GGG MXR Envelope Filter BASS mods? Mark Hammer? Anyone?

[Mark Hammer]

With decay pots, you have to think in terms of noticeable ratios. The difference between a max of 1300k and 300k represents an approximate 4:1 ratio (when you factor in pot and resistor tolerances).  Is a 4:1 ratio noticeable?  Depends on the parameter.  You would probably notice a 4:1 change in filter cutoff.  You would probably notice a 4:1 change in volume at some levels.  Noticing a 4:1 difference in decay time, given what the minimum decay time is will be hard, primarily because the detectability of decay time will depend on your picking.

Keep in mind that each picked/strummed note that generates a sizeable enough envelope to get past the series diode (not the one going to ground) is going to charge up the cap to ground.  That cap will then discharge, affecting the rest of the CMOS circuitry as it does so (in this case it alters how long the 4066 switches stay open or closed by affecting the duty cycle of the HF clock produced by the 4069 sections).  How quickly it discharges will depend on: a) how charged up it was in the first place, b) how leaky the cap is, and c) how easy a path to ground the charge has via the parallel resistance.

If another note comes quickly after the first, there will often be insufficient time for the cap to completely discharge from the preceding note.  The only way that could be noticeable would be if the decay time was VERY fast and one left just a little more time between notes.  Conversely, if the cap is larger and the resistance to ground very high, the decay time will be long enough that the circuit will seem to be insensitive to the next note, and will not generate a sweep.  BOTH of these instances, however, will still depend on how much space/time you leave between notes for the effect to be noticeable.

In my own experience, I find that, at the sweep rates we normally expect from envelope controllled filters, a 10:1 (or better) ratio is called for in order for adjustments of decay time to be noticeable.  So in the case of the EF/Agua, you would probably want to make that 1meg fixed resistor 100k and probably even 47k if you want variations in a 1meg pot added to it to make an audible difference.

The caveat is that, with ANY sidechain-based circuit (limiter, compressor, noise-gate, filter, etc) faster decay times can often mean more envelope ripple if one chooses to use fast decay but let a note linger.  Think of slower decay as being like anti-aliasing on a computer screen.  Short decay reveals the inherent choppiness of a simple half-wave rectified envelope.  Extending the decay longer makes something inherent choppy seem smoother.

So, while I would recommend faster decay for bass simply because of how bass players use envelope controled filters (VERY percussively), it shouldn't be TOO fast.  The likelihood of variations between "just fast enough" and "too damn slow" being noticeable is slim.  It may be simply better to stick in a 220k fixed resistor instead of 1meg, and leave it there, or at most have a "slow/fast" toggle that goes between the standard 1meg and something maybe as low as 100k.

Hope that makes things clear enough for you to experiment productively.