The AF-4 is a virtual-analog lowpass filter module for Reason, based on the well-known transistor ladder design.

In order to obtain highest quality, this lowpass filter employs a topology-preserving, zero-delay feedback network with 4x oversampling, yielding results comparable to industrial circuit simulators. As a result, all prominent features of the analog prototype are preserved- the filter can be driven into a warm, musical saturation, as well as into self-oscillation. Additionally, the resonant tuning is correct across the entire frequency range.

The filter can be toggled between a 12dB/oct and 24dB/oct lowpass response, and supports both mono and stereo processing.

Full integration into the Reason environment and workflow is provided by the CV Inputs on the backside, allowing to control all front panel parameters.