The lineage 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, due to the architecture of the recording console, uses an unbalanced input. 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 it 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.
The input XLR is connected to an active balanced circuit. The output XLR is driven from a transformer coupled output and can drive any load from 600 ohms or greater to full output capability. The polarity is such that there is no change from input to output, so it is suitable in studios using either pin 2 or pin 3 as the "hot" connection. In addition, there is a 1/4 inch input connector that interrupts any signal that is present 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 gain or operating level.