converting multiple piezo pickups to one input

Here's where we have to really stretch the analogy: you and your arm are the instrument, the skipping rope is the signal going down the cable if not the actual cable, the ring and wall are your preamp, which in this case is included inside your amp. What's the catch? Well if you know that your signal is nice and strong then a brick wall is a waste as the small movements aren't as easy to measure. This is the high input impedance scenario and the movement of the ring captures all your nuanced wiggles on the skipping rope in exquisite detail. The ring is barely moving now, but when it does it's a much more accurate representation of your output frequencies of your arm. Also another problem with the jelly is that there will be some reflections as the ring starts to move the skipping rope, creating an echo (not really a problem in musical instruments, but thought I'd mention it). However, what happens when you make the signal small? Your small movements are going to be lost in all the noise of external factors, and in particular your high frequencies are just not being seen in the movement of the ring. That wall has a nice low input impedance, and it's really easy to measure what the ring is doing, which is great. Now, imagine that wall is made of jelly, that ring is going to be wibble wobbling up and down like mad when you're really yanking that skipping rope. You want to see what frequencies you're pumping out with your arm, and you do so by measuring and plotting the up and down movement of this ring over time. Imagine you tie one end of a skipping rope to a ring drilled into a wall, and wiggle the other end of the rope.

I think I have an analogy that's just about right enough to help understand impedance.

Converting multiple piezo pickups to one input