DOD™ 440 Enevolpe Filter

This is a DOD Envelope Filter 440 replica called EF440. It is a very nice little filter circuit. Probably the simplest to build filter we have on the site. We made the layout of the PCB very small so that it will fit in a box the size of the original unit.

Modification Information from Mark Hammer:

1) Sensitivity is determined by the gain of non-inverting stage IC1a. When gain is altered by changing the pin 2 to ground resistance, bass rolloff changes as gain is increased, such that at max sensitivity there is less response to lower notes. As with the MXR Dist+, loss of low end in this stage can be fixed by increasing the value of the cap. Let me emphasize, though, this does not change the bass output of the pedal/circuit, just how much the envelope follower responds to lower notes.

2) If you want the sweep to linger, make C4 bigger (e.g., 4.7 or even 10 uf)

3) If you want to slow the attack, stick a small-value resistor (<1k) between D2 and C4

4) If you want to use if with bass, stick a small-to-medium value (say, 10k) resistor in series after C8, and run another similar-value resistor from the R1/R2 junction to the free end of the resistor after C8. Now run a 100k terminating resistor to ground. That will mix in some straight signal with the swept filter so you don’t lose bottom during the sweep. Note that since the dry signal is tapped before processing, and since the filter build around IC1B is inverting, the filter will be subtracted from the dry signal at the output, yielding a swept notch. Very funky in its own right. You may have to tinker with mixing resistor values to get a decent balance or simply stick in a trimpot and tweak.

5) R15 injects some DC from the supply into the LED to adjust the default brightness and the starting point of the sweep. Not having used one I can’t say how much useful variation this provides. However….the min and max DC offset available at the R6/LED junction is set by resistors R13/R14. To move the range downward so that initial starting point is lower, make R14 smaller by the same amount you make R13 bigger. To move the starting point upwards, do the opposite. To allow finer start-point adjustments, make R15 a 47k pot and divide the “missing” 50k between R13 and R14.

The ways of Wien Bridge filters are a mystery to me. I have no sense of what needs doing to adjust Q. It may be worth making any switching of filter caps independent, though, to see what that does to the passband. So, use two toggles, one for C6 and another for C7, for rangeswitching purposes. Certainly tinkering with the passband would have implications not only for filter-inly sounds but for dry+wet mixtures.