Role of phono preamps
In 1954, the Recording Industry Association of America (RIAA) created a standard based on the curves used by RCA and Victor. Shortly after, all the companies applied this pre-emphasis, now known as 'RIAA'. Most records pressed from 1955 onwards were recorded using RIAA filters.
That's why a phono preamp is required to play a record. The dedicated preamp uses an inverted curve: in broad outline, a bell curve is applied on recording, and an anti-bell curve is applied on playback. This brings the frequency response of a record down to around 20 Hz (bass). However, the phono preamp has other roles, such as adding gain to the cartridge, boosting the very low signal (around 3 mV for MM cartridges and 0.4 mV for MC cartridges) at the cartridge output into a line level signal between 0.3 and 0.5 V. In addition, it also addresses certain electrical issues. In fact, MM cartridge manufacturers combine various parameters to obtain the flattest response curve possible, but this result is dependent upon the capacitative and resistive value of the cartridge and cable combination. MC cartridges are also designed to operate with a specific load in terms of resistance and capacity. The preamp must automatically or manually adapt, via a selector switch (Pro-ject Phono Box SE) or a remote control (Audio Research PH6), the capacity and impedance to be as close as possible to those of the equipment in use. Considering the low values present, it is obvious that the cables (with MC cartridges in particular) have a significant influence, and it is strongly recommended not to replace them with interconnect cables that are not specifically designed for record players. Standard interconnect cables designed for high output sources such as CD are often too capacitive and resistive, which leads to a drop in high frequencies with peaks and troughs in the frequency response.
Phono preamp technology
Before the compact disc made its glorious entry on the market, ALL commercially-available built-in amps and preamps featured a phono input. Depending on the brands and ranges, it was more or less sophisticated and of varying quality level. High end products often had a switchable MM and MC input, others had two separate inputs, a high output and a low output. Amid the drop in analogue sales in the early 90's, many manufacturers decided to remove the precious input from their products. It must be said that many of them saw the supremacy of the CD as an opportunity to discard the phono preamp without reducing the overall price of their equipment. Although a phono stage might only cost a few centimes (in mainstream products on the Asian market, it often boils down to a simple operational amp), it can amount to several hundred euros for high end-models, via the use (past and current) of discrete components (Class A polarized transistors, etc.) or tube stages (12ax7, 6922, 6H30…).
Nowadays, 97 % of commercially-available turntables don't have a built-in phono preamp. Only very few manufacturers offer it as an option. So you'll need to purchase a phono preamp to be able to play your records. They come in different prices, sizes and design.
1. Transistor preamp
Entry level products use relatively basic but well tried circuits. A combination of resistors, condensers and transistors and operational amplifiers (Pro-ject PHONO BOX). The quality of the components and their combination, as well as the contacts and power supply are key features. Some use high end discrete components (REGA IOS, EAR 324) and standalone (NAIM AUDIO Stageline, Superline) or battery-powered power supplies (SUTHERLAND PH3D).
2. Tube phono preamp
This technology applies to high end preamps, which are therefore more expensive (AUDIO RESEARCH REFERENCE PHONO and PH8). They have however become more prevalent in recent years, with affordable tube products available on the market (PRO-JECT TUBE BOX, EAR 834P). They are very popular with audiophiles because of their impressive musicality. They are still expensive to build and require maintenance as the tubes have a limited life, which varies depending on mount and model (between 2,500 and 4,500 hours). The most common tubes are triodes or dual triodes (12ax7/ecc83, 6922/ecc88, 6H30…).