JackKnife Foldback Box
Turn Any Dirt Box Into a Foldback Dirt Box
Overview
In the world of synthesizers, foldback distortion has been around a while. However, in the guitar world, it simply isn't that common. What is foldback distortion? Well, there is a fantastic writeup on it here. The one issue that most foldback distortions have is that they only allow for a single clipping section. The JackKnife, however, does all the necessary signal stuff while allowing you to put any dirt box in the loop and it will do the folding back. In fact, you don't even have to use dirt in the loop, you can use whatever and create previously-unheard-of sounds and textures. Give it a try!
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The JackKnife also has a couple of fun options available to create even more weird sounds. The mode toggle switch makes it so that the bypass switch does one of two things. First, it can act as a standard bypass where you have your clean signal when bypassed and the folded back dirt signal when engaged. In the second mode, though, you can have your regular dirty signal when bypassed and the folded back version when engaged. Another fun feature is that there are two sets of pads for foot switches so that one can be latching and one momentary. This means you can play with regular dirt and just punch in folded back dirt for texture momentarily. The final feature is the Mix control, which adjusts just how much folding back there is, from none to full on.
How It Works
At it's heart the JackKnife is two parallel signal paths that get blended together at the end. An input signal is buffered and split. One path creates the inverse of the signal while the other path sends the signal out of the loop and then buffers it upon return. I'll show you what I mean.
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Let's start with the input buffer. This is a simple inverting input buffer with pulldown resistor. The pads shown are for the two foot switches.
JackKnife Input Buffer
After the input signal is buffered, the signal is split by using two more unity gain, non-inverting buffers. This is to make sure that there is no interaction between the two paths, since allowing any circuit in the loop means that we can't know exactly what is safe to assume. The output from one of the buffers goes to the Loop Send, marked LS.
JackKnife Signal Splitter
The loop return is a straightforward non-inverting opamp stage. The gain is determined by the one side of the dual-ganged Mix control. Mix controls the levels of the two parallel signal paths, resulting in full folded back dirt at max and increasingly less folding back until there is none happening at minimum Mix.
JackKnife Loop Return Buffer
The phase inverted signal path is a simple inverting opamp gain stage with the other side of the Mix control setting the gain of this path. An inverting opamp stage can go to zero gain, which is what happens at minimum Mix, with a maximum gain set the parallel combination of Mix and R7, which is approximately 6 dB.
JackKnife Phase Inverting Stage
The outputs from the two parallel signal paths are then summed together and sent to the output. This is a simple unity gain summing amplifier with an output Volume control, since it is possible for the signal to get up to 6 dB louder than the input (louder depending on the volume of the signal coming back in the loop).
JackKnife Output Buffer
As mentioned earlier, the JackKnife has a Mode toggle switch. In one direction, the footswitch bypasses the pedal entirely, but in the other mode, it simply bypasses the signal splitting, phase inversion, and summing, so that whatever is in the loop is always active. The actual schematic for this is a little ugly, but read the description while looking at it and it helps.
JackKnife Switching
The last bit of the circuit is the power section. It's super simple, just some filtering and a reference voltage for biasing the opamps.
JackKnife Power Section
So with all this power at your fingertips, what will you do with it? Put an octave pedal in the loop? A fuzz? A modulation?! What kind of a mad man (or woman) are you?! Get everything you need to build on here (coming soon).