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I finally fired up my soldering iron again! :icon_biggrin:

Someone had drawn my attention here to the possibility of providing a lowpass filter output from the DOD FX-25, in addition to the stock bandpass filter. (see http://topopiccione.atspace.com/PJ11DODfx25.html for project details)

Since it was a simple mod, I figured I'd re-enter the world of modding nice and easy, and went for a toggle that would select between bandpass and lowpass.  The lowpass is obviously more bass-friendly.

The FX-25 uses a 2-pole active filter formed out of a pair of OTAs in a single LM13600.  The bandpass filter output is tapped after the first of the two LM13600 sections.  The lowpass can be tapped after the 2nd.

Here's what I did:

1) Cut the trace between C11 and pin 8 of the LM13600.

2) Run a wire from the common (middle lug) of a SPDT toggle to the + side of C11.

3) Run a wire from one outside lug of the toggle to pin 8 of the LM13600.

4) Run a wire from the other outside lug of the toggle to pin 9 of the LM13600.

That's it.  One position is bandpass, the other is lowpass.

Because the decay of the FX-25 is not really suitable for bass, I stuck in a bleed resistor in parallel with C7 to quicken the decay.  I used a 33ok resistor, but in retrospect, a 220k might have been better.  I put it in permanent, but feel free to use a toggle to switch it in circuit or take it out for slower and faster decay.

Is it a direct trace? I don't understand...pix?

hi everybody, long dead thread i know. but since i was searching for fx25 mods this popped up.
just in case somebody wants to try this, mark swapped the numbers of the capacitors :-) c7 is the one that goes to the lm13600 and c11 controls the decay. the lowpass mod is perfect for bass. i will now attempt a sidechain input, should be fairly straight forward.

Thanks for digging it up, and for the correction.  I gave the one I modded to an old friend who is a bass-player (who gave me a vintage TC Chorus/Flanger/Pitch Modulator in gratitude), and was starting to get a taste for having one for myself, again.

I'll just mention that the lowpass output is noticeably louder than the bandpass output, and bypass.  So you may want to include some level adjustment.

mark, yeah i noticed the lowpass is louder, but i will mod it further and maybe add a level control at some point. actually i just found out which schematic you were referring to, there c7 and c11 is as you described! you used this schematic, right?

http://topopiccione.atspace.com/pjimages/DODfx25.sch.gif

and i was looking at:

http://files.effectsdatabase.com/docs/schematics/dod_fx25.gif

funny that the cap numbers are exactly opposite...

edit: to clarify and why i even said that marks caps were "wrong". i'm working on an actual dod fx25 not on my own pcb. on the dod pcb the parts are labelled and as in this schematic: http://files.effectsdatabase.com/docs/schematics/dod_fx25.gif
hence the confusion...

I've been working on the fx25b circuit for several weeks now.

Attack control:
Add  5k pot before the diode that fills the decay cap (D5 in fx25b schem)

Whispy "Q" control: replace R21 with a 25k pot

I put 100k across the sensitivity pot, 500k was too big

If you're feeling adventurous, try different sizes for the decay cap, some weird stuff happens when you get fairly low.....

Use a dpdt switch to toggle between lp and bp
(Just select output from pin 8 or 9 into c11 i think)
Use other half of dpdt to send final output through a trimpot, so when you switch on LP, the trimpot compensates for higher volume (from fewer filters)

Quote from: jez79 on August 08, 2015, 03:05:38 AM
[...]
Use a dpdt switch to toggle between lp and bp
(Just select output from pin 8 or 9 into c11 i think)
Use other half of dpdt to send final output through a trimpot, so when you switch on LP, the trimpot compensates for higher volume (from fewer filters)

I'm building this pedal. I'm trying to cut some bass frequency, I want to use it with the guitar.
Your internal volume mod is interesting. The value of the trimpot could be...?
Would somebody to draw a schematic? ;)

Thank you!

Quote from: jez79 on August 08, 2015, 03:05:38 AM
I've been working on the fx25b circuit for several weeks now.

