Thought I would start a topic to share the custom units I’ve built. None of them are extremely fancy or clever or groundbreaking, but I like them and find them pretty useful. 
These were all built on firmware 0.4.11.
Feedback Keeper: Put this inside a feedback loop along with a reverb or delay, to automatically maintain “safe” infinite sustain. There’s a fixed HPF in case anything within the loop adds a DC offset, an envelope follower inversely controlling a biased VCA, and a further limiter and manually/externally controlled VCA. Because every feedback loop is unique I can make no guarantees – sometimes you’ll need more limiters or attenuation in other places throughout the loop – but I’ve used the technique many times in hardware and successfully used this custom unit in a few patches.
FB Keeper.unit (7.1 KB)
Complex Osc: this is a simple ER-301 take on the style of complex oscillator I prefer to work with, with options for both linear and exponential FM, AM, automatic tracking between the two oscillators with the offset exposed at the top level, and a feedback control for other timbral variation. (I considered a sine-triangle crossfade, but in practice I work mostly with sines anyway and I wanted to keep this a simple “complex” oscillator 
You can easily dig in and substitute the aliasing triangle or something else in place of a sine oscillator, of course.) Exponential FM does not generally combine well with linear or dynamically changing the depth, and you have to match depth and frequency if you want it to be harmonically nice – but it can give somewhat different results and I think is worth playing with.
Complex Osc.unit (21.2 KB)
LPG: it’s got some quirks, and is possibly not the most natural of gates but I kind of like its characteristics. The envelope is a Skewed Sine that feeds back into its own level for an interesting shape, but honestly I assign an external envelope or a gated ADSR as often as not. The business end is just a simple low-Q ladder LPF and VCA in series.
LPG.unit (12.5 KB)
Crossfade: simply enough, a 2-Band unit (not a Custom Unit preset) with a mix control that opens one VCA while closing another. I used it in the Wavefolder unit.
xfade.unit (4.7 KB)
Wavefolder: based on a sample scanner with a single-cycle sine, plus the plumbing to control the folding mix level as well as intensity and “symmetry” (sample scanner width). It will expect the file \samples\Adventure Kid\AKWF_bw_perfectwaves\AKWF_sin.wav to be on the card, but you can redirect it to any single-cycle sine you happen to have, or replace the sample for different folding character. I like it as it is though, and it sounds pretty nice after the LPG with no need to modulate it (as non-traditional as that may be). But one could also dig into the modulation osc from the complex oscillator and assign that to fold amount – or to go much crazier, the “symmetry” control.
Wavefolder.unit (13.9 KB)
Wavefolder 2: this is an improved version. Instead of a sine, it uses a hand-drawn approximation of the transfer function of the middle stage of a Serge wavefolder. It’s probably a little bit disturbed by the crossfade setting I used, but that helps stop really harsh clipping (in a “real” circuit that function would not wrap around as it does here) There are two stages, because that sounded good to my ears, but you can copy and paste the Sample Scanner if you want to try more. I also merged the mix and fold controls into a single one for convenience so that no settings on it are silent.
Wavefolder2.unit (15.1 KB)
The required /ER-301/samples/serge_stage.wav
