API 550b Discrete 4-Band EQ — 500 Series

A 500-series compatible 4-band version of the renowned API 550A and inheritor of the classic API 550 EQ designs

The API 550b is a modern design based on the classic API 550 and 550A. The 550b came about when API’s then owner and chief designer, Paul Wolff (1985 – 1999) was commissioned to build 75 550-style EQs for O’Henry Studios (Burbank, CA), where feature film scores and albums by notable artists such as Ray Charles, k.d. lang, The Wallflowers, and R.E.M. were recorded. The studio requested a 4-band version of the 3-band 550-type equalizers that would fit in a normal 550 chassis—and so the API 550b was born. A modern reissue of the original 550 EQ, the 550b incorporates API's exclusive circuitry innovated by API founder Saul Walker, plus proprietary components, such as the legendary API 2520 op amp. Once word got out about the API 550b, it began to outsell the 550A dramatically.

API 550b discrete 4-band EQ unique features:

• Four bands of API signature equalization
• "Proportional Q" narrows filter Q at extremes
• Traditional API fully discrete circuit design
• Re-issue of 1967 API 550 EQ with an extra band

API 550b discrete 4-band EQ key features:

• Each band offers seven frequency centers
• Reciprocal and repeatable filtering
• 12dB of boost/cut per band
• EQ band 1 and 4 offer shelf/peak switching
• High headroom, +30dB clip level

Don’t forget to check out RSPE’s Rack Revolution, a virtual 500-series rack builder that lets you custom-configure the perfect 500-series lunchbox for your studio, DAW, or mobile recording rig. RSPE carries every 500-series module imaginable, including mic preamps, compressor-limiters, equalizers, and 500 power racks.

API 550b Inside The Box

The API 550b has four EQ bands with seven switchable filter frequencies that span four to five octaves. The selectable frequencies are significantly overlapped to provide the 550b with dual functionality as both problem solver and sweetening device. 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, including Paul Wolff himself.

What makes the API 550b unique and musical, apart from the additional mid frequency band is the combination of Saul Walker's incomparable 2520 opamp and “Proportional Q" circuitry. Proportional Q narrows or widens EQ bandwidth depending on the amount of boost or cut applied to a selected frequency. With greater amounts of boost or cut, the bandwidth narrows, allowing for surgical correction without compromising the overall sound. For example, if you wanted to remove muddiness from an instrument, greater amounts of cut to -12dB narrows the bandwidth, removing the unwanted frequency while preserving the body of the instrument. Less gain applied to boost/cut widens the Q for a more musical sweetening effect. Another advantage of API’s Proportional Q is the near-elimination of phase shift, a malady found in many parametric equalizers.

In addition, the reciprocal nature of the 550b enables the user to "undo" what has been done previously with exact precision. With a surprisingly wide range of tonal variations, the 500b is an invaluable and professional audio tool with great flexibility and excellent sonic ability.

API 2520 Op Amp—key to the API sound

API’s 2520 Op Amp found in the 550A is given most of the credit for the API sound. Designed by API engineers, the 2520 is a high-gain, wide-band, direct-coupled amplifier specifically designed for audio applications. The circuit derives its characteristics mainly from the performance of its passive elements, being connected into a feedback loop, which provides predictable, stable performance. Furthermore, the discrete 10-transistor amplifier is encapsulated in a thermally conductive epoxy that protects against thermal shock, vibration, and humidity, assuring long life and reliable performance

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.

API 550b Outside The Box

Operating the API 550b is simplicity itself, which is what makes it so effective as an audio tool, since you can get creative without fighting complicated controls. The API 550b has four dual-concentric, custom API aluminum rotary knobs that select frequency and apply gain. The stepped center knob selects one of seven center frequencies in each band. The outer knob boosts or cuts the selected frequency by ±12dB. The low and high bands are peak/shelf switchable EQ, which are selected by positive-action, military grade toggle switches. The 550b’s low-mid and mid-high bands are peaking type EQ. At the bottom of the front panel is a silent EQ in/out switch that allows EQ to be switched in and out of the signal path silently.

API 550b filter frequency centers:

• Low: 30, 40, 50, 100, 200, 300, 400Hz
• Low-Mid: 75, 150, 180, 240, 500, 700, 1kHz
• Mid-High: 800, 1.5k, 3k, 5k, 8k, 10k, 12.5kHz
• High: 2.5k, 5k, 7k, 10k, 12.5k, 15k, 20kHz

Do or undo, there is no try

The reciprocal nature of the 550b enables the user to “undo” what has been done previously. The benefits of the API 550b are most obvious to those who work with EQ on a continuous basis. The 550b’s ability to affect or reverse tonal modifications is perhaps the most important feature. If major tonal restructuring is required, the extraordinary headroom made possible with API’s 2520 opamp offers predictable and gentle analog performance under duress. With a surprisingly wide range of tonal variations, you can do no better than the API 550b—the force is strong with this one.

API Audio's VPR Alliance

The VPR Alliance is a program of standardization and consistency guidelines for approved manufacturers wishing to design products for API's 500-series rack format. The program provides complete design specifications for manufacturers interested in producing third-party modules that physically fit and electronically conform to API's rack specifications.

The creation of the VPR Alliance was encouraged by the overwhelming popularity of API's 500-series racks, including the 10-space 500V and the six-space lunchbox®, which led to a proliferation of third party modules that fit the API format. Typically installed into API racks, these third party modules had raised issues of warranty and interaction with API-manufactured modules in the same rack.

Because of the variables related to those third party modules, API had previously been forced to declare that inserting any third-party modules into a 500V or lunchbox voided the warranty on that rack. Through the VPR Alliance, API is now able to eliminate confusion as to which third-party products definitively do and do not void the API warranty. The VPR Alliance is a straightforward resolution that saves the company time and money while promoting goodwill to API customers.

API’s customers benefit from the VPR Alliance with the knowledge of exactly which products are approved for placement in API racks; customers have a wider range of module choices for their racks that have been tested by API staff for specification compliance.

An Abbreviated History of API

It all began with a dream. It was 1968. The company: Automated Processes Inc., formed by men with a vision. Best known for their now legendary "2520 amplifier", this unique amp has been and will continue to be the heart of all API discrete products. It continues to provide reliability and sonic purity unmatched by the competition. The resulting console met the needs of the music, commercial and broadcast industry.

API became the leading audio broadcast console manufacturer for radio and television networks and high profile stations. In addition, recording studios, large and small, began using API...and receiving rave reviews from engineers and producers. There are over 700 API consoles across the nation and around the world, including the three major networks. Over thirty years later, many of these consoles are still in daily use in some of the most prestigious recording and broadcast facilities in the world because, to date, there are few, if any consoles of equal sound quality at a cost-effective price.

Why settle for three bands of amazing API EQ when you can have four bands with an API 550B. For more information, call or chat with an RSPE representative today.