Why do I need a phono preamp?

Phono Preamp - What's It For?

A phono preamp? What's that for? Why do I need a phono preamp to play my vinyl?

The phono preamp serves the purpose of "de-emphasizing" the information sent to it from a magnetic phono cartridge mounted on your turntable.

When you drop the "needle" on the record, the stylus (as it's properly known) on the phono cartridge tracks the record groove. The record groove spirals from the outside of the record to a continuous circle at the record label. On its travel when it encounters music or vocals (a track) the record groove wiggles horizontally: The bass is a gradual wiggle, the treble is a fast wiggle and all the other frequencies are in between.

Contrary to popular belief, the output from a record does not rise in proportion with frequency - it is the output from a magnetic phono cartridge which rises with frequency!

Prior to magnetic phono cartridges there were ceramic phono cartridges which required a little frequency correction, but apart from that the ceramic cartridge was a "line-level" output device (but only when operated into a high impedance input of 1-2 Meg Ohms which a valve amp of that day featured). It needed a little frequency correction because the output of a record was "shelved down" between 500Hz and 2kHz - this in turn because of the "new orthophonic standard" otherwise known as the "RIAA Curve" of the mid 1950's.

If you rotate the graphical example of a magnetic phono cartridge output clockwise by 45 degrees (below), that is the frequency response of a record (an RIAA record that is).

The reason why the properly orientated graph (below) goes upward to the right is because the output of a magnetic cartridge goes up with increasing frequency. Therefore the magnetic cartridge "emphasises" the upper frequencies (the ceramic cartridge didn't).

The output of a moving magnet phono cartridge (the most common or budget type of magnetic cartridge) when playing a regular LP record is 5mV on average (usually between 2.5 and 10mV) and that's quoted at 1k Hz (the middle of the above graph)

A "line level input" however, is somewhere between 100mV and 1V - let's choose 500mV. From that you can deduce the output of the moving magnet phono cartridge needs to be amplified 100 times.

But at the lowest frequencies it's output is going to be 20dB less (-20dB in voltage terms is another way of saying a tenth) and at the highest frequencies it's output is going to be 20dB more (+20dB in voltage terms is another way of saying ten times).

Therefore that 100 times amplification becomes 1,000 times at the lowest frequency and 10 times at the highest. The kink in the middle of the curve on the graph also needs straightening.

Obviously a preamp is needed between the turntable output and the amplifier "line-level" input - your "line level" input could also be called "aux" and inputs for "CD", "radio/tuner" etc are also line level and can be used.

The type of preamp required is a phono preamp. A phono preamp combines high gain amplification with frequency equalisation to "de-emphasise" the output of a magnetic cartridge, making it sound right - and to make it sufficiently big to drive a "line level" input on your amplifier or mixing console.