Have you ever wondered what the phono input is on your audio equipment?

Have you ever tried to plug a record player into a line level input?

LP records and record players must adhere to a standard EQ curve for playback. This was developed by the Recording Industry Association of America in the 1950’s. Before this standard was established, each record company applied their own processing which resulted in dozens of different playback combinations. A major consequence of this was that broadcasters would routinely be forced to adapt to many various recording characteristics as certain albums could only be played back on a proprietary turntable.

The reason a special EQ curve is applied is due to the long wavelengths of low frequencies. The grooves that these low frequencies would create are too difficult for the phonograph’s cartridge to playback. Additionally, the playing time of an LP would be limited to only a few minutes in order to accommodate these large grooves.

The RIAA standard implemented a recording process that reduced low frequencies by 20dB and boosts high frequencies by 20dB. The playback devices then incorporate an inverse of this curve to make the audio sound the way it was intended. By doing this, the grooves for the low frequencies are much smaller. As a bonus, the drastic high-end cut serves to reduce hiss and noise from playback making big improvements to the recording fidelity.

The phono input contains the appropriate preamp circuitry to manage the playback of the record. Line level inputs are designed to be as flat as possible, which is why plugging a record into these inputs is not a good idea.

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