In this post, I’m going to show you exactly how to tell whether a Class AB or Class D dual‑channel amplifier will sound better in your system and why the answer is almost always about measurements and implementation, not the letter on the spec sheet. I’ve seen the exact same amp spec produce great results in one car and problems in another. You’ll get: a concise technical primer, the measurements that predict audible differences, a reproducible bench and audition checklist, and what to ask manufacturers. Let’s dive right in.
How Class AB and Class D Work a concise technical primer
Class AB is analog linear with bias between push and pull devices; Class D is a switching topology that reconstructs audio with an output filter.
Why? Class AB operates with output devices partially conducting to reduce crossover distortion; Class D toggles MOSFETs on/off and uses an LC filter to recreate the waveform.
Class AB trades efficiency for simplicity: expect linear operation around quiescent bias points and rising heat at higher outputs. Class D trades switching complexity for compact power and high efficiency.
Typical numbers: switching carriers for Class D commonly run from ~200 kHz to ~1 MHz. Practical efficiencies: Class AB ≈ 60-75% at useful operating points, Class D ≈ 85-95% peak under similar conditions. CLASS D IS NOT DIGITAL it’s analog PWM/PDM in practice.
Key Takeaway: Class AB = simpler linear path and heat; Class D = high efficiency + switching/filter complexity.
This foundation leads us to the specific measurements that actually predict audible differences.
Common implementation choices that change sound
Feedback topology, output device type (MOSFET vs BJT), and PSU stiffness make huge differences.
Why? Because closed‑loop feedback, output device linearity, and supply sag determine distortion under load and transient behavior.
Examples: a Class D based on a high‑quality module (Purifi, Hypex) with tight feedback will measure orders of magnitude better than a budget Class D chip without robust feedback. Conversely, a well‑designed AB stage with proper biasing can outperform a cheap D in midband linearity.
Key Takeaway: Topology labels hide implementation differences vendor and circuit choices matter more than class alone.
Which brings us to the measurements you should trust.
Key Measurement Metrics what really predicts audible differences
THD+N, IMD, SNR, damping factor, and load‑dependent frequency response are the metrics that map best to what you actually hear.
Why? Because harmonic and intermodulation distortions create perceivable coloration; noise floor limits resolution; damping factor impacts bass control; and load‑dependent FR shows if the amp interacts with real speakers.
THD+N should be read at specified conditions: 20 Hz-20 kHz, both channels driven, continuous RMS. Good ranges: many solid Class AB designs publish 0.01%-0.1% THD+N at moderate power; modern high‑end Class D designs can publish sub‑0.001% figures under ideal test conditions.
IMD (SMPTE or CCIF) predicts coloration that THD alone can miss. SNR (A‑weighted) below −100 dB is audible in quiet rooms; high‑end lab traces often show >110 dB SNR.
Damping factor is useful but often over‑emphasized above a point (roughly DF > 100-200 at low frequency) speaker/room interaction and crossover design dominate bass solidity.
LOAD‑DEPENDENT FREQUENCY RESPONSE is CRITICAL for Class D: the LC filter interacts with speaker impedance peaks and phase to create HF bumps or rolloffs.
Key Takeaway: Read THD+N and IMD at realistic conditions both channels driven and with real‑world speaker loads.
Next: here’s a bench test suite you (or a reviewer) can run to expose the telling weaknesses.
How spec presentation can mislead (peak vs continuous, narrowband vs fullband)
Don’t trust single‑tone, single‑channel, peak numbers demand full‑band, both‑channels‑driven, continuous figures.
Why? Manufacturers can present best‑case 1 kHz single‑tone curves that understate distortion under realistic program material and dual‑channel stress.
Action: insist on THD+N vs output plots (20 Hz, 1 kHz, 10 kHz) with both channels driven. Narrowband harmonic probes hide noise and switching artifacts outside the test tone.
Key Takeaway: One‑line specs are marketing; graphs reveal real behavior.