Attack control:
Add  5k pot before the diode that fills the decay cap (D5 in fx25b schem)

I used to do that, and can lightly endorse it.  However, in simple, unbuffered envelope-followers like this one and so many others, I found that such Attack controls have a way of reducing the current of the sweep control, such that seeking slower attacks requires cranking the sensitivity.  And sometimes the sensitivity settings simply can't compensate for the sorts of slower attacks that are audibly different from the faster ones.  In short, I just stopped implementing it because I didn't get enough in return for what I had to invest.  Variable decay-time, on the other hand, is sort of a near cost-free thing, so that's what I install instead. YMMV

QuoteWhispy "Q" control: replace R21 with a 25k pot

The Range control already is a Q control, except that it has the side-effect of shifting the range downward as Q is increased.  I gather your version alters Q in a manner that doesn't do that?

QuoteI put 100k across the sensitivity pot, 500k was too big

I think that's what I did too.  Depends where you put it in your signal chain,  The 500k is for those situations where you're hitting it with a big signal and only want gentle sweep.

QuoteIf you're feeling adventurous, try different sizes for the decay cap, some weird stuff happens when you get fairly low.....

Small-value averaging caps yield more ripple.  Sometimes that ripple is perceived as, or can be made to sound like, distortion.  So yeah, monkey around.  Just know that smoother decays want bigger caps that drain and charge off slowly.

QuoteUse a dpdt switch to toggle between lp and bp
(Just select output from pin 8 or 9 into c11 i think)
Use other half of dpdt to send final output through a trimpot, so when you switch on LP, the trimpot compensates for higher volume (from fewer filters)

Yep, that's how to do it.

ok, just some notes, while i'm still modding.

opamp does matter for the envelope follower part! i replaced the stock 1458 with a tl072 and the envelope got very strange in comparison... i then tried 4558 still bad. once i popped an lm358 in there i got a very nice envelope again!

i also added a sidechain input and this is a great mod! this also gives you an expression input "gratis". (just use a volume pedal and a steady signal, or think about a tremolo) i just copied the input buffer and cut the existing trace before c8. the newly added input buffer connects then to the positive end of c8 (c8 in petrinis layout). i used a switched jack socket, so if there is no additional input present, the pedal behaves normally.

next up, a second lm13600 for parallel bandpass fun...decay control, reverse sweep...

Keep us posted please!

ok, here are some updates:

i added an attack control, as per jez79 suggestion, works great! no really long attacks possible, but still a big difference.
i upped c5 to 0.3 as mark noted that some low end gets lost when sensitivity is fully cranked (low end for the envelope follower).

i replaced r9 with a 1M Pot with a 5k resistor in series. mark had thought that this would give variable decaytime. it does not, but it does something else: it "sets" the lowest frequency of the filter, less resistance gives higher base frequency. you can get some very nice sounds!

this pedal is so "moddable" i don't know when and where to stop :-)

Quote from: Elijah-Baley on August 08, 2015, 05:54:03 AM

Quote from: jez79 on August 08, 2015, 03:05:38 AM
[...]
Use a dpdt switch to toggle between lp and bp
(Just select output from pin 8 or 9 into c11 i think)
Use other half of dpdt to send final output through a trimpot, so when you switch on LP, the trimpot compensates for higher volume (from fewer filters)

[...]
The value of the trimpot could be...?
Thank you!

Excuse me, I'm preparing an order for my building. I ask again what could be the value of the trimpot.

Thank you!

It might be more useful to simply use a 100k log pot, mounted to the chassis, instead of the 100k terminating resistor on the stock drawing output.  Turn it up full, and you have no attenuation, as per the original.  Flip the filter mode to lowpass, and you can simply turn the output down to whatever level you want.

If I got, are you suggesting me to replace R22 100k with a pot?
I wanted avoid to add a fourth pot, and the trimpot solution seems to me a perfect final solution. But I will consider both the solutions.

Thanks Mark! ;D

Well, technically, you could install a 100k trimpot in place of R22, and use a DPDT toggle to simultaneously select between bandpass/lowpass and the input or wiper of the trimmer.  The input lug of the trimmer will be full output, and the wiper will be whatever division of max output you have it set to for the lowpass mode.

