![]() ![]() Now to consider impedance, the problem here can be simplified without having to know everything there is about impedance itself. That is the first high-pass filter problem if the cable from the piezo to the preamp is too lenghty there is then enough additional picofarads of cable capacitance (in parallel with the internal piezo picofarads) to where it adds up enough to roll off audible highs and then you get a dull sound. Any piezo element which is suitable over the full audio range as an instrument pickup will intrinsically have low enough picofarads to keep from rolling off the highest audio frequencies to ground at the source. Keep in mind that a piezo transducer IS a capacitor, but it actually has quite small capacitance itself. Unfortunately, nothing that I've experienced will make a piezo pickup sound like anything else so no matter what circuit you use you've got to deal with the "honk" and "quack." You could reverse the windings on something like this and a 2,500 ohm load on one of the 50 ohm windings should reflect a 4M impedance.Īs I noted when responding to the Bogen CHB35 post the grid leak type bias has a really high input impedance and couples well to piezo pickups - if you're into cobbling up tube circuits - as does the "boot strapped" input which utilizes a bit of NFB to up the input impedance. I've got an octal base Triad input tranny in front of me with two 50 ohm input windings and an 80K secondary. The result would probably be very noisy and subject to all sorts of stray electromagnetic fields as well as UFO communication signals but you never know. ![]() But for experimentation purposes you could find the highest impedance OT - I've seen them around 20 K or so - and then up the secondary impedance to reflect a primary impedance in the mega ohm range. David mostly nailed it - any tranny would have to have a 3-10M ohm input winding and crafting a decent frequency response with this much distributed capacitance along with the inductance of this much wire would wind up costing lotsa bucks.
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