Phono input

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A phono input is a connection to an amplifier to which a record player can be connected without a built-in equalization preamplifier .

The phono input on hi-fi amplifiers (mostly cinch connections ) differs from other signal inputs in that it is designed for a different voltage. For turntables with a magnetic pickup system, it is 2 mV, while “normal” inputs have a voltage of 200 mV (each at 1 kHz ). A preamplifier is therefore required to connect these turntables to other inputs . Conversely, the signals from other sound sources cannot be connected to a phono input.

Equalization

The music information, which is stored on a micro-groove plate, is linearly distorted in the frequency response during the cutting process. This is necessary in order to be able to use the surface of both sides of the record more effectively for a longer playing time. With this distortion, the low-frequency signals, which statistically have greater amplitudes, ie deflections in the side and depth modulation of the stereo record, are attenuated. This is done using internationally standardized characteristics, the best known and most widespread of which is the RIAA characteristic.

To restore an undistorted frequency response, the input signal must pass through a filter whose filter characteristic is a mirror image of the recording characteristic. The "under-represented" low frequencies are raised. The RIAA characteristic curve must be implemented as precisely as possible, as otherwise a noticeable discoloration of the sound image and phase errors affecting the stereo sound image will result.

Unfortunately, turntables with their mass-spring-oscillator analog drive (motor, belt or string, massive turntable) and a mechanical-electrical converter of the same analogy (dynamic and static mass of tonearm and cartridge, needle stiffness) generate interference, especially at very low levels Frequencies ("rumble"). If these were appropriately raised in the equalization amplifier and amplified in the further signal chain, the result would be an impermissible deflection of the loudspeaker diaphragms that would at least lead to high non-linear distortions. For this reason the equalizer contains a high-pass filter that strongly attenuates frequencies below 20 Hz.

Connection of other pickup systems

Other pickup systems deliver voltages that do not have to be amplified ( crystal pickups ) or by a factor of 10 ( moving coil system).

Only a few integrated amplifiers offer connection options for moving coil systems, as their extremely low output voltage should ideally be increased or amplified in the immediate vicinity of the turntable. In addition, the preamplifier required for this makes considerable demands on freedom from noise and insensitivity to electrical and magnetic interference. Since moving coil systems (also due to their price) are less common than conventional moving magnet systems, only a small target group would be approachable for this feature.

literature

  • Gustav Büscher, Alfred Wiegelmann: Little ABC of electroacoustics (= Radio-Praktiker-Bücherei. Vol. 29 / 30a). 6th, completely revised and expanded edition. Franzis-Verlag, Munich 1972, ISBN 3-7723-0296-3 .