In the past, almost all hi-fi preamplifiers, integrated amplifiers, and receivers had a built-in phono-type input with the RIAA equalization. In modern designs, phono inputs are often omitted, due to the gradual obsolescence of vinyl records.
For those still enjoying playing vinyl records in classic turntables, there are some commercially available add-on phono preamplifiers with the RIAA equalization. The main purpose of a generic add-on phono preamplifier is for adapting the magnetic phono cartridge of the turntable to an unbalanced consumer line-level input. Such a device, can also be built by yourself at a fraction of the price of a commercially made one.
How it works
The stereo RIAA preamplifier circuit is based on NE5532. NE5532 is a dual operational amplifier, designed with particular emphasis on performance in audio systems. Alternatively, you can also use LM833 on TL082.
Phono preamplifier schematic
RIAA equalization is actually a low pass filter having transition points at frequencies of 2122Hz, 500Hz and 50Hz. Implementing this characteristic is not especially difficult, but is more involved than a simple amplifier.
Apart from R1, R2, R3, R4, R5, C3, C4, C5 and C6, all other components implement a stereo filter. This filter have a frequency response very close to the ideal RIAA curve. R1, R2, R3, R4, R5, C3, C4, C5 and C6 are used for biasing the non inverting inputs of the dual op-amp. This is essential, since the circuit is powered from a single power supply source. C2, C1 and C13, C14 are used for ac-coupling at the input and the output, respectively.
Construction
Using the PCB pattern provided below, you'll be able to build the RIAA preamplifier in no-time. Just remember to use low tolerance components for achieving the same performance on both Right and Left audio channels. In the prototype, we use 1% tolerance resistors and 5% tolerance capacitors. However, C13, C3, C4, C5 and C6 can be of much higher tolerance, since there are not part of the audio filter. They are only used for providing a path to the ground for the AC-signals and also for removing noise from the power supply lines. All resistors are of 1/4W type.
For using the circuit, you will definitely need a stabilized power supply unit. Power supply can be anything from 8 to 20V DC. Current consumption is no more than 35mA.
Using the RIAA preamplifier
Just connect the stereo-output of your classic turntable to the R and L inputs of the preamplifier. Then, connect the R and L outputs to a line-level input of your amplifier or mixer. Power-on the circuit and enjoy the sound from your beloved vinyl record collection!
Attachments
Stereo RIAA preamplifier Printed Circuit Board details
