circuitbenders - AC-CPL

Quad DC offset removal / AC coupling euro rack module

The AC-CPL module is continuing our apparent policy of making products for a very niche demographic, simply because we needed one in our system and couldn't find anyone else selling them, so we had to design our own!

This module isn't very exciting, but it is a useful tool to have. All it does it remove DC offset voltages from audio frequency signals, and recentres them around zero. We designed this specifically because we had three modules where the VCO's outputs didn't appear to have any kind of AC coupling. This meant that the VCO waveform outputs had a massive DC offset of up to 5v and using them in a eurorack system was extremely problematic with all kind of unwanted distortion and other unexpected effects. Just running them through three channels of this four channel module immediately cleared up a whole lot of issues. The module can be used to remove DC offset in an audio frequency signal from any source.

DC offset can be inadvertently introduced to an audio frequency signal by many different modules and processes. Its often a result of poor circuit design, but sometimes its just unavoidable. Without actually viewing a waveform on a scope or in a sample editor it can be difficult to diagnose a DC offset issue, but It can be effectively removed and the waveform recentred around zero by using a passive high pass filter circuit with the cutoff point set below audio frequencies. Any DC voltage that is either static or moving slower than the cutoff frequency will be effectively blocked and removed.

The AC-CPL module is 2HP wide and as it is passive it needs no power. It has four independent channels of DC offset removal. The unmarked sockets are the inputs, and the sockets with concentric rings surrounding them are the outputs.