Referring to the original DX-25 schematic (Petrini), and using Mark's mod of choosing the output from pin 8 or pin 9 of the TCA, it seems you could play with R21 values to give the same output level from either TCA out- the original R21 (1K) for TCA-8 out, and a higher value for TCA-9 out. Breadboard it and use a pot to determine what value of R gives an equivalent output to R21, call that Rx. You could wire the components on a DPDT (on/on), so that flipping the switch will give a change in sound, not volume:

(http://i1362.photobucket.com/albums/r688/russrutledge/DSCN2814_zpsmxj5wymp.jpg)

did anybody else see that the filter section on the different schematics (the one i posted and petrinis) is slightly different? the range pot and a couple of resistors are different, so no wonder the different versions sound different...

Thanks Mark! Thanks Cozybuilder! ;)

I hope to try some of these solutions soon!

ok reverse sweep as per: http://www.diystompboxes.com/smfforum/index.php?topic=70362.0

kind of works... the sweep is not very pronounced and very slow. (i connected the 47k resistor to a pot, as per slackers suggestion) but i like it for slow "synthy" stuff.

Quote from: Elijah-Baley on August 08, 2015, 05:54:03 AM
I'm building this pedal. I'm trying to cut some bass frequency, I want to use it with the guitar.
[...]

Hello. Finally I resume this circuit. I have too much "blasting" bass in the decay with this circuit. In both the modes. Sometime I have a blow of bass frequency with the first note after I didn't play for a while or when the circuit is just powered.
I built the veroboard layout: http://vulcanofx.blogspot.it/2013/10/dod-fx25.html (http://vulcanofx.blogspot.it/2013/10/dod-fx25.html)
I tried to replace C9 e C10 both 10nF to 6,8nF and 5,6nF. Even 2,2nF, but I got a reduction of amount of bass frequency and got thin sound, but not in the tail of the decay of the notes.

On the board the caps is right, so the resistors. No contatct between the tracks and no track cut missed. The pots wiring is ok. As my usual: no footswitch, led and jacks with my tests.

Quote from: Mark Hammer on June 23, 2009, 07:43:28 PM
[...]
Because the decay of the FX-25 is not really suitable for bass, I stuck in a bleed resistor in parallel with C7 to quicken the decay.  I used a 33ok resistor, but in retrospect, a 220k might have been better.  I put it in permanent, but feel free to use a toggle to switch it in circuit or take it out for slower and faster decay.

Could C7, a 22uF capacitor cap, to be responsible of this bass? (The resistor is 220k).

The sound is good, but this defect, that is pretty annoying, and I'm scaring for my amp.

Thanks!

Eli
C7 is not in the audio path.
Its part of the envelope detector. I very much doubt its responsible for the extra bass.

The OTA lm13600/ lm13700 is used to make the filter through which the audio signal passes,
the envelope detector detects a signal and sends a voltage/currant to the filter which alters freqauncy of said audio.

I hope this helps
Rich

Thanks you. I also was starting to doubt about C7.

I'm finishing the ideas. I didn't touch the output cap, but seems to me the problem is somewhere else.

Has someone verified this layout: http://4.bp.blogspot.com/-m3EQYiHWXKA/UlbObQx7G-I/AAAAAAAAA18/6OEJwqZwHA8/s1600/DODFX25_rev8a.png (http://4.bp.blogspot.com/-m3EQYiHWXKA/UlbObQx7G-I/AAAAAAAAA18/6OEJwqZwHA8/s1600/DODFX25_rev8a.png)

Or the layout is right so (and I want to correct it) or I made a mistake something.

What happens when you use the range pot? Still too much bass?

Cant help you with the vero layout. Its doing my swede in just glancing at it sorry.......
I dont do vero

Try to explain. When I set low the range pot I get a "less wide frequency", a bit less treble, and so less low.

I have to verifiy the decay pot or some near to it, because it seems so subtle, almost useless. ???

