Chewborg Envelope Filter

From the vendor

Chewborg Envelope Filter

Aug 6, 2023 9 min read

Updated: Aug 2, 2024

Introduction

On my quest through re-imagining effects pedals, I felt like envelope-controlled filters deserved some attention. As it is usual, my motivation comes from the unsatisfaction with existing circuits: from the widespread use of OTAs, with their drawbacks in terms of distortion and noise, and limited availability in some cases; to one-trick pony circuits; to circuits that require matching; circuits whose complexity isn't reflected in the features offered. Ripple is another common issue, at least in my experience with the Mu-Tron Micro V, and I find it very noticeable on a resonant filter. The Mu-Tron III and its improvement, the Q-Tron, get a mention for almost doing things right: LDRs weren't my first choice, and there are still some things worth changing (the Q-Tron fixed the awful input stage of the Mu-Tron). Still, I liked the choice of a State-Variable Filter, which is the filter topology I've ended up using. There are many other possibilities even just between two-pole filters, like Sallen-Key types or Multiple-Feedback, but the ability of the SVF of providing three filter modes while being easily tunable independently from Q and gain are hard to beat.

Another dilemma was in the control element to use; almost everything has been tried to voltage-control analog filters: LDR, JFET, BJT, diodes, PWM, switched capacitors, compander chips... and MOSFETs! While the limitations in terms of distortion of this last option are well known, my previous experience with the PUP was encouraging, and the advantages are just as obvious: when packaged in an IC, CMOS FETs are a cheap and easy source of matched devices. After this bit of background, it's time to introduce the circuit and continue from there.

The circuit

Here's my realized example of an envelope-controlled two-pole SVF with CD4007 FETs as variable resistors. Since only two VCRs are necessary, this classic chip is the perfect choice, and comes with one to spare. As I mentioned, I have some first-hand experience using CMOS chips for VCR applications with the PUP, but more specifically there is one documented use in VCFs already. This last schematic served as inspiration for my own inverting SVF filter, but even without considering the sidechain, there was still a lot to be done to adapt it to single supply, to the measly 9V usually available for stompboxes, and in the lack of a parallel resistor to the FETs, which makes it more of a proof of concept than a working circuit.

The signal path

The first thing in the signal path is, alas, a buffer, which is made necessary for the filter input and for the rectifier, which require both a low impedance source. The SVF topology that follows is a pretty common one, with two N-MOSFETs in parallel to resistors tuning the frequency from 159 Hz to about 10 kHz. The lower limit can be made lower by increasing the parallel resistor, up to 47k, but has been chosen to be limited