Index VinylRIAA Equalisation

RIAA Equalisation

The playback curve at any frequency and the output of a phono stage.

Playback EQ here+16.95dB

Stage gain here56.9dB

Output at 1 kHz500mV

Cutting EQ here−16.95dB

RIAA curves

Playback (phono stage)
Recording (cutting lathe)

Input

Frequency to inspect

f

IEC subsonic roll-off (7950 µs)

A 1976 amendment that cuts warp energy below 20 Hz. Many phono stages leave it out.

Cartridge output at 1 kHzmV

Typical moving magnet: 3 to 6 mV. Moving coil: 0.2 to 0.6 mV.

Phono stage gain at 1 kHzdB

How it works

Why records need equalisation

A cutting lathe driven with a flat signal would carve bass grooves so wide that they run into each other, while treble would be buried in surface noise. So the bass is cut and the treble boosted before cutting, and the phono stage applies the mirror image. The Recording Industry Association of America standardised the curve in 1954.

Three time constants

The playback response is fully defined by three time constants:

\tau_1 = 3180\,\mu\mathrm{s}

\tau_2 = 318\,\mu\mathrm{s}

\tau_3 = 75\,\mu\mathrm{s}

H(s) = \frac{1 + s\,\tau_2}{(1 + s\,\tau_1)(1 + s\,\tau_3)}

Each corresponds to a turnover frequency $f = 1/(2\pi\tau)$ : 50.05 Hz, 500.5 Hz and 2122 Hz. The magnitude, normalised to 0 dB at 1 kHz, is

\left|H(f)\right| = \frac{\sqrt{1 + (2\pi f\tau_2)^2}}{\sqrt{1 + (2\pi f\tau_1)^2}\,\sqrt{1 + (2\pi f\tau_3)^2}}

The result is about +19.3 dB at 20 Hz and −19.6 dB at 20 kHz, a total swing of almost 40 dB.

What that means for a phono stage

A stage with 40 dB of gain at 1 kHz must supply nearly 60 dB at 20 Hz, where hum and rumble live, and stay accurate to a fraction of a decibel across the band. An error of 0.5 dB in one time constant is audible as a tonal tilt.

Output level

V_{out} = V_{cart} \times 10^{G/20}

Aim for roughly 0.3 to 1 V at 1 kHz. Loud passages on a record can exceed the reference level by 12 dB or more, so leave headroom.