I should probably add that using a Schottky diode for the series diode in the rectifier (not the one that goes to ground, but its partner) will get you a little more sensitivity and responsiveness.  Keep in mind that the higher the forward voltage of that diode, the stronger the amplified signal from the op-amp has to be to "break through" and force a filter sweep.  I can't say with any certainty, but the difference between how long it takes for an amplified string to reach the forward voltage of the 1N418, vs a Schottky type, probably adds a couple of milliseconds to when the filter starts to sweep.

Thanks for joining Mark Hammer.
About the sensitivity the sound is alright, I'm using 1N4148. By the way, the ICs are LM13700N and NE5532P.
I hear no effect when I turn the decay pot, it could be meaning, but it is wired exactly like the layout show, and the multimeter said me it is well connected.

Just a detail, I don't know what it could be considerable. The Sensitivity pot (it works) has the lug 3 connected on the board. I started to build the circuit months ago, then I have to stop, so I don't know why but I connect lug 1 and 2 together, and connected it to the ground. Do I misread the schematic?
http://3.bp.blogspot.com/-zfoc_H5RK04/UiGU3o2QKiI/AAAAAAAAApk/FoqIFM8sBQs/s1600/DODfx25.sch.gif (http://3.bp.blogspot.com/-zfoc_H5RK04/UiGU3o2QKiI/AAAAAAAAApk/FoqIFM8sBQs/s1600/DODfx25.sch.gif).

The op-amp should probably be a 1458 or 358 to get a proper sweep.  The Decay control mod needs to have an appropriate ratio of change.  So, it there is a 330k fixed resistor and a 500k pot in parallel with that 22uf cap, you likely won't hear much effect of the added control because (500k+330k)/330k is only a little more than a 2:1 ratio of change.  100k and a 1M pot will give you 11:1 and 47k plus a 1M pot will give you almost 22:1.  The bigger the ratio, the easier it will be to notice differences in decay time.

You can also skip the pot and just go for presets with a 3-way toggle.  In the middle position, it doesn't connect anything in parallel with the cap.  On one side position it connects 330k, and on the other side position it connects 47k, giving you slow, medium, fast....which may be enough for many users.

I used NE5532P because it was suggest on topopiccione web site. But I tried a JRC4558D with no changing. I don't have other ICs.

I follow the layout, so I have a Decay pot 1M and a 220k resistor. I'll do further tests in different combination.

And about the wiring of the Sensitivity? Somebody had take a look?
I have lug 3 to the board and lug 1 and 2 to the ground, but now I'm not sure if it is correct.

As long as the total resistance of the pot to ground gets smaller as you rotate clockwise, you're in good shape.

I'm sorry, but I need really help. I didn't find anybody who used this layout, so I wish this thread could be useful to someone else.

I didn't get my built working fine. I tell again: I got the wah effect, but sometimes I have a sort of bump of bass on the first note when the circuit is just powered, and I have this bump of bass frequency, while I play, in the decay of the wah just before it fade.
It's almost impossible play it at max range. I can take off this bass lowing the range.

I noticed the pot decay do nothing. And I have some doubt about the wiring of the Sensitivity pot.
I want to attach the scematic and the vero layout (it includes filter mode switch and the decay pot).

(http://3.bp.blogspot.com/-zfoc_H5RK04/UiGU3o2QKiI/AAAAAAAAApk/FoqIFM8sBQs/s1600/DODfx25.sch.gif)

(http://4.bp.blogspot.com/-m3EQYiHWXKA/UlbObQx7G-I/AAAAAAAAA18/6OEJwqZwHA8/s1600/DODFX25_rev8a.png)

Could be useful this PCB with not mods:
(http://1.bp.blogspot.com/-KvLpNtyG4Ks/VakP50055pI/AAAAAAAAFdQ/PoDjT4cCfYA/s1600/DOD%2BFX25.png)

The layout show just a wire on the lug 3 of the Sensitivity pot, but not the other lugs.
How you would connect the other lugs?

About the bass bump, thought I tried low values for C4, C5, C9, and C10, I lost bass, but never the bass bump in the decay. Could be the problem around the no working decay pot?

