Model Phi – Phaser

Inspired by a recent modwiggler conversation about the somewhat obscure Symterix Phase Filter, I became interested in toying around with a SSI2140-based setup. There are great Eurorack phasers, for the 2140 too, and Neil Johnson has some great SSI2164 phaser design, yet curiosity about the specific Symetrix take got the better of me. So, below, you find an in-progress report on what’s going to be the Model PHI.

First Sounds – toying around with mix out, L/R outs, and basic CV modulation:

Module Description

This is a dual phaser on a 16HP module with manual as well as voltage control over various parameters. There are two parallel 4-stage phasers. The Symetrix had 8 in series, but after testing, I like this better for stereo work. Both parallel phasers share one input, one resonance control (with cv over resonance as well), several modulation sources (manual, two inbuilt LFO with Speed CV, and external CV), all of which pass attenuverters. The latter means you can determine for each modulation source if and how much of a positive or negative effect it has on the phaser frequencies. The coolest feature is probably the “spread” knob, by means of which the modulation intensity between both phasers can be tilted. When using the mono MAIN out, this results in one phaser modulating more, one less. When using L/R outs, however, this makes for wonderful stereo effects. The MAIN out itself is a simple mix of the input signal and the two 4-stage phasers. The extra feature here is that MAIN out passes a dedicated VCA, so you can, for instance this phaser like a filter + VCA module. If no CV s plugged, the MAIN signal just passes through. All CV inputs respond to voltages between zero and +5 volts.

As for components, nothing special is used: two SSI2140, two LM311 (used to turn the each QVCA of the 2140s into an voltage controlled LFO – see Figure 21 of the 2140 datasheet), an LM1370o for CV-able feedback and depth duties, and a couple of TL072 OPAs.

The Module is a sandwich construction with a front PCB hosting controls and a rear PCB hosting most of the sound processing and power supply

Preliminary Schematic

Although this version is up and running, I still want to make some changes, so read this as preliminary only.