Gramophone Pre-Amp: A Pre-Amplifier with RIAA Response Curve
Recording Industry Association of Ameria (RIAA) has developed the recording standard of response curve for their recording. This curve is developed to optimize the gramophone recording response for lowest signal-to-noise ratio for gramophone technology. Because the tape recording response is not linear, the pre-amp for gramophone playback pickup should have such response that restore the original response, a flat response of played back signal. The schematic diagram of common RIAA pre-amp is shown in the figure below.
The op-amp used in the circuit is recommended using LM833, but any low noise op-amp will works. There are two types of the circuit in the figure. The first one is non-inverting, and the second one is inverting. In the non-inverting circuit, the 47k resistor shunting the input signal provide damping for the moving magnet phono cartridges. The input is also shunted by a capacitance equal to the sum of the input cable capacitance and Cp. This capacitance resonates with the inductance of the moving magnet cartridge around 15 kHz to 20 kHz to determine the frequency response of the transducer, so when a moving magnet pickup is used, Cp should be carefully chosen so that the total capacitance is equal to the recommended load capacitance for that particular cartridge. [Circuit’s schematic diagram source: National Semiconductor Application Notes]