If you’ve connected a turntable directly to a standard line input on an amplifier and been disappointed by the faint, thin, bass-light result, you’ve experienced firsthand why a phono preamplifier is not optional equipment. It is a required part of any vinyl playback chain, and understanding why illuminates a great deal about how record playback works.
Why You Need a Phono Stage
A turntable cartridge generates an electrical signal by converting the physical motion of a stylus tracing a record groove into voltage. That voltage is extraordinarily small — between 0.1 and 5 millivolts, depending on the cartridge type. A standard line-level signal is 200 to 300 times stronger. Simply plugging a turntable into a line input produces a barely audible, high-pitched whisper.
That’s the gain problem. But there’s also an equalization problem.
When vinyl records are manufactured, the audio is intentionally processed with the RIAA equalization curve: bass frequencies are reduced (by as much as 20 dB at 20 Hz) and treble frequencies are boosted. This is done for practical reasons — recording bass at full amplitude would require enormously wide groove excursions, making records physically impractical.
When you play the record back, this equalization must be reversed. The RIAA playback curve boosts bass and cuts treble by equal and opposite amounts. A phono preamplifier applies both functions: it amplifies the tiny cartridge signal to line level, and simultaneously applies the RIAA inverse equalization curve.
Without RIAA equalization reversal, records played through a flat amplifier sound painfully bright and thin, with almost no bass presence.
Moving Magnet Cartridges
A moving magnet (MM) cartridge uses a small permanent magnet mounted at the end of the stylus cantilever. As the stylus moves, the magnet moves within a pair of fixed coils, generating a current.
MM cartridges produce a relatively strong output signal — typically 2.5 to 5 millivolts — and have higher coil inductance. They require a phono stage with standardized MM gain (typically 40–46 dB) and a standard 47 kilohm input impedance.
Advantages of MM:
- Strong output signal, tolerant of phono stage noise
- User-replaceable stylus — when the stylus wears out, you replace just the needle, not the whole cartridge
- Wide selection at every price point
- Most integrated amplifiers and receivers with phono inputs are MM-compatible by default
Disadvantages of MM:
- The moving mass is slightly higher than MC designs, which can limit high-frequency tracking ability in the finest styli
For most vinyl listeners, especially those new to the format, a moving magnet cartridge is the ideal starting point.
Moving Coil Cartridges
In a moving coil (MC) cartridge, the design is inverted. Tiny coils are wound around the cantilever and move within the field of fixed magnets. Because coils can be wound with far fewer turns and less mass than a moving magnet assembly, MC cartridges can achieve lower moving mass — which theoretically improves transient response and high-frequency tracking.
The tradeoff is output level. MC cartridges typically produce 0.2 to 0.6 millivolts — roughly 10 to 20 times less than an MM cartridge. This requires much higher phono stage gain (typically 60–72 dB for low-output MC) and lower input impedance to properly load the cartridge.
Advantages of MC:
- Lower moving mass can mean better transient accuracy
- Many of the world’s finest cartridges are MC designs
- Can offer a more “immediate” and detailed presentation in high-quality implementations
Disadvantages of MC:
- Requires a phono stage with MC gain capability (higher gain, lower impedance)
- Non-replaceable stylus — when a low-output MC wears out, you typically send it for re-tipping or replace the whole cartridge
- Generally more expensive
- More sensitive to phono stage noise due to lower output
There is also a category of high-output MC cartridges that produce 1.5–2.5 mV outputs and can be used with standard MM phono stages. These sacrifice some of the moving mass advantages of true low-output MC but offer a simpler entry point to the MC world.
What to Look for in a Phono Preamp
MM-only vs. MM/MC: If you’re starting with vinyl or using an MM cartridge, an MM-only phono stage is perfectly appropriate and allows manufacturers to optimize for one cartridge type. If you want to explore MC cartridges now or later, choose a unit with adjustable MC loading.
Gain: Verify the phono stage’s gain matches your cartridge output. Most MM stages provide 40–46 dB. MC stages typically offer 60–72 dB for low-output cartridges. Some offer switchable gain.
Loading: MC cartridges are sensitive to the impedance load presented by the phono stage. Most MC stages offer adjustable loading — common values range from 50 to 1,000 ohms. The correct loading depends on the specific cartridge; the manufacturer’s recommended value is a starting point, but listening comparisons often reveal what sounds best in practice.
Subsonic filter: Record warps produce low-frequency oscillations far below audible sound — typically 3–10 Hz — that can overload your amplifier and move your woofer cones without producing any useful output. A subsonic (or infrasonic) filter below 20 Hz prevents this. This feature is especially important with older or warped records.
Noise floor: MC phono stages require much more gain, making noise performance more critical. A noisy phono stage will audibly hiss through your speakers at normal listening levels. Specifications below 80 dBV referred to input are generally fine; the best designs measure significantly lower.
The Phono Stage in Your Amplifier
Many integrated amplifiers and receivers include a built-in phono input. These range from adequate (in midrange Japanese solid-state units) to excellent (in some specialized phono preamplifier stages built into premium integrated amplifiers). If your amplifier has a phono input and you’re happy with the results, there’s no obligation to add an external phono stage.
External phono stages become worth exploring when you’re using an MC cartridge and your built-in stage doesn’t support it, when you’re getting more background noise than you find acceptable, or when you’re simply curious whether a better phono stage changes what you hear from vinyl. For many listeners, it’s one of the more revealing upgrades in a mature vinyl system.