October 11, 2004
After 20 years, the API 550A discrete, three-band equalizer will start rolling off the production line.
API's Saul Walker designed the 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 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.
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 15kHzbandpass filter.
A feature 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.
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