MXR Envelope Filter Modification Help

[Dafatcheeken]

I have been trying to build the mxr envelope filter listed here: http://tonepad.com/project.asp?id=4  I would also like to do all the mods listed but have no clue how to wire them up. I wanted wondering if anyone could shed any light on it. Thanks in advance -Jonathan

[bluebunny]

Welcome, Jonathan.

For the "Emphasis control" mod, you need to replace the wire link from F to E with a 500K pot^* (and make the R19 resistor change as advised).

For the "Reverse sweep" mod, take a DPDT toggle switch and wire the middle lugs to G and H.  Wire one set of the outer contacts to C and D.  Finally solder a wire to connect the remaining two lugs:



For the "Filter range" mod, make the capacitor changes as advised and wire points B and A to one side of another DPDT toggle switch and points I and J to the other:



^*One connection to the middle lug and the other to one of the outer lugs.

[Dafatcheeken]

I cannot even tell you how greatful I am to you for this reply it is everything I required. Good luck in your future ventures -Jonathan

[Mark Hammer]

Let's start simple.

The resonance/emphasis of the filter can be increased by increasing the value of the resistance shown as R19.  This is normally 200k (although I recall liking my second purchase of one more than my first and the second used 240k).  Francisco has included pads for adding this mod, labelled E and F.  The stock build simply bridges these two points with a piece of wire.

While adding more resistance to the 200k is useful, there is actually some value to be gained by dropping the 200k a bit, to 100-150k.  Francisco suggests 47k, but I think that might be a bit too low.  When combined with the additional resistance, this will span a range from a nice swell to something far more pronounced.  Coupled with a slow attack and some chorus, I found the low resonance does a pretty passable emulation of the drone of a fretless bass - Pastorius-style.

So, start by swapping out R19 for a lower value.  The wire bridge between E and F will now go to a pot.  Since you want the resonance to increase in a clockwise direction, the lugs to use will be - with the pot shaft facing you and the lugs pointing down - the leftmost and center lugs.  It doesn't matter which of those two the E and F wires go to.  Either connection will result in more emphasis in a clockwise direction.

The second mod to be interested in is the "range" mod.  The range the filter sweeps across is set by the value of C6 and C7.  The stock range and cap value is .001uf (1000pf).  A useful higher range can be produced by use of a smaller value.  The build document indicates 560pf, which in my experience is a good choice.  To get it back to the stock range, we add 470pf.  Mathematically, that doesn't add up to 1000, but given capacitor tolerances it probably wasn't exactly 1000 to begin with to 1030 is close enough.

Much like the resonance/emphasis pot, it doesn't matter which wires from the A/B or I/J contacts go to which lugs on the switch, so long as the A/B wires are used on one half of the switch, and the I/J on the other, and both halves of the switch close and open those contacts at the same time in the same direction.

As clever as the sweep-direction mod is, it probably requires some circuit changes to work effectively.  Unlike the emphasis and range mods that work nicely no matter where the other controls are set, the sweep direction mod is fussy and requires setting the other controls just right, in order to be musically pleasing.  So my advice is to work on the emphasis and range mods first, and save the sweep-direction until you've lived with the pedal for a while.

[Dafatcheeken]

Once again the kindness and commitment on this forum amazes me. Thank you so much for using your time to help me.

[Mark Hammer]

It's what we do here.

Get used to it.