Quasar brings 3D spatial audio positioning to the Eurorack world. It is a two-channel audio mixer that uses binaural algorithms. You can seamlessly move sounds around your head, dynamically change the angle, height and distance in real time. There is also a room reverb available. CV-ins allow for external control of parameters.

In a simplified way this module can be seen as a very advanced stereo processor.

Here's how to use the Gain Switch Multi module with the Transistor Ladder Filter, Discrete Cascaded VCA, and Gemini 2412 VCF.

With the Ladder Filter and VCA it allows convenient access to the gain jumpers on the back of the module, so that you can simply turn on and off the increased gain level feature, and add musical overdrive to your sound - this simulates the old Minimoog trick of connecting the headphone out put to the audio input to add some desirable distortion to leads etc. With the Gemini, the module will connect to it's two jumpers which determine the resonance available on each of it's two filters - the normal SEM-style level of resonance, or a much higher level, allowing either or both of the filters to self-oscillate. Also there's a passive 6-way multiple to further make use of it's tiny 2hp Eurorack footprint.

Amazona did a good review of the Doepfer A-126-2 Frequency Shifter. 

Read it here: https://www.amazona.de/test-doepfer-frequency-shifter-a-126-2-eurorack/