Better tone control on a DOD Grunge distortion pedal

[Fr_3_aK]

Hi guys,

I've started starting modifying ever single piece of music equipment i have, im addicted and cant stop.
One change i made to my guitar was replace the tone pot with a 1M ohm from a 500k. This boosted the treble.
If i had've put in a 250k pot, it wouldve made it warmer (or bassier).

So on my DOD Grunge pedal, ive noticed when you turn it on, alot of the bass gets cut out of the tone.

There are 4 knobs on this pedal:

Volume, bass, treble, distortion gain.

Using the tone pot change theory, can i replace the 'bass' pot with a lower value and get back some of this lost bass tone?
Or could there be something else responsible?

Any advice is appreciated 

[Somicide]

Somewhere a cap could be changed to a lower value; not sure where, when I modded this pedal it was simply IC swaps.

[Mark Hammer]

Looking at the schematic, it's a touch on the complicated side.  There are a whole lotta capacitors in there that can have an impact on the bass, so changing one will not have any drastic impact on increasing bass.

If you want to mod, I would suggest something simple like swapping out the two silicon diodes (D2, D3) for red LEDs.  That won't give you more bass, but it will give you more volume.

[Stompin Tom]

I had an old DOD dist box I tried to mod some time ago... It really needed all the help it could get. I don't know why they insist on taking all the lows out of their distortions... I couldn't figure out how to get it sounding good. I gutted it and put a fuzz in the box... I recommend you do the same. The distortion plus is easy to build and mod and understand.

Unless, of course, you like the way it sounds (but then why mod!).

[pyrotek]

Tone pot change theory is not applicable since it is an active tone control circuit.

Do what Mark Hammer & Somicide suggest.

Changing the tone pot of your guitar from 500k to 1M did not actually boost the treble since it is a passive tone control circuit, what really happened is that by changing to a higher value you are  allowing more treble to pass thru (lesser signal is shunt to the ground). The same with the volume pot, if you have a lower value then more of your sound gets shunt to the ground. Your are actually lossing treble not boosting your bass.

[runmikeyrun]

agreed.  get rid of the circuitry and put something else in the housing.  That's exactly what i did to my grunge pedal.  It sucked.  Interestingly, i had one of those tec 8 pedalboards back in about 2000 and the grunge distortion with speaker sim sounded fantastic on bass.  I used it in the effects loop and mixed it about 50/50%.  I could never get the pedal to sound good.

[Fr_3_aK]

Thanks for the responses guys.

I do actually like how it sounds, i just wish it didnt cut the bass out.
Its not my only source of distortion, my Marshall amp does a great job, but for thrash and heavy metal i need this pedal.

Maybe i'll just buy (or make if there's plans here) an EQ pedal.

[Mark Hammer]

The Grunge doesn't really cut the bass out.  Rather, it would seem that there are a couple of resonances added along the way which shine too much of a potlight on the mids for your tastes.

Here is the schematic, or at least one version of it drawn by someone.
http://www.freeinfosociety.com/electronics/schematics/audio/pictures/dodfx69b.jpg

In the upper left hand corner, you can see an op-amp with a 120pf ap and 220k resistor in the feedback loop.  The little subcircuit hanging off it, built around a transistor, is a gyrator circuit that behaves like a a preset EQ section.  Though built around a transistor, you will note its similarity to the subcircuit built around an op-amp for the "Low" EQ section over on the right.  Read the document on simple and easy parametric EQ over at www.geofex.com and you'll understand it better than I can explain it.

Immediately after the pair of back-to-back clipping diodes, you can see another op-amp section with two of these resonant subcircuits.  So, in total, we seem to have one resonant boost before the clipping and two selective resonant boosts after the clipping.  If the accentuation that they provide does not warm the %^&*les of your heart, then perhaps the bestthing to do is to simply change them from selective boost to full boost.

So, cut the trace in the pre-clip resonant boost section between the 27n cap and the other stuff shown below it.  Stick a 100k resistor between that and ground (what is shown as -4.5V), and your first op-amp becomes a gain stage of x3.2 with a low end rolloff of just under 60hz.  That, times the max gain of the op-amp stage immediately after it (x235), gets you an overall amplification factor of x752, which is, um, more than enough to make the diodes clip, and certainly more than enough to make red LEDs clip pretty hard.

