A Guide to Phono Stages

Magnetic cartridges, including MM, MI and MC cartridges produce a tiny electrical signal, measured in millivolts. A typical moving magnet or high output moving coil cartridge will output anywhere between 2-7MV, while the output of a low output moving coil cartridge is even less, around 0.5MV or so. To put this into perspective, the output of a hi-fi CD player or the headphone output of an iPhone is 2 volts, almost 800 times that of a moving magnet cartridge and 4000 times that of the moving coil.

When a record is cut, it is done so with a frequency curve which reduces the bass frequencies and boosts the treble frequencies. Known as the RIAA curve, named after the Recording Industry Association of America, this process increases the amount of playback time available over the side of a record by decreasing the width of the record groove and reducing physical movements of the stylus, reducing the potential for distortion and or groove damage that would otherwise arise during playback.

When playing back a record, the RIAA curve must be reversed or the sound would be tinny and shrill with a complete absence of bass. A turntable preamplifier, sometimes known as an RIAA preamplifier or phono stage, reverses the RIAA curve, while boosting the signal of the cartridge to a line level signal which can then drive an amplifier.

When vinyl was the most prominent audio format, most amplifiers featured an internal RIAA preamplifier usually labeled as a ‘phono’ input. These days while a great many amplifiers do feature an internal phono stage, an equal number omit the phono stage altogether usually offering some digital connectivity in its place, or a further line level input to which an external phono stage can be connected.

External phono stages come in all shapes and sizes. The cheapest take the form of a tiny box with input, output and power connections with no user settings for cartridge loading. These phono stages usually offer a single stage of amplification suitable only for moving magnet and high output moving coil cartridges, though some do offer basic provisions for moving coil cartridges. Some may also offer a USB output, allowing you to create digital copies of vinyl records using a turntable and software package of your choice.

More advanced phono stages will allow the user to adjust their electrical loading characteristics to match the cartridge in use, and will feature better circuitry offering lower residual noise. Before the introduction of the RIAA curve in 1954, each record company applied its own equalisation meaning that their records could only be played back on equipment able to reverse the frequency curves used during the cutting process. Some preamplifiers offer the ability to playback records recorded before the standardisation of the RIAA curve, applying the correct de-emphasis equalisation during playback to reproduce them accurately.

Many budget turntables on the market today feature an internal phono stage, often with USB capability. These phono stages allow you to setup an extremely simple system consisting of a turntable and a pair of powered speakers, and the preamplifiers are often at least comparable to the cheaper external units. They’re usually switchable too, meaning you can use an external phono stage with your turntable if you wanted to, for example, install a moving coil cartridge.

Due to the extremely high amounts of gain required to boost the signal of a phono cartridge to line level, a poorly designed preamplifier can introduce a large amount of hiss, hum and other residual and unpleasant noise into the system. This is especially a problem with cheaper moving coil preamplifiers as the tiny output of a moving cil cartridge requires an enormous amount of amplification using low noise components. It is also of crucial importance that a RIAA preamp is able to accurately reverse the RIAA equalisation to achieve an accurate reproduction of a record.

This article is part of our Guide to Turntables and Vinyl series. Continue reading: Part 4, Should You Buy an All-In-One Turntable?.