DOD FX-25 filter-type mod confirmed

[Mark Hammer]

I finally fired up my soldering iron again!

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.

[bleachedfan]

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

[lokki]

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.

[Mark Hammer]

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.

[lokki]

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...

[jez79]

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)

[Elijah-Baley]

[...]
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!

[Mark Hammer]

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

Whispy "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?

I 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.

If 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.

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)

Yep, that's how to do it.

[lokki]

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...

[jez79]

Keep us posted please!

[lokki]

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 :-)

[Elijah-Baley]

[...]
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!

[Mark Hammer]

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.

[Elijah-Baley]

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!

[Mark Hammer]

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.

[Cozybuilder]

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:

[lokki]

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...

[Elijah-Baley]

Thanks Mark! Thanks Cozybuilder!

I hope to try some of these solutions soon!

[lokki]

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.

[Elijah-Baley]

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
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.

[...]
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!