MXR Envelope Filter variable range mod

[TrashCandy]

Hi all,

I just finished building the MXR Envelope Filter on a pcb from EPK. I have changed some values and implemented the emphasis mod, sweep mod, release mod, and the range mod.

I think the range mod is a bit limited and was wondering if it would be possible to have a dual gang pot to gradually change the value of both filter caps. I would like to do this by connecting the input of the filter to the wiper of the pot so I can gradually change from a 560pf cap to a 2200pf cap which I'll connect to lug 1 and 3 of the pot. Can anybody confirm if this will work?

- Thom

[TrashCandy]

I found a picture by Dano from Beavis Audio Research that visualizes the idea I have.

[Mark Hammer]

It's an interesting idea, I suppose, but I'm not sure that continuous control of range is either necessary, or best done via the means shown.  The EF uses the lowpass output of a state variable filter.

Note that design uses switched resistance to sweep the filter, essentially mimicking the function of the two LDRs in a Mu-Tron/Neutron/Q-Tron.  It does so by forcing the duty cycle (pulse width) of a HF clock to change.  When the duty cycle stays positive/high longer, keeping the switches on, that is treated as a smaller/lower average resistance in series with each of those 47k resistors.  As the envelope decays, the duty cycle drops back down, with the CMOS switches in their "off" setting for a longer period of any unit of time, functioning as a higher average series resistance.

In theory, if one had a means to adjust the "starting" duty cycle of the clock, that would set the resting/default corner frequency of the filter - the "starting point of the sweep". I have no idea just how much it could be adjusted.

As I've blathered on about before, downward sweep is not simply the inverse of upward sweep.  So, for my money, I'd rather have a mod that optimizes the EF for downward sweep, rather than dicker with ranges.

Why is downward sweep different?  Pluck a string and it starts out with lots of harmonic content, which quickly declines as the string decays.  A lowpass filter that sweep upward from a low starting point begins its sweep restraining the harmonic content, and letting it pass after a few milliseconds, again pulling it back as the string decays and the filter sweeps back down.  The result is a sweep that can sound pleasingly vocal.  A downward sweep starts at the point where there is all this harmonic content to let pass, and then removes harmonic content as that content dies out of its own accord. The result is that, even though it is simply "the inverse", downward sweep sounds annoyingly abrupt and more extreme than upward sweep.  Steve's mod to produce downward sweep, using the unused inverter section to flip the duty cycle, was elegant if not brilliant.  But it starts the sweep higher up than is pleasing.  One can compensate a bit by slowing the Attack time, but the starting point (initial duty cycle) of the clock needs to be lower in the spectrum.  I should add that this is a fatal flaw in pretty much every autowah that includes an up/down direction switch.  The weak link is not the design, per se, but rather the string's behaviour, and the failure of many designs to take that into account.