Now: here’s the exact bench test battery I use on the truck and in the shop.
Bench Tests Every Buyer/Audiophile Should Look For
A small, repeatable test suite separates well‑implemented amplifiers from the rest.
Why? Because the right graphs catch thermal, stability, and filter interaction problems before you buy or install.
Minimum recommended tests:
- THD+N vs output at 20 Hz, 1 kHz, 10 kHz (both channels driven).
- IMD (SMPTE or CCIF) with two‑tone tests to reveal intermodulation products.
- Frequency response 20 Hz-20 kHz into both a pure resistive load and a real speaker impedance curve.
- THD spectrum harmonic content at several output levels.
- Switching noise spectrum up to several MHz to detect carrier leakage and spurs.
- Damping factor measurement at 20 Hz and 100 Hz into specified loads.
- Stability into low impedances 4 Ω and 2 Ω tests (or the amp’s rated minimum).
How to read the graphs: flat FR into real loads is good; rising THD at high frequency or under low impedance is a RED FLAG. A THD+N ≤ 0.01% at moderate power is “good”; sub‑0.001% is exceptional but often only achievable in narrowband conditions.
Key Takeaway: Request THD+N vs power, IMD, FR into real speaker loads, and a switching spectrum before you trust a claim.
Which tests specifically expose Class D implementation flaws? Read on.
Which bench test catches Class D implementation flaws?
Load‑dependent frequency response and switching‑spectrum scans are the ones that reveal filter and EMI problems.
Why? Because the LC output filter can interact with impedance peaks and phase to add HF gain or distortion that only shows with reactive loads.
Actionable test: run frequency response with an impedance overlay of the speaker, and run an FFT of the output up to several hundred kHz while driving program material. If you see HF spurs or FR anomalies correlated with impedance bumps, the Class D filter is interacting poorly.
Key Takeaway: Use real speaker impedance curves, not just resistors, to expose filter issues.
That evidence is what I look for in published case studies examples next.
Bench Measurement Case Studies representative examples & what they show
Measurements explain the audible behavior you hear in the car or room.
Why? Because a single trace ties distortion, noise, or FR anomalies to the listener’s complaint.
Case study 1 Classic Class AB: a well‑engineered AB amp measured THD ≈ 0.02% at 20 W into 4 Ω, moderate SNR (~100 dB), and rising dissipation above 50 W. Audible correlate: warm, stable midband and predictable thermal throttling at high power.
Case study 2 Modern high‑end Class D (Purifi/Hypex‑based): published lab traces show THD+N <0.001% at 1 kHz midpower, SNR > 110 dB, and stable behavior into 2-4 Ω. Audible correlate: extremely transparent mid/highs and tight bass without excessive heat when implementation is correct.
Case study 3 Budget Class D module: THD spectrum shows elevated odd harmonics above 10 kHz and switching‑spur energy in the 200-500 kHz band that folds under non‑ideal loads. Audible correlate: perceived “brightness” or grain on top end and potential interaction with sensitive source gear.
Key Takeaway: High‑end Class D can outperform AB on measured metrics; budget D often reveals HF artifacts and load sensitivity.
These examples point directly to the most important mechanism: the LC filter + speaker impedance interaction next section.
Output Filter Interactions & Speaker Load why Class D can behave differently
The LC output filter in Class D amplifiers interacts with a speaker’s complex impedance and can change frequency response and distortion in real systems.
Why? Because speakers present reactive impedances: peaks at resonances, phase swings at crossover points, and dips from driver behavior. The LC filter sees that complex load and can boost or attenuate certain bands or change stability margins.
Practical lab test: plot speaker impedance and FR side‑by‑side. A HF rise in FR that lines up with an impedance bump signals filter interaction. A contrasting test into a pure resistor that looks fine is not proof of safety the real speaker is the truth.
For installers: if an amp shows HF gain with certain speakers, try a different speaker load or use a snubber/compensation network in the output stage if the design allows. MATCH AMP TO SPEAKER is a rule, not a suggestion.
