Turntables - Equalization


Early "mechanical" gramophones used the stylus to vibrate a diaphragm radiating through a horn. Several serious problems resulted from this:

  • The maximum sound level achievable was quite limited, being limited to the physical amplification effects of the horn,
  • The energy needed to generate such sound levels as were obtainable had to come directly from the stylus tracing the groove. This required very high tracking forces that rapidly wore out both the stylus and the record on lateral cut 78 rpm records.
  • Because bass sounds have a higher amplitude than high frequency sounds (for the same perceived loudness), the space taken in the groove by low frequency sounds needed to be large (limiting playback time per side of the record) to accommodate the bass notes, yet the high frequencies required only tiny variations in the groove, which were easily affected by noise from irregularities (wear, contaminates, etc.) in the disc itself.

The introduction of electronic amplification allowed these issues to be addressed. Records are made with boosted high frequencies and reduced low frequencies. This reduces the effect of background noise, including clicks or pops, and also conserves the amount of physical space needed for each groove, by reducing the size of the low-frequency undulations.

During playback, the high frequencies must be rescaled to their original, flat frequency response—known as "equalization"—as well as being amplified. A phono input of an amplifier incorporates such equalization as well as amplification to suit the very low level output from a modern cartridge. Most hi-fi amplifiers made between the 1950s and the 1990s and virtually all DJ mixers are so equipped.

The widespread adoption of digital music formats, such as CD or satellite radio, has displaced phonograph records and resulted in phono inputs being omitted in most modern amplifiers. Some newer turntables include built-in preamplifiers to produce line-level outputs. Inexpensive and moderate performance discrete phono preamplifiers with RIAA equalization are available, while high-end audiophile units costing thousands of dollars continue to be available in very small numbers.

Since the late 1950s, almost all phono input stages have used the RIAA equalization standard. Before settling on that standard, there were many different equalizations in use, including EMI, HMV, Columbia, Decca FFRR, NAB, Ortho, BBC transcription, etc. Recordings made using these other equalization schemes will typically sound odd if they are played through a RIAA-equalized preamplifier. High-performance (so-called "multicurve disc") preamps, which include multiple, selectable equalizations, are no longer commonly available. However, some vintage preamps, such as the LEAK varislope series, are still obtainable and can be refurbished. Newer preamplifiers like the Esoteric Sound Re-Equalizer or the K-A-B MK2 Vintage Signal Processor are also available. These kinds of adjustable phono equalizers are used by consumers wishing to play vintage record collections (often the only available recordings of musicians of the time) with the equalization used to make them.

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