MDMA bass mod
- Thread starter mywmyw
- Start date
Chuck D. Bones
Circuit Wizard
I have not built one, but I know what I would do if I wanted to retune the BASS control.
But first, are you positive that you used the correct parts for R9, C6 and the BASS pot?
Do you overdrive this pedal with something else, or go straight in with your guitar?
Where do you like to set the GAIN pot?
R9 & C6 roll-off the gain below 3.4KHz, but the BASS pot limits the gain drop to -20dB.
Depending on how you answer the three questions above, the recommended mod might be to increase the BASS pot to C50K or even C100K.
Another option is to decrease C2. C2 and R2 set the low freq roll-off at 16Hz. Lowering C2 might get you where you want to go. Best thing about that mod is you can try it without even opening the pedal. Just insert a 1nF capacitor in series in the input cable. If that gets you in the ballpark, try other values. Smaller cap = less bass.
But first, are you positive that you used the correct parts for R9, C6 and the BASS pot?
Do you overdrive this pedal with something else, or go straight in with your guitar?
Where do you like to set the GAIN pot?
R9 & C6 roll-off the gain below 3.4KHz, but the BASS pot limits the gain drop to -20dB.
Depending on how you answer the three questions above, the recommended mod might be to increase the BASS pot to C50K or even C100K.
Another option is to decrease C2. C2 and R2 set the low freq roll-off at 16Hz. Lowering C2 might get you where you want to go. Best thing about that mod is you can try it without even opening the pedal. Just insert a 1nF capacitor in series in the input cable. If that gets you in the ballpark, try other values. Smaller cap = less bass.
I also haven't built it but Chuck's ideas are spot on. I'll offer 2 more ideas.
Off the 1st op amp, there are 2 paths that affect the gain at different frequencies. Path 1 is made by R9 and C6. Path 2 is made by R9 + position of Bass pot, and C7. For each path, less resistance equals more gain. Also for each path, the cutoff frequency is defined by 1/(2piRC). This is discussed in this article in this section. https://www.electrosmash.com/tube-screamer-analysis#non-inverting
My ideas are:
1. Reduce C7 to 470nF (adjust to taste). This will mean that when the bass pot is cranked, the cutoff frequency for the bass frequencies will be 340Hz (previously 72Hz), so you will be boosting low mids instead of bass.
2. Increase C6 to 100nF or 220nF. Currently, that path has a corner frequency of 3.4kHZ, which is quite high for guitar. Everything below that is being cut (including your mids). If you move this corner frequency lower, you'll get more mids, so you'll have more differentiation between mids and bass, which might be what you want (especially combined with Chuck's idea to use a larger bass pot).
Off the 1st op amp, there are 2 paths that affect the gain at different frequencies. Path 1 is made by R9 and C6. Path 2 is made by R9 + position of Bass pot, and C7. For each path, less resistance equals more gain. Also for each path, the cutoff frequency is defined by 1/(2piRC). This is discussed in this article in this section. https://www.electrosmash.com/tube-screamer-analysis#non-inverting
My ideas are:
1. Reduce C7 to 470nF (adjust to taste). This will mean that when the bass pot is cranked, the cutoff frequency for the bass frequencies will be 340Hz (previously 72Hz), so you will be boosting low mids instead of bass.
2. Increase C6 to 100nF or 220nF. Currently, that path has a corner frequency of 3.4kHZ, which is quite high for guitar. Everything below that is being cut (including your mids). If you move this corner frequency lower, you'll get more mids, so you'll have more differentiation between mids and bass, which might be what you want (especially combined with Chuck's idea to use a larger bass pot).
mywmyw
Active member
Yes i've built a few of these, compared them with online demos of the original. brian wampler even made a video addressing the bass control because it's a notoriously bass heavy pedal. It's just inherent in the design.
I go straight in with my guitar, or at most a wah (the compression of the mdma softens the wah quack a bit, it's nice for live playing where you don't wanna spank people in the face with treble). in situations where i need a spanky but dirtyish clean, this is a very very nice setting, but sometimes when im playing some ripping country for example, it would be nice to get a little more gain and sizzle during a solo. as it stands, i get a great low gain rhythm tone with the pedal. i've tried stacking with the tumnus ahead of the mdma, and that works okay, but it is still quite bassy. this pedal eats up quite a bit of treble which is generally useful, but it makes it hard to stack with other pedals. perhaps i should dig out the pantheon clone i built and put it ahead of the mdma, it has gobs of treble.
I usually set the gain pot about half, or at most, 2 or 3 oclock, but that's mostly because of the nature of the bass overloading the circuit at that point. I think increasing the bass freq to 160 hz or so would clean up the sound considerably, just my gut instinct. i'm a fairly decent audio engineer but i am still learning how to read schematics and understand these types of circuits so there may be more at play that i don't understand. ill try your guy's ideas and report back. thanks yall. i appreciate it
Fantasticky
New member
Hey, did you make those mods ? Which ones did you do and how did it turn out ? Building one up soon.