This project came off the back of the Rotund Robot. It is going to be an octave overdrive based on the original BOSS OC2 circuit.

If you haven’t read the previous posts about the Rotund Robot and can’t be arsed -Basically I started off with the idea of doing a pedal based on the BOSS OC2 synth mod. After breadboarding the full OC2 circuit, I realised it would be much easier to just start from scratch and do a square wave synthy CMOS fuzz with octave from scratch. The OC2 has a bunch of filtering in it to get the clean (though fun and wobbly) octave down. The synth mod basically bypasses all this, so why bother with it!? Well, I did start to bother with it, and I like the results, so now we have sausage fingers too.

So this here is the continuation of Rotund Robot (4), where I basically left off with the chopped OC2 circuit on my breadboard. In case you haven’t seen it here’s the vid from that last post

also, here’s the schematic of the chopped OC2, which is almost identical to what I have on the breadboard in the video above.

First things first, we need a new plan and something to aim for.

Sausage fingers in going to be an overdrive/ distortion with octave, so I’m planning on adding more gain, I’ll look at some clipping perhaps, and I have 1/4 of a TL074 going to waste, so we’ll see what I can do there too. So here’s it is:

version 1.0 – the chopped OC2 (done)

version 1.1 – add more gain

version 1.2 – add some diodes and see what happens

version 1.3 – use the last op amp stage of the TL074

version 1.4 – do we need a version 1.4? I don’t know.

So, let’s get on with it. Here’s my updated schematic

adding gain

Very simple. You see that resistor (R16) going from the output to the inverting input of U1A in the top left of the schematic? That fella determines the gain of that first op amp stage, if you think about it R16 and R15 are voltage divider right! To increase the gain, all I need to do is pop a pot in series with R16.

and here’s how it sounds now

 

I am thinking about changing the filtering on gain stage, but then I remembered that the filtering on this stage (R15 and C8) will affect the the signal going to the second octave, and that they’re probably best not tampered with yet, best to keep it simple for the time being. I’ll be coming back to this though. Especially as one thing I did try was popping a couple of red LEDs across the output and inverting input of the op amp though like this…    … and yes the schematic is getting a bit messy…

As you can see from the schematic I’ll pop a switch into the PCB to make this optional. Here’s how red LEDs sound.

It sounds promising, there’s plenty of gain, and the diodes definitely change the sound, but there’s more work to be done on that section.

In the next part, I’ll see if I can use that last section of the TL074 as a summing amp and cut out that last transistor stage. Should be simple enough (fingers crossed). I’ll also come back to the filtering on the the gain stage and do a brief analysis of what’s going on with some simple maths.