Two very different pickups, one instrument

My latest ukulele is an exercise in versatility: short enough to go inside a suitcase, long enough to be remain playable under heavy capo and, of course, both acoustic and electric. And not just merely electric, but featuring both a passive piezo pickup and a magnetic pickup, which can be mixed in any ratio. I looked for a wiring diagram that could do this, and found nothing simple. They all required a switch to select the main pickup, with perhaps the ability to add a bit of the other, or had something weird about them. Since the two pickups are very different electrically, there was no assurance that they would mix well. I took a guess, and it worked, though theory predicted that it shouldn’t have. Read on for the solution.

The aforementioned wiring that allows you to mix in a bit of a second pickup after you have selected one is in the picture at left. The blend pot adds from a trace to 100% of the unselected pickup, but it still requires a switch. The one at right, which is actually for a bass guitar, does not require a switch, but if you look carefully you’ll notice something weird: the hot wire from either pickup goes to the wiper (center electrode) of its respective pot, not to one of the ends. This means that the resistance in series with the signal path (when the pot is not at 100%) is after the bypass to the ground, not before. In other words, the pots don’t work as typical voltage dividers.

I thought this was too weird, so I ended up making a circuit patterned after the third diagram, which differs from the second only in that the signal from each pickup goes to the end of its respective pot, and the output is picked up in the middle, as is normal for a volume control.

I tested it with all pots set to zero, and there was no sound as expected. But then, when I moved just one of the pots to get some sound from one pickup with the other volume still at zero, still there was no sound. What?

Fortunately, I found that everything works fine if you give a little juice to the pot at zero. After that, the more you give to either volume pot, the louder the contribution of that particular pickup. You can mix them 50-50, or give 100 percent to both, if you want. Better still, the whole thing works although the impedances of my two pickups (magnetic and piezo), are very different.

So  why don’t I get any sound when either volume pot is at zero? Because then I’m grounding both pickups, not just the one whose hot wire is connected directly to it. Since the output electrodes of both volume pots are connected to each other, grounding one by setting the pot to zero grounds the other as well. The amazing thing is that just a little bit of resistance allows the other pickup to come through without any problems. Likely a consequence of the high 500 kOhm value of the pots, so that a little separation from ground voltage allows sufficient current to go to the output. The fact that the amp has a high input impedance doesn’t hurt, either.

So give it a try for your builds and see how you like it, or try the one with the hot wire going to the center electrodes, and tell me how they compare.

Before I finish this post I want to bring you attention to the wonky bridge of the instrument, seen on the right side of the very first picture. Normally, this type of homemade instrument would have a metal bolt or a wooden dowel for bridge, but I couldn’t find anything fat enough for this particular one. On top of that, I wasn’t sure of what the correct height of this bridge should be. So I found a piece of scrap galvanized steel, bent it into a V shape, filed some grooves at the apex to locate the strings, and gave it a go. It worked perfectly from the very beginning and I was able to adjust its height several times to set the action up or down to the precise height I wanted. Quite a discovery, and it was super-simple.

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