DOD FX-25 filter-type mod confirmed

Started by Mark Hammer, June 23, 2009, 07:43:28 PM

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Nice! The 'speed' mods - are they decay or attack? I've built the YerayM version on overboard, (posted earlier by Elijah-Baley) and was wondering whether I just replace the 'decay' pot (1MΩ in parallel with C7) with an on/off/on switch and add the suggested 330kΩ and 47kΩ on the outside pairs of lugs to achieve the same results, as that sounds awesome!


If it sounds awesome, good for you! ;)

Quote from: Elijah-Baley on November 05, 2015, 03:50:03 AM
This could be my first failed project. :'( I don't understand if there's something wrong or the effect is right so.

I was thinking about to Snow White Auto Wah, it should be pretty similar, and I can use recover some components.
I didn't want to abandon this DOD FX25 because I like it, but I really don't know what I have to do.

In the end I built a Snow White Auto Wah. More components, the biggest board ever for me. But it worked at first attempt, and seems better and versatile than DOD FX25. :D

«There is something even higher than the justice which you have been filled with. There is a human impulse known as mercy, a human act known as forgiveness.»
Elijah Baley in Isaac Asimov's The Cave Of Steel


Quote from: Elijah-Baley on October 29, 2015, 08:46:21 AM

In the topopiccione web site there is a recommendation about this IC, because the FX25 could have a bit of distortion.
This is the desription: «As previously stated, the sweep is very powerful, maybe too much... cause my unit slightly distort the guitar signal. Changing the op-amp is reccomended, and I had a major improvement in the sound using an NE5532P by Texas Instruments: the unit seems more quiet now.
Unlike Dr. Quack, the DOD FX25 seems not very IC sensitive, so it will work with many op-amps: I tried TL072, TL062, NE5532, LM1458, RC4558, LM358 all with good results (they all make both filters sweep, but there are some differences about distortion).

This is interesting to me.  I have a DOD FX-25, and it does distort some when I hit the strings hard.  Should I change out the 1458?  On hand I have TL072, various 4558 types, and I think NE5532.  Is the NE5532 maybe the best option?


When I was building it I used immediately a NE5532P, never had distortion problem, neither with a 4558.
«There is something even higher than the justice which you have been filled with. There is a human impulse known as mercy, a human act known as forgiveness.»
Elijah Baley in Isaac Asimov's The Cave Of Steel

Mark Hammer

One of the misperceptions that regularly comes up with envelope-controlled devices is that of "distortion".  This is particularly true when the control-element is something that responds quickly, like a JFET or OTA, as opposed to something that is a little sluggish, like a photocell.

Envelope ripple produced by fluctuations in the string can result in rapid fluttering in the envelope voltage in the audio range that sounds for all the world like harmonic or intermodulation distortion.

That's not to say there couldn't be actual distortion from the headroom of something in the circuit being exceeded.  But ripple-based artifacts are a fairly common occurrence.  One of the solutions for it is to either extend the decay time with a much bigger averaging cap - which can interfere with responsiveness - or to shorten the decay time such that the filter goes back to its starting point before any ripple in the string decay starts to make itself apparent.


I went ahead and put in a TI RC4558P, because I have alot of those, and only one NE5532.  I can't get it to distort at any setting, hitting the strings hard.  A big improvement for sure.  There might be some ripple thing as Mr. Hammer was mentioning, but not the distortion I was hearing before.

Overall it's a pretty goofy effect that I wouldn't use outside the privacy of my own home  ;D  But it's kinda fun sometimes, and the way it reacts to picking can be a good exercise tool.  I usually run it into a dirty TS, but now it's good for clean too.


Hey you Guy's sorry to seem like i'm barging into this uninvited but i was very intrigued with this thread... and sorry if i'm kicking a dead donkey bubububu,,but ...i need help!
I'm trying to do this mod to my DOD FX25 and getting notta
>>>introduction: I'm a relatively new member but i have successfully started and completed one thread so far having to do with a tube screamer that I was having some problems with and yes I took the advice from what i considered to be a few of the more sage'y members who helped me figure out where the problem was and kerrblammo! my TS8 ri came back to life!. I think i passed an initial snarky detector test so wisely sprinkled into the priceless knowledge that was being imparted to my unlearned soul as well so thanks again for that!

ok, trying to avoid irritating others can irritate tf out of still others so i'll stop submission of my waiver and launch right in here, now.

I have a DOD FX25 Envelope Filter pedal that I recognize it's inherent possibilities and went looking for mods on the web and found this thread and found it to be purdy much exactly what i was looking for in terms of the direction for modding this pedal.

so i went ahead and did the mod verbatim offered by Mark who started the thread (so i think anyways) and it's not working. 

