Bionic Lester

HM1873: Dual 12db/oct switched capacitor multimode filter with mode selection and clock disruption

"Knowledge rests not upon truth alone but upon error also."

Bionic Lester is a dual multimode filter capable of many operational modes. Each side has dedicated lowpass and bandpass outputs, and the third output for each side can be switched between highpass, allpass, or notch. The input amplifiers for each filter are scaled to allow some distortion. The character of this distortion changes with the mode setting.

Bionic Lester has some normalled signal paths for convenient patching. The sum of the first channel's manual cutoff control and CV input are normalled to the CV input for the second channel. This allows you to control both filter sections with a single hand (or CV), useful for dual-peak or allpass phasing applications. The audio input for the first channel is also normalled to the second to facilitate parallel filtering. The filter cutoffs are determined by dividing down a high-frequency clock generator.

For the second channel, you can choose either the first or second channel's generator as a clock source. If you select both clock generators, absolutely horrifying audio garbage will result. A pushbutton control for selecting the desired amount of aliasing (minimum or maximum) applies to both sections. Resonance is voltage-controllable.

Because Lester is a switched-capacitor filter, the user must be warned of some of its characteristics. The aliasing, third output mode selection, and resonance settings are shared between both units. At lower cutoff frequencies, some aliasing may be present in the output signal depending on your resonance, aliasing, and mode settings, as well as the type of input signal. Bionic Lester does not self-oscillate using the onboard resonance control.

Expander

An expander unit will be released soon. It is a 2x3 bipolar mixer (with mutes) that invisibly attaches to Bionic Lester for patchcord-free mixing of the six outputs, with dual VCA feedback paths back into the filter sections. It also functions standalone as a performance mixer, with front-panel knobs and jacks that are also useful for injecting external signals and effects into the Lester core.

Overview

Features:

Twin multimode filter
Dedicated LP (12db) and BP outputs on each side
Third output switchable from HP/allpass/notch (setting shared on both sides)
Q setting shared between both sides
Filter clock for channel A can optionally control channel B as well (for identical cutoff frequency on both sides). You can also combine both clock signals at channel B for some absolutely horrifying effects.
Aliasing at low cutoff frequencies can be optionally exaggerated.
Mode switching also causes the character of the distortion to change. There are a bunch of sounds in this thing.

So, imagine a filter companion to the Hertz Donut and you have the right idea. I never built this thing because I was a student back then and couldn't afford to get a quantity of these chips, and I wasn't even sure if it would sound good. I noticed that the chips were available, more or less, so I dusted off the design and added my usual buttons-and-LEDs interface design language. Fortunately it sounds great if you like the rest of the Harvestman designs. The disadvantages are that there's a touch of aliasing at lower cutoff frequencies (depending on the characteristics of the input signal and your selected modes), and it doesn't self oscillate.

I've also designed an expansion output mixer/distributor that does everything you'd expect in order to mix the outputs and feed them back into the filter, with some VCAs thrown in for good measure. Also runs standalone.

power tested in 2021 with a joranalogue test3 under heavy cv control and as many things lit up as possible...