This is a long story. It begins at the turn of the century when my pal Christoph traveled New Zealand and was gifted an old Jim Dunlop Crybaby wah-wah pedal. After playing around with it and realising that he was not a reincarnation of Jimi Hendrix, the thing went into the parts bin where it stayed for one and a half decades. Chris is still a bassist and uses electronic effects in his music. And he decided that he wanted to have a device that allowed him to gradually blend these in.
Of course this isn’t an exotic problem and there are commercial solutions readily available. They are called ‘parallel loop mixers’ and higher end units can be controlled with an external foot pedal:
The catch here is the ‘higher end’ part. Converting the old wah-wah into an expression pedal is trivial, but the parallel loop mixer would have to be bought and then clutter the floor. These things cost money. Chris put his superpower (obsessively researching stuff on the internet) to work and came up with a cheap kit for a parallel loop mixer. The kit didn’t offer the ability to hook up an external pedal but used a regular potentiometer to fade between wet and dry signal. The Crybaby features a potentiometer in a rack and pinion assembly controlled by the pedal movement, so we decided that we would put the mixer inside the pedal and replace the mixer’s pot with the one in the pedal.
The paraLoop is a simple device, and well documented (pdf). It consists of an input buffer stage followed by a simple splitter, followed by one buffer stage for each signal path and an adjustable output amplifier. The wet and dry signals are combined by a potentiometer acting as voltage divider before they are fed to the output amplifier. Besides the buffers this is the simplest possible design. Since audio signals are DC, the buffering would require a power source that provides negative and positive voltages for the OpAmps to work. Alternatively one can add a DC bias to the signal and remove it again after buffering. This avoids adding a possibly noisy negative voltage source and is the solution used in the kits circuit.
Chris ordered the kit, soldered everything in place and after replacing the power LED everything worked as expected. We tested it with a cheap bass driver pedal in the effect loop and found the paraloop working fine when the blend pot was at either end but audibly attenuating the output when the blend pot was half way. This didn’t happen when we replaced the bass driver with a pach cable and thus we suspected the bass driver to invert the signal. With a smartphone as signal generator and my nScope we found out that this was indeed the case. This behaviour is not uncommon among effect pedals and so we decided to upgrade the paraloop to be able to deal with this.
Chris and me being total noobs at analog circuits didn’t help and it took us two days of breadboarding intermingled with occasional handwringing to come up with the final solution. The most helpful tool turned out to be the falstadt circuit simulator. We cut a trace on the board to insert a fifth opAmp behind the buffer stage of the return line that was wired as inverting amplifier with unity gain. In this location the new stage shares the filter capacitor with the return buffer, so we didn’t have to take care of biassing and debiassing. We also added a switch and tapped into the VCC and VVC/2 lines of the board.
With just three parts the pedal gained the ability to invert returning signals at the flip of a switch. The previous owner of the wah-wah had already modified the device by adding a switch to change the frequency response of his pedal, so we already knew where our switch would go. Two components and a switch don’t justify their own board, so we hooked them up dead bug style. The opAmp lost some of its legs in the process.
The circuit was tested and we crammed everything into the wah-wah’s enclosure. Which required some drilling, since the pedal originally only has two jacks: In and out, not even one for power as it is supposed to run on a battery. We needed additional holes for send, return, power, the boost knob and the indicator LED. Note that although people may tell you otherwise, a hole enlarging drill bit and a cheap Dremel knock- off are no satisfactory replacement for correct size drills. At least nobody got hurt while discovering this.
With everything in place, Chris decided that the pedal lacked functionality. As said above, turning a wah pedal into an expression pedal isn’t that hard. All that’s required is a TRS connector wired to the pot. The paraloop features such a connector at its input side although the device is mono. A plugged in mono jack shorts the ring and sleeve and in the process closes the circuit that provides battery power. This is to prevent the battery from being drained when the device is not in use. Since we had ditched the battery (You had to unscrew the enclosure to change it originally), this function was no longer required. We added another switch that allowed to route the pedal’s pot directly to the input jack. It also cuts the power to the mixer to avoid frying the circuit when used as expression pedal. The extra switch is the 4P2T thing in the upper right corner of the photo above.
Now Chris has a blend pedal with a lot of character that can also be used to control his virtual analog synth or anything else that has an expression port. Additionally we now have the circuitry of an antique wah-wah lying around which may or may not get its control pot replaced with a digital potentiometer and a microcontroller to add a MIDI interface. Until then have some more pictures of the modified pedal, which Chris hasn’t named yet (While I like to call it the Frankenpedal).