the bridge pickup improved
A couple of months ago, I tried making a Cortado-based bridge pickup for the octave mandolin. It worked okay - better than the ad-hoc clamp arrangement I’d bodged together for last January’s Conflikt show, but not what I’d hoped. It was a lot more stable, but still needed lots of equalisation help.
I’ve built a new one, with a new design! It’s much better. Here’s an mp3 of the previous design alternated with the new design, on octave mandolin - no eq of any kind, no effects, just raw output from the old design vs. the new. (Old design is first.)
While working with the previous attempts, I’d figured out what really improved things was the right kind of pressure on the piezo disc itself. My thumb was pretty optimal, but you can’t exactly do that and play at the same time. The clamp wasn’t bad, but it was slippery and awkward and actually came off on me during rehearsal, so I didn’t trust it. Most piezo-style pickups live under the bridge of an instrument, but you can’t do that the usual way with this one, it’d be destroyed by the pressure.
So I went about trying to fix that.
First was to take the bridge plate and add a wide, flat channel - one wide enough specifically to contain the entirety of the Cortado piezo element. I made it by wrapping sandpaper around a flat piece of metal, and scrubbing back and forth to excavate out the wood I needed removed.
This is actually a new bridge plate.
But it’s made of the same material, so no real diff.
You need to sand away enough wood to make room for the piezo and all the tape wrappings, and some extra. But you do not need to sand away enough for the wires soldered to the disc - you want to avoid those entirely.
Keep sanding away wood until the bridge slides freely over both the new channel and the piezo, like so:
What this makes is basically a wide clamping chamber around the pickup element itself. It doesn’t do any clamping yet; it just creates a space for it. At this stage, in fact, if you hook it up and try it, there’ll no change in sound from the previous version.
(In fact, the “old design” recording I used in the sample is actually this version at this point in the process. I verified that it sounded exactly the same as the previous version, as predicted, which means I’d re-established the old baseline. Important for science!)
But now, of course, I have a clamping chamber! We just need something to apply pressure.
So what’s our clamp? Pieces of paper. Post-it notes, to be specific, just because they were handy. The right number of sheets in this exact case turned out to be four.
Five also worked, and did not feel like too much pressure inserting the papers under the bridge. But it did sound like a bit much compression, tonally, so I went back to four.
(Here’s that sample track again, alternating old design and new, old first.)
The beauty of this is that since it takes several thicknesses of paper, and since that paper be changed out without taking apart the pickup, you can use any number you like. You could even adjust the tone on the fly.
Interestingly, the pickup didn’t even get quieter with more paper. I’d worried about that, but didn’t need to. In fact, adding more sheets made it louder, meaning that the pressure is not so much “damping down treble” as it is pulling up bass. Which, in turn, makes me wonder if it’s not so much “resonating better” as moving the zero/no-vibration point of the crystals’ charge state from all-electrons-in or all-electrons out (doesn’t matter which) to a more middle-range position, which…
…hm. Actually, that’s interesting. No, that’s really interesting. That would explain why the pickup got louder with more clamping, rather than muffled or…
…huh. This is an hypothesis. If I’m right, I can make my next crystal mic substantially more modern sounding, by enclosing the piezo in a small clamping chamber, which is, like this, attached to the resonating disc of the microphone, and possibly…
…possibly I should take my SRMD meds now or I’m going to be up until 5am next Thursday playing with crystals and possibly taking over the moon again, aren’t I? Yeah. I am. Okay. BRB.
So. Yeah! I’m super-pleased with this result. I’m also thinking that maybe this could be used on other items that have flat surfaces which need pickups - like, a piano, maybe - and instead of the bridge, as here, you use a weighted flat bar of some sort across the pickup plate to create the clamping chamber. Then you’re off with tonal control via paper again. I have no need for this functionality at the moment, but it strikes me as legitimate nonetheless.
And most importantly for me, I now have a much more conventional DIY pickup for the octave mandolin. Here y’go, doc - just plug ‘er in, and we’re off.
Much better.
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