Signal-to-noise ratio — SNR — expresses the difference in level between the amplifier’s full output signal and its noise floor, typically measured in decibels. A high SNR sounds desirable: more signal, less noise. But the way SNR figures are measured and presented in manufacturer specifications conceals several important variables that determine whether the number has any practical relevance to your listening situation.
The first issue is the reference level. SNR is measured relative to rated output power — the amplifier’s maximum undistorted output. An amplifier rated at 200 watts per channel will naturally produce a higher SNR figure than one rated at 10 watts, because the noise floor is the same but the signal reference is much larger. Comparing SNR figures between amplifiers of different power ratings is therefore misleading without normalisation. A 10-watt single-ended tube amplifier with an SNR of 80 dB and a 200-watt solid-state amplifier with an SNR of 100 dB may have similar noise floors in absolute terms — the difference in the specification reflects the power rating, not superior engineering.
The second issue is weighting. SNR figures are frequently presented as A-weighted values, in which the measurement is filtered to approximate the frequency sensitivity of human hearing. A-weighting reduces the contribution of low-frequency noise, which the ear is less sensitive to. An unweighted SNR figure for the same amplifier will typically be lower — sometimes significantly. Presenting only the A-weighted figure flatters the specification.
The third issue is the measurement bandwidth. Measuring noise across a narrower frequency range produces a better figure than measuring across the full audio band. Without knowing the measurement bandwidth, a direct comparison between two SNR specifications is not valid.
In a high-sensitivity speaker system, noise floor becomes audible during quiet passages. In such systems, SNR deserves careful scrutiny — with standardised measurement conditions, not headline figures.