Under the hood of Chow Phaser

The original Compact Phasing ‘A’ made by Gerd Schulte Electronik Audio (left) and Chow Phaser plugin (right)

The Schulte Phaser

The original Schulte phaser is a pretty rare effect but with a somewhat large cult status, owing to its use by several notable artists including Kraftwerk, Pink Floyd, Tangerine Dream, and more. While I won’t do a full circuit analysis here, I will try to explain the overall function of the phaser at a higher level. The general signal processing structure of the Schulte phaser is shown below:

LFO

The typical digital phaser might use an LFO made up of a pure sine wave, square wave or other standard digital waveform. While the Schulte phaser does use a sine-like oscillator circuit, the way this oscillator affects the allpass sections is a little bit more complex. The Schulte LFO circuit is used to drive a light bulb, which then affects the resistance of a light-dependent resistor (LDR) present in each allpass section. LDR tend to exhibit an interesting nonlinear characteristic, of the form:

Feedback Stage

While the Schulte phaser uses some of the same allpass sections for the feedback and modulation processing, separating these two processes can be convenient in the digital implementation. With that in mind, I developed a dedicated feedback stage with the architecture shown below:

Modulation

With the feedback stage separated out, we can now develop a dedicated modulation stage made up of a chain of allpass sections modulated by the phaser LFO. While the original circuit uses a chain of 8 allpass sections, in the digital realm we are free to change the number of allpass sections in real-time, even allowing the user to fade between the number of stages continuously.

Putting It All Together

In the end, all these components need to be brought together as a cohesive, musical effect. With that in mind, I’ve made a mono version of the effect with a similar signal flow to the original Schulte phaser, as well as a stereo version, with a dedicated LFO, feedback stage, and modulation stage for each channel. There are also “drive”, “thrash”, and “dirt” controls that affect the nonlinearities present in the feedback stage. A video demo of the effect can be seen on YouTube. To download the plugin or checkout the source code, check out the ChowPhaser GitHub repository.

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