OCD style gain section - gain reduction

[Asdrael]

Hi there. First day, first thread! I decided to try my hand at building a pedal kit after building my first guitar as a change of pace. Since I really like the OCD, I am going for a AionFx Titan. I wanted to take it one step further and make it more my own by changing something I don't like on the original - it has too much gain for me, and feels like there is always a tiny bit of gain even with the pot at zero.

Now my electronic classes date back slightly, and I tried looking around, but thought I would engage here to double check and maybe even receive encouraging feedback (or bananas thrown at me because I am an idiot, I am fine with it too).

The gain section of an OCD should be a negative feedback op-amp looking like this (http://zadvitsky.com/index.php/projects/pedals/ocd-overdrive-diy.html)

Which would make the gain range follow this formula: k=1+(R6+R7)/R17. If I am not mistaken, that would mean the gain is always >1 due to R7 being >0 Ohm. I would like to:
- change R6 to a A500k to reduce max gain
- jump R7 to be able to set the actual min gain to 0
- not touch R17 because I think this would mess up the OP frequency response (?)

Does this even make sense? Will it break everything down? Will my house catch fire? I am simply trying to ease myself slowly into this to slowly make more and more customized stuff...

Cheers

 

[Cybercow]

Hi there. First day, first thread! I decided to try my hand at building a pedal kit after building my first guitar as a change of pace. Since I really like the OCD, I am going for a AionFx Titan. I wanted to take it one step further and make it more my own by changing something I don't like on the original - it has too much gain for me, and feels like there is always a tiny bit of gain even with the pot at zero.

Now my electronic classes date back slightly, and I tried looking around, but thought I would engage here to double check and maybe even receive encouraging feedback (or bananas thrown at me because I am an idiot, I am fine with it too).

The gain section of an OCD should be a negative feedback op-amp looking like this (http://zadvitsky.com/index.php/projects/pedals/ocd-overdrive-diy.html)

Which would make the gain range follow this formula: k=1+(R6+R7)/R17. If I am not mistaken, that would mean the gain is always >1 due to R7 being >0 Ohm. I would like to:
- change R6 to a A500k to reduce max gain

This is fine.

- jump R7 to be able to set the actual min gain to 0

No, leave R7as is, it seems to establish minimum gain at ~x1 (unity)

- not touch R17 because I think this would mess up the OP frequency response (?)

Changing R17 will change the frequency response. I'd leave it or or socket it and see. Altering the value of C7 will also change the frequency response.

Does this even make sense? Will it break everything down? Will my house catch fire? I am simply trying to ease myself slowly into this to slowly make more and more customized stuff...

Cheers

Makes sense. Nothing will break and the house won't catch fire. (But your riffs might. )

 

[Asdrael]

Cheers, thanks a lot!

No, leave R7 as is, it seems to establish minimum gain at ~x1 (unity)

What I meant with it but failed in expressing myself correctly was to keep the minimal gain value at 1 -unity, clean boost style- (so the R6+R7 term at 0). Right now with R7 as is, it would give k=9.18 which is still non negligible gain. Or is there something I am overlooking?

 

[Cybercow]

Cheers, thanks a lot!



What I meant with it but failed in expressing myself correctly was to keep the minimal gain value at 1 -unity, clean boost style- (so the R6+R7 term at 0). Right now with R7 as is, it would give k=9.18 which is still non negligible gain. Or is there something I am overlooking?

Don't think you're overlooking anything. My ears can't tell the difference between a unity gain and a gain of 9.18 using that math. To be sure, socket things before committing them to solder and see/hear.

 

[Asdrael]

Thanks a lot for taking the time to reply. There are a few things on the schematic I want to try out the values of before comiting so I'll order a few sockets. Learning is cool

 

[Chuck D. Bones]

You must be looking at an earlier version of the build docs, because the current version (1.0.1) has different ref des and at least one added feature.

Min gain = 1 + (R6 + R7) / R17 at mid frequencies.
As-designed, the midband gain varies from 9.2x (19dB) to 455x (53dB).
R17 works with C7 to control bass roll-off. If you like the bass response as-is, then leave R17 & C7 alone.
C6 works with the GAIN pot to roll-off high freq. Cutoff freq varies as you turn the GAIN knob.
If you're not using the top end of the GAIN knob's rotation, then consider reducing GAIN to A500K or even A250K. If the bottom-end of the GAIN knob's rotation is too touchy, then reducing the GAIN pot will fix that too.

You might want to try all of this on a breadboard first, rather than wreck the PCB changing parts.

 

[Asdrael]

I took the schematic from the website I linked as it was explained there, making it easier for my noobish self to grasp. The Titan has basically the same gain structure with as you mention a few more options.

Thanks a lot for the rundown. From the Aliexpress clone I have, and assuming the circuit is the same, it seems like I like the overall bass and treble response. I am just not liking how the gain ramps up. I took some measurements from my settings and I set the gain pot at 400k or so as a max usable value but my classical use is with the gain pot at 0 and it's sometimes even too much (I use it as a boost on a gain channel mostly). I am going to order a A500k, which should help, but will also experiment with R7 as you mention.

Cheers!