I would say, start at the value that gives the frequency desired for a simple first-order crossover, for both caps. This can be tuned to give a pronounced "tilt-up" to the drive of the peizo, which cancels out its falling response. a cap, then a resistor to ground, then another cap, then the L-pad/resistor/piezo driver. The way to correct this, is to use a low-electrical-Q quasi-third-order highpass crossover. In these cases, the Powerline drivers oftentimes wind up having TOO MUCH output in their lower registers, due to the "horn gain" (horns have a tendency to cause a "tilted down" response as frequency increases, due to their pattern narrowing at low frequencies down to the horn cutoff, since more of the sound is projected on axis and less off-axis). many people wind up using the 1" throat screw-on piezo drivers, with various horn flares. after the resistor to ground.Īlso, with Powerine type piezos. If using a KSN1005-type piezo with the above circuit diagram, I'd place a 4 ohm resistor (the value isn't critical, anything from 4 to 10 ohms would be quite reasonable) just before the driver. this damped and alleviated the "resonant circuit" created by the capacitive load on the amp output. I've personally heard MANY cases, where a harsh "gritty" sound was ELIMINATED, by the insertion of even 4 ohms in series. Especially with vintage amps (which have less high frequency stability), it's a VERY GOOD idea to use a series resistor. they go to as low as ONE OHM at extremely high frequencies (ie, 100KHz region), and can cause amp oscillation. the "fluted bullet" type, like the ones in the Dahlquist DQ10, basically. Many people using piezos, are using the KSN1005 type. The piezo will not know the difference, it works off voltage only, drawing almost no power. But if you put an 8 ohm resistor across the terminals (follow this whole discussion now.) now it looks like an 8 ohm load. The CTS/Motorola paper says you can put a small CAPACITOR in series with the piezo to reduce the level, and that can work. If you put a 20 or 50 ohm resistor in series with a 1000 ohm speaker, well, it is a miniscule change in power to the speaker. Do you know why? Because a piezo tweeter has an impedance of 1000 ohms or greater. If you do put a resistor in series (and even Martin Sound says for Lpad use a 20-30 ohm resistor) I say, go ahead, but it will NOT make a difference in sound. These are once again available through Martin Sound.Īnyway, don't use a resistor with the Powerline Series piezos for power protection purposes it is simply not needed. This is very mild and almost inaudible, and certainly not noticable. What this does is give "soft clipping", that is, a sort of compression of the highs. As the bulb heats up it increases resistance. At about 100 watts the thermal breaker heats up, opens and now the power goes through the light bulb. This protection circuit uses a resistor and thermal breaker paralleled with a tiny light bulb. The CTS/Motorola Powerlines were the very high quality ones for pro use, and contained built in protection circuits, quite sophisticated. (The Motorola piezo speaker division was purchased by CTS some years ago) They did not recommend using resistors for any of the Powerline series, which include the KSN1142a, KSN1165a, KSN1188a, and a few other in the series. This gave protection to the amp from high freq (100khz or so) oscillations as well as some protection to some of their tweeters. Years ago with some of the Motorola line, not all, just some, and I have the paper from them about this, Motorola recommended a 20-50 ohm resistor in series with SOME of the tweeters. Some will tell you to place a resistor in series with the piezo tweeter, some say parallel. Most all are quoting each other and MISQUOTING the manufacturer. Ignore everything you have read about resistors and piezos. By reducing the tweeters output by 4 db it matches the woofer very well and makes for an excellent speaker. Using the KSN1165a with a 92 db SPL woofer, the tweeter does sound very bright. I have used the KSN1165a, rated at SPL = 92 db 1w 1m, with woofers in the 96-97 db range and they seem to have excellent balance. But with the Motorola/CTS units I have come to see (and others agree) that they have higher sensitivity than the published 92-93 db/wt SPL ratings. First, if they aren't a real Motorola/CTS unit, they are junk. Sometimes it is said that piezo tweeters are "harsh" or "bright". If you pay more than $2 for one made by anyone but CTS, you are being ripped off. I have seen them selling for as low as 75c each to dealers. The ones made by other companies are junk. The only piezo tweeters worthy of high fidelity or pro audio use are the Motorola/CTS units, and of those, only the Powerline Series are suited for pro audio.
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