Mod for DOD 440 Envelope Filter and 690 Chorus

[Mark Hammer]

I was flipping through my stack of POLYPHONY magazines this morning and came across some mods for DOD pedals from David Di Francesco, who I believe worked for DOD as an engineer for a while.  He submitted a couple of mods for their pedals in 1981.

The stock 440 envelope filter (you can see a project for it at generalguitargadgets) has controls for sweep width (envelope sensitivity, actually) and initial frequency.  David notes that you can vary the resonance of the filter by altering the value of the 430k feedback resistor in the filter stage.   He suggests adding a 100k trimpot in series with this resistor (which apparently is 470k in some units), although if you are building your own, a chassis-mount pot is a smarter option.  See http://www.generalguitargadgets.com/v2/diagrams/ef440_sc.gif for the schematic.

Though he does not mention it, the standard recommendation from me is to dicker with the time constants of the envelope follower.  The 440 appears to be set up for a very fast response by having no resistance in series with the diode in the envelope follower, and a very small cap (C4).  A slower attack time can be achieved by inserting a 500R to 1k pot as a variable resistor between D2 and the positive side of C4.  A slower decay time can be achieved by tacking another cap of 4.7uf to 10uf in parallel with C4, making sure to observe proper polarity.

Although there is no posted schem for the 690 chorus, David indicates that modifying it is quite simple.  There is a single LM324 quad op-amp on the board, and a single 470k resistor on the left side of it (parallel with pins 1-7).  Apparently, if one solders a 33k in parallel with the existing 470k resistor, you can add some flanging range as you adjust the width control.  Normally at maximum clockwise setting of the width control, the pedal sweeps from 14-26msec.  With the altered resistance value, that range is provided when the width control is set to around 9 o'clock, and full width gets you a sweep from 4-12msec, which is more in the range of slow Leslie tones from a flanger.  

Doesn't get much easier.

[Dan N]

Thanks Mark!

I tried the resonance pot and it works great! I used a 100K pot in series with 430K resistance. Now I wonder about maybe a 350K pot in series with maybe 330K? More and less resonance? Excess? I think not!

When I added a larger cap across C4 it changed the attack time. Normal sounds like bwow, flip in 14u7 (10uf and 4u7 caps) and the sound goes to boowow. I'll use 22uf for the final value for a healthy change from normal.

I used a 1K pot between C4 and the diode, but can't tell much difference. Maybe the tone is affected a little, or maybe I am imagining it. On a scope it may just round the shoulder of the attack?

Anyway, cool stuff to try and thanks again!

Dan

[mr_doyle]

i'd jump in the topic of 440 mods too

i'd like to improve an old one (red chassis)

i was thinkin to upgrade resistors with 1% toleraance ones, and i'm trying different opamps too

do you have any other suggestion? i' dlike to reduce the overall noise and improve the sonic performance

last. my model has a low input impedance, and i know that the 2nd model (the green one) had 1MOhm, so i'm wondering if i could modify th einput section in some way

thanks in advance!

D.

[WorkBench]

I just did the resonance mod, and it seemed to just induce a distorted noise with the decay.  It makes the sound more aggressive, big, but brings out that grrrruumphggg, distortion, again near the end of the decay???  I have a 100k pot in series with the 430k resistor.  The pot is marked W100k, that I pulled out of an old DOD Chorus pedal.  So, any ideas, or is this the way this mod sounds?

Chris

[chemosis]

. I had high hopes that some one has sucessufully added a couple parameters like a attack knob or resonance or sweep knob and I find nothing. Anyone please???

[Mark Hammer]

1) If you were trying to build one from a schematic that used to be posted on the Basic Audio site, that schematic is incorrect.  The drawing shows one photocell, and it should have two.

2) In the dozen years since the original post, I finally made myself one of these, and quite like it.  I initially used a superbright white LED for the homebrew optoisolator, but the high forward voltage of that LED resulted in an annoyingly loud all-or-nothing sweep.  Improved immensely when I switched out white for red.

3) Since 2003, I've found that there is limited return on attack mods.  The variation in attack time attainable is so small that its just not worth the effort.  In simple circuits (which this is), you get far more return by doing a variable decay mod.

• replace the series diode (NOT the one going to ground) with a schottky type to charge up the cap a little faster
• swap out the 1uf cap to ground for a 10uf-22uf with a variable resistance to ground in parallel; I'll suggest a 22k fixed resistor in series with a 500k pot

This should get a wide range of decay rates.  Faster rates achieve a more synthlike feel.