Riaa curve

The RIAA curve for a phono preamp has a 40 dB swing from 20 Hz to 20KHz. Half of that is a 20 dB boost of the bass and the other half is a similar cut in the treble. It crosses the zero point at 1 KHz.

However, it is not a completely straight line from boost to cut.

Obviously the more accurate the curve, the closer your LPs' frequency response will be to flat when you play records.

The frequency response will also be influenced by the interaction of the phono cartridge with the preamp. One can have a preamp with a very flat response that becomes a lot less flat if mismatched to a cartridge.

It bears repeating, though, that this one parameter is not the only one that affects a phono preamp's sound quality. Distortion, noise, headroom and other factors are also very important. The ultimate test is what the user thinks of the sound when he plays a record.

Ultimately, the accuracy of the inverse RIAA equalization on a phono preamp is only meaningful to the point it is equal to the accuracy of the equalization used on the cutter in the production process.

Look at it this way. I can't add "about 2" plus 2.00000 and expect an answer of 4.00000. The "about 2" in the left hand side of the equation limits my answer to "about 4."

If one looks back to the "golden age" of LP production - the 1960s - which produced in particular jazz and classical recordings that are still revered to this day, I seriously doubt that much of the production equipment was much more accurate than 1 dB.

One has to be careful to not chase one specification to the exclusion of others. That happened in the 1970s when amp designers went after ultra low distortion numbers. They overused feedback which gave good looking static test numbers but gave audibly poor results with dynamic music.

Yes, a flat frequency response is a worthy goal. But then so are lots of other characteristics. The goal is not how each one tests by itself in a static environment but rather how they operate in unison when playing music.