Key Takeaway: Always test Class D into a real speaker impedance curve that’s where filter problems appear.
Which brings us to EMI and switching noise detection a close cousin of filter issues.
EMI & Switching Noise how to detect it and what to ask manufacturers
Switching carriers and spurs can leak into the audio chain or nearby electronics and become audible or disruptive.
Why? Because switching energy above the audio band can intermodulate down into the audible band or couple into sensitive inputs and electronics.
Measurement methods: use an oscilloscope with a wideband probe, run an FFT up to several MHz, and use an RF sniffer around the chassis and outputs. In practical listening, try quiet program material and sensitive tweeters at nearfield; switching artifacts often show up as a thin, metallic grain during low‑level passages.
Ask manufacturers for: switching carrier frequency, filter cutoff, EMI suppression measures (common‑mode chokes, snubbers), and measured switching‑spectrum plots. If they can’t provide scans to several hundred kHz, consider that a transparency issue.
Key Takeaway: Require switching‑spectrum plots or bench scans; HF energy that folds into audio under some conditions is actionable.
Now: how all this maps to what you actually hear and how to audition properly.
What You’ll Hear subjective differences tied to measurements + audition checklist
Measured behaviors map predictably to subjective descriptions: warmth, brightness, tight bass, or grainy highs.
Why? Because harmonic balance, noise floor, and FR errors produce consistent perceptual effects across listeners and rooms.
Common mappings:
- “Warm / musical” mild even‑order harmonic balance and midband linearity (often AB or well‑tuned D).
- “Bright / clinical” HF rise or switching artifact; often tied to filter interaction or unfiltered switching spurs.
- “Tight / fast bass” high current delivery and effective damping (good DF under test conditions).
Audition checklist (ABX style):
- Level‑match use an SPL meter to match playback to within ±0.2 dB.
- Use reference tracks six short clips across genres: a capella, acoustic guitar, kick drum, orchestral transient, synth bass, and a jazz cymbal shimmer.
- Run ABX blind tests swap amps behind a curtain with a helper or a box that reverses inputs automatically.
- Listen for switching noise use quiet material and sensitive tweeters; listen near crossover points and at low levels.
- Test bass control play low sinusoidal tones and real program bass; note decay and control.
Key Takeaway: ABX, level‑matching, and targeted tracks reveal real audible differences tied to bench results.
Which leads to a compact buying/evaluation checklist you can use at the shop or on the truck.
Practical Buying & Evaluation Checklist (quick reference)
Short rules that get you the right amp for the job fast.
Why? Because you often need a quick decision without full lab access.
- If you need compact power and low heat → favor well‑implemented Class D; demand load‑dependent FR and switching spectrum scans.
- If you want midband tonal density and have room for cooling → Class AB at the same price can be preferable.
- Insist on third‑party bench tests or on‑site ABX listening with your speakers.
- Level‑match for auditioning and test bass behavior at realistic SPLs.
Key Takeaway: Choose by implementation and measured behavior, not by class letters alone.
That’s the end of the checklist now a short wrap‑up.
Conclusion
Class letter alone doesn’t decide sound implementation, measurements, and speaker match do.
Quick recap the fixes and checks that matter most:
- Request THD+N vs power, IMD, FR into real speaker loads, and switching‑spectrum scans.
- Audition level‑matched ABX tests with reference tracks and listen for switching grain and bass control.
- Match amp to speaker impedance and power needs; consider thermal/ventilation constraints.
- Insist on both‑channels‑driven measurements and real‑load tests for Class D units.
- Trust measured behavior over single‑line specs when deciding.
Get these fundamentals right, and you’ll solve the majority of perceived class‑related issues before they become callbacks. After 14 years on the truck and thousands of installs, I can tell you that disciplined measurement plus blind auditioning is the only reliable way to pick the amp that actually sounds better in your car.
