Tag: Phase 90

I’ve recently connected with Dillon Sharlet, who is the main author of the open source LiveSpice software. Our approaches to modelling are quite similar, we’ve both written our own CAS (computer algebra system) to symbolically solve the non-linear system of equations, and then generate code that is compiled to run the circuit in realtime. The main focus for Dillon is having a fast turnaround time between editing the circuit and being able to run it using a just-in-time compiler, at the cost of taking extra CPU to run. Cytomic focuses on taking longer to generate the code, which uses an offline C++ compiler with SIMD support to make the end simulation as efficient as possible for products, but takes longer to turn the circuit into something you can run and process audio through.

In our discussions we used the TS808 circuit as a baseline to try different things out, and we have both improved upon our previous solving methods. I wanted to get another circuit to try out, so decided to have a go at the one Dillon said LiveSpice was the least efficient at solving, the MXR Phase 90. It’s a phaser circuit, and uses JFETs as voltage controlled resistors to change the cutoff frequency of 4 all-pass filters in series. The JFETs also add considerable distortion to the signal, which sounds great. For a rundown on the circuit head over to ElectroSmash.

I always like to hear what happens when you push the input signal really loud into a circuit, and in this case the input signal saturated the JFETs until it leaked into the Gate of the JFET, which is where the LFO signal is connected to wobble the phaser. Although it was interesting having the LFO wobble differently I also wanted to just listen to how it would sound if that didn’t happen. I removed the parasitic pn-junction diodes from the JFET model and had a listen, and it sounded much better to me, you could drive the signal into warm clipping without interrupting the slow wobble of the LFO. Normal guitar signals don’t get this loud, but in plugin land gain staging is easy, so why not?

Here are a few audio examples of my Phase 90 model. I don’t have an analog pedal to compare against to match things properly, but it’s already sounding great to me. Here is how it sounds on some classic washy minor key electric guitar chords:

And here are a few examples on a 303 sample courtesy of the great people at Acid Voice: