Pro-Ject Phono Box Phono Stage

Pro-Ject Phono Box is a great value solution for moving magnet (MM) and moving coil (MC) phono cartridges. Many amplifiers and AV receivers today, do not feature a phono stage for turntables. The Pro-Ject Phono box is a popular and high-performing preamp solution, that is affordable and simple to use.

The Phono box has a sophisticated internal design that uses low noise IC’s and precise RIAA equalization. There is also a moving coil or moving magnet cartridge switch. Simply connect the unit between your turntable and the line level input of your amplifier or powered speakers.

The Phono Box

The Pro-Ject phono box is a small well made unit that is available in silver or black. Just connect the unit to an amplifier with a high grade RCA lead and then connect the power supply. Set the cartridge switch to match your turntable cartridge type.

Why a phono stage?

A phono stage is needed as the output from a moving magnet cartridge is only a few millivolts. An amplifier needs at least 500mV to give a good output. Additionally, the output is even less, with a moving coil cartridge. In this case it can be less than 1mv. That is why the Phono box has a switch to boost the gain when used with a moving coil cartridge. That is not the end of the story, however. Records are not mastered with a flat frequency response. The bass frequencies are reduced to prevent over-modulation of the groove. Additionally, high frequencies are boosted to reduce surface noise. These requirements mean that the design of the preamp needs careful consideration.

The needs of a phono preamp

Firstly the PCB has to be designed to use the shortest possible signal paths to avoid noise pickup. Also, the input and output cables need to be shielded. Another source of noise is the power supply. That is one of the reasons that the Phono Box has an external PSU. As a result, there is no hum pickup from the transformer. Another issue is the noise present in the amplifier chip or transistors. Normally very low noise op amp IC’s are used. Finally, the RIAA response curve has to be accurate to give a flat frequency response.

Moving magnet or moving coil?

There are two types of magnetic cartridge. Moving magnet and moving coil. Both are known s transducers and perform the same function. They convert the modulations of the record groove into an electrical signal.

A moving magnet cartridge works by mounting magnets onto the cantilever that reads the vibrations from the groove. The stylus is mounted on the opposite end of the cantilever. Each magnet is placed between two coils, one for each channel. The movement of the magnet creates a tiny electrical signal. Moving magnet cartridges have the advance of interchangeable styli. However, they are relatively heavy, with a higher mass cantilever. This makes it more difficult for the groove vibrations to move the assembly. As a result, there is some high frequency loss.

Moving coil cartridges work by mounting the coils on the cantilever rather than the magnets. This creates a lower mass assembly, with less tracking weight. The main advantage is less inertia. This allows the groove to be tracked more accurately. Moreover, transients are tracked better as is the high frequency information. The disadvantage is manufacturing cost, along with a lower output and a non-replaceable stylus.