| Audio amplifier |
A circuit that increases the power of an audio-frequency signal (approximately 20 Hz–20 kHz). The complete audio signal chain typically includes a preamplifier (voltage gain), signal processing stages (tone control, equalization), and a power amplifier (current gain for speaker drive). Each stage adds gain, potential noise, and potential distortion. |
| Preamplifier |
The first and most critical stage in an audio signal chain, which raises a weak source signal (microphone or instrument) to line level (approximately 0 dBV) with minimum added noise and distortion. Requires high input impedance, low noise figure, flat frequency response (±0.5 dB from 20 Hz to 20 kHz), and often variable gain of 20–60 dB. |
| Power amplifier |
The final stage in an audio signal chain, which delivers sufficient current to drive a loudspeaker to useful volume. Input is typically at line level; output power ranges from watts to kilowatts. Characterized by output power, efficiency, and distortion class (A, AB, D). Does not increase voltage significantly — it increases current (and therefore power). |
| Signal-to-noise ratio |
The ratio of the desired signal power to the background noise power, expressed in decibels: \(\text{SNR} = 10\log_{10}(P_{signal}/P_{noise}) = 20\log_{10}(V_{signal}/V_{noise})\). High SNR indicates a clean signal. CD audio: ~96 dB SNR. Professional systems: 100–130 dB. Listening fatigue and perceived quality improve with higher SNR. |
| Noise floor |
The minimum detectable signal level in a system, determined by thermal noise, circuit noise, and electromagnetic interference. All signals below the noise floor are masked and unrecoverable. Expressed as a power level (dBm) or voltage density (nV/√Hz). Lowering the noise floor requires better components and careful layout. |
| Thermal noise |
Noise generated by random thermal motion of electrons in any resistor at temperature above absolute zero. Also called Johnson-Nyquist noise. RMS noise voltage: \(V_n = \sqrt{4kTRB}\), where k = 1.38×10⁻²³ J/K, T is absolute temperature in kelvin, R is resistance in ohms, and B is bandwidth in Hz. Unavoidable — sets the fundamental noise limit. |
| Audio distortion |
Any unwanted alteration to an audio signal as it passes through a circuit. Causes include nonlinearity (harmonic and intermodulation distortion), clipping (amplitude limiting), frequency response errors (linear distortion), and noise. Distortion degrades audio fidelity and is measured as a percentage or in dB. |
| Harmonic distortion |
Distortion produced when a circuit's nonlinearity creates output components at harmonics of the input frequency (2f, 3f, 4f, ...). A pure sine wave input produces a distorted output containing harmonics. Total harmonic distortion (THD) is the RMS sum of all harmonic amplitudes divided by the fundamental amplitude, expressed as a percentage. |
| THD |
Total Harmonic Distortion — the standard measure of nonlinear distortion in audio systems. \(\text{THD} = \sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots}/V_1 \times 100\%\), where \(V_1\) is the fundamental and \(V_2, V_3, \ldots\) are harmonic voltages. High-fidelity systems have THD < 0.1%. Audible above approximately 1% for most listeners. |
| Intermodulation distortion |
Distortion produced when two or more signals at different frequencies pass through a nonlinear circuit, creating sum and difference frequency products (\(f_1 + f_2\), \(f_1 - f_2\), \(2f_1 - f_2\), etc.) that were not present at the input. IMD is particularly objectionable because these products fall at non-harmonic frequencies, sounding discordant against the original signals. |
| Clipping |
The distortion that occurs when a signal's amplitude exceeds the output voltage swing limit of an amplifier or system. The output waveform is "clipped" flat at the supply rails, converting smooth waveform peaks into flat plateaus. Clipping is severe nonlinear distortion that generates strong odd harmonics and is readily audible as harshness or buzzing. |