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A phono stage performs two distinct functions: it amplifies the cartridge's signal (which is far smaller than a line-level source) and it applies RIAA equalization to correct for the frequency-response curve used in vinyl mastering. Both functions must be executed accurately. Errors in gain, loading, or RIAA correction are audible as tonal imbalance and reduced dynamic range.
Gain — MM vs MC
Phono Stage Gain (dB)
Moving-magnet (MM) cartridges produce an output of approximately 3–6 mV. A phono stage for MM use must deliver around 35–45 dB of gain to bring this signal to line level (typically 150–200 mV for driving a preamplifier). Moving-coil (MC) cartridges produce much lower output — typically 0.2–0.5 mV for low-output MC designs, and 1.5–2.5 mV for high-output MC. Low-output MC cartridges require 60–70 dB of gain. Using an MM stage with a low-output MC cartridge will give severely reduced output levels and a poor signal-to-noise ratio.
Gain requirements can be calculated precisely. If your cartridge outputs 0.3 mV and your preamplifier input sensitivity is 150 mV, you need at least 54 dB of gain (20 × log(150/0.3)). A phono stage providing exactly 54 dB will drive the preamp to full output with that cartridge — leaving no headroom for dynamic peaks. In practice, choose a stage with 6–10 dB more gain than the theoretical minimum, then attenuate at the preamplifier volume control as needed.
Gain mismatch symptom: If the volume control must be set above 12 o'clock to reach a comfortable listening level, the phono stage gain is insufficient for the cartridge. Below 9 o'clock for comfortable listening means the gain is excessive — the volume control resolution is poor and noise may be audible.
Input Loading — Resistive
Input Impedance / Load Resistance (Ω)
The resistive load at the phono stage input affects cartridge behavior. MM cartridges are designed to work into 47 kΩ — the industry standard since the 1960s. Deviating significantly from this alters the cartridge's frequency response, typically causing a treble rolloff at lower load values. MC cartridges are load-sensitive in a different way: the correct load resistance is typically specified by the cartridge manufacturer and is usually 10–200 Ω. The general rule for MC loading is to use a value at least 10× the cartridge's internal impedance, up to the manufacturer's recommendation. Overloading (too low a resistance) damps the MC generator coils and rolls off high frequencies. Underloading (too high a resistance) can cause ringing at high frequencies in some designs.
Many phono stages aimed at MC use offer adjustable loading, typically via DIP switches or plug-in resistors. Common settings are 100 Ω, 200 Ω, 470 Ω, 1 kΩ, and 47 kΩ. Some audiophiles prefer to adjust by ear, noting that loading affects both frequency response and the perceived character of a cartridge's treble. If the manufacturer specifies a loading value, start there.
Input Loading — Capacitive (MM Only)
Input Capacitance (pF)
MM cartridges use an inductance-based generator. The cartridge inductance interacts with the total capacitance at its input — the sum of the phono cable capacitance, the phono stage's input capacitance, and any added capacitance — to form a resonant peak near the top of the audio band. The manufacturer specifies a total load capacitance (typically 100–300 pF) that places this peak at a frequency where it provides a slight high-frequency lift, compensating for stylus and cantilever rolloffs. Exceeding this capacitance moves the peak downward in frequency, causing a treble shelving effect. Too little capacitance can cause an audible brightness peak above 15 kHz. Tonearm cables typically contribute 100–200 pF; phono stages add 50–200 pF. Choose cables and a phono stage that together stay within the cartridge's specified total load capacitance.
RIAA Equalization Accuracy
RIAA Deviation (± dB from 20 Hz to 20 kHz)
The RIAA curve boosts bass and attenuates treble during vinyl mastering; the phono stage must apply the exact inverse. The standard is precise: time constants of 3180 µs, 318 µs, and 75 µs define three specific turnover frequencies. Deviations from this curve are heard directly as tonal imbalance. A phono stage with ±0.5 dB RIAA accuracy is very good; ±0.2 dB is excellent. Inaccuracies above ±1 dB are audible to most listeners as bass heaviness or treble thinness. Budget phono stages sometimes cut costs in the RIAA network components, resulting in audible deviations.
Noise Floor and Overload Margin
Equivalent Input Noise (EIN) and Overload Margin
A phono stage's noise is usually expressed as equivalent input noise (EIN) in nanovolts per square root hertz (nV/√Hz), or as a signal-to-noise ratio referenced to a specific input level. For MM use, an EIN of 10 nV/√Hz or better is good; for low-output MC, target 3–5 nV/√Hz. Overload margin refers to how much signal the input stage can handle before clipping. A budget MM stage may clip with a 10 mV input; a well-designed stage should handle 30–100 mV without distortion. High dynamic range recordings and loud passages can momentarily drive MM cartridges to 10–15 mV, so adequate overload margin matters more than the datasheet figure suggests.
Quick Reference: Phono Stage Specs at a Glance
| Specification | What It Means | What to Look For |
|---|---|---|
| MM Gain | Amplification for MM cartridges | 35–45 dB; must reach line level with your cartridge's output |
| MC Gain | Amplification for low-output MC | 60–70 dB for 0.2–0.5 mV cartridges |
| MC Load Resistance | Resistive termination for MC | 10× cartridge internal impedance; use manufacturer spec |
| MM Load Capacitance | Total capacitive load for MM | Match cartridge specification; cable + stage combined |
| RIAA Accuracy | Equalization curve precision | ±0.5 dB or better, 20 Hz to 20 kHz |
| EIN / Noise | Self-noise relative to cartridge signal | ≤10 nV/√Hz for MM; ≤5 nV/√Hz for MC |
| Overload Margin | Headroom before clipping | ≥20 dB above nominal cartridge output |
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