DOD 440 Envelope Filter "Q" point question

[Krinor]

I'm playing around with a DOD 440 replica, and somehow it doesn't "see" my neck pickup.
I only get that auto wah sound using the bridge pickup (which I never use)...
Anyone have any idea on how I could lower the "q" point in this circuit so that it responds easier and at a lower frequency ?
Thanks.

[Krinor]

Bump

Sorry, just couldn't resist. 

[RickL]

I think if it works on the bridge pickup and not the neck pickup it is likely a level issue. Are you using the neck pickup with the volume turned down a bit? Is the neck pickup adjusted a bit further from the strings than the bridge pickup? Are you using a single coil in the neck position and a humbucker in the bridge position?

Try adding a booster between the guitar and the 440. If that fixes the problem you could modify the 440 to send a hotter signal to the envelope detector. I bet Mark Hammer could tell you how. I might even be able to if you'd provide a link to the schematic.

[Krinor]

Thanks for answering.

I use humbuckers in both neck and bridge positions. I play an SG with stock 490's.
I tried to alter the height of the neck pickup, but it only get's too loud and doesn't do anything for the envelope filter.
The schematic is here:

http://www.generalguitargadgets.com/pdf/ggg_ef440_sc.pdf

[Mark Hammer]

Two things to consider here: the frequency response of the envelope detector and the frequency response of the filter itself.

The most energy in a note will be at the fundamental, but the composite of the lower order harmonics will also add energy to be detected by the envelope follower/sidechain.  As you realize, the neck pickup will emphasize the lowest order harmonics, and the bridge pickup will emphasize middle and higher-order harmonics.  So, to the extent that the envelope follower has a frequency response, it may be more sensitive to some content than to other.

You will note that C2 = .1uf.  When the sensitivity or gain of the envelope detector stage is at maximum (R16 =  zero), the low frequency rolloff point of the gain stage is 1 / (2*pi*C*R), which in this case is estimated at 159hz.  If that cap is under spec (e.g., .009uf), then that rolloff point goes higher.  Now I doubt that people run this thing with the Sensitivity pot flat out all the time, so with a bit more resistance contributed from R16, the rolloff point moves down a bit and the fundamental should be within the "flat" portion of the envelope follower's passband regardless of the pickup used.  Still, it is worth considering upping the value of C2 just a bit to lower that rolloff point.

The other aspect, as noted, is the frequency response of the filter itself.  From what I understand, this filter is a bit "throatier" than some, so it doesn't seem that there is great need for shifting the range in a downward direction.  Still, what shows up as harmonic content in the neck pickup in guitar A may be lower than what shows up in guitar B.  So, for argument's sake, try tacking on a 470pf cap in parallel with both C6 and C7 to bump them up to around .0027uf and lower the range a bit.  If that seems to work, then consider changing the .0022uf caps for .0027 or maybe even .0033uf.

If those two suggestions don't work, then I am convinced the problem lies elsewhere.

[Krinor]

Thanks Mark!

That was very helpfull. I'll try these suggestions tonight and see how it works.

[Krinor]

Okay, for the record - I tried some different values for C2. I went up to 0.22uF, and while there is a difference it is quite subtle. It also takes away some of the funky coolness in the upper register (sorry about the layman terms) , so I went back to 0.1 and will keep it that way for now.

[Mark Hammer]

Fair enough.  At least we know it's not that.

[johngreene]

Two things to consider here: the frequency response of the envelope detector and the frequency response of the filter itself.

The most energy in a note will be at the fundamental, but the composite of the lower order harmonics will also add energy to be detected by the envelope follower/sidechain.  As you realize, the neck pickup will emphasize the lowest order harmonics, and the bridge pickup will emphasize middle and higher-order harmonics.  So, to the extent that the envelope follower has a frequency response, it may be more sensitive to some content than to other.

You will note that C2 = .1uf.  When the sensitivity or gain of the envelope detector stage is at maximum (R16 =  zero), the low frequency rolloff point of the gain stage is 1 / (2*pi*C*R), which in this case is estimated at 159hz.  If that cap is under spec (e.g., .009uf), then that rolloff point goes higher.  Now I doubt that people run this thing with the Sensitivity pot flat out all the time, so with a bit more resistance contributed from R16, the rolloff point moves down a bit and the fundamental should be within the "flat" portion of the envelope follower's passband regardless of the pickup used.  Still, it is worth considering upping the value of C2 just a bit to lower that rolloff point.

But even at the rolloff point, the gain is still over 100. Plenty to give enough voltage to rectify.

The other aspect, as noted, is the frequency response of the filter itself.  From what I understand, this filter is a bit "throatier" than some, so it doesn't seem that there is great need for shifting the range in a downward direction.  Still, what shows up as harmonic content in the neck pickup in guitar A may be lower than what shows up in guitar B.  So, for argument's sake, try tacking on a 470pf cap in parallel with both C6 and C7 to bump them up to around .0027uf and lower the range a bit.  If that seems to work, then consider changing the .0022uf caps for .0027 or maybe even .0033uf.

If those two suggestions don't work, then I am convinced the problem lies elsewhere.

You might want to mess with the level offset resistors as well. The 22k might be limiting it such that the LDR isn't hitting minimum resistance. The VTL is a substitute for the original so you may need to re-center things to get optimal results. Somewhere I have a schematic of the mods I described here back in 1999.
http://archive.ampage.org/articles/1/fxgd/002167/Re_Dod_envelope_filter_440_HELP.html
If you put a 100K pot in series with a 390K resistor in place of the 430K resistor you will have an adjustable 'Q' control. If you increase the value much above 500K it will oscillate on its own as it is a Wien Bridge oscillator configured with too little gain to oscillate. Once the gain reaches 3, it will oscillate on its own.

--john

[Krinor]

Wow! Thanks John! This mod worked wonders!

I didn't have the exact values you suggest, but used a 270K resistor in series with a 250K linear pot. Now I can really fine tune this thing and it works all over the scale even with the neckpickup.

[johngreene]

Wow! Thanks John! This mod worked wonders!

I didn't have the exact values you suggest, but used a 270K resistor in series with a 250K linear pot. Now I can really fine tune this thing and it works all over the scale even with the neckpickup.

Cool, glad it worked for you. I had a lot of fun with it back then. I put a BMP and a DOD440 in the same box. With the Q control it was a pretty wild pedal. Still have it, stuffed in a closet somewhere.....

--john