According to wikipedia, A Phaser is a “an electronic sound processor used to filter a signal by creating a series of peaks and troughs in the frequency spectrum. The position of the peaks and troughs of the waveform being affected is typically modulated so that they vary over time, creating a sweeping effect. For this purpose, phasers usually include a low-frequency oscillator.” This sounds like a lot of fun.
There are plenty of those pesky boxes available for guitar and hard-ware multi FX processors also tend to have one around, there are a few in the modular realm. I have one in my Intellijel Polaris, but while the filter is quite good – although lacking a bit in character – the phaser is rather dull and uninspiring IMHO. So I dug a bit through the wealth of schematics available for guitar pedals to get some ideas, and seems there is plenty out there. Thanks Google.
In the end I settled for a 4-stage phaser loosely based on the MXR 90 phase guitar pedal. More stages would give a more profound phase character though, but 4 will already do the job and cuts back on the complexity of the circuit. I still had some home baked vactrols around from my VCF projects, so I decided to go with these instead of the mosfets used in most circuits because, for some reason, whatever I do, I never get the hang of these mosfet things. I didn’t felt like going down that rabbit hole so I stuck with vactrols and all their unreliability and unpredictability, hoping it would add some fun to the whole phaser thing.
I’ve added the simple LFO as a modulation source, which can be overridden by a CV input if desired, so you can have a lot more variety in modulation, but a phaser without an LFO simply isn’t a phaser.
I’ve also added a feedback circuit with 2 opposing diodes to add some ‘crunch’ to the whole lot. The whole thing behaves rather erratically so I’ve added a trimmer to tame the feedback. From my tests a 8.2k resistor was ideal to get the feedback going without it galloping off into total mayhem – not the metal band, although the sound is similar. A 10k trimmer and a 4.7k resistor should do the trick to find that sweet spot so things don’t explode when opening up RV1 completely.
You should aim the offset trimmer at about 6V. RV4 will adjust the working range. Getting the whole thing right is kind of an exercise in patience and persistence as it involves a bit of going back and forth. Getting those 4 vactrols to play ball takes a bit of elbow grease, but in the end, well it works.
Don’t really expect a clean phaser. You’ll get the notches in the spectrum and can move them around, so there’s that. That’s what phaser are intended to do. But it’s nowhere as clean as the phasers you’ll find in your nice multi-effect rack with the fancy backlit display. It wasn’t really the intention either though so I consider it a feature.
I’d like to build a version with some more stages, but we’ll see about that. Who knows, maybe I join some of the remaining PCB boards together and see what that does.