Total Harmonic Distortion

Drive gear hot and it invents harmonics at 2x, 3x, 4x the original pitch; THD% is the size of that unwanted pile.

What it is

A percentage that measures how much extra, unwanted harmonic tone a piece of gear adds that was not in the original signal.

Key facts

• THD% = (square root of the sum of all harmonic powers, divided by the fundamental power) x 100. Harmonics are whole-number multiples of the fundamental.
• If fundamental = 100 Hz, harmonics are 200 Hz (2nd), 300 Hz (3rd), 400 Hz (4th)... Gear invents these when driven hot.
• 2nd harmonic = 1 octave up (warm/musical). 3rd = octave + a fifth (harsh). Even harmonics flatter, odd harmonics sound aggressive.
• THD is often quoted as THD+N (Total Harmonic Distortion + Noise), which also folds in the gear's noise floor.
• Clean pro power amp: under 0.1% THD (0.001 ratio) at rated power. Quality preamp: under 0.01%. Top converters: ~0.001%.
• Audibility: under ~0.1% THD is inaudible to most listeners on music; 1% is the classic onset-of-clipping reference.
• Tube/tape gear runs 1-5% THD on purpose and sounds warm because it is mostly even-order 2nd harmonic. Guitar overdrive hits 10%+.
• Clipping is extreme THD: peaks flatten against the supply rails into a square-ish wave, dumping massive odd harmonics.
• A perfect square wave (full clipping) contains only odd harmonics at amplitudes 1, 1/3, 1/5, 1/7 of the fundamental.
• Reference numbers: speed of sound 343 m/s at 20 C; +6 dB = double voltage; -3 dB = half power; 0 dBu = 0.775 V RMS; +4 dBu line level = ~1.23 V. THD spikes near max output, so always read it at the level you actually run.

How it works

1. Gear takes your clean input signal (a single fundamental tone).
2. Push it past its linear range: gain too high, hot input, or near the supply rails.
3. The transfer curve bends, so the output is no longer a faithful copy of the input.
4. That bending mathematically equals adding harmonics at 2x, 3x, 4x the original pitch.
5. A little = warmth/grit. A lot = the wave squares off into harsh fuzz and mush.
6. A THD meter sums those added harmonics and reports them as a % of the original.

Real examples

• Drive a tube preamp hot and the vocal gets a warm, thick edge: that is low-order THD doing its job.
• Crank a power amp into clipping and cymbals turn to harsh fizz: high odd-order THD eating your headroom.
• A guitar overdrive pedal is a THD generator on purpose, deliberately squaring the wave.
• An interface spec reads 'THD+N 0.0008%': that is a clean, transparent converter.
• Push a small PA top past its limit and clipping THD cooks the tweeter (squared waves dump excess HF energy).

How it helps in live sound

• Watch amp/console clip LEDs: first red flash = THD spiking toward 1%+, so back off the gain 3-6 dB.
• Leave 6-10 dB of headroom on drive racks so transients do not clip; clipping THD is what kills tweeters.
• Set gain structure with the input pad/trim, not by slamming the master into the rails.
• Use a limiter before the amps (threshold a few dB under clip) to cap THD on peaks.
• For deliberate warmth use a tube or tape emulation at 1-3% THD; do not chase it from a clipping power amp.
• On a spec sheet, only trust THD figures quoted at rated/full power, not at a soft -10 dB.