W as heat. THAT heat difference matters in racks and cars.
THD+N and IMD High‑quality Class D modules (Hypex Ncore, Purifi, ICEpower variants) report THD+N figures down to 0.002%-0.01% at specific power points. Datasheet figures are useful, but watch THD vs power and THD vs frequency graphs. IMD and wideband noise tell more about perceived artifacts than a single 1 kHz number.
Damping factor / output impedance Modern Class D implementations often deliver very high damping factors (hundreds to >1000 in some designs) which improves bass control into difficult speaker loads. Class AB typically runs lower damping factors (tens to low hundreds) depending on output stage and feedback.
Switching frequency & bandwidth Commercial Class D switching spans roughly ~400 kHz-2.1 MHz, with advanced/GaN designs pushing into the multi‑MHz range. Higher switching reduces LC filter size and can improve linearity, but it increases EMI risk and demands tighter layout and filtering.
For installers: prioritize continuous all‑channels‑driven ratings and thermal numbers over single‑channel bench figures. Measure or request THD vs frequency and THD vs power graphs when possible.
Quick comparison table
Here’s a conservative spec comparison for multichannel expectations.
| Spec | Class AB (typical) | Class D (typical) |
|---|---|---|
| Efficiency | ~60% | ~85-90%+ |
| Heat at 100 W out | ~67 W lost as heat | ~11 W lost as heat |
| Typical THD (high‑end) | 0.005%-0.05% (depends on bias and design) | 0.0018%-0.02% (top modules can be extremely low) |
| Typical switching freq | Linear (no switching) | ~400 kHz-2.1 MHz (GaN up to 2-5 MHz+) |
| Damping factor | ~50-200 | ~200-1000+ |
| Size/weight | Large heatsinks, heavier | Compact, lighter (higher density) |
Key Takeaway: Efficiency and all‑channels continuous ratings predict rack and vehicle cooling needs more than peak single‑channel numbers.
This leads us directly into how thermal power loss changes enclosure design and placement.
Thermal Management and Multichannel Installations
Heat is the silent killer of multichannel reliability PLAN for the heat the amp will generate under sustained multi‑channel use.
Why? Because thermal limits determine whether an amp will throttle, fail, or quietly degrade over months in real installs.
Use the 100 W example: a 4‑channel Class AB system running all channels at moderate levels can easily produce hundreds of watts of continuous waste heat. For example, four channels at 100 W out each → ≈268 W heat if each channel is AB @60%; the same four channels in Class D @90% → ≈44 W heat. That changes enclosure, ventilation, and fan requirements entirely.
Design targets I use on jobs: allow a conservative thermal margin of 20-30% above your expected continuous dissipation. Put another way: size heatsinks, vents, and airflow so the amplifier never sees more than its specified ambient + rise when all channels are driven simultaneously.
Practical installer guidance:
- Space leave clear vertical space between stacked amps; stacked AB channels must not be tightly enclosed.
- Venting force convection or dedicated fans for AB racks; passive vents may suffice for many D banks but check continuous spec.
- Thermal monitoring verify amp surface temps during commissioning and after 10-15 minutes of multi‑channel pink noise; note throttling behavior.
Key Takeaway: Size cooling for ALL‑CHANNELS‑DRIVEN dissipation, not single‑channel peaks.
Which brings us to EMI the other practical tradeoff for high‑density Class D amps.
EMI and Switching Noise Risks and Mitigations for Class D in Multichannel Systems
Class D switching creates real EMI and RFI risks if you ignore layout, routing, and filtering.
Why? Because fast dv/dt switching and common‑mode currents couple into power, signal, and chassis, producing audible hiss, radio interference, or digital glitches.
Common symptoms I see on installs: HF hiss on sensitive inputs, AM/FM radio hash in cars, occasional digital artifacts on cameras or telemetry, and ground hum created by unexpected ground paths. For example, I fixed a fleet vehicle where the radio suffered alternator‑related noise by adding ferrites and re‑routing speaker runs the noise dropped immediately.
Prioritized mitigation checklist start at the top and work down:
- PCB & module choice use modules or amps with proven EMI control (internal layout, dead‑time control, input filtering).
- LC output filters ensure inductors and capacitors are rated for peak currents; DO NOT undersize L or C to save cost.
- Ferrite beads/cores place on power rails and speaker leads, as close to the amp as possible.
- Common‑mode chokes effective for reducing radiated emissions on long speaker runs.
- Grounding star ground where practical; minimize loop area between input and power grounds; in vehicles, isolate chassis ground if the amp maker recommends it.
- Cable routing separate power and low‑level signal runs; avoid long parallel runs of speaker wires and RCAs.
- Shielding & enclosures metal enclosures and internal partitions reduce radiated noise in multi‑channel banks.
- Verification perform simple RFI checks (AM/FM radio sweep, walk test) and try ferrite cores on suspect cables before major rework.
