sfcs: lpg mixer

About

Do you like Buchla synthesizers? Do you like the famous quad low pass gate module with the nice nice vactrol ringing it has? Of course you do. Well, you came to the right place - this one module is a ~~much crappier, almost passive and barely similar~~ close analog to the Buchla quad low pass gate!

Jokes away, this is a very simple, audio-aimed module that has 4 separate low pass gates. Each has a classic in-cv-out, as well as an additional tone switch (switches between dull and bright response) and a super original SYNTHFOX-proprietary Ping input (seriously though, i spent a few hours figuring out the circuit and the values), which does exactly what we all love and know.

Additionally, i make use of the normalise leg of the output 3.5mm jack, making the 4 separate LPGs into an LPG mixer. I made it so that if nothing is patched, then channel A is added to B, channel C is added to D, and output B (which is A+B if not patched) is also added to B. This makes it usable in different configurations: two 2-channel mixers (output A+B at channel B and C+D at channel D), one 2-channel stereo mixer (use some mults, and then B is left out and D is right out. Stereo pair input are A/C and B/D), or, naturally, a 4-channel mono mixer. It always is possible to take just one of the gates for your own needs and the remaining 3 can still form a mixer.

This is an extremely easy to make device, and if you want easy dynamics control, this is for you. Imperfect design warning, though! Each tone switch will affect the overall connected cluster of LPGs rather that only the one it's tied to, except if the LPG is used separately. The outputs can be re-summed via an active summator to make up for this. I don't find this much of an issue, though, as i'm not planning to always use it as a 4-channel mixer, or rather as a flexible LPG bank.

Also: i was bored, powerless and wanted to make something really cursed when i was building this. So it's almost entirely point to point. It holds on well, but if i were to squish it, it would be no more. Don't be like me. Better do it all on a veroboard.

Schematic

This is your classic passive LPG, but better. The bare vactrol resistance and fixed capacitor LPF design stays in place and is the same as with a ton of other (often passive) low pass gates. However, the CV input section is different! There's the cool subcirtuit to extract a pinging pluck from gate signals, a CV input and the initial level pot, all summed up via a non-inverting amplifier with gain of about 3. The mixing action is purely passive, though, and only works because even when vactrol is fully open, it has about 10K of resistance still, so the vactrols connected via the unpatched jacks (or just connected, if you only want the mix output), form a simple passive mixer, except the resistors' values are CVable. This explains the capacitor stack-up imperfection: when the vactrol outputs are connected, the capacitors to the ground get connected, too. This makes them add up and form a bigger capacitance, making the overall sound duller.

Media

One LPG used to modulate the FM amount from one half of the 40106 Dual Oscillator to the other, other three are mixing the second half of the 40106 osc (pinged), a half of VC sampler playing a steady tone (faded in by an LFO) and the quad VC resonator processing another VC sampler half (stepped random volume).

All 4 LPGs getting pinged by the different Shift Core Generator's gate outputs, gating various sound sources.

Shift core generator pinging the LPGs. Each LPG is processing one of the 4 outputs of the quad resonator. The resonators are all processing the same FM pair made up from the 40106 Dual Oscillator, which is also being pitch modulated by the SCG. The output is in stereo: one side is A+B and other side is C+D of the LPG Mixer. This makes for some wicked stereo!

Pictures

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