The MXR Envelope Filter // a great invention in itself, and a great sounding auto-filter
But if you go in the forums you’ll find many peeps having a hard time making this circuit work well, or even at all // and none of the FX guru’s know well enough how this thing works to help … this is because peeps have failed to notice what MXR did when they switched to different IC’s thru their manufacturing run
Here’s the hint: in the ’76 schematic we see the current limiting resistor R1 in line with the battery to be 4.7k ohm. While in the ’79 schematic we find it listed as R4 and with a 3k resistor value. The ’79 schematic shows a CD4016 (QUAD BILATERAL SWITCH) and a CD4069 (Hex Inverter) used. While in the ’76 a CD4066 and CD4069 are used. the total load, and even swap to other vendor IC’s would likely result in altered bias conditions as well.
(looks like I’m wrong about the ’79 schematic, see comment below)
The moral of the story, that current limiting resistor can (and maybe should) be tweaked … the question is, what effect does it have on the way the overall circuit operates // I made a video that provides some key specifics …
of course, I discovered all this by trying to fix a dead (original) unit and in the process of changing the IC’s ran into the same problem that everyone else seems to be having (those that do) … by replacing the resistor by a trimmer one can even “fine-tune” the response of the pedal … milking the circuit for all it’s got
as far as other component values differences, notice R14 (62k) in the ’79 version versus the same resistor listed as R16 (100k) in th e’76 version … I have not studied the oscillator in any real depth yet, but plan to do so in the future … and with the aim of bringing the Duty-Cycle minima down below 5%, so the filter can reach a little lower in frequency … the top end response is perfect the way it is IMO // probably the main reason why some people dig it so much
you can download my resource files “LINKED HERE”