Author Topic: Increasing the sensitivity of the DOD FX25 Envelope Filter?  (Read 2658 times)

TheFantod

Increasing the sensitivity of the DOD FX25 Envelope Filter?
« on: October 19, 2009, 11:33:26 PM »
Hi, this is my first post here.

I have a couple of the older, two knob versions of the DOD FX-25. I've heard these things sound great on bass. My problem is I have to turn the sensitivity all the way up to the max, and really hit the strings hard in order to get the filter to engage. I've got a very hot signal, an Ernie Ball Music Man HH 4-string, so I don't think it's my signal. It's the same on both pedals, so I know it's not just one bad pedal.

What I'm trying to do is mod the pedal so the sensitivity is much, much lower, so I don't have to "force" the sound. I want the filter to engage at a "normal" string pluck, if that makes sense.

I'm using this schematic:
http://topopiccione.atspace.com/pjimage ... 25.sch.gif

A friend of mine suggested dropping R7 from 4.7 to 1k. I tried that and it seemed to help, but only slightly.

Any other suggestions? If I need explain my goal in further detail, please let me know.

Many thanks,

Tim

Thomeeque

Re: Increasing the sensitivity of the DOD FX25 Envelope Filter?
« Reply #1 on: October 20, 2009, 04:26:51 AM »
Hi, this is my first post here.

I have a couple of the older, two knob versions of the DOD FX-25. I've heard these things sound great on bass. My problem is I have to turn the sensitivity all the way up to the max, and really hit the strings hard in order to get the filter to engage. I've got a very hot signal, an Ernie Ball Music Man HH 4-string, so I don't think it's my signal. It's the same on both pedals, so I know it's not just one bad pedal.

What I'm trying to do is mod the pedal so the sensitivity is much, much lower, so I don't have to "force" the sound. I want the filter to engage at a "normal" string pluck, if that makes sense.

I'm using this schematic:
http://topopiccione.atspace.com/pjimage ... 25.sch.gif

A friend of mine suggested dropping R7 from 4.7 to 1k. I tried that and it seemed to help, but only slightly.

Any other suggestions? If I need explain my goal in further detail, please let me know.

Many thanks,

Tim

 Hi!

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

 You could try to enlarge value of C4 as well, 1nF is probably to "tight" for bass signal (1.6MΩ @ 100Hz), try 10nF or even more..

 T.
« Last Edit: October 20, 2009, 04:39:22 AM by Thomeeque »
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Mark Hammer

Re: Increasing the sensitivity of the DOD FX25 Envelope Filter?
« Reply #2 on: October 20, 2009, 10:24:46 AM »
....and if that isn't enough, try reducing R7 to 3k9 or 3k6.

TheFantod

Updated: Made some changes, got some distortion.
« Reply #3 on: October 21, 2009, 08:44:30 PM »
Okay, here's what I've done, still using  http://topopiccione.atspace.com/pjimages/DODfx25.sch.gif.

Lowered R7 from 1k to 470

Raised C4 from .001 to .01

Raised both C9 & C10 from .01 to .022

The result is much better sensitivity. I can dial the knob back about 1 o'clock and the filter engages. However, I have lost some of the wet/high envelope effect. I think that might be a result of upping C9 & C10.

Also, there's a bit of distortion now. Not a terrible amount, but noticeable. Would sloppy soldering cause that, or something else?

Thanks!

Tim

Mark Hammer

Re: Increasing the sensitivity of the DOD FX25 Envelope Filter?
« Reply #4 on: October 21, 2009, 09:38:29 PM »
I think that part of the distortion may stem from envelope ripple produced by the added low-frequency input into the envelope extractor portion.  While it is true that you want it to be sensitive to low notes, it is also true that beats and things "live" in the low end, and these can add a ripple or "burr" to the sweep that many people describe as "distortion".

The advice to double the caps use to set the filter range is normally sound, but in this case you may not need to do so.  Try .015 and see if that helps a bit.

The other thing is that the filter used is actually a state-variable filter.  This means that you can get a bandpass function if you tap in one place (pin 8 ), but you can also get a lowpass function if you tap the signal at pin 9 instead.  I've done it and it works.  This will ket you keep the top end but always keep the bottom, since you are only sweeping the upper end of the filter, not the upper and lower as you would with a bandpass.

Just a word of warning, though.  The lowpass output is noticeably louder than bandpass, and also noticeably louder than bypass.  So, I would suggest omitting R22, and feed R21 to a 100k log or linear pot to adjust output level so that you get the level you want when you kick the effect in.

So, to recap (pardon the unintended pun): 
  • reduce C9/C10 down to .015,
  • change C4 to something like .0022 or .0033, but not much higher,
  • connect C11 to pin 9 instead of pin 8 (this means cutting a trace)
  • replace R22 with a pot instead of a fixed resistor


If you wish, you can install a toggle to select between bandpass and lowpass.  If you do, you'll be glad you installed the output level pot.  I didn't and I wish I had.