piezo super tweeter

operaman

Member
2016-02-22 11:58 pm
I want to install a Goldwood GT-1005 Piezo Super Tweeter on my 3-way. Is it best in parallel with an existing Polk SL5000 Soft Dome Tweeter w/ a low cutoff @ 5000Hz or across the 3-way's input? I want a 8500Hz low cutoff for the piezo. I'm not sure what size series cap that will do the trick? Do I also need a series resistor before the piezo in either case?
 
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A series cap by itself will not work as a crossover with a piezo. Look up the piezo crossover posts from djk on the subject. By coincidence, I am wiring up a crossover for a dual piezo today based upon what he has written. The piezo is pure capacitance, so a capacitor in front of it reduces it's output but does not cut the lower frequencies. This is what I am doing: Put a 75 ohm (give or take) resistor in series with the piezo. Before the 75 ohm resistor, put a resistor across the + and - terminals, or better, a resistor and small inductor. (I am using 7 ohms and .1 mh in series) Then before that, put your crossover. I am going to use a 12db/octave crossover at about 10k for my setup. The crossover "sees" the resistance across the terminals, but is shielded from the piezo by the 75 ohm resistor. At least that is my understanding. Maybe somebody with more knowledge will respond.
 
Sounds great. I haven't measured the change in frequency response yet. For an 8k crossover frequency, 12db/oct crossover, Linkwitz-Riley response, the crossover components are a 1.24 mfd cap in series, then a 3.13 mh (or as close as you can get) inductor across + and-, then the 8 ohm resistor or 7 ohm resistor and .1 mh inductor in series across + and -, then a 60-80 ohm resistor in series with the + terminal of the piezo. For 9 k, change the first cap to 1.1 mfd and the first inductor to .283 mh ( or thereabouts). I'll run 8.5 k later if I get a chance. Don't forget to reverse the phase of the piezos because it is a 12 db/oct crossover. I wouldn't use a 6db/oct crossover with a piezo. (or any other tweeter)
 
Motorola piezos are excellent, Goldwood are not.
Supertweeter make sense only if the regular tweeter doesn't reach 20kHz and only if crossover is based on measurements and simulation. Obviously, that is not a case here. Theoretically devised high-pass filter for the piezo, with no knowledge of the phase and frequency response of the Polk tweeter is a recipe for disaster. High-pass filter for the piezo supertweeter must blend well with the natural low-pass response of the tweeter. That is impossible without measurements.
 

operaman

Member
2016-02-22 11:58 pm
Thanks again to Decker for the specific values that I was looking for. Are Motorola piezos still available? As for testing, I am a novice who wants to add a bit of airiness to my sound system - I don't have test equipment to tell me specific results other than what I hear.