Here, we present a high quality, stereo, single power supply voltage RIAA preamplifier circuit which is based on operating amplifiers.
Since the vinyl records have passed, modern audio amplifiers and mixers do not have any phono inputs. However, turntables have always been popular among audiophiles and music lovers. There are many people still enjoying the time spent in playing a vinyl record and the interaction with the music and the album art. If you are looking back to enjoy the sound from your beloved vinyl record collection, all you need is a classic turntable with a moving magnet or a moving coil cartridge and a phono preamplifier (known also as a RIAA preamplifier).
A phono preamplifier's primary task is to provide gain (usually 30 to 40 dB at 1 kHz) and accurate amplitude and phase equalization to the signal from a moving magnet or a moving coil cartridge. In addition to the amplification and equalization functions, the phono preamp must not add significant noise or distortion to the signal from the cartridge. Phono preamps follow the RIAA equalization curve, established by the Recording Industry Association of America (RIAA). TheRIAA equalization curve was intended to operate as a de facto global industry standard for records since 1954.
The RIAA equalization results in low frequency amplification, which means that any RIAA preamplifier is very sensitive to low-frequency noise, even from 50 or 60Hz noise coming from the public power supply network. It is therefore very important to place the preamplifier in an earthed metallic box in order to ensure good shielding against electromagnetic noise.
The RIAA preamplifier circuit is very simple. It is based on a low noise dual op-amp IC, the NE5532. The RIAA response is implemented from the feedback network of resistors R4, R5, and capacitors C5, C6 for the Right audio channel and from R12, R13 and C13, C14 for the Left audio channel.
The input impedance of the circuit is about 47KΩ and the voltage gain is about 35dB at 1,000Hz. These characteristics match the standard requirements of a typical moving magnet or a moving coil cartridge of a turntable, and the circuit is able to amplify the signal of a cartridge to a few hundred mV in order to drive any “line”- type input of an audio console, of an amplifier or the sound card of a computer.
Resistors R2, R3 and R10, R11 are used to DC-bias the non-inverting inputs of the operational amplifiers at a voltage level equal to the half of the supply voltage. This is essential, in order to the circuit being actually able to operate from a single supply voltage.
The supply voltage should be in the range of 12 to 30V according to the specifications of the NE5532 op-amp. Maximum supply voltage ensures greater dynamic range and better linearity.