The beginnings of the 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 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.

The 5500's input XLR is connected to an active balanced circuit. The unit's output XLR is driven from a transformer coupled output and can drive any load from 600 ohms or more to full output capability. There is no change from input to output in the unit's polarity, so it is suitable in studios using either pin 2 or pin 3 as the "hot" connection. Additionally, the 5500 has a 1/4 inch input connector that interrupts any present signal on the input XLR. It is balanced and can be driven from either balanced or unbalanced sources. A positive signal on the tip will deliver a positive signal on pin 2 of the output XLR. Using the 1/4 inch input does not bypass any internal circuitry and does not change the gain or operating levels.

The 5500 features API's industry exclusive five-year parts warranty and exhibits the reliability and long life characteristic of all API products.