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Connect line-level signal to phono input

2017-09-08 05:02  
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Circuit description


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. The RIAA reproduction curve:

RIAA equalization 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.

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.

Circuit specifications

Input impedance: > 10 kohmOutput impedance 470 ohmsNominal input signal level: 500 mVNominal output signal level: 2.5 mVAttenuation: 46 dBFrequency response: inverse-RIAAPhono input compatibility: moving magnet and high output moving coil type inputs (= compatible with most receivers and integrated amplifiers)

Circuit operation

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:

Inverse-RIAA equalization curve

Circuit diagram

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 the RCA audio connectors.

Adapter circuit diagram

This circuit is a simplified version of circuit published in Elektor Electronics (T.Giesberts, Pick-up input becomes line input, Elektor Electronics, December 1995, page 99). The basic circuit idea is same, but my circuit is simpler and it does not use high-precision components. The performance is not the same, but I think that this circuit build even from 5% components is adequate for many purposes.

Component list

C1   2.2 nF
C2   1.5 nF
C3   1.5 nF
R1    68 kohm
R2     1 Mohm
R3   147 kohm
R4   470 ohm
and two RCA connectors
This part list is for the one channels described in the schematic. Usually you want stereo sound, in which case you have make two identical circuits: one for left channel and one for right channel.

Using the circuit

When you connect this circuit to your amplifier, set the phono input to MM or MD position (if it has a selector). Keep in mind that phono input is much more prone to pick up interference, because signal levels in it are much lower than in line level inputs.

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