RIAA Standard Hi-Fi Phono Preamplifier

Overview

The circuit was designed to produce a hi-fi phono preamplifier circuit with RIAA equalization standard suitable for moving magnet cartridge that produces good sound.

Terminology

• Recording Industry Association of America (RIAA) – the trade group that represents the recording industry in the United States
• Phono Preamplifier – an electronic circuit that equalizes and amplifies the analog output of the cartridge in a phonograph turntable where the output is increased to intensity equal to other audio sources like CDs and tapes and to restore the original signal, RIAA equalization is required
• TL072 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• BC212 – a PNP complementary Silicon planar epitaxial transistor used in AF small signal amplifiers and drivers, as well as in low power universal applications
• BC639 – an NPN medium power transistor used in driver stages of video or audio amplifiers due to its high current with 1 A maximum, low voltage with 80 V maximum, and enclosed in a TO-92 SOT54 plastic package
• BC183 – a complementary silicon planar epitaxial transistor used in AF small signal drivers and amplifiers as well as in low power universal applications due to its feature of good linearity of DC current gain

Circuit Explanation

The main purpose of RIAA is to create, manufacture, and distribute around 85% of all legitimate sound recordings sold and produced in the United States, since its members consists of a large number of private corporate individuals and record companies like the record distributors and labels. They are involved in administering and creating technical standards of music reproduction and recording which includes magnetic tape, CDs, and digital technologies that are software-based. Their main purpose is to monitor and review relevant policies, regulations, and laws, to perform research about the music industry, and to protect the worldwide intellectual property rights and the first amendment rights of artists. They are also involved in the distribution, administration, and collection of music licenses and royalties, which makes them responsible for certifying gold, platinum, multi platinum, and diamond albums and singles in the United States.

The hi-fi phono preamp should have an accurate RIAA equalization, which is a technical standard of frequency response used on vinyl records during the playback and manufacturing. Once vinyl masters are cut, below 1 KHz frequencies are attenuated to prevent the stylus from crossing over into the groove. Higher frequencies are amplified in order to prevent noise from the disc surface. The RIAA equalization of phono preamps increases the lows and attenuates the highs so the original analog signal can be recreated.

The above circuit is based on the design of RIAA preamplifier with the first stage using visible components while the second stage contains an operational amplifier NE5532 or TL072. The standard imposed by RIAA filter is applied to the negative feedback. The use of capacitors C2 and C5 should be suitable with the moving magnet cartridge. The moving magnet refers to the process used to regenerate the music signal from the record grooves where a small and powerful magnet is attached to the cantilever with a diamond tip. This is used for tracing out the music signal from the record grooves. The movement of the cantilever is transmitted to the magnet that moves in a coil formed by very fine wires. The electrical signal is generated in the fixed coils due to the movement of the magnet. This signal will be input into a special phono circuit to amplify it to a level that is usable.

The transistors Q1 & Q2 have the same characteristics with Q8 & Q9 in that they have similar thermic behavior. This is essential especially in measuring with hfe meter. The input of differential amplifiers is regulated by the trimmer TR1 and TR2, to void the DC voltage in the output stage. The capacitors C4 & C17 is matched with C11 & C24 using a capacitance meter to avoid purchasing precise capacitors. The resistors are made of metal film. The supply of the circuit comes from the power supply which can be found in the amplifier or from external supply with 15 V and 100 mA. The circuit should be enclosed in a small external box to prevent noise from the environment.

Part List

Application

The phono preamplifier is normally built into an A/V or audio receiver and can also come as external devices for components without phono inputs in addition to the audiophiles that require the highest quality of restoration. For turntable to computer applications, external preamps are being used. A phonograph turntable is used for conversion of tracks on vinyl records into a digital format like the MP3 or CD audio. It converts the analog signal from the cartridge into digital PCM, since the turntable contains a built-in phono preamp that is required for equalization and amplification. The RIAA preamplifier is the best solution in getting a signal from an old turntable and into the mixing board, whether to obtain a pair of loops from a 70’s LP record or just to archive the collection of record.

Source:users.otenet.gr/~athsam/phono_2.htm