The API 550B is one of those EQs that feels almost impossible to overthink—and that’s a huge part of why it’s become such a studio staple.

It takes everything engineers love about the classic API sound—punch, presence, and musicality—and expands it into a slightly more flexible format than the 550A. With four bands instead of three, you get a little more control, but it still keeps that fast, instinctive workflow API EQs are known for.

What really makes the 550B special is how musical the frequency selections feel. You’re not endlessly sweeping around looking for the “perfect” spot—every option was chosen because it works in real-world sessions. Drums get more impact, guitars push forward in the mix, vocals gain clarity and presence—it just gets you there quickly.

The Proportional Q design is a big part of that experience. Small moves stay broad and natural, while more aggressive boosts and cuts tighten up automatically. It’s one of those features that makes the EQ feel incredibly forgiving, even when you’re making bold decisions.

Sonically, the 550B has that unmistakable API attitude. The midrange is forward and energetic, the low end feels tight and solid, and the top end adds bite and clarity without getting brittle. There’s also a certain depth and headroom from the discrete 2520 op-amp design that helps everything feel bigger and more “record-ready.”

What we love most about the 550B is that it encourages commitment. It’s not a surgical EQ for endless tweaking—it’s a tone-shaping tool built for making confident, musical choices fast.

For engineers who want an EQ with character, speed, and decades of proven studio history behind it, the API 550B remains one of the most satisfying and reliable tools you can put in a rack.

Features

• Four bands of our famous sounding equalization
• Each band offers seven API selected frequency centers
• Reciprocal and repeatable filtering
• 12 dB of boost/cut per band
• EQ band 1 and 4 offer shelf/peak switching
• "Proportional Q" narrows filter Q at extremes
• Traditional API fully discrete circuit design
• High headroom +30 dB clip level
• Re-issue of 1967 API 550 EQ with an extra band

Based on API's original 550 from the late '60s, the API 550b is a continuation of the EQ that played a major role in the history of music recording, but with an additional filter band and several new frequencies. Incorporating API's exclusive circuitry and proprietary components (such as the legendary API 2520 Op Amp), the 550b artfully blends the past with the present. So many hit records still depend on the unique 550 sound that engineers around the world find it to be an invaluable tool. In fact, the 550b design has been taken from the original blue prints and spec control drawings from the API archives. It is unlike any other EQ you will ever use.

Rather than offer a huge assortment of complex features, the API 550b provides exactly the right number of controls. Its four EQ bands are overlapped significantly to aid in dual roles as problem solver and sweetening device with each band offering seven switchable filter frequencies that span four-to-five octaves. These frequencies, purposely selected to be musical rather than numeric, were selected by an experienced "who's who" list of the industry's most proficient engineers.

Making use of API's "Proportional Q," an innovation designed by Saul Walker in the '60s, the 550b intuitively widens the filter bandwidth at minimal settings and narrows it at higher settings without the need for additional bandwidth controls. This unique feature minimizes the "phase-shift" sound found in many equalizers. In addition, the reciprocal nature of the 550b enables the user to "undo" what has been done previously with exact precision.

The benefits of the API 550b are most obvious to those who work with EQ on a continuous basis. If major tonal restructuring is required, the extraordinary headroom made possible with API's 2520 Op Amp offers the predictable and warm analog performance, even under duress. With a surprisingly wide range of tonal variations, the 500b is an invaluable and professional audio tool with great flexibility and excellent sonic ability.

Warranty

This item is sold without the Sound Pure 3-year warranty (used / B-stock / dealer-exempt). Coverage terms for items in this category live in Terms & Conditions, Section 27.

Returns

Try Before You Buy: 48-hour audition. Program details →