The phono stage is perhaps the most underestimated component in an analogue system. It performs two functions simultaneously — amplification and equalisation — and it does so at a point in the signal chain where the signal is at its smallest and therefore most vulnerable to noise, interference, and the phono stage’s own distortion characteristics.
The amplification requirement arises from the low output level of phono cartridges. A moving magnet cartridge typically produces 2 to 5 millivolts of output — far below the 200 to 500 millivolts that a line-level input expects. A moving coil cartridge may produce as little as 0.1 to 0.5 millivolts. The phono stage must amplify this signal by a factor of 100 to several thousand without adding significant noise of its own. The noise performance of the phono stage’s input stage is the critical parameter: any noise introduced at the input is amplified along with the signal and becomes part of the output. Low-noise transistor or valve designs at the input stage are therefore a genuine engineering priority.
The equalisation requirement arises from the RIAA curve — the standardised frequency response contour applied to all LP recordings during manufacture. Bass frequencies are attenuated during cutting and treble frequencies are boosted, allowing the groove to be cut without the bass modulation becoming physically too wide and without the treble being buried in surface noise. The phono stage must apply the inverse of this curve — boosting bass and attenuating treble — to restore flat frequency response. The accuracy of this RIAA correction is a significant performance parameter: deviations from the correct curve produce tonal imbalance that travels through the entire system. A phono stage with inaccurate RIAA response sounds tonally wrong regardless of its noise floor or distortion figures.
Cartridge loading is the third critical variable. Moving coil cartridges in particular are sensitive to the resistance and capacitance of the phono stage’s input loading. Different cartridges perform optimally into different loading values, and a poorly matched load can produce high-frequency roll-off, resonance peaks, or brightness. Adjustable loading — allowing the phono stage’s input impedance to be matched to the cartridge — is a feature that pays for itself in optimised cartridge performance.
The phono stage deserves as much attention as any other component in an analogue system. Its contribution is irreversible — whatever it adds or subtracts cannot be corrected downstream.