Vifa XT25 is rolling off too soon? - diyAudio
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Old 2nd January 2006, 01:52 AM   #1
K-amps is offline K-amps  United States
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Thumbs down Vifa XT25 is rolling off too soon?

I have a system with the XT-25's as the HF driver hoping to get at least a response flat to 25kHz plus... however when I measure with a radioshack SPL meter, the sinewaves show a gradual roll-off after 10kHz.

20kHz is about 13dB lower than 10kHz. A-weighted

Granted my measuring equipment could be better but this level of roll off is a little unnerving.

I am placing the SPL meter about 6" away from the tweeter, I have tried on axis as well as 30 degrees off and get same results.

Both Tweeters have the same issue.

The crossover is a 3rd order with a 4.7uF cap in series and 0.2mH coil in parallel and another 4.7uF in series with the XT-25. So basically cap/coil/cap then tweeter on the live terminal and nothing on the gnd terminal (except the other end of the coil).

Schem attached... apologies for the lousy MS paintjob

Anyone have any ideas?
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Old 2nd January 2006, 03:37 AM   #2
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A quick look at the manual for your radioshack SPL meter tells me that with an A weighting the meter is more responsive in the 500Hz - 10KHz range than other frequencies. You need to set the meter to a flat frequency response curve (although technically that's not a curve, I know).

It seems that your meter is only really of use up to 10KHz anyway. I wouldn't worry about your tweeter's response, I'd be more worried about the response of the meter at that frequency.
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Old 2nd January 2006, 05:18 AM   #3
simon5 is offline simon5  Canada
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You should use the C-weighting with a compensation chart.

It's not perfect, but it's better than what you're currently doing.

Here's the compensation chart :

http://forum.audiogon.com/cgi-bin/fr...6&openfrom&3&4
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Old 2nd January 2006, 02:57 PM   #4
K-amps is offline K-amps  United States
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Thanks Guys!

The thing is I took a second reading with C-weighting and the 10k to 20k diff was just as bad. There is no "Flat curve" setting on this meter.

I did have some metal film resistors in series with the Tweet which I though may have been inductive enough to roll off the highs, however being only 1.3ohms, I didnt think they'd make such a difference and removed them, the response is just the same.

I am using MIT Terminator2 cables... perhaps they have some roll-off in that black box of theirs attached to the Cable...

I am thinking that the meter is no good beyond 10kHz... but then why do the ears want more 16kHz?


Anyway I will also try C-weighting with the compensation curves (wonder what 1db compensation will do lol).
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Old 3rd January 2006, 12:25 AM   #5
simon5 is offline simon5  Canada
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It's a bit hard to measure that high, the microphone needs to point directly at the tweeter, it needs to be perfectly aligned vertically and horizontally with the tweeter.

The RadioShack meter is far from perfect also, but it's great for the price.
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Old 3rd January 2006, 06:12 AM   #6
K-amps is offline K-amps  United States
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I wonder .... if because of the XT25's phase plug if the reading needs to be mase slightly off center at 6"....
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Old 3rd January 2006, 07:15 AM   #7
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Quote:
I am using MIT Terminator2 cables... perhaps they have some roll-off in that black box of theirs attached to the Cable...

Ha,ha. They do, but not at 10kHz, more like 300kHz.
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Old 3rd January 2006, 04:38 PM   #8
K-amps is offline K-amps  United States
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Quote:
Originally posted by analog_sa



Ha,ha. They do, but not at 10kHz, more like 300kHz.

Would that be factory literature or your own measurements?
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Old 3rd January 2006, 11:00 PM   #9
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K-amps,

The meter is suspect, to some extent, without equalization (I have one), but with a test disc and limited bandwidth frequency increments, you can spot general trends.

I also wonder at the component values you have shown in your crossover. These are not something I would recognize as a common 3rd order xover, with the second cap value the same as the first. Ordinarily the second cap is around 3X the value of the first in an 18 dB high pass circuit. In a 24 dB LR xover, this 2nd series capacitor is exactly double the first value (along with a second coil). If you have something near 15uf, I'd give that a try in this position, along with any padding resistors you will using, if needed to match levels with the woofer.

Just for testing purposes, you might want to try just a single 4.8 uf capacitor in series with the tweeter alone. Apply power carefully, however, as the tweeter has less protection in this simple 6dB circuit, and do not feed any low frequencies(below 1K Hz) to it.

Tim
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Old 3rd January 2006, 11:20 PM   #10
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Thanks Tim, some good info.

I actually also at one time had the arrangement at a simple 2nd order, i.e. cap/cap/coil wth the two 4.7uF caps being the same as a 2.35uF single, however Now I am not sure if I made readings while the caps were set in 2nd vs. 3rd order.

I will short out the second cap, making this a second order and re-test it.

I use a sinewave generator so I will not be using a full spectrum as such.

But that apart, the next issue I run into is, how do I determine polarity of individual drivers where the tweet is 3rd order, mids second order, midbass first order and Bass drivers that are actively crossed over...
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