Stereo/Dual Mono 550 EQs with Range Switch
The beginnings of the API 5500 circuit can be traced back to the original 550 equalizer designed by Saul Walker, the founder of Automated Processes Inc. The 550 was designed as a console equalizer, which uses an unbalanced input due to the architecture of the recording console. Because the ergonomics of a console dictate that the controls take up little space, sometimes the number of included functions can be limited. The 5500 is specifically designed to address these limitations; it has a balanced input, a true straight-wire bypass, an integrated power supply with noiseless muting and a range control that expands its versatility to mastering applications.
Typical of API products, the 5500 contains no integrated circuits in its signal path. The gain comes from two hand-built 2520 Operational Amplifiers in each channel. The balanced input is handled by a 2510 Discrete Operational Amplifier, which is similar to the 2520, but without the high-current output stage. A key component of the API sound, the 2520 coupled with the API 2503 output transformer, is capable of delivering +30dBm before clipping.
An extremely useful feature in the API 5500 is the range control. The range of the amplitude controls can be reduced to 1/2 or 1/4 of their stated scale, providing a means to adjust the tonal balance with finer resolution in an even gentler manner. This is especially useful for complex program material as contained in stems or submixes and is ideally suited for mastering purposes.
API's 5500 also features a true hard-wire bypass which wires the output connector directly to the input. Shortly after the power is first applied, or immediately after it is lost, a special circuit enables "bypass mode" so that power thumps are never heard.