Zlob Modular graciously sent me schematics for the VC F3DB. I can’t share them here. Zlob schematics are available directly from Zlob, if you ask. One reason I wanted them was to explain the curious fact that Input 1 can produce an output signal (as I described in the functional video). You get an output on Input 1 only when the Gain Feedback switch is engaged. And the level of the output depends both on the Gain pot and the Gain Feedback pot. The schematic showed me how this was happening, and I got an idea for a simple modification.

Purpose of the modification

The idea is to separate Gain Feedback from Input 1. A very simple way to do this is to repurpose Input 1 to be the direct output of the distortion circuit. That’s it! Now Input 2 is the only input to the gain/distortion circuit. And Input 1 is always the output of the gain/distortion circuit. (And remember, the output of gain/distortion goes into the filter bank.) Gain feedback works exactly as before.

Advantages

The gain/distortion circuit can now easily be used independently of the filters. As before, the initial gain is unity, i.e. the output signal is about the same level as the input when the gain knob is all the way down. Turn up the gain and there is a good deal of amplification available. Good for boosting any signal in the modular synth. Then you have the soft and hard clipping switches to add distortion. And you can still add gain feedback, though it’s best to combine it with clipping or the signal rapidly reaches the power rails. You can even use the filter Feedback path to add in signal from the filter output mix.

Disadvantages

You can’t mix two signals together into the gain/distortion circuit. That’s about it.

How to make the modification

The goal is to disconnect the jack for Input 1 from the Gain Feedback and connect the output of the gain/distortion to the jack. You have to cut one circuit trace and add one resistor. That is all. I’d recommend you have basic DIY skills before doing this mod.

The trace to be cut is on the side of the jack PCB that the jack is mounted on. (The most tedious part of the job is taking the panel off and putting it back on. Patience helps.) I removed the Input 1 jack to be completely sure I was cutting the right trace. But it turns out you can cut the trace without taking off the jack.

I used an X-Acto knife to cut the trace. If you’re used to DIY work, it’s not difficult.

One more thing I learned from the schematic

The Feedback In jack does not override the feedback from the filters. Rather, it sums with them. When you patch to Feedback In, the signal from the filters is halved. You can easily test this by patching a dangling cord to Feedback In. You’ll see that you still get filter feedback. I was rather surprised by this. It would take another mod to make Feedback In replace filter feedback. But I’m not sure it’s important.