GGG MXR Env. Filter - pros/cons?

[Bucksears]

I've got a PCB finished for this unit, but was wondering about the pros/cons on the performance. I'll be doing the extra mods for the switches, etc.
Is it a decent-sounding Envelope Filter? I don't use wah or filtering all that much, but I'm hoping for something fun and worth the build.

Thanks,
Buck

[Mark Hammer]

I bought one in 1978 and it has remained one of my favourites.  Perhaps not as outrageous as the Mu-Tron and all its derivatives, but very musical.  Its chief virtue is the variable attack control, which few other swept filters have.  That allows it to be matched for feel to whatever song you are playing.  This is distinct from many filter pedals where you have to search for a song that fits the pedal's feel.  The variable Q/emphasis is a very nice addition, as is the range switch.  The drive direction mod is of moderate value.  It is certainly a simpler mod to implement than how it's done on the Mu-Tron, but the Mu-Tron also compensates for how the upwards and downwards drive need to be a little different from each other, not just opposite.  You may find the downward drive trickier to get working right in the EF.

There have been some issues around which brand of CD4069UB to use, but I think these have been resolved by adjusting the values of R14/R16.  Those two components provide a bias voltage, and it would seem some brands need a higher/lower bias voltage to kickstart that clock circuit than others.

Note that while the GGG board is nice and compact.  Very similar to the original board so I assume it fits in a Hammond 1590B.  The Tonepad board is larger and will not fit in the smaller chassis, but it has extra pads for accommodating the mods, whereas the GGG board does not.  The variable Q mod does not really require extra pads.  You can simply run two wires from the pads reserved for R19 to the offboard components.  Those should be a 47k fixed resistor in series with a 250k-500k pot, wired as a variable resistor.

I had made myself a clone of the original years ago, and installed a range switch, but it was a little awkward.  What you need to do with the GGG board is again run wires from the pads normally reserved for C6 and C7 to offboard components.  Essentially what you are going to want to do is have two identical pairs of capacitors in series, and use a toggle to selectively shunt one cap in each pair.  How you do it will depend on your preferences.  On way is to have two 1000pf-1200pf caps in series, and wire up a DPDT toggle to shunt one cap in each pair for a high/low range.  The other way is to have 2 different cap values in conjunction with a 3-position DPDT centre-off toggle to select a high/medium/low range.   The first way is easier io terms of calculation and is generally enough for most people.

In my own noodlings, I have found that setting the EF for a slow attack and low resonance, when used in conjunction with a chorus pedal, can deliver up a nice approximation of that classic Jaco Pastorius fretless bass sound found on so many Joni Mitchell albums.  The combination of slow attack and less pronounced filtering gives a nice swell to the tone that downplays the initial attack enough that the notes just seem to emerge from your thoughts rather than from your fingers.  Mmmmmm.