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MXR Envelope Filter..?how it works?

11/29/1998 5:38 AM
Paul Perry
MXR Envelope Filter..?how it works?
Looking at a ckt for this (bad photocopy of the original 1979 company one). Can't see how it works, I assume that it is a switched resistor type with the pulsewidth of the signal to the CD4016 being modulated by the envelope of the input signal... but how is this all happening?  
In particular, what is the 'attack' pot doing, and how? All comments gratefully received.
11/29/1998 11:31 AM

Your assumption on how it works is correct. I haven't delved deeply enough into the thing to tell you much more than that. I'll put it into the queue to look at.
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11/30/1998 2:21 PM
Mark Hammer

Near as I can tell, it probably isn't the pulse *width* of the CMOS switch sections that is being modulated, but the clock frequency (turn the light switch on and off faster, and the room generally appears to be brighter than if you turn it on and off slowly). There are 12, count em, 12 inverter sections in the MXR-EF, only 2 or 3 of which go unused. Two or three are dedicated to a variable frequency VCO.  
I have a hand-drawn schematic, and scans of the PC board and parts layout, which I am happy to send to interested parties, but all three need some touching up before they can be guaranteed not to lead you astray (fixingup a bad Bosstone schematic is one thing, but this baby is a little more complicated).  
If you own one of these delights (one of my all time favourite pedals), you can switch to a higher value attack pot (1M instead of 500k) to provide somewhat longer attacks, and can try swapping the caps in the 4016 switch sections for different frequency ranges. The existing caps are .01uf (or maybe .001uf, I forget). Lowering the two in value will raise the range into something more Dr.Q-like, and increasing the value will yield something more bass-appropriate. I stick a 3-position toggle switch in to give me the original range, a deeper, and a higher setting.
12/1/1998 2:45 AM
Paul Perry

Thanks Mark.. if the envelope is modulating the frequency of the switch, then it must have a constant pulse width (like a one-shot). Hadn't thought of doing this before, althoufh it IS how a theremin design I use works.
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