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.  

Bart Hopkins, http://windworld.com