THD Calculator

Harmonic distortion as a measurable quantity emerged in the 1920s as vacuum-tube audio amplifiers became widespread. Harold Black\'s 1927 invention of negative feedback at Bell Labs reduced distortion by orders of magnitude — linear amplifiers finally became practical. Black\'s feedback amplifiers were the first circuits to achieve THD below 1%, enabling high-fidelity broadcast and telephony systems.

The transistorized "Wien bridge" audio oscillator, followed by Hewlett and Packard\'s HP 200A (1939) and its successors, provided the low-distortion source needed to measure amplifier THD accurately. By the 1960s, solid-state audio amplifiers routinely achieved <0.01% THD — well below human hearing thresholds. The audiophile market subsequently split into camps: "straight-wire" solid-state designers chasing lowest distortion, and tube-amp revivalists arguing that even-harmonic distortion (characteristic of tubes, 2nd and 4th harmonic dominant) sounds more musical than the odd-harmonic distortion (3rd, 5th) of solid-state amps at the edge of clipping.

IEEE 519 established harmonic distortion limits for power systems in the 1980s, driven by the rise of non-linear loads (computers, variable-speed drives) that injected harmonics into utility lines. Modern measurement uses FFT spectrum analyzers rather than analog harmonic distortion analyzers; software like Audio Precision APx555 integrates signal generation and analysis into a single instrument.

Enter the fundamental amplitude (V₁) and harmonic amplitudes (V₂ through V₅) from an FFT or spectrum analyzer. The tool computes THD as a percentage and in dB, along with total RMS and a categorical grade (audiophile / hi-fi / typical / tube-like / saturated).

Two conventions exist for THD: IEC (referenced to fundamental, shown here) and IEEE (referenced to total RMS). They give slightly different numbers; always note which is being used when comparing specs. Some amplifiers also quote "THD+N" which includes the noise floor in addition to harmonic products — always higher than THD alone. All math runs client-side.