I cut the trace connection between pin 8 of the LM13600 and C7 closer to the chip in a way so no other components to that etch are shorted, then proceeded to solder a SPDT the 3 points instructed but the only thing it seems to have done is become a cut switch
i saw the correction having to do the C7 vs C11 thingy and made the adjustments specifically to my DOD pedal and i'm fairly certain of my cross reference because there's no other way as C11 does Not connect to pin 8 of the chip in any way directly. so i ruled out any confusion of that part but obviously i've done something wrong or perhaps i found a bad resistor or something..(??)

anyway i welcome any help i can get and i will continue trouble shooting and report as a i go...


dyno - we always ask - can you post your voltages, and some photos of the modded mod and switch wiring? please.

this simple request has been known to scare pedals back to life, you know.
the circuit output is not the input. the reverse holds true for the circuit input.


these are the voltages taken at the switch and input of pedal. i can take more if it helps i assume it would be helpful to check v at all pins of the LM13600N as well as the opamp (1458) but hopfully this will shed light. Thnx!
Input power 9.4v
switch in "Band Pass". pin-8 4.7v / pin-9 4.7v / cap-7 4.7v
switch in "Low Pass". pin-8 1.5v / pin-9 1.5v / cap-7 1.5v


Is the center yellow wire from the switch (connecting to C7) touching another track while connecting to C7? Snip the end of that wire


no that wasn't it :icon_frown: i did a continuity check then yeah, snipped it as it was running a bit over... thanks Jez79.

only reason i haven't started checking caps and resistors is that the pedal seems to work perfectly normal aside from the mod.

Would i be mistaking to assume that #2 buffer output of OTA (pin #9) is dead but the first one (pin #8) is ok??


from what I can see, throwing the switch should not change any DC voltages. however, if you have mistakenly cut the resistor to ground from pin 8 (which I think is what I can see - R14 on the Petrini schem), all bets are off. you need to check continuity from pin 8 to its pair of 22k resistors (R14/15), and from pin 9 to its pair (R19/20, Petrini).
the circuit output is not the input. the reverse holds true for the circuit input.



you cut the trace near pin 8, so pin 8 is only connected to the switch. this may be your problem.
Should the trace be cut at the + leg of C7?
There should be a noticeable volume difference between the two settings

Try cutting the trace at the + C7 leg, and jumper your initial cut with a wire from pin 8 to the appropriate leg of R5, R12 or R4. I'd do that component side rather than under the board


chacking on both those suggestions ...will check back in with results. duck, the voltage thing makes sense... gives me something to chase down and jez i'll check that, i thought i cut it in the best possible place but you might've nailed it. thnx!


Right on both accounts^^... that was it! now it's working and quite nicely i might add.  :)
i had misinterpreted the rewire and cut the trace at pin 8 instead of at the cap thus cutting it off from the rest of the circuit it's dependent upon hence the suspicious drop in voltage when switched to LP because the two 22K weren't able to feed  the leg of that circuit. little bit of a green move  :icon_redface: i know but i could've gone the duration not realizing it had i know brought it up so thanks again.
also, just to confirm what others have said... "low pass" mode indeed has a nice lil boost in the signal witch is a nice bonus for the effort.
now i'm curious about the "bleed"  220k resistor or pot at cap11 (i believe).. Q: what sort of effect does this have exactly.. does it sustain the envelope longer or the opposite. because i'm looking for a way to elongate sustain a micro second or so as i believe this would help balance the attack between the high strings vs low strings (non-wound vs wound resp. )
i find the wound strings little too silent when i've dialed in how i like the higher strings to sound. btw, i always start at a Garcia (estimated prophet) and/or Zappa (Inca roads solo) classic envelope filter sound first and foremost as it's my favorite for envelope filter sounds, fro there i do occasionally venter past that to other types of sounds.
That said i have it purdy darned close especially with the LP mod. now a few fine tunes here and there and i believe this pedal can cop a Mutron 3 or a Maxon AF-9 with relative ease... what do you think...??


well that is interesting. the only 220k I could see on the ^ dia is hanging off a fet gate, and appears near-to a 1M, "bleeding" an envelope filtering cap. seeing as the other fet shows the more tradiational 1M at gate, I wonder if those 2 resistor values are transposed on that diagram. the Petrini doesn't have the bypass fets, but shows 1M bleed and 22uF filter.

does anyone have the real thing we can check against? and as for elongated sustain, the easist way would be to tack an extra couple of microfarads across that cap, see if it gets you there.
the circuit output is not the input. the reverse holds true for the circuit input.


Hey Duck... sorry to be misleading. i meant the the suggestion from Mark's first post about adding a 330k "Bleed" resistor (or better 220k) in parallel to cap #7 (transposed in my case to cap #11... i think  :-\  )


Mark quotes this page, but I don't seem to ever be able to contact it, I don't know why:

cross referencing the circuit I have from that page [Petrini] with this circuit:

would put the C in question as C11. that is the envelope filtering cap. you said you wanted elongated sustain - if this means more/longer/increased/sustained sustain, then you want more capacitance, not more bleed (which would result in a faster decay). as for a resistor across that cap, I dunno, it would form a voltage divider in a spot I don't fully understand, being the feed to the Iabc pins.

so, increase that cap for a more, decrease that cap for a less sustained envelope.
the circuit output is not the input. the reverse holds true for the circuit input.


gonna have to stop there guy's... it sounds amazing! ok, so it's not a mutron III but a happening little device nonetheless. love the lo pass feature very much. A friend is in need of my help so i've decided to sell this pedal to seed puting together a lil scratch to send him to help him and his wife get moved to another state as it's a little rocky for them at the moment. asking 95 plus shipping on Reverb but to anyone here who's helped me figure this mod out or a friend of a friend ... you have a huge discount coming, just pm me(hope this isn't in violation of any rules here). I have other projects im doing so you'll be seeing me pop up here and there some more. thnx again! 
p.s. i will also make a donation to this website/forum. peace out!


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 10k volume trim on the LP side to have unity volume between both)
- 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.