The Ratstortion (Modified Kokko Distortion clone)

[Cybercow]

After finding the Kokko Distortion schematic, I traced the Kokko unit I had to check if the one I found is correct. My tracing matched the found schematic perfectly. But when I breadboarded it, it didn't sound like the original. I repeated the breadboarding a few times to be sure I was doing it right. But it just didn't sound right. So I had to tweak some things to get it to sound like the original. The most impactful change was to use an LM358 (0.3V/µs slew-rate - like an LM308/µA308/OP07) in place of the original LM4558 (1.7V/µs slew-rate). I also tweaked few resistor and a coupla cap values.

Original schematic . . . . Traced and updated schematic . . . .


Then, some help from @rwl and Bill Carrigan, I was able to get a set of Gerbers for PCB fabrication. (Thanks fellas!)
Then on to the builds themselves . . . .




I also wanted to have one where the Gain control was actually two separate controls instead of a dual-ganged pot. I used a 9mm Alph concentric pot so I could use the same PCB and drill template. Having the Gain pot split into two controls adds a bit more dimension to the adjustability of the two separate gain stages. And I think the finished build still looks good too.



Thanks for looking. Happy building folks!

 

[dcfvgb]

Love the graphics.

 

[jwin615]

Nice! Haven't heard this pedal but I remember you mentioning it a while back.
It looks like the original used 4v as virtual ground/Vref, which would/could cause asymmetrical opamp clipping. Would give you 4v and 5v headroom, clipping the negative portion of waveform sooner and more drastically. its then inverted and repeated. Wonder if that is some of the difference heard?
You could just tweak R19 and R20 to test it if so inclined.

 

[jcpst]

The concentric pot is rad!

 

[jimilee]

After finding the Kokko Distortion schematic, I traced the Kokko unit I had to check if the one I found is correct. My tracing matched the found schematic perfectly. But when I breadboarded it, it didn't sound like the original. I repeated the breadboarding a few times to be sure I was doing it right. But it just didn't sound right. So I had to tweak some things to get it to sound like the original. The most impactful change was to use an LM358 (0.3V/µs slew-rate - like an LM308/µA308/OP07) in place of the original LM4558 (1.7V/µs slew-rate). I also tweaked few resistor and a coupla cap values.

Original schematic . . . . Traced and updated schematic . . . .
View attachment 100815 View attachment 100816

Then, some help from @rwl and Bill Carrigan, I was able to get a set of Gerbers for PCB fabrication. (Thanks fellas!)
Then on to the builds themselves . . . .

View attachment 100817
View attachment 100818 View attachment 100819

I also wanted to have one where the Gain control was actually two separate controls instead of a dual-ganged pot. I used a 9mm Alph concentric pot so I could use the same PCB and drill template. Having the Gain pot split into two controls adds a bit more dimension to the adjustability of the two separate gain stages. And I think the finished build still looks good too.

View attachment 100820 View attachment 100821

Thanks for looking. Happy building folks!

What does the optional mod do?