The 5500 circuit has is origins in the classic original 550 EQ but with added functionally and a range of controls that expands its versatility to mastering applications.
Like all 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. Besides being a key component of the API sound, the 2520 coupled with the API 2503 output transformer is capable of delivering +30dBm before clipping. With this much headroom, it is unlikely that the 5500 can be driven to distortion unless perhaps another API unit is driving the input!
A new feature in the 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 of adjusting the tonal balance with finer resolution in an even gentler manner. This should be especially useful for complex program material as contained in stems or submixes, and is ideally suited for mastering purposes.
Also new on the 5500 is a true hard-wire bypass. In this mode the output connector is wired directly to the input. Shortly after the power is first applied, or immediately after it is lost, a special circuit enables bypass mode so signal is never lost and power thumps are never heard.