Why a Phono Equalizer Is Needed: RIAA Curve and Gain

Why the signal must pass through a phono equalizer | The physics and standard of the RIAA curve

When playing a vinyl record on a turntable, connecting the electrical signal from the cartridge stylus directly to a standard line input (such as an AUX or CD jack) on an amplifier produces only a faint whisper of sound, and a tinny, high-pitched tone with the bass completely missing. This is not a malfunction in the amplifier, but an inevitable consequence of the physical recording standard used for analog records.

When a cutting lathe engraves sound waveforms into the groove of a lacquer master disc, attempting to record low frequencies at their original amplitude would make the lateral excursion (the side-to-side width of the groove swing) physically enormous. Excessive excursion not only causes adjacent grooves to collide and break through each other’s walls, but also makes the playback stylus lose tracking and skip out of the groove. Worse still, wider groove spacing severely reduces the playing time that can fit on one side of a record.

At high frequencies, on the other hand, wavelengths are short and amplitudes are tiny, meaning the signal is easily buried in surface hiss caused by friction against the polyvinyl chloride compound and microscopic dust particles. To solve both problems at once, the Recording Industry Association of America standardized the international specification known as RIAA equalization.

Under the RIAA standard, every record undergoes a uniform process during mastering: low frequencies are substantially attenuated before cutting, and high frequencies are heavily boosted. During playback, a phono equalizer circuit placed between the turntable and the amplifier passes the signal through the exact inverse filter, boosting the lows and rolling off the highs. This inverse correction restores a flat overall frequency response while simultaneously pulling down the high-frequency surface noise that sits in the boosted treble band, achieving both a high signal-to-noise ratio and long playing time.

For the fundamentals of handling the stylus tip and cartridge across a record playback setup, see our guide on stylus lifespan and tracking force. Right after the mechanical vibrations of the stylus tip are converted into a minute voltage, the phono equalizer takes over electrical frequency correction and gain amplification.

The correction curve defined by three time constants

The frequency response of RIAA equalization is not a loose tone-control circuit, but a transfer function strictly defined by three time constants specified in the standard. Determined by the product of resistance and capacitance in circuit design, those three time constants are 75 μs, 318 μs, and 3180 μs.

Calculating the cutoff frequencies (turnover and rolloff frequencies) corresponding to each time constant yields three reference points.

Taking the gain at the 1 kHz reference frequency as 0 dB, a playback phono equalizer applies a massive boost of roughly 20 dB at 20 Hz in the deep bass (unverified estimate). Conversely, at 20 kHz in the upper treble, it applies roughly 20 dB of attenuation (unverified estimate). Between the low-frequency and high-frequency extremes lies a slope of roughly 40 dB (unverified estimate). The amplifier circuitry inside a phono equalizer must trace this complex frequency slope across the entire audible band (20 Hz to 20 kHz) with high precision, typically within an RIAA deviation of ±0.5 dB.

As summarized in Vinyl for beginners: the first three components to buy and where to place them, placing the phono equalizer in the wrong spot in the signal chain prevents proper signal correction.

An amplification stage that raises tiny cartridge outputs to line level

The second decisive role of a phono equalizer is high-gain amplification, raising the minute voltage generated by the cartridge up to the line-level signal that a standard audio amplifier expects.

Digital playback sources such as CD players and network streamers, as well as analog audio outputs on televisions and smartphones, are designed around a standard rated output of roughly 150 mV to 250 mV. By comparison, the output voltage of a cartridge tracing a record groove is orders of magnitude smaller.

For the most common MM (Moving Magnet) cartridges, the nominal output voltage is typically around 2.5 mV to 4.5 mV. For MC (Moving Coil) cartridges, which move tiny coils directly, the output drops to a microscopic level around 0.28 mV. To raise these minute voltages to line level (roughly 150 mV to 250 mV), a phono equalizer circuit must provide the following voltage gain:

• For MM cartridges: Approx. 35 dB to 36 dB of amplification
• For MC cartridges: Approx. 59 dB of amplification

Because the circuit magnifies a tiny signal by 100 to more than 1,000 times in voltage, the slightest power-supply ripple, electromagnetic interference, or thermal noise on the circuit board is amplified by the exact same factor. When evaluating a phono equalizer, how effectively it rejects background noise while maintaining high gain (its signal-to-noise ratio) is a critical design parameter.

Published specification comparison: built-in phono stage vs. standalone external unit

Many entry-level turntables on the market today include a built-in phono equalizer circuit inside the plinth. At the same time, standalone external phono equalizer units are widely available as dedicated audio components. Below we compare the design philosophy and physical performance differences between the two using published specifications from Audio-Technica.

Because a built-in phono equalizer shares the same plastic chassis as the drive motor and power transformer, space and cost constraints usually dictate a simplified circuit built around a single-chip op-amp IC. As a result, its signal-to-noise ratio stays around 50 dB.

By contrast, a standalone phono equalizer such as the AT-PEQ30 is housed in an independent extruded-aluminum shielded enclosure and isolates its power supply via an external AC adapter. Populating the RIAA filter network with tight-tolerance passive components allows it to achieve low-noise performance of 100 dB on MM input alongside strict RIAA curve tracking within ±0.5 dB across the audible spectrum.

Connecting mismatched cartridge and amplifier sensitivities will not improve sound quality

When choosing or connecting a phono equalizer, the most important mistake to avoid is an impedance or gain mismatch between components. Even if you invest in an expensive external phono stage, failing to match input sensitivity and output level will cause severe distortion or elevated noise rather than better sound.

The classic wiring mistake is double equalization: leaving a turntable with a built-in phono stage switched to “LINE” output while plugging it into the “PHONO” input jacks on an integrated amplifier. Doing so applies RIAA correction twice in series, producing a muddy, bass-heavy, heavily distorted output that can overload and damage the amplifier’s input stage.

Similarly, if you use a low-output MC cartridge (0.28 mV output) while the phono equalizer is left in MM mode (roughly 35 dB gain), you will have to crank the amplifier volume far up to reach normal listening levels, letting residual hiss and hum dominate the speakers. Conversely, plugging an MM cartridge into an MC input overloads the front end, clipping the waveform peaks and generating harsh distortion. Unlocking the benefits of a phono equalizer requires matching the cartridge’s generator type and load impedance strictly, typically 47 kΩ for MM and around 100 Ω to 120 Ω for MC.

A record worth hearing through a precise equalization chain

Accurate restoration of the RIAA curve paired with a high-SNR phono stage makes the biggest difference on ambient, contemporary classical, and wide-dynamic-range acoustic recordings where delicate sonic textures sit inside quiet space.

On a flat playback chain where low-frequency distortion is kept in check and high-frequency surface hiss rolls off smoothly, room reverberation and the decaying tails of notes emerge without murkiness. We explore the sonic architecture of Brian Eno’s landmark studio work in our liner notes on Brian Eno’s Another Green World. Only when the electrical signal picked up from the record groove is accurately equalized does the three-dimensional sense of air unique to vinyl come into focus between the speakers.