Thanks!

e-b - can we see photos of your work? have you posted "the" circuit diagram (I can't connect to anything at atspace.com, including topopicione)? that vero layout is to be frowned upon, as it shows four places with two links or component leads sharing a single hole. did you make those four in a goodly manner?

and the perf layout sugests that "the other end" of the sens pot is grounded, which agrees with the boxed note in the vero dia. (SENS: 1 and 2 to ground {the rest needs me to scroll down, then across, so I can't see it}.)

I had resume the schematic, now we can see it.

I checked the contact of the jumpers and of those resistors. Seems alright.
In the boxes...  >:( I missed it! Now I know why I had connected the wires like that. Indeed, that pot works! Ok that is ok.

http://i.imgur.com/KC0IXDa.jpg (http://i.imgur.com/KC0IXDa.jpg)

http://i.imgur.com/0cOVwhn.jpg (http://i.imgur.com/0cOVwhn.jpg)

Some caps are on socket, because I made some tests. No changing after I add the sockets. Diodes on socket, too.

I stared and checked it along. No contacts between stripes, wires well connected and in the right place, caps are ok... Also the resistor seems right, I measured it, but some are hard to measure, so I compare the colours with the other. I didn't find no mistake.

I really can't hear any effect with the decay pots, anything it should to do it doesn't. I think there could be the problem, or some mistake, but I really can find it.
This circuit are driving me crazy.

Decay knob is wired normally, in parallel with the envelope cap shunting it to ground.

EDIT:
Instead, connect the decay 1 wire to the =9V spot just above it, and remove R9 (1M). Essentially replace R9 with the pot/R24.
Your decay will become an attack knob, and it will make audible differences, much more than any decay mod i've tried.

Decay on an envelope-governed circuit is a bit like an alley-cat: if you keep feeding it, it won't go away.  As a result, despite being far more effective in my experience than unbuffered attack mods, it is easy to mistakenly interfere with a good decay mod.  If one lightly mutes the strings with the butt of your picking hand, you'll find that the decay mod results in clearly audible differences in sweep.

None of that is to take away from the value of a good attack-time mod.  The trouble is that, in most unbuffered contexts (which the FX-25 is) it is difficult to achieve audibly different attack times without detracting significantly from the sensitivity of the circuit.  Remember that both attack and decay revolve around the time it takes to charge up, and discharge, that electro cap to ground.  And any resistance placed in series with the output of the rectifier gain stage reduces the current feed to that cap. in a manner akin to having less envelope signal.

Anyway, if I really can't hear no effect with this pot. I like have the Sensitivity pot much high, else I have less attack. It is like an attack control.
I have the sensation could be something wrong around this no working pot.

If I right, this decay control includes the pot and the 220k resistor R24. So I think I can take off this components getting the "standard" Alex Petrini's schematic. And, I wish, the circuit should work.

This bump of bass I have on the first note when the circuit is just powered could be some charge caps? (Maybe the 22uF cap?) I had low C4, C5, C9 and C10, but this bump had never go away.
Sorry, I'm trying to guess.

I take off the Decay pot and the 220k resistor, but no changing.
I replace C11, the output cap with a 100nF polyester cap, I get a bit less bass, but I have the bump. I have this bump in the decay of the notes but also on the first note when the circuit is just power. I'm using a NE5532 instead of 1458, but I think thet is not the problem. Sorry if I repeat myself.

Should I disconnect even the filter switch? Where this bass bump coming from? It is not normal, it is like an earthquake.

I wouldnt expect an earthqauke.
The 1458 is a compensated chip not sure the 5532 is...........
pm me ill send you a 1458.

Quote from: Kipper4 on October 29, 2015, 07:41:30 AM
[...]
pm me ill send you a 1458.

pm sended ;).

Quote from: Kipper4 on October 29, 2015, 07:41:30 AM
I wouldnt expect an earthqauke.
The 1458 is a compensated chip not sure the 5532 is...........

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).»

I'm beginning to think could be some ground problem. There's no more the "Decay section", I change the value of several caps, I have this bump of bass on the any first note. I connected the ground from the board and that from the Sensitivity pot together, adding the negtive of the snap battery. Am I missing something there's no in the layout?

