A piezo is a contact pickup that captures the vibration of the instrument. It is typically connected to a preamp of sorts and the signal is processed like a microphone. But anyone who has tried a piezo pickup will tell you that, for the most part, they do not sound all that great. They tend to sound peaky, and with violin, they can sound shrill. The problem is not so much the piezo transducer, but the way it is loaded.
During our research, we discovered that when you apply the typical load of a mixing console – say 10 k-ohms – on a piezo, it causes the bass and high frequencies to roll off, narrowing the response, and generates peaks in the mid-range. As you increase the load, it begins to flatten out. For years, electronic manufacturers have employed a one-size-fits-all 1 m-ohm input impedance as a means to satisfy as many sources as possible. As the impedance rises above 4 m-ohms, the response extends and flattens out further and seems to really sound great at around 10 meg-ohms.
Rush FOH engineer Brad Madix continues: “We knew that impedance matching was going to be a problem right from the get-go. In the world of DIs, there are really not a lot of options for 10 m-ohm transducers. The preamps that the pickup manufacturers offer have too much tonal control for our purposes. We just wanted to get the signal into the console without mucking it up by mismatching the impedances. I thought we were going to have to have something custom made but is so happened that there was a solution in the works. Our first tests with the PZ-DI were impressive. The pickups required some EQ (I would say “the usual”) but the frequency response was great. Good frequency response, good dynamic range, good signal to noise… none of the issues that become apparent when impedances are mismatched.”
Properly loading the piezo magically transforms the device into a truly functional transducer. In fact, it works so well, a new problem sets in: you get so much bottom end that you have to keep it under control. By inserting a variable high-pass filter into the signal chain, you can eliminate excessive bass and size the instrument to suit. Cleaning up excessive resonance eliminates low frequency modulation and enables the various instruments to better sit in the mix, making it easier to balance them as a whole.
So whether you are mixing Celtic band in a club, a 50-piece pop orchestra in a park, or an eight-piece violin section next to the biggest drum kit in the world, piezo pickups can prove to be an excellent choice.
Peter Janis is President of Radial Engineering Ltd., A manufacturer of professional audio products used for live touring and in recording studios around the world including the PZ-DI direct box for acoustic and orchestral instruments. Special thanks to Brad Madix for his added input on this article.