PUP: a host of Perfectly Usable Phasers

From the vendor

PUP: a host of Perfectly Usable Phasers

Oct 22, 2022 15 min read

Updated: May 13, 2023

I’ve always liked phasers, in fact one of my first pedals I’ve ever built was a Phase 45 clone. Not only I liked how it sounded, another main reason for choosing it (other than the complexity) was that I only needed two matched JFETs instead of, for example, four for a four-stage phaser.

I’ve been sitting there, content with my box, for some years, until recently I’ve heard of the Project 447 phaser on an old ETI magazine. It wasn’t perfect by most accounts (the long string of 741 op-amps in series with the signal probably didn’t help noise-wise), but it exploited a CD4049 unbuffered hex inverter as the source of Voltage Controlled Resistors, with the idea behind it that FETs on the same die will be practically matched by nature. Specifically, the ETI phaser uses the NMOS devices: the PMOS are in parallel, since the supply rails are shorted, but they won’t conduct since the gates are biased up from the source voltage.

This amazed me. Not only I hadn’t thought before of using the MOSFETs inside a CMOS inverter this way, but I was happy to find a working example of MOS as VCR in a phaser, something I had contemplated before, since their use as cheaper VCR than JFETs is not unheard of, especially in synth projects, but which I had some doubts about because of the presence of the body diode which strongly harms linearity with anything other than small signals.

Before I knew it, I discovered even more phasers using the same principle, one being the Jen KPS-900 and another being the EHX Bad Stone, a circuit unjustly shadowed by the similar-named Small Stone, but with a beautiful schematic I suggest analyzing, once you can bring some order into it. The former uses the PMOS in the CD4007, the latter the NMOS in the oddball inverter CD4009, but the principle is the same.

Next, I even found a nice series of articles on EDN about FET VCRs, analyzing and demonstrating JFET and MOSFET equally in different applications, including a phaser effect (there’s 5 parts to the series, make sure to use the search function to find them all!).

This is to say that, while the idea isn't certainly new, it's surely a bit neglected, and I hope that in doing this I'm sharing a way for the DIY enthusiast to make cheap and easy to build phasers or other effects requiring multiple VCRs: this was a long journey made up of uncharted territories and violated assumptions, but it brought its results in the end.

The basic principle

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The above pictures show the internal schematic of a 4069 section, as shown in the datasheets, and what it turns into when wired as in the following circuits: leaving Vdd disconnected excludes both the PMOS and a protection diode, while the other is welcome to stay. The bulk terminals are connected the same as shown in the datasheet. The inverter inputs are the gate terminals, the output is the drain connection, Vss the common connection to all sources when