Phono Preamp Gain & Impedance Explained

Most phono preamp advice stops at “get one that matches your cartridge” without ever saying what matching means. It means two numbers: gain, measured in decibels, and loading, measured in ohms. Both are printed on the cartridge’s spec sheet and both are printed on the preamp’s. This page shows you how to compare them.

Why a phono signal needs special treatment at all

Two things happen when a record is cut. First, the signal is tiny by the time it comes back off the groove — a cartridge generates its output from a magnet moving a fraction of a millimeter, and the result is measured in millivolts, thousandths of a volt.

Second, the signal is deliberately distorted. Cutting an unmodified signal to vinyl would give bass so wide it cut into the neighboring groove and treble so quiet it disappeared into surface noise. So the cutting engineer applies the RIAA curve: bass reduced, treble boosted. Playback has to apply the exact inverse, which is the second job every phono stage does.

That is why you cannot simply turn up the volume on an aux input. The level would be wrong andthe tonal balance would be wrong — the familiar thin, tinny sound of a turntable plugged into the wrong socket.

Gain: the arithmetic

Gain is amplification expressed logarithmically. The useful conversions:

• 20 dB = 10 times the voltage
• 40 dB = 100 times
• 60 dB = 1,000 times

Now the two ends of the problem. A typical moving-magnet cartridge outputs 2.5 to 5.5 mV— the Ortofon 2M Red is specified at 5.5 mV, the Audio-Technica AT-VM95E at 4.0 mV. Line level, what an amplifier’s aux input expects, is around 300 to 500 mV.

Divide: 400 mV ÷ 4 mV = 100. That is 40 dB. So a moving-magnet cartridge needs about 40 dB of gain, and that is why nearly every MM phono stage is built around that figure.

A low-output moving-coil cartridge produces 0.2 to 0.8 mV— roughly ten times less. Ten times less signal needs ten times more amplification, which is 20 dB more: about 60 dB. That single fact is the whole reason MC cartridges need a different phono stage, and why plugging one into an MM-only input produces a faint, hissy sound no volume knob can rescue.

Working out your own number

1. Find your cartridge’s output voltage on the manufacturer’s spec sheet.
2. Divide 400 mV by that figure. A 2.5 mV cartridge gives 160; a 5.5 mV cartridge gives 73.
3. Convert to dB: 73 times is about 37 dB, 100 times is 40 dB, 160 times is about 44 dB.
4. Compare with your preamp’s published gain. Within about 3 dB is fine. More than 6 dB out and you will notice.

This is exactly why a stage with switchable settings — 39, 42 or 45 dB, or the four-position 38/48/56/66 dB arrangement on some units — is worth more than its price difference suggests. It lets one box suit whatever cartridge you end up with.

What a mismatch sounds like

• Gain too low:everything is quiet. You run the volume knob much higher than for streaming or CD, and as you do, background hiss comes up with the music. Dynamics feel flattened. Nothing sounds broken — it just sounds small.
• Gain too high: hard, gritty distortion on loud passages that vanishes in quiet ones, and a volume knob you can barely move off zero. On heavily-cut records the effect is obvious; on quiet ones it may never appear, which makes it confusing to diagnose.
• Loading wrong (MM): a treble that is either notably bright and edgy or slightly dull and closed-in, without any accompanying distortion. This is subtler than gain problems and much rarer, because nearly every MM stage uses the standard value.
• Loading wrong (MC): more consequential. MC cartridges are far more load-sensitive, and a mismatch produces a ringing brightness or a conspicuously lifeless top end.

Loading: why 47 kOhm is on every spec sheet

Loading is the input impedance the phono stage presents to the cartridge. It is not an arbitrary number: a moving-magnet cartridge is a coil, and a coil has inductance. That inductance, the load resistance and the total capacitance of the cable and input together form a filter that shapes the cartridge’s high-frequency response.

Cartridge makers design around 47 kOhmbecause it became the industry convention decades ago, so their published frequency response assumes it. That is why a fixed 47 kOhm input is not a limitation for a moving-magnet user — it is the correct value, and adjustable MM loading solves a problem most people do not have.

Moving coil is different. MC cartridges have far lower inductance and their makers specify a wide range of load values, sometimes as low as 100 ohms. Here adjustable loading genuinely matters, and its absence on a budget MC-capable stage is a real compromise rather than a specification detail.

Capacitance, the third number

MM cartridges also specify a total load capacitance, typically 100-200 pF, and “total” includes your interconnect cables. A long or high-capacitance cable can push the total well past what the cartridge expects, shifting the treble response. If your system sounds bright or dull in a way that cables seem to affect, this is usually why. It is a real effect and a small one; fix gain first.

A worked example

Suppose you own a Fluance RT81 with its stock elliptical cartridge (a 4.0 mV Audio-Technica type), running into a receiver’s phono input with a fixed 36 dB gain.

You need about 40 dB; you have 36. That is a 4 dB shortfall — about 1.6 times too little signal. Not catastrophic, but enough that you will run the volume noticeably higher than on other sources and will hear more of the receiver’s own noise floor. A stage offering a 42 dB setting closes that gap and drops the noise you were amplifying alongside the music.

Now suppose you upgrade to an AT-VM95ML or an Ortofon 2M Blue — both moving magnet, both in the same output range. The gain requirement does not change. That is the useful part: within moving magnet, upgrading the cartridge does not obsolete your phono stage. Moving to moving coil does.

What to do with all this

1. Look up your cartridge’s output voltage and recommended load. Both are on the manufacturer’s page.
2. Look up your phono stage’s gain and input impedance. If neither is published, that is informative in itself.
3. If gain is within a few dB and loading is 47 kOhm for an MM cartridge, you are matched — stop, and spend your money on speakers instead.
4. If gain is well short, a switchable stage is the cheapest fix, and it fixes your next cartridge too.

For the practical shortlist, see the best phono preamps under $200 or the under-$100 picks. And if the symptom you are actually chasing is hum rather than level, that is a wiring problem, not a matching one — see turntable troubleshooting.