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Some details about SoftDrive GV

AXP's picture

Some details about SoftDrive GV

This is a repost of a comment I made a while ago. Stumbled upon it now and decided it's worth keeping here.

Pickasso> I don't understand the LED clipper. (I know it sounds killer, I prefer it active) Is that part of the Guv'nor pedal's circuit? What would happen if you were to apply 30v dc to the actual hardware pedal? I imagine all sorts of components melting....

AXP> The Guv'nor has 3 points where nonlinear distortion can occur. They are the two Op-amps and a pair of reverse-parallel LEDs connected to the ground in front of the tone stack. Some people might think that LEDs are the only part responsible for distortion in this pedal, but the op-amp gain is so huge the signal gets clipped by supply rails (roughly 0 and 9v by default). The tricky part is that those LEDs work together with tone stack and can't be separated from it without losing finer details in the sound. It makes this stage both non-linear, frequency-dependent, and time-variant! It was a challenge to model it in real-time but it's there and no sacrifices were made on the way. I've decided to make LEDs switchable because they affect the overall distortion character a lot. The complete gain structure is like this (this is very simplified!):

Gain ->Opamp clip (0..9v) -> Gain -> Opamp clip (0..9v) -> LED clip (-1.8..1.8v)

The higher the supply voltage, the more headroom op-amps have and the more they overdrive LEDs. So the power supply voltage effectively controls the balance between op-amp and LED distortion. There is an "Overdrive Stages" indicator in the plugin's interface just for that.

The real-world op-amps would likely take 30v dc well, however electrolytic caps are probably rated for lower voltages and would blow up. I was tempted to make the upper voltage bound to something like 220v but it was useless sound-wise. The whole DSP model is parametric, i.e. there are no precomputed look-up tables. It would be stupid not to make at least some of the parameters adjustable in the final plugin.

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