Types
| Type | Cause | Character |
|---|---|---|
| Harmonic (HD) | Non-linear transfer | Integer multiples of input tone |
| Intermodulation (IMD) | Non-linearity mixing two tones | Sum / difference frequencies appear |
| Clipping (hard) | Output hits rail | Flat peaks, rich odd harmonics |
| Clipping (soft) | Gradual saturation | Rounded peaks, warm overtones |
| Crossover | Class-B amp zero-cross dead zone | Kink at zero — audible hiss on quiet parts |
| Slew-rate limit | Amp can't follow signal | Triangle-ish at high frequencies |
| Quantization | ADC / low bit depth | Granular noise |
| Group delay / phase | Non-flat phase response | Transient smearing |
Audibility thresholds (ballpark)
| Condition | THD threshold |
|---|---|
| Expert listener / reference gear | ~0.1% |
| Trained listener / good gear | ~0.3% |
| General listener / music program | ~1% |
| Heard as obvious distortion | > 3% |
| Tube amp "warm" territory | 1–5% |
| Overdriven guitar | 10% – 50%+ |
Measurement tips
- THD+N includes noise — always report which (THD vs THD+N) and at what level.
- Test at multiple frequencies — distortion rises with output level and frequency.
- Harmonic content matters: even harmonics (2nd, 4th) sound warm; odd (3rd, 5th) sound harsh.
- Masking: intermediate 2f_1 ± f_2 IMD products are less masked than harmonics — often more audible.
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