Line signal to phono adaptor circuit

Posted on Feb 1, 2013

Many amplifiers have phono inputs for connecting record players to the amplifier. Phono input is designed to take a up to few millivolt signal from phono pickup and amplify it. The amplifier stage does also some equalization based on standardized RIAA curve. That RIAA equalization is used in the playback to reduce the high pitch noise and maximize bass dynamics in the phono playback. The audio material which is rocorded to the record has been pre-equalized so that the frequency response of the whole chain from the mixing desk to your speaker will give flat frequency response.

Line signal to phono adaptor circuit
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Nowadays phono inputs are largely unused because vinyl record players are getting rare. This circuit is is a simple converter to convert line level signal (0..1 V) to phono input levels (0..5 mV), which makes it possible to use those inputs as an extra line input. This circuit does the level conversion, impedance matching and inverse RIAA filtering. The inverse-RIAA filtering network in this circuit is needed, because all phono inputs have RIAA filter in them. Input impedance: > 10 kohm Output impedance 470 ohms Nominal input signal level: 500 mV Nominal output signal level: 2.5 mV Attenuation: 46 dB Frequency response: inverse-RIAA Phono input compatibility: moving magnet and high output moving coil type inputs (= compatible with most receivers and integrated amplifiers). The circuit does two functions: signal level attenuation and inverse RIAA filtering. The signal attenuation is needed to convert the 500 mV signal to 2.5 mV signal. The inverse-RIAA filtering is needed to make the frequency response of the system flat (same equalization that is used when music is transferred to vinyl in the studio). The picture blow shows the frequency response of the ideal inverse-RIAA filter: The picture below shows the circuit diagram of one channel. The other channel (for stereo) is identical. The circuit is so simple that you can easily build it by just soldering the components together and fit them to small metal box with...

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