The T4BX Photocell is an optical coupler significantly responsible for the sound of the 2LA compressor. It operates on the principle of optical negative feedback, which reduces audio amplification. Simply put: larger input signal, more light in the photocell. More light, deeper gain reduction of the audio track. This physical phenomenon may seem simple, but the optimal projection of the time constants and reduction levels is not that easy. These are the parameters you will hear the most. Minor changes to the attack and release influence the unique sound features of the device.