API Audio 550A 500 Series EQ, from API

The 550A Saul Walker Edition

The unit is identical in circuit design and parts complement to the current re-issue version, with the addition of a special custom face-plate. The unit will also feature the signature of its designer, Mr. Saul Walker, founder of API. The Saul Walker 550A will be built in small quantities, with limited supply. The 550A Saul Walker Edition: MSRP: $2, 195

The 500A Production Model

This new unit features the original 550A circuit design, maintaining the integrity of the circuits and the sound, while replacing the hand wired, resistor pack switches used in the re-issue version with the 6 pole boost/cut switch we have used for many years in the 550b and the 7600.

The 550A Production Model: MSRP: $1,195

- 3 bands of classic API equalization
- Each band offers 5 API selected frequency centers
- Reciprocal and repeatable filtering
- maximum 12 dB of boost/cut per band
- EQ Band 1 and 3 offer shelf/peak switching
- "Proportional Q" narrows filter Q at extremes
- Traditional API fully discrete circuit design
- High headroom +30 dB clip level

Jessup, MD: The 117th AES Show in San Francisco is the time and place for the long-requested re-issue of the API 550A discrete, three-band equalizer.

API's legendary Saul Walker designed the "timeless" 550A in the late 60s. Originally a modular OEM equalizer, the 550A quickly found it's way into many custom console designs by Frank DeMedio and other leading engineers, many of which are still in use today. This often-copied but never duplicated equalizer became API's standard channel module EQ when they started manufacturing consoles in 1971.

Providing a true replica of the famed API 550A equalizer was no small undertaking for API. Jeffrey Bork, head API engineer, comments: "We've gone to great lengths to ensure that the new 550As are exactly (and I mean exactly) the same as the original. For starters, API constructed exact duplicates of the metal work and circuit boards from the original artwork. We made silk screens from the same original artwork for the new front panels. The 550A uses the same output transformer that API has used throughout the years. And to keep everything exactly the same, we went to the original manufacturer for the exact same rotary switches and the same Schadow switches for the bypass. What you end up with is an exact replica even down to the gauge of the wiring and the harnessing- even the original documentation is included. Everything is so exact that people with older 550A EQs will now be able to buy matching units or purchase parts to refurbish and fix their older units."

"It's basically the next production run of API units from the early eighties. And its worth mentioning that we never even considered trying to make the 550A 'better,'" added Bork. "There just wasn't any need to do that. The industry holds the 550A in pretty high regard and wanted it "the same as it ever was." It's been an honor, and a tremendous amount of fun, leading up this project."

The 550A uses bridge-T filters (RC networks), two of API's 2520 discrete amplifier modules, a 1:3 output transformer, and provides up to +28dBm output level. All three of the frequency bands overlap and there is up to +/-12dB of boost or cut in 2dB steps available for each band. There are choices of shelving and peaking for the Hi and Lo Band and a switchable, 12dB/octave, 50Hz to 15kHz bandpass filter.

A very desirable feature and so important to the classic sound is the proportional Q design where the bandwidth (or Q) of the equalizer narrows as boost or cut is increased. The Q is three octaves wide at 2dB of boost/cut and narrows down to one octave with 12dB of boost/cut.