API 550A 500 Series Discrete 3-Band EQ

A long-awaited reissue EQ that has had such a part in the history of recording is continuing to make history in today's music

There’s a reason why platinum engineers back in the ’70s, ’80s, and ’90s (and today) would categorically state, “I won’t mix without my API 550 EQ.” Only a handful of equalizers (assuming you have really big hands) have achieved the legendary status of the API 550A. The also legendary Saul Walker designed the 550A in the late ‘60s before the days of commercial consoles. The discrete API 550A was first used as a modular equalizer for custom consoles in the days when recording engineers spec’d out the designs for their studio’s desk. In 1971, the 550A became API's standard channel-module EQ when the company began manufacturing consoles—and with virtually all units in existence already spoken for, popular demand prompted API to resume production in 2004. The 500-series API 550A reissue brings the coveted sound of API equalization to both professional and home studios alike.

API 550A unique features:

• Proportional Q narrows filter bandwidth at extremes
• Proprietary API fully discrete circuit design

API 550A key features:

• 3 bands of classic API equalization
• Each band offers 7 API-selected frequency centers
• Reciprocal and repeatable filtering
• Maximum 12dB boost/cut per bands
• EQ bands 1 and 3 offer shelf/peak switching
• High headroom +30 dB clip level

The sound of the API 550A spread like wildfire in the industry. Custom consoles using API modules, including the 550AEQ module, were built for recording facilities such as CBS in-house studios, Fox sound stages, Sunset Sound, and Hollywood Sound Recorders—not to mention the Dal-Con at Oceanway, to which Alan Sides added a bank of 40 API 550A EQ modules. Many of these consoles are still in use today, including the two custom API consoles designed by Frank De Medio, Eric Benton, and Deane Jensen at Sunset Sound.

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 550A — Inside The Box

The combination of Saul Walker's incomparable 2520 op amp and “Proportional Q" circuitry is what makes the API 550A unique and musical. 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.

The 550A provides reciprocal equalization at 21 points in five steps of boost to a maximum of 12dB gain at each point. The fifteen equalization points are divided into three overlapping ranges. The high and low frequency ranges are individually selectable as either peaking or shelving, and a band-pass filter may be inserted independently of all other selected equalization settings. Three dual-concentric switches select frequency ranges and boost/cut, and a pushbutton "in" switch allows the EQ to be silently introduced to the signal path. A small toggle switch is used to insert the band-pass filter into the 550A.

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.

API 550A — Outside The Box

The front panel of the API 550A has three dual-concentric rotary knobs for high frequency, midrange, and low-frequency bands. The API custom aluminum, center-frequency selector knobs each have seven selections for each band. The inner knobs are stepped gain control ranging from -12dB to +12dB in 2dB increments. Below the rotary controls are a band-pass filter on/off toggle switch (-3dB down at 50Hz and 15kHz) and LED indicator, plus three buttons for LF shelving or peaking EQ, HF shelving or peaking EQ, and silent in/out button. The 550A fits into the API Lunchbox, 500VPR frame, and API 1608 console slots.

API 550A controls:

• Dual Concentric Rotary: HF select: 2.5k, 5k, 7k, 10k, 12.5k, 15k, 20kHz and stepped degree of equalization –12 dB to + 12 dB
• Dual Concentric Rotary: MF select: 200, 400, 600, 800, 1.5k, 3k, 5kHz and stepped degree of equalization –12dB to +12 dB
• Dual Concentric Rotary: LF select: 30, 40, 50, 100, 200, 300, 400Hz and stepped degree of equalization –12dB to +12 dB

API 550A in use

The 550A is especially popular fattening up snare drum while emphasizing attack and boosting high-end clarity. To bring out the sound of a snare’s shell, boost the 200Hz peak by 4dB on the low band. Boosting 1.5kHZ in the mid band by 6dB will bring out the “crack” of the snare, and a 4dB boost at 10kHz will improve clarity.

The 550A is especially good on electric or acoustic guitar tracks, a 2dB boost at 200Hz widens the bandwidth to round out the body, while the same boost at 5kHz adds presence. A +4dB boost at 10kHz brings out the “air.” While your mileage may vary, this combination of settings will sound good almost all the time, plus it brings the guitar forward in the mix.

One of the favorite tricks of Grammy winner Vance Powell, chief engineer at Blackbird Studios, is to use the 550A to accentuate the midrange in bass guitar and help it sit better in the mix. To bring out the “Powell growl,” Vance sets the API 550A’s middle band to boost a full +12dB at .8kHz (800Hz).

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.

Forty years later, the 550A remains the standard against which other EQs are measured, and has played a major role in the recording industry for decades. For more information, call or chat with an RSPE representative today.