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Super tweeter for FullRange Driver

I have a pair of Audio Nirvana 15in Alinco Classics that I just got done getting into cabinets. After some listening impressions, I've realized I cant live with the high frequency response these things put out by themselves. They only go to about 15k hz. and it's made music lose its sparkle and sense of space and spacial cues. I was thinking of buying a pair of Fostek T90As and pointing them at the ceiling for a more even dispersion and better integration with the Audio Nirvana driver. My main question is what  capacitor volume would you suggest? Where in the frequency response would you crossover to the tweeter? Would wiring the 8ohm tweeter and the 8ohm Audio Nirvana in parallel make me need to change my amp(Audio Note Kit 1 300b Set) from 8ohm to 4ohms? Any other brands of tweeters recommended for a fullrange driver that is around 96bd efficent? Sorry for all the questions, incredibly new to trying to implement crossovers with full rangers. If this sounds like a bad idea. Please speak up. 
akwilson501

Showing 4 responses by audiokinesis

audiokinesis
2,755 posts
 
Your plan makes a lot of sense to me, and imo the T90A is a good choice. My first commercial effort used a fullrange driver augmented by a rear-firing T90A.

In a case like this, where you don’t have the benefit of measurements, you find the capacitor value for the supertweeter by trial and error. There is simply not enough information available to reliably predict what the correct value should be. I’d recommend you not go over 3 microfarads, and I’ll go out on a limb and toss out 1.5 microfarads as a suggested starting point.

Capacitor values add when you wire them in parallel. If you want to end up with a boutique capacitor, find the correct value using inexpensive film capacitors before you spend the big bucks on a boutique cap. Capacitor quality cannot make up for not using the right value.

I don’t think you’ll end up with an impedance curve that will cause your amp any distress. There is very little energy in the top octave so an impedance dip up there is unlikely to be an issue, and the capacitor in series with the tweeter will keep the net impedance from dropping very low anyway.

Duke
audiokinesis
2,755 posts
 
Alexberger wrote: " Back of ceiling firing tweeters sound horrible!!! It completely destroys focus and sound-stage."

I disagree. Quite the contrary in fact when done right, and I can go into detail if you’d like.

The reason for keeping the supertweeter out of the first-arrival sound is so that it doesn’t screw up the phase of that first-arrival sound coming from your fullrange driver.

By increasing the amount of high-frequency energy in the reverberant field while leaving the first-arrival sound intact, you are fixing the tonal balance without degrading anything else, as long as the reflection path length is long enough. And a ceiling bounce usually is long enough at high frequencies.

Regarding whether or not to use a resistor or an L-pad, again that’s "play it by ear". DO NOT put an L-pad or any parallel resistor (or inductor) legs AHEAD of the crossover capacitor. Any resistor (or inductor) leg in parallel with any driver’s coil NEEDS the crossover capacitor in between it and the amplifier to filter out the rest of the signal, otherwise your resistor (or inductor) leg is seeing a full-power, full-spectrum signal from the amplifier.

If your crossover (more precisely, high-pass filter) is first order - i.e. just a capacitor - and if you’re just using a series resistor to adjust the level, then it does not matter whether you place that resistor before the capacitor or after the capacitor. Either way, the resistance that the capacitor "sees" is the supertweeter + the resistor.

Duke
audiokinesis
2,755 posts
 
mijostyn wrote: "Duke, I think the L pad or resistor goes in series in front of the capacitor and driver. No?"

If the input power levels are low then you can probably get away with placing the L-pad in front of the capacitor, but you LOSE the L-pad’s feature of not shifting the crossover point. This is because the impedance of the load that the capacitor "sees" is no longer constant as you turn the knob on the L-pad.  At high power levels, you run the risk of cooking the L-pad because it's seeing a full-range signal from the amp. 

The L-pad is designed to go just in front of the tweeter, where it maintains an 8-ohm load regardless of how much attenuation is cranked in so the crossover point doesn’t change. Also in this location, it is protected by the crossover capacitor.

Duke
audiokinesis
2,755 posts
 
akwilson501 wrote:

"I’m trying to cross my tweeter over around 15khz, wouldn’t a .33uf capacitor be ideal, in a 1st order filter? Considering the 106 db on the fostex, and the 96db for the fullrange...

"3uf and 1.5uf seem as little high doesn’t it."

By all means go with whatever works! You may well be right.

My thinking was this: The T90A’s contribution will all be going into the reverberant field, and its radiation pattern is fairly narrow so that on-axis 106 dB doesn’t apply. What does apply is the power response - that is, the sum of the driver’s output across all angles. So when you eyeball the factory curves, the off-axis curves are more representative of what the T90A’s contribution will be:

https://www.madisoundspeakerstore.com/bullet-tweeters/fostex-t90a-top-mount-horn-super-tweeter/

Mentally average the 30 degree and 60 degree curves and imo THAT is a reasonable approximation of what you’re starting out with.

Duke
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