The Rotund Robot (4) – diverging plans and sausage fingers

So! This post is great example of how things change when you’re experimenting. My original plan with the Rotund Robot was to make a synthy octave pedal starting with the BOSS OC2 synth mod I talked about in part 1. In the last post I mentioned my plan to start with the original OC2 circuit and slowly go from there, changing bits as I go along.

I started off as planned (see part 3) with version 1.0 (the full and original BOSS OC2). I noticed there were a couple of small errors in my layout, tweaked them accordingly and got the breadboard populated. For some reason I couldn’t get the second octave down to work at all. I’m still not sure why, but I wasn’t too concerned about that. I wasn’t 100% about whether I wanted to use it anyway, and as you’ll see I’ll be coming back to that later. So, I took out all the bits and bobs for the second octave.

The next issue was that octave 1 seemed a little quiet. That’s why I decided to skip ahead with my plans and include the mid boost section from the chopped OC2 (see last post), which worked well, it makes the octave 1 sound much better. I also took the out from 12 (not Q) instead of 13 (Q), I’m not sure why bit this also sounded better. At the minute I can’t explain this confidently, but I ‘think’ the difference between Q and not Q (Q with a little line over it) is phase. It’s possible that the two outs from the LM324 are the wrong way round and that caused me to have to use pin 12 instead of pin 13. I’ll have to check this before I get the PCB done. More on this at the bottom of this post.

Here’s a quick vid of the Chopped OC2.

After making this video I jumpered the direct out from some other parts of the circuit. The distortion you hear in the video vanished, but the signal had been through some fairly heavy low pass filtering so it sounded pretty muffled. I also tried taking ground from different areas of the breadboard for that first op amp, but that didn’t do a thing. BUT! I did realise that as I had a lot of long jumper wires from the LM324 right next to the input jumper (yellow), so I moved the input away from those, and it became much quieter (see below). So I’m putting this down to the peculiarities of breadboarding and moving on. The thing that baffled me was that the noise was definitely coming from the first op amp gain stage, but I was sure the noise was linked to the LM324 or the CD4013 because it sounds like a square wave, and those two ICs were nowhere near that first TL072. Anyway, problem solved for now, and something else to bear in mind when I design the PCB.


I also tried bridging the section in the original synth mod, it sounds OK, but there is some noise when you stop playing, but I don’t need to worry about this for reasons easily explainable and explained below.

My new findings gave me plenty to think about. I had also been re-reading some of the material on CMOS octave up from Parasit Studios (highly recommended reading) for another project.

You can probably hear in the video that when you boost the mids you’re getting into overdrive territory. That’s why I’ve decided to separate this into two projects. The Rotund Robot will continue but it will be a dedicated CMOS project, and this circuit will become an octave overdrive/ distortion and will now be known as ‘sausage fingers’ unless I can think  of a decent name for it. I’m going to continue with it and try to get some prototype PCBs done in the next batch.

I’m going to come back to the Rotund Robot concept shortly and it will be a full on synth octave fuzz. I’m planning on octave down (probably two octaves down), octave up, some pulse width modulation (PWM) and I also read something a while ago about portamento in CMOS, so I’ll definitely be giving that a shot!

I had a lot of ideas for the Rotund Robot, probably too many to include in this circuit. It’ll be much simpler to have a square wave fuzz without all the filtering included in the OC2 circuit. It also makes more sense to continue with ‘sausage fingers’ in a way that’s more natural to the OC2 circuit. There’s still plenty of learning to be done here, but this feels like the neatest way to proceed.

Anyway, I’m now planning to do a separate series of posts specifically about experimenting with CMOS that will run parallel to a couple of projects, including sausage fingers and the rotund robot. I’ll try to get into the nitty gritty of how CMOS works through experimenting. I’ll also find out what the craic is with Q and not Q.



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