For the post-clip resonant stage, you may simply want to cut the traces that link the 330n and 15n caps to the 120pf/3.3k network and that pin of the IC.  This will turn that op-amp into a unity-gain stage with no selective boost.  From there you can simply apply whatever bass boost/cut the tone controls provide.

If you do any or all pats of the suggested changes, certainly let us know how it turns out for you.

[Fr_3_aK]

this link doesnt work 

http://www.freeinfosociety.com/electronics/schematics/audio/pictures/dodfx69b.jpg

[Stompin Tom]

Go to freeinfosociety.com... I can access it through their website.

[Fr_3_aK]

For the post-clip resonant stage, you may simply want to cut the traces that link the 330n and 15n caps to the 120pf/3.3k network and that pin of the IC.  This will turn that op-amp into a unity-gain stage with no selective boost.  From there you can simply apply whatever bass boost/cut the tone controls provide.

If you can explain how i "can apply whatever bass boost/cut the tone controls provide" after i do this mod, i'll give it a shot.
I finally found the schematic, and with my limited knowledge of circuit jargon i think i figured out what you wanted me to cut and where.
Finding the spot on the actual PCB will be another story 

Here's where i think i have to sever the the traces

[Mark Hammer]

You understand more than you think. 

Although a correspondance between parts and part numbers would likely make it easier to find stuff on the board, happily the caps in question tend to be unique values on the board.  As well, when you can buy stuff in large volumes, as DOD can, then it is a reasonably safe thing to say that you can use cap size as a clue for locating them, since .027uf  (27n) and .33uf (330n) will be larger than many of the other plastic nonpolarized caps there.

So, to summarize, cut the trace between the 27n cap and the 10n/2k parts, NOT betweenthe 27n cap and the feedback loop.  Then solder a 100k resistor between the 27n cap and ground.  I would suggest that: a) you use a 1/8W resistor that will lie nice and flush against the solder side of the circuit board, and b) you fit a piece of heat shrink tubing over that resistor and its exposed leads so that it doesn't short out anywhere.  If it should turn out that you like the resonant boost in that section, then restoring it is a simple matter of scraping away enough of the coating over the traces that you can re-bridge what you cut with a small wire link soldered in place.

The other cuts you highlight are also the ones suggested, and can be restored in the same way.  You might want to cancel one of those resonant sections at a time.  In view of your complaint about excessive treble and thinness, cut the trace to the 15n cap first, so you will still have the other resonant section working.

[Fr_3_aK]

omg i spent 4 hours staring at this thing and i finally get it!!

Now all i need is a tiny drill bit to sever the trace (or what would you guys recommend?)

Big thanks to this website for my crash course in resistor IDing
http://www.dannyg.com/examples/res2/resistor.htm

[Fr_3_aK]

Done and done!

I just heated up one end of the 15n cap (the yellow one that says 0x 1533 on it) and pulled it out.


What does it sound like?
Exactly what i wanted, the bass is back (i dont care how or why  ).  Only one problem, i burnt my finger on the soldering iron and cant use that finger to play with now!!!


I'd like to express my love to Mark Hammer for being such a genius! Kudos!!!

[Mark Hammer]

Done and done!
I just heated up one end of the 15n cap (the yellow one that says 0x 1533 on it) and pulled it out.
What does it sound like?
Exactly what i wanted, the bass is back (i dont care how or why  ).  Only one problem, i burnt my finger on the soldering iron and cant use that finger to play with now!!!

a) You're welcome.
b) Thanks for reporting back on what precisely produced the desired improvement.
c) You'll just have to learn to play like Django Rhinehardt now.
d) My thanks to all those folks, especially RG, for helping me to easily identify resonant sections such as the ones used in so many DOD distortion pedals.

A little theory, a usable schematic, and you can get downright dangerous, can't you?

[Fr_3_aK]

lol why didnt i use this schematic?

http://www.diystompboxes.com/pedals/dodfx69.jpg


rofl

[Mark Hammer]

Wait, it gets better.  I actually had that one and would have referred to it (because the parts are numbered), but I didn't know where I got it from.