ProcirelTHD Explained: Reading Distortion Numbers Correctly
THD sums the harmonic energy an amplifier adds to a pure tone, expressed as a percentage or dB below the fundamental. It is the most quoted and most misquoted audio specification in existence.
> At a glance: 6 minute guide · part 9 of 10 in the amplifiers complete guide track · includes a worked example and a quick-reference table.
## The measurement
Here is the working theory in one pass. Feed a sine, notch the fundamental, measure what remains: harmonics plus noise. THD+N includes broadband noise the honest version. Distortion analyser or audio analyser sweeps frequency and level to map the behaviour, since THD varies with both.
| Metric | Means | Credibility cue |
| — | — | — |
| THD | Harmonics only | Check bandwidth & level |
| THD+N | Harmonics + noise | More honest |
| THD vs frequency | Sweep | Reveals edges |
| THD vs level | Sweep | Reveals crossover/cliff |
| IMD | Two-tone interplay | Catches high-order sins |
## How numbers get gamed
Quoting THD at 1 kHz only, at 1 W into 8 Ω, at low bandwidth each narrowing flatters. Crossover distortion (high-order harmonics at low level) hides in a single mid-band figure yet dominates listening at whisper levels.
## Audibility context
Below roughly 0. 1 %, simple low-order THD is broadly inaudible on music. High-order, low-level distortion is detectable far lower. Spec sheets win arguments; sweeps at multiple levels win engineering.
## How to apply this in your build
Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Note the frequency, level and load behind any THD number
2. Compare THD+N at matched conditions only
3. Look for level sweeps, not single points
4. Trust measurements over adjectives
### Worked example
Two amplifiers both “0. 008 % THD”: one at 1 kHz/1 W, the other at 20 kHz/half power. The second figure is far harder-won identical numbers, different engineering. Run the numbers yourself with the opamp-gain and the result should agree to within rounding.
> Practical note from the bench. Our amplifier measurements always state the full condition set frequency, level, load, bandwidth because a naked percentage is marketing.
## Pitfalls that cost real hardware
– Comparing THD measured under different conditions
– Assuming lower THD always sounds better at every level
– Ignoring IMD, which stresses linearity harder than single tones
## Key takeaways
– The measurement the foundation of this guide; revisit it if any measurement here surprises you.
– How numbers get gamed the foundation of this guide; revisit it if any measurement here surprises you.
– Audibility context the foundation of this guide; revisit it if any measurement here surprises you.
## Who this guide is for
Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the amplifiers complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.
## What you need before starting
Nothing exotic: the parts or tools named in the guide, a multimeter. The opamp-gain open in a tab. Note the frequency, level and load behind any THD number before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.
### Quick reference card
| Aspect | Where to find it in this guide |
| — | — |
| Core theory | The measurement |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Pitfalls that cost real hardware |
## How this fits the amplifiers complete guide track
This guide is one stop in the structured learning path. Start from the [amplifiers complete guide](/tutorial/amplifiers-complete-guide) pillar page for the full map, or continue with [frequency response](/tutorial/amplifier-frequency-response) and [Class AB design](/tutorial/class-ab-push-pull-output). For the arithmetic, open the [opamp-gain](/tools/opamp-gain).
## Frequently asked questions
What THD is audible?
Context-dependent: ~1 % low-order on bass may pass unnoticed; 0. 05 % high-order crossover can annoy on quiet passages.
Why publish IMD too?
Two-tone tests generate difference and sum products real music contains a sterner linearity exam.
Is there a calculator for this?
Yes the [opamp-gain](/tools/opamp-gain) tool runs the formulas from this guide instantly, client-side, with no signup.
## Your next step in this track
– The complete amplifiers & audio guide: [Amplifiers & Audio complete guide](/tutorial/amplifiers-complete-guide)
– Read next: [inverting op-amp: gain, virtual ground and design](/tutorial/inverting-op-amp-configuration)
– Also in this track: [non-inverting op-amp: high-impedance gain stage](/tutorial/non-inverting-op-amp-configuration)
– Continue with: [the op-amp voltage follower: small circuit, big leverage](/tutorial/op-amp-voltage-follower)
– Calculate as you go: [op-amp gain calculator](/tools/op-amp-gain) · [power dissipation tool](/tools/electrical-power) · [gain to dB converter](/tools/opamp-gain)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.
## Practical working notes
Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.
The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
## Formulas and checks from this guide
Verification checklist for this track: measure bias at idle before signal testing, check heatsink temperature under load. Confirm gain across frequency rather than at 1 kHz alone. Amplifiers forgive nothing at their edges.
Bookmark this page against your next build in the track. The checklist above is the same one used across 19 guides in this series.
## Experience notes
Thermal drift in bias. Check the bias tracker is bolted to the heatsink and re-set per the service values.
Across the emitter resistors at idle, no signal. Compare each output device.
## How to revisit this guide
Second readings work best with a purpose. Pick one section from The measurement,How numbers get gamed,Audibility context and rebuild only that part at the bench, predicting each value before measuring. Prediction errors mark exactly which concept needs the next pass, and the linked amplifiers calculators resolve any arithmetic doubt in seconds. Keep the marked sections in your notebook: after a month of builds, that list becomes your personal Amplifiers syllabus.