Piezo Film Pickups: How many can be wired together?

by Bart on December 14, 2009

We sell a lot of piezo films, ready for our customers to wire them up as musical instrument pickups. In most applications, a single piezo film pickup is all that’s needed to capture an instrument’s sound. But there are also a lot of situations in which it’s useful to have two or three, and sometimes much larger numbers of piezo film pickups on a single instrument. (For example of an instrument calling for many, think of a xylophone-like instrument in which a pickup is needed on each bar.) The question arises: is it OK to have multiple piezos feeding into a single output? My response to this question, up until recently, has been based on book-learning and not on direct experience. So the other day I finally sat down and did a whole lot of piezo hook-ups in different configurations in order to see first-hand how many piezos you can wire together with good results.

In this posting I’ll first tell you what I learned in terms of numbers, then I’ll describe how I did the tests, and then I’ll provide additional how-to information for hooking up multiple piezos (including ideas for what to do if the number of piezos you need is too large for a single group) .  But first let me review the basic information that was already known before I did my series of tests. The ideal situation is to have a single piezo element operating independently, sending its signal through a cable to its own amplifier input.  It’s also possible to have two or more piezos wired together and going into the same input; however, the more piezos you have in the system, the weaker the signal from each piezo will be.  The question to be answered is, how many separate piezos can you wire together before the loss becomes a problem?

I experimented with both the very small piezo films we sell (piezo “tabs,” they’re sometimes called) and the big 6″ ones.  Here’s a rough summary of what I found.

 RESULTS

 1″ piezo tabs

With the tabs, I was happy to find that you can get away with more pickups in a group than I expected: the signal strength deteriorates as you add more piezos, but not as badly as I had feared. In the chart below, the numbers in the left column are the number of piezos wired together in a group; the comment on the right describes the resulting signal strength and quality.

1          Excellent

2-3       Almost no discernable loss

4-5      Very little loss

6-10    Increasingly noticeable loss, but still functional

10-20   Increasingly serious loss

 Recommendation for multiple piezo tabs: The fewer piezo tabs wired together in a group the better, but anything less than five will be OK in most applications. Depending on your requirements you may be able to work with as many as nine or ten. Use more in a group only if you can accept compromised signal strength and sound quality.

6″ piezo films

With the larger 6″ piezos, the signal strength deteriorated more rapidly as the numbers increased.

            1          Excellent

            2-3       Very little loss

            4-5       Increasingly noticeable loss, but still functional

            5-10     Increasingly serious loss


Recommendation for larger piezo films: The fewer piezos wired together the better, but two or three of the large piezos is probably OK, and you may be able to get away with up to five.  Use more in a group only if you can accept compromised signal strength and sound quality.

2.5″ piezo films

I didn’t test these in-between-sized films, but if you’re working with them you can assume the results will fall somewhere between the larger and smaller ones described above.

TEST SET-UP

Managing such large numbers of temporary hook-ups (up to 20 for the piezo tabs) was slightly chaotic in a fun sort of way. The test set-up was pretty informal. I did the hook-ups using lengths of hook-up wire with alligator clips at the ends. I’ve got a large supply of these convenient little connectors for just this sort of purpose.  I started by hooking up a single piezo film, running its output to an amplifier, and testing it simply by flicking the end, noting the signal strength and tone quality.  I then added a second piezo, and a third, and so forth, testing by flicking after each new one was added.  After I had hooked up and tested twenty of the small piezo tabs, I then went back and tested a single piezo once again, in order to directly compare the full multiple set-up with the single piezo. I did the same for a total of ten of the 6″ piezos.

Something worth noting about this set-up: the hook-up wire I used isn’t shielded.  This means that as I was adding more and more hook-ups, it was to be expected that increasing noise would appear in the system from stray electromagnetic frequencies in the air.  This did occur, especially when I switched on certain lights nearby, but it wasn’t as bad as I feared. In a real installation, of course, you’d use shielded wires.

HOW-TO INFORMATION FOR MULTIPLE-PIEZO INSTALLATIONS

 You can find full information for hooking up single piezo films here.  This is the sheet that comes with the piezo films when you buy them from us.  The basic idea is that you run two wires, called the hot wire and the ground wire, from two terminals on the piezo, through a cable and to the preamplifier. (The preamp is often incorporated into a regular amplifier input.) The shorter the length of cable from the piezo to the preamp, the better. 

In hooking up multiple piezos, you have a choice of whether to join them in series or in parallel. Because of the electrical nature of piezos, series connection yields poor results; parallel is the way to go.  In practice this means: connect the hot lead from each of the piezos in the group to a common wire for the hot side of the output, and connect the ground from each peizo to a common ground wire.

What if the number of piezos you need for an instrument is larger than the number that can work well wired together in a single group? Example: imagine you’re putting pickups on a home-made xylophone with 12 bars, but to prevent signal loss you want to keep the number of piezos grouped together to five or less? The answer is to wire the piezos in two or more smaller groups, and keep the groups electrically “buffered” from one another. In this case, for the twelve piezos needed you might create three groups of four, or perhaps two groups of six.  To buffer the groups from one another, they need to go to separate preamps before their signals are mixed.  If you’re an electrical whiz, you can build miniature op-amps into each circuit before mixing them.  If you’re not an electrical whiz, the easy solution is to send them to separate mixer inputs. This is quite feasible because nowadays there are very compact and affordable mixers on the market with as few as four inputs – you may even be able to affix a mini-mixer to the body of the instrument somehow.  This also has the advantage of giving you separate volume and tone controls for each of the groups. 

Addendum (March 26, 2010): This later post has information on another consideration in multiple piezo installations, phase cancellation.  

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