Since first reading about the Parker Fly in the early 1990s, i was captivated by the idea of having access to both electric and acoustic sounds on one guitar, and being able to flip between them (or blend them) at will. In many ways, this was “the sound” i had been hearing in my head for years, and finally there was a guitar that could do it.

Years later, i finally acquired a stunning Parker Fly Classic, and it did not disappoint. Well, except for the fact i found that i wanted to be able to move between the magnetic and piezo sounds while playing, and with two hands already occupied with picking and fretting, that seemed like an ideal task for one of my feet.

Read on…

I was surprised to learn that nobody had made a true “blend” pedal that worked the same way the magnetic/piezo balance pot worked on the Fly. At one end of the knob’s travel, it was full magnetic. At the other end, it was full piezo. In the middle, it was 100% of both, with any blend available between the two extremes.

Stereo volume pedals tracked two inputs and two outputs in parallel. Pan pedals usually swept one input between two outputs. And the occasional “blend” pedal i found either allowed the player to add the piezo sound to a fixed magnetic sound (like the Fishman PowerBlend pedal) or didn’t allow 100% of both signals in the middle position.

So i took it upon myself to design and build the pedal i envisioned.

My design criteria were that the pedal had to work exactly as i wanted and be a 100% passive design, so i wouldn’t have to mess around with preamps, buffers, batteries, impedance matching and so on. I wanted it to be universally workable for anyone with a magnetic/piezo equipped guitar like a Parker, a Music Man, or any guitar equipped with an aftermarket bridge and preamp from Fishman, L.R. Baggs or GraphTech.

I had to make a few assumptions, the first of which was that the guitar’s magnetic signal would be buffered by the piezo’s preamp. I know that GraphTech’s AcoustiPhonic preamp does. The advantage of this is that a 25 kOhm pot can be used because both circuits are active. If the magnetic signal is not buffered, then a 250 k, 500 k or 1 Meg pot would have to be used.

I found empty wah pedal shells available at Small Bear Electronics, bought blend pots through StewMac, found various jacks locally, and set to work creating the first prototype.

In my R&D process, i discovered that the automatic stereo/mono switching used in some preamp circuits (such as the AcoustiPhonic and newer Parkers) presented a problem. From my testing, it appeared that a tiny amount of voltage is sent down the piezo (ring) signal wire, and when that voltage is detected on the ground circuit (which would happen when a mono TS plug is inserted in a stereo TRS jack… the ring would be shorted to ground), the preamp switches into mono mode, sending both magnetic and piezo signals down the “tip” of the connector. It’s my understanding that newer Parkers that lack the red mono/stereo button next to the output jack also work in this way.

Wiring the pot backwards succesfully isolated the voltage from the guitar’s onboard preamp, but resulted in a faint “zipper-like” sound as the pot was moved through its range.

I worked around it by putting an effects insert in the piezo circuit, where i placed a Tech21 Acoustic DI. This active preamp effectively isolated the voltage from heading back up the ground wire to trip the preamp into mono mode. But it didn’t meet my “100% passive” rule, so i knew i wasn’t done.

I left it alone for a while, because in my pedalboard at the time, it worked. But as my rig evolved and changed over time, i knew i’d have to revisit it. Further research into balanced and unbalanced circuits gave me an idea to try a “pseudo ground lift” whereby a resistor and a capacitor in line with the ground, which filters any voltage from travelling back up the ground wire. Eureka!

And here i present to you the schematic diagram for the pedal:

Click here to download it in PDF form.

I don’t believe in keeping what seems like a common-sense solution a “secret”, or trying to apply for a patent for what is essentially a blend pot stuffed into a wah pedal casing. (Chances are, a patent would not be granted for such an obvious solution.) So i’ve published the schematic here for anyone who wants to DIY their own pedal together, or improve upon my design.

On customer request, i will be including one of these pedals with each Decibel guitar that’s equipped with a GraphTech AcoustiPhonic piezo system. In my opinion, it’s the simplest, most elegant way to handle the switching or blending between magnetic and piezo sounds in a hybrid guitar.

Currently, i’m using a Dunlop-style wah pedal shell from Small Bear, but i would love to be able to retrofit this into an Ernie Ball VP or VP Jr. pedal. Unfortunately, the Ernie Ball pedals use a long-shaft pot, and i have not been successful in sourcing a long-shaft blend pot that would make this retrofit possible. If anyone can point me in the right direction, that would be great!