The DOD FX25 Envelope Filter is an envelope filter, the automatic wah style of pedal where the filter opens and closes in response to how hard you play, rather than being swept by foot like a wah-wah pedal. It was made by DOD, and the later FX25B version carries a blend control that RockBoard's PedalPedia describes as making the pedal equally responsive to guitar and bass1.
The FX25B and the blend control
DOD's own description of the FX25B frames it as a revival of the original circuit with one addition: "You still get that great automatic wah sound, but we've taken it one step further by adding a blend control making the FX25 equally responsive to both the guitar and bass guitar."1 The blend control is the practical difference for a bass player: it lets you mix the filtered signal with the dry one, which is what makes the pedal track bass material usefully1.
DIY layouts: regular and modded
Guitar FX Layouts published stripboard (vero) and tagboard layouts for the FX25 in July 2018, with a schematic available via a linked image2. There are two layouts:
- Regular version: the stock circuit. A builder reported in August 2018: "Built the 'Regular' version today. Sounds very cool!"2
- Modded version: a second layout incorporating Mark Hammer's mods from the DIY forum2. A builder reported in February 2021: "I made this moded version. Works fine."2
The site tags the layout for bass2.
Builders have reported two issues worth knowing before starting a build. One builder of the modded version heard "a more than noticeable distortion" at the hardest effect hits, possibly from using an LM13600 in place of a 137002. Another reported "a huge loss of volume" when the effect was engaged2.
Substitutions and fixes
For builders sourcing parts, substitutions have been tried and reported working: "I used NE5532 instead 1458 and NE5517 instead LM13600"2. If the filter is not sensitive enough, with the sensitivity knob sitting at full all of the time, Guitar FX Layouts suggests changing D2 for a 1N34 or OA90 germanium diode2.
Related DOD envelope filters
The FX25 has an active DIY following, with vero (stripboard) and tagboard layouts and a schematic shared online2. Builders report both the regular version and a modded version with Mark Hammer's mods working, and the layouts are tagged for bass2. Two practical notes come up in build reports: one builder found a large volume drop when the effect is engaged, and another reports noticeable distortion at the hardest effect hits on the modded version, possibly from using an LM13600 in place of a 137002. For sensitivity problems, changing D2 for a 1N34 or OA90 germanium diode is reported to help2.
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Questions
Does the DOD FX25 envelope filter work on bass?
Yes. DOD says the FX25B version adds a blend control that makes the pedal equally responsive to both guitar and bass guitar, so the automatic wah tracks low-frequency playing as well as six-string material. The DIY community has also tagged the FX25 layout for bass, and builders report the modded version working fine on bass.
What are Mark Hammer's mods for the DOD FX25?
Mark Hammer posted mods for the FX25 on the DIY forum, and Guitar FX Layouts published a second stripboard layout incorporating them alongside the regular version. Builders report the modded version working fine, and one builder used an NE5532 in place of the 1458 and an NE5517 in place of the LM13600 in that build.
How do I fix low sensitivity in a DOD FX25 build?
Guitar FX Layouts notes that if the filter is not sensitive enough, with the sensitivity knob sitting at full all of the time, changing D2 for a 1N34 or OA90 germanium diode addresses the problem.
Can I use an NE5532 or NE5517 in a DOD FX25 build?
Builders on Guitar FX Layouts report doing exactly that in the modded version: an NE5532 substituted for the 1458 op-amp and an NE5517 substituted for the LM13600. The build was reported working fine as of February 2021.
Is there a schematic for the DOD FX25?
Yes. Guitar FX Layouts, which published vero (stripboard) and tagboard layouts for the FX25 in July 2018, links to a schematic for the circuit alongside the layouts.
Why does my DOD FX25 build lose volume when engaged?
A builder on Guitar FX Layouts reported the same symptom in December 2022: the build worked but there was a huge loss of volume when the effect was switched on. No fix is documented in that report, so it stands as a known issue other builders have run into.
Sources (2)
- FX25B Envelope Filter by DOD, RockBoard PedalPedia, 2026
- Guitar FX Layouts: DOD FX25 Envelope Filter, Guitar FX Layouts (tagboardeffects.blogspot.com), 2021-02-02
