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PostPosted: Wed Jul 06, 2016 7:27 pm 
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Joined: Wed Jul 06, 2016 7:22 pm
Posts: 9
Here’s a great couple of mods for the Envelope Filter that convert the Auto/Manual switch into selecting either a band pass (the current default for the EF) or a low pass output from the filter and convert the Manual control into a Bias control that sets the start frequency for the filter sweep. The functionality of the old Manual control can still be had by simply setting the Sensitivity to minimum. The Bias control will then work just like the old Manual control (only better).

Completely disconnect the Auto / Manual switch from all circuit tracks. Rewire the switch so the centre pole goes only to the output of the pedal via the 10K and 100nF. One side of the switch should be wired to where the output 10K used to go (cut this track) which effectively is the band pass output from pin 8 of the LM13600. The other side of the switch should be wired to pin 9 of the LM13600. That completes the Band Pass / Low Pass mod.

Rather than having the switch selecting one or the other, the auto sweep current from the emitter of Q1 and a bias current from the Bias (was Manual) control must be blended together at the control current inputs of the LM13600.
To do this remove the two 10Ks that go to pins 1 and 16 of the LM13600 (separate control current resistors for each half of the LM13600 aren't necessary) and connect pins 1 & 16 together with a short wire link. Connect 1 & 16 to the clockwise end of what was the Manual pot and, via a 4K7 resistor, to the emitter of Q1.

For both mods cut any tracks that may foul up these connections / connect to other places.

To get a smooth frequency sweep the Manual pot should be swapped out for an Antilog 100K. Alternatively the old Log pot can be wired in reverse, in which case it works in reverse, with the filter frequency falling as the knob is rotated clockwise.

You may also want to swap out the two 10nF filter capacitors for 22nF. With 10nF the Envelope Filter tends to run high and this is even more noticeable following these mods. With the mods and 22nF caps the filter operates in the same range as a Cry Baby Wah.

These modifications have been checked against the data sheet for the LM13600 and have been tested, both in simulation and in hardware on a real BYOC pedal.


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