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API 550

Sweeten your mixes and masters with proportional-Q curves
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Shape Your Tone with Exceptional EQ Curves


The API 550 introduced visionary EQ curves. Q varies with gain, so boosting adds a more focused musical quality—while reducing gain provides natural, subtle-to-steep tone shaping. Developed in close collaboration with API, the 550A/B are loving re-creations of these late-60s legends.

Expertly selected frequencies lift instruments up, or place them further back—just as easily as they can sweetly shape a mix’s contour. For individual tracks, buses, or full masters, the 550 defines musical transparency.

  • 550A: 5 selectable frequencies for 3 bands, and 50 Hz – 15kHz filter
  • 550B: 4 overlapping EQ bands with multi-octave ranges
  • Ideal for general problem-solving or sweetening
  • High/low bands for 550A and B switch between peak and shelving
  • Switchable analog harmonic distortion and hiss
  • Convenience features like phase switch, output trim, and metering
Watch It in Action

EQs That Are Both Transparent and Musical

  • Ahead of the Curve

    API’s pioneering proportional-Q curve made it a favorite for high-end studios that demanded musicality with transparency, rather than the vintage sound that was the mainstay in the 60s and 70s.

    Features: Glue Mixes without “Squashing”
  • Two EQs, One Coherent Goal

    The 550A and 550B are designed to the same exacting standards and sound quality, but offer different EQ philosophies—the 550A’s more general slant, or the 550B’s surgical, problem-solving approach.

  • All That Changes Is the Tone

    API’s EQs bring clarity to the zing of a violin string, the snap of a snare, the intimacy of a vocal, or the contour of a mix. These EQs were engineered to bring out the very best of whatever sound you put through them.

    Features: Fast and Efficient Operation
  • Harness API 500’s Symmetrical Boost and Cut

    With Proportional-Q, boost and cut are symmetrical—which works wonders for instruments. You can differentiate two layered guitars solely with EQ: a medium boost on one guitar, a medium cut by the same amount on the other. Now the two "add up" to the same energy in that part of the spectrum. This is easier to do with API than with any other EQ.

    Features: From Vocals to “The Ringo Drum Sound”