API AUDIO 5500 Dual Equalizer 2-Channel EQ

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. That three-band EQ was originally used in the earliest API consoles built in the 1960s. Engineers were immediately captivated by its fully reciprocal boost/cut characteristic and its proportional Q design, one of API’s many industry firsts.

The 5500 is specifically designed to continue the tradition of API’s precision equalization. It features 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.

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, and the balanced input is handled by a 2510 discrete operational amplifier.

Besides being a key component oft he API sound, the 2520 coupled with the API 2503 output transformer is capable of delivering +32 dBu before clipping. With this much headroom, it is unlikely that the 5500 can be driven to distortion.

The Twin T topology is found in many equalizer circuits, but the legendary Saul Walker included a novel Proportional Q section that makes it truly musical. Proportional Q works by spreading the equalizer’s action over a broad bandwidth at low degrees of boost or cut. This provides delicate shading and subtle coloring of the signal’s tone, without leaving the impression that signal processing was applied.

As the amplitude is advanced to more extreme settings, the equalizer’s bandwidth narrows to provide the user with surgical precision. This allows the equalizer to be useful in removing undesirable noises or radically emphasizing an aspect of an instrument’s character, such as its attack.

Before the 1960s, few equalizers had the same characteristics in boost mode as in cut mode. Today, it is common for equalizers to be reciprocal, but again, Saul Walkers design is special.

When switching an API EQ from boost to cut, the exact same components are used, reorganized around the amplifier for the appropriate function. This guarantees 100% reciprocity. The 5500 Dual Equalizer API 7 benefit becomes apparent when a track is re-equalized to remove the previously applied equalization. The engineer can be confident that, because of the 5500’s fully reciprocal design, a truly flat frequency response will be restored.

The 5500 features a true hardwired bypass. In this mode, the output connector is wired directly to the input. Shortly after power is first applied, or immediately after it is lost, a special circuit forces this mode, so signal is never lost and power thumps are never heard. The bypass button illuminates in red when the 5500 is in bypass mode, whenever either the special circuit or the operator put it into bypass.