modding a dod fx25b

[BigTexas]

anyone know how to disable the envelope section on one of the dod fx25b enevelope filters so that one could manually sweep its cutoff? From what i understand it uses 2 ic's and i should probably take a good look at a schematic.

just wondering if its been done.

[Mark Hammer]

Now, I can't vouch for this, but I suspect it is very close to the requested mod if not the mod itself.

Robert Penfold had an autowah project a number of years back (Practical Electronics, if I recall) that also used a dual OTA chip for the filter (LM13700 instead of 13600, but same deal and pinout)  You can find a copy of it as a zipfile at my site (hammer.ampage.org) around page 7 or 8.

Penfold provides an initial frequency control to complement the sweep width control.  The initial frequency control is a 47k pot that divides down the supply voltage and feeds it via a 100k current-limiting resistor.  This is essentially like what happens when one wants to feed a bias voltage voltage to the input of a BBD.  If the sensitivity is set to minimum, then this "bias" control can be used to sweep the filter manually or via foot-pedal.  

Despite a number of similarities, there ARE differences between the FX25B and the Penfold design.  One is that where the FX25B uses individual 10k fixed resistors going to each of the Iabc pins (1 and 16) and varies the voltage fed to those resistors, the Penfold circuit uses a single variable resistor to adjust the current (470k, going into a fixed 22k resistor), and feeds the two Iabc pins directly from that resistor.  Penfold mixes in the additional control current coming through the 100k resistor, so that the 13700 sections are essentially "seeing" two simultaneous current sources - one from the bias circuit and one from the envelope follower - setting the filter frequency wherever that combination tells it to.

What this says to me is that there are two potential ways of doing what you describe.  One is to mix in a second bias current/voltage using Penfold's method, and inserting it at the junction of the two 10k resistors going to pins 1/16.  A second method might involve mixing in a bias voltage at the input to the envelope follower gain stage (U1a), in much the same way Craig Anderton did it in the Bi-Filter Follower.

Something very close to either of these is bound to work.  Okay, I'm going to stand back and wait for someone smarter to correct some of these kooky ideas.

[El Cap]

Old thread but maybe someone has tested out these designs or figured out other way to make it?

[Gila_Crisis]

I got again after ages a DOD FX25B, and I did few mods to it.
- changed C11 up to 22uF to have a better, not so quick decay.
- changed R5 from 20K to 10k, this solves the volume drop and gives the filter the bite many says is missing in the FX25B.
- BP/LP switch (with a volume trim on the LP side to have unity volume)
- since this filter is very similar to the Korg MS20 VCF, I added a switch to select betwenn the DOD envelope, or flipped it becames a fixed filter with the additional pot to set the cut frequency.