Taming DOD Death Metal

[MetalUpYerEye]

Hey folks, just finished my Tubescreamer (sounds f-ing great, and worked after only 5 minutes of debugging  )

Anyway, I decided that instead of jumping head-long into another pedal that this time i'd try modifying an existing pedal to make it (in my eyes) better. Enter the DOD Death Metal distortion. My younger bro abso-freakin-lutely loves it (i, however, disagree) but its a very harsh pedal. Like rediculously harsh. Harsh doesn't even begin to describe it. In fact, its so harsh that even my bro sometimes wishes it was less harsh. Well, i like to make wishes come true, so I was wondering if there's a good way to add a gain/distortion control to this bad boy so that its bad-ness could be controlled.

The schematic for it is right here: http://filters.muziq.be/files/schematics/dod_fx86.gif

A few suggestions on where to start would be more than appreciated (as always).
-M

[Connoisseur of Distortion]

i would say pull that 220k resistor in the feedback loop of the opamp, and replace it with a 250k pot.

BEWARE: do not assume i know what i am doing. If you drill a hole in the chassis only to find you don't like what it does, your life will suck.

BTW, anyone see that tonestack? why the trannies?

[MetalUpYerEye]

Would replacing the 220k with a 250k pot do it? Would be a variable gain/distortion or more like a guitars volume knob?

[cb]

Actually, if you want a gain control (which these things sorely need) replace the 120k (on the 2nd op amp from left) with a similarly valued pot. You can also reduce the hi's to reduce harshness. I would first try increasing the value of the 50p (same op amp). Try 2x (i.e. 100p), 5x, 10x and beyond - it's a trial and error sort of thing. You can also try increasing the value of the 47n just in front of that op amp. You can increase the value of the 8.2n on the Scream (hi's) control. Then you would roll off more hi's when turning Scream down.

BTW, a cool trick you can do when experimenting with larger values of caps is go to Radio Shack and get those leads with a miniature alligator clip at each end. Use 2 of those cables, find the cap whose value you want to increase, clip one end of each lead to a leg of the cap, and clip the other ends of each lead to the legs of a 2nd cap (from your bag o'capacitors). This connects the 2nd cap in parallel with the cap on the board. The effective capacitance is the sum of the values of the 2 caps. So when you find a sound you like, add the 2 caps' values, unhook the alligator clips and replace the cap on the board with a cap whose value is the sum of the 2 caps.

Good luck, keep us posted.

[MetalUpYerEye]

Well earlier today I took CoD's advice and replaced the 220k in the feedback loop and it (somewhat) did the trick. I was able to tune out a low-fair amount of crappiness, but it just didn't give me the control i'm looking for. Tomorrow i'll put the 220k back in its place and replace the 120k with a 100k pot. That way (i figure) it'll be a little less harsh than the stock pedal even when the new gain control will be set at max, and i should be able to achieve full control of the gain setting. As for reducing/modifying highs, well I dont want to alter my bro's sound without his consent so i'll leave that up to him, but thanks for the suggestion     i'll post at you sometime tomorrow and let you know how the *new* gain control is working.

[slacker]

BTW, anyone see that tonestack? why the trannies?

The tonestack's a 3 band graphic EQ, there's a similar arrangement explained here
http://www.geofex.com/Article_Folders/EQs/paramet.htm

[Elektrojänis]

As for reducing/modifying highs, well I dont want to alter my bro's sound without his consent so i'll leave that up to him, but thanks for the suggestion   

Hmmm... If that schematic is correct it has practically nothing to cut any harsh high treble content that the clipping produces (well the scream control will if you turn it down). The feedback caps on the opamps after clipping set the upper cutoff well over the audio band (over 20kHz that is). You might be able to have a bit smoother tone even on high gains and still keep the essential parts of the tone if you make the thing cut treble at 5kHz (experiment with this though) and over.

Well... if the ultra high ear bleed treble is not essential part of your brothers tone... It might be, but for most of us it isn't.

[Transmogrifox]

As for reducing/modifying highs, well I dont want to alter my bro's sound without his consent so i'll leave that up to him, but thanks for the suggestion  

Hmmm... If that schematic is correct it has practically nothing to cut any harsh high treble content that the clipping produces (well the scream control will if you turn it down). The feedback caps on the opamps after clipping set the upper cutoff well over the audio band (over 20kHz that is). You might be able to have a bit smoother tone even on high gains and still keep the essential parts of the tone if you make the thing cut treble at 5kHz (experiment with this though) and over.

Well... if the ultra high ear bleed treble is not essential part of your brothers tone... It might be, but for most of us it isn't.

This high cut-off frequency is what keeps it tight on the palm mutes, and adds the "fine grain" (as opposed to ratty) texture to the tone.  If you want to add some chunkyness that's more amp-like, I found that adding a diode in series with one of the clipping diodes does a nice job of making the sound a little more interesting.  At this time my Death metal pedal is unstable because I tried a different kind of clipping idea that the op amps didn't like much.

The feedback resistors in the gain stage are your key to decreasing gain.  If you get into messing with the resistors that are parallel/series to the caps, then you risk changing the sound of the pedal altogether (which may not be a bad thing).

[MetalUpYerEye]

So you mean the 220k that CoD mentioned and the 120k that cb mentioned are the keys to controlling the gain?

[Connoisseur of Distortion]

graphic eq in a stompbox... farther than i'm willing to go 

the feedback resistors set the gain in an opamp stage. I only took a quick glance and chose the 220k because i thought it would get rid of the rail-rail distortion. A closer look, and i see that the 120k stage gives a lovely 250+ gain. and is probably a more noticeable problem.