I'm afraid there's no much things to do.
I like this effect, but I can't use it. :'(

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.

Finished some mods to an FX25b unit.
Cool synthy sounds possible!

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

Cheers!

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.

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!. ...plus 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...
thanks!

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.

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
(https://c2.staticflickr.com/8/7737/27579855371_be245dc965_c.jpg)
(https://c2.staticflickr.com/8/7210/27375615520_eb0cb314d5_c.jpg)

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).

(http://pages.suddenlink.net/fantod/DODfx25.JPG)

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.

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:
http://topopiccione.atspace.com/PJ11DODfx25.html

cross referencing the circuit I have from that page [Petrini] with this circuit:
(http://pages.suddenlink.net/fantod/DODfx25.JPG)
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.

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.
(https://i.imgur.com/CA8kRkpl.jpg)

im having issues getting a noticeable result from the decay mod also. it seems so sutle if working at all. ive had the same problem with a decay mod on my bassballs. I built 3 bassballs over the course of 2 years all with decay mod but only one worked effectively like in the video. I tried marks suggestion. maybe a 2m pot with a 22k resistor this time

The decay time depends on how much current is stored by the cap, how quickly it charges up, and how quickly it discharges.

I'm wondering if the 1M resistor between V+ and the 22uf cap is the better path to pursue.  It is reminiscent of what happens in the Dynacomp with the 10uf cap and 150k resistor to V+.  That resistance sets how log it takes the storage cap to "recover".  Try throwing a 470k in parallel with the 1M and see if that shortens the decay time.

Hope you guys don't mind me reviving this thread. I'm gearing up to build one of these and have some curiosities.

What would happen if you mismatch the resistance (and thereby current) going into each 'Amp Bias Input'?

On this schematic, R12 & R13 are both 10k. What should we expect if 5k and the other 15k?
(https://i.postimg.cc/QxrkJzJr/DODfx25-sch.gif)

> What would happen if you mismatch the resistance (and thereby current) going into each 'Amp Bias Input'?

It becomes even more unpredictable than it is. Especially at very low current (low frequency).

I don't know why you would want to offset the two filters, but surely a safer technique would be to offset values of C9 C10.

Changing the ratio of the currents will make the gains of the integrators different.   Off hand, that's one way to change the Q of the state-variable band-pass filter.

There's some stuff here on the analysis which you could modify,
https://www.diystompboxes.com/smfforum/index.php?topic=125170.0

However you could poke around of the web for state-variable filters for a web page showing the effect of changing the gain terms, the k's.


EDIT:
Here's the idea,
https://www.electronics-tutorials.ws/filter/state-variable-filter.html

- circuit about 10% down,
- then at about half way down, there's an expression for Q.

If you imagine increasing the gain of the last integrator, is similar to decreasing R4, and that shows Q increasing (I think).
(The difference between the integrator change and the R4 change is it predicts a different gain at Vout.)

I'm going to try this 'Balance Pot' idea in the next week or so. I'm thinking of trying a 10k pot with series with a 10k fixed resistor.

                                        [Env Follower]
                                                  ↓
                                        [10k Resistor]
                                                  ↓
Amp Bias Input A (Pin1)  ← [b10k Pot] → Amp Bias Input B (Pin16)

Do you guys think that will have a noticeable effect? Can we think of the 'Amp Bias Input' as having an input impedance?
My understanding of OTAs is weak and I can't find that info on the LM1700 data sheet (https://www.ti.com/lit/ds/symlink/lm13700.pdf?ts=1607068884477).

The Iabc pin (called ABI here) is a "zero impedance".

Why do you need to know?

Quote from: PRR on December 04, 2020, 10:17:00 PM
The Iabc pin (called ABI here) is a "zero impedance".

Why do you need to know?

Just making sure I understand it correctly and I'm trying a mod that will actually do something.

Based on what PRR's last post, the scheme above would probably not work. Going to try this instead.

                            [Env Follower]
                                      ↓
     [10k Resistor] ← [b10k Pot] → [10k Resistor]
               ↓                                           ↓
          [ABI-A]                                 [ABI-B]
           (Pin1)                                  (Pin16)