So now we have a firm starting point and some ideas, let’s get on with it! The immediate plan is to cut out that last transistor stage and use the last section of the TL074 – which is currently just hanging about doing nothing – as a summing amp.
Before anything else, I redrew the schematic (I also changed the references on the components to keep things logical) thusly.
As you can see I had to make a decision about moving the octave 1 pot. It was before the mids boost, but I figured if I was going to be using a summing amp as an audio mixer for the direct level and octave 1, then I would need to have both of those pots going into the summing amp, otherwise I’d have to add an extra pot or do some experimenting with balancing the two signals. So I took out the pot, but left the cap, as so.
So now the full octave signal goes straight into the mids boost and the level of octave 1 is controlled by a pot that comes after the mid boost and straight into the summing amp. I moved the direct level pot on the schematic to neaten it up too. I found as many pictures of summing amps (audio mixers) as I could for an idea of values and configurations. Here’s what I settled on for my first try. I also added a volume pot for the sake of it.
After playing around I made a couple of very simple, common sense adjustments. I always believe it’s worth trying things out, even if you think it won’t work. Especially if like me you’re approaching this from the perspective of learning from your experiments rather than an EE (electrical engineering) background. It’s always best to try it out!
Those adjustments are as follows:
- I took out the volume pot, and replaced it with a 100K resistor to ground. It occurred to me that it’s not really loud enough to warrant a volume control. You can manage the levels with the gain control and the two pots going into the summing amp.
- I changed the direct and octave 1 pots from B100K to A100K
Anyway, it worked so here’s a wee video
In part 3, I experiment some more and simplify the circuit.