Car‑specific tips: use automotive‑grade modules, clamp ferrites on both speaker and power leads, and watch alternator and CAN bus coupling. Home installs: prefer balanced inputs where available and keep long signal runs shielded.
Key Takeaway: Layered EMI controls (layout, filters, ferrites, routing) remove >90% of real‑world switching problems.
Next: how those metrics translate to perceived sound quality in multichannel systems.
Sound Quality Measurements vs Listening in Multichannel Systems
Measurements predict problems; listening confirms impact. Use both.
Why? Because a low THD number at 1 kHz tells you something, but not everything and subjective differences often come from interaction with speakers and room, not the amp alone.
Which metrics matter most in practice:
- THD+N at realistic power look at THD at 1/10th, 1/3, and near full rated power; low‑power THD is not the whole story.
- IMD reveals intermodulation artifacts that matter for complex program material.
- SNR and distortion floor audible noise and hiss under quiet passages.
- Damping factor and output impedance affects bass control and decay, especially with low‑impedance speakers.
- Thermal stability does THD drift as the amp heats under multi‑channel load?
For example, I’ve compared high‑quality Class D modules with mid‑tier AB amps. In lab charts the D could show THD+N = 0.002% and the AB = 0.01%. In a living room, most listeners heard no meaningful difference; differences showed up with certain speaker loads and under sustained multichannel peaks.
Quick field checks for integrators (non‑lab):
- Channel balance check at low and moderate levels across channels.
- Distortion floor swap sources and listen for added HF hiss or odd harmonic tinge.
- Bass control play tight bass tones and listen for decay and cabinet control.
- Thermal test run all channels for 10-15 minutes and recheck THD/floor if possible.
Key Takeaway: Use THD/IMD/SNR and thermal stability together not one spec alone to predict perceived quality.
Which brings us to real‑world matching: which class for which use case.
Implementation Guidance Matching Class to Multichannel Use Cases
Match the class to your system constraints and priorities there’s no universal winner.
Why? Because space, ventilation, EMI sensitivity, and speaker impedance determine what will work reliably and sound best.
Practical recommendations I use on jobs:
- Home theater / AVRs Class D is usually the best fit due to power density and low heat; choose well‑engineered modules and verify EMI behavior in your room.
- Car audio multichannel Class D is the go‑to for amps and subs; AB still appears in high‑end 2‑channel front ends where amplifier character and thermal allowance exist.
- Pro audio / installed systems Class D for multi‑zone and powered speakers; AB in studio monitors or specific places where an engineer prefers linearity.
Headroom guidance: prioritize an amp’s all‑channels‑driven continuous rating. A single‑channel spec at clipping is useless if the PSU sags when all channels demand headroom simultaneously.
Key Takeaway: For most multichannel installs prioritize quality Class D modules and plan EMI/thermal mitigation; reserve AB for 2‑channel audiophile or specific studio needs.
Next: common myths and troubleshooting quick wins I use on service calls.
Common Pitfalls, Myths, and Quick Troubleshooting Pointers
Don’t fall for shorthand myths they cost time and callbacks.
Why? Because misunderstanding class characteristics leads to the wrong amp choice or the wrong fix when problems show up.
Myth busters and quick pointers:
- Myth: “D = digital” FALSE. Class D is switching analog; it’s not a digital codec.
- Myth: “All Class D sounds bad” FALSE. Implementation matters far more than the class label.
- Pitfall: Ignoring switching frequency/filter interaction with certain speaker crossovers; this can create HF artifacts.
- Pitfall: Underestimating AB heat in stacked multichannel racks you’ll get thermal shutdowns or shortened life.
- Troubleshooting quick wins: isolate the suspect amp, put ferrite on speaker/power leads, swap source/preamp to rule out signal chain, and check thermal throttling during long‑play tests.
For example, on one call a client complained of “grit” from the rear channels. Swap tests showed only one amp produced artifacts; adding a ferrite to the speaker lead and re‑routing the RCA away from power cabling solved it in 15 minutes.
Key Takeaway: Diagnose systematically: isolate channel, swap known‑good cable, add ferrites, and recheck thermal behavior.
That wraps up the practical comparison here’s a concise summary to lock it in.
Conclusion
Main takeaway: Class D gives better power density and MUCH lower heat for multichannel systems, while Class AB still has roles where linear analog behavior or specific tonal character matter.
Quick recap the fixes and checks that actually reduce callbacks:
- Check continuous all‑channels ratings and thermal dissipation before mounting.
- Measure THD/IMD and listen for HF switching artifacts on suspect channels.
- Mitigate EMI with ferrites, LC filtering, and careful cable routing.
- Provide 20-30% thermal margin for heatsinks or forced airflow with AB multichannel racks.
- Prioritize amps with documented all‑channels performance and proven module designs for Class D.
Get these fundamentals right, and you’ll solve the majority of multichannel amp headaches before they become callbacks. I’m confident this approach will save you time on installs and keep systems sounding clean and reliable.
