Continuous +/- filter overdrive for smr4

I was playing around on a friend’s elektron analog 4 and I liked being able to tweak the filter overdrive with positive and negative values, and was wondering if I could achieve something similar with the shruthi.
My first guess would be to put an attenuverter in a feedback loop for the filter, but I’m not sure if this is all that’s needed or if one would need to include some kind of clipping circuitry as well…
Also thinking about tying this in with a few SDE mods, would love to hear thoughts.

Also I have a bunch of LME49740na and LME386 DIP chips lying around, would either of these be suitable for an attenuverter at shruthi levels/voltages? Would love to know some outline of the schematic/implementation

:slight_smile:

ah I just did a bit of reading and it seems simple feedback will just alter the resonance of the filter…

so then… any ideas for implementing continuous positive/negative filter drive on the shruthi smr4ii board? similar to analog4

You could experiment with replacing R13 with a pot and a resistor in series. Maybe 1k resistor and 100k pot.

shiftr’s suggestion wouldn’t really work, because the signal will hit the op-amp rails and clip. You don’t want that to happen - instead, you want the distortion to occur when the signal hits the first OTA. OTA distortion is more interesting than op-amp clipping!

You can replace R36 by a low resistor (say 2k) in series with a 10k pot.

would this give the positive/negative behaviour I’m after or would this be more similar to the SDE output drive mod?
I’ll see if I can find a video to show what I mean

I don’t own an analog 4, but I suspect that the 0 setting correspond to a “nominal” level with moderate drive, the - values correspond to less drive and thus a more linear behaviour ; and the + values overdrive the filter.

In any case, this is completely different from the SDE output drive: the mod I describe applies the distortion before the filter cascade, so any added harmonic will be processed by the filter.

ah elektron have a handy signal flow diagram in the a4 manual, and it looks like the overdrive is not part of the filter architecture… oops
it’s between the 4 pole lpf and the 2 pole svf.
Overdrive is right next to the filter params in the menu so I thought it was filter drive…

Interesting behaviour though because the signal is cleanest at zero, and has two different flavours of drive for negative and positive values.
IIRC negative drive was more bass-heavy and uhh… wooly? while positive brought in more midrange and brought out the filter resonance.

Thanks Olivier, I’ll try out your suggestion

On the A4, the 0 ovedrive setting gives a clean signal, + values gives you hard clipping, - values give you a somewhat softer overdrive.

The overdrive is hardwired after the ladder filter, but before the multimode filter.

You can drive the ladder filter by setting osc levels above 100.