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Old 5th March 2002, 01:54 PM   #1
Higo is offline Higo  Argentina
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Question Tweeters!!!

Hello. Thanks to advises that some of you give me
I decided to build a 2 way closed system.
But now a have a problem with the tweeters:
I have a woofer (8"/8 Ohms/80W/Qts 0.67) which bandwith from 40 Hz to 4Khz.
I measured and I found that only reach 3 Khz. Beyond this frequency it's so unlinear.
Due to this I decided Xover in 2K5. Here the problem. It's imposible
to found in my country (Argentina) a good 8 ohms tweeter (BW from 2K to 20 K /50W).
However It's possible to get these specifications in 4 ohms.
But I have a A/V receiver (Sony STR-DE475) that It's 8 ohms/80W stereo/5.1.
I have to options:
1- Buy 4 tweeter. Put 2 in serie in each channel. Then good for the receiver,
but what about the sound? Will be sound piercing? Therefore It's
expensive...
2- Buy 2 tweeter. Put one in each channel. I have to pray that the out stage of
the reciever don't be harm or It will be dangerous only to high levels???
But what abut the sound? 'cause Iīll have a 8 omhs woofer (6.5 ohms with zobel network) and a 4 ohms tweeter? I'm changing the impedance that the reciever "see".

Any comments, advices and ideas will be appreciated.
Thanks in advance.
Marcelo
from Argentina.
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Old 5th March 2002, 02:18 PM   #2
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Default solution

If you were to find a 4 ohm tweeter with at least 3dB more sensitivity than your woofer you could simply use an L-pad on the tweeter. Another solution, even more simple than the L-pad, would be to place a single 4 ohm resistor in series with the tweeter. Both of these options can give you a tweeter network with approximately 8 ohms of impedance.

Stephen
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Old 5th March 2002, 02:49 PM   #3
Higo is offline Higo  Argentina
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Question Doubt

Thank you Stephen.
But I have a doubt. Is It advisable to use a R in serie? Iīll be wasting power. Am I right???
L-Pad??? I have the idea that you only use it when you have SPL different. I donīt have any idea of use it like "impedance adapter"
Sorry, but Iīm "Neophyte"
Thanks in advance
Marcelo
From Argentina
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Old 5th March 2002, 03:39 PM   #4
Higo is offline Higo  Argentina
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Ok. Sorry, now I understand L-Pad (Z=R1+(R2//Ztweeter))
If I have a A/V reciever of 80 W, what is the power recommendable for R1,R2?
Thanks.
Marcelo.
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Old 5th March 2002, 04:18 PM   #5
CHRIS8 is offline CHRIS8  United States
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"If I have a A/V reciever of 80 W, what is the power recommendable for R1,R2? "

HF drivers draw very little power. 10 watt and up non-inductive resistors will be fine in most circumstances.

-Chris
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Old 5th March 2002, 08:42 PM   #6
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Quote:
Originally posted by CHRIS8
HF drivers draw very little power.
For this same reason you might well be able to get away with the 4 ohm tweeter, even if it needs no padding.

dave
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Old 10th March 2002, 10:06 PM   #7
Wizard of Kelts
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I go along with Chris and Planet10. You should be alright with the right 4 ohm tweeter.

I think adding another tweeter can be tricky, though it can be done. Just for the heck of it, what is the 4 ohm tweeter that you have in mind, and do you know it's DC resistance? Actually, if I know the tweeter, I should be able to look that up.

The reason I am asking is that the impedance of the tweeter rises the higher the frequency, so you will be above 4 ohms at the high end. At the low end, the crossover is likely to be adding impedance, so again, you should be above 4 ohms.

You don't have to be at 8 ohms to have an 8 ohm speaker. Just be something over 4 ohms and you should be alright. Some manufacturers have speakers with a 2 ohm DC resistance and call them 4 ohm speakers. Usually, impedance is 25% or so above DC resistance. Crossovers can change that.
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Old 11th March 2002, 02:32 PM   #8
Higo is offline Higo  Argentina
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The tweeter are TW-18W (Made in china I guess)
Check http://web.singnet.com.sg/~rochor/.
This weekend I measured the impedance from 1KHz to 18 KHz and I found that Itīs very linear. The frequency response Itīs very linear too. They have a very small ripple in 18K.
Maybe Iīll buy a pair of AW010E1 (OEM) from Audax to my rears speakers. I donīt want to use it in the frontal speakers īcause they have Fs in 3 Khz and I want to Xover in 2K5 o 2K8. Anyway they are 25 W and my reciever Itīs 80 W (I donīt measure it and I donīt believe It reachs this power).
Thanks for your comments.
Marcelo
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Old 11th March 2002, 05:53 PM   #9
Wizard of Kelts
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Default An Interesting Matchup

Quote:
Originally posted by Higo

This weekend I measured the impedance from 1KHz to 18 KHz and I found that Itīs very linear. The frequency response Itīs very linear too.
Very good. As long as you measured the impedance, what was it? 4 ohms? 6 ohms?

I am very happy that the frequency response is very linear. Frankly, I am a little surprised. Your tweeters are of the horn type, and are rated at 98 dB @ 1W/1M. That is about 8 or 10 dB higher than most tweeters, and about 8 or 10 dB higher than most woofers.

Most tweeters like that are designed for professional use-DJ systems and such-and tend to give irregular response as a trade-off for such high efficiency. Remember, the DJ or band has to fill a whole hall with music, so PA equipment emphasizes efficiency at the expense of frequency response. PA speakers are often rated 95 dB to 100 dB, while most home woofers are rated 87 to 90 dB @ 1W/1M. Remember, the more efficient the speaker, the less amplifier power is needed to fill a hall with sound.

Regardless, it is excellent that your tweeter gives a smooth frequency response.

One thing that concerns me is the issue of dispersion. Good sound should radiate not just straight ahead, but off to the side somewhat. High frequencies especially tend to radiate out in a straight line.

One inch dome tweeters do a very good job dispersing the sound off to the side. Your tweeters are horns. Depending on how they are designed, horns can sometimes give good dispersion as well, and sometimes not. Your efficiency indicates that your tweeter may be designed for PA use, where dispersion is not a high priority. But then, the smooth frequency response is a good sign. It indicates that the engineers know what they are doing-so maybe you DO have decent dispersion. Without a frequency response chart giving the response 30 degrees off axis, I cannot say for sure.

At any rate, your tweeters are very efficient and will likely require an L-pad or homemade network to bring their output down in relationship to the woofer in order to make a match.

I could make some recommendations, but when it comes to crossovers, I tend toward the "let's-try-this" school which requires me to be there.

You have an interesting matchup situation here, and there are people on this forum better than I at crossovers. Might I suggest that you start a different thread, maybe entitled "Matching 98 dB Tweeter to Woofer" where you give the impedance of the tweeter you just measured, (the website listed it as "4-16 ohms"), state it's efficiency which you also measured, and state the woofer's efficiency as well. Include the make and model of your woofer. I think you are going to get some excellent recommendations from the people here who really know crossovers.

PS: Since you like to make speaker measurements, might I suggest you download the Audua Speaker Workshop at www.audua.com ? It is free. Many people here have raved how it makes speaker measurements so easy to do. To be honest, I have not got around to using it myself just yet, but it has recieved many glowing reviews from people on this forum. Nothing like being able to measure your speakers yourself to get the job done right.

Good luck. And hope to hear from you soon.
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Old 11th March 2002, 08:23 PM   #10
Higo is offline Higo  Argentina
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Thank you very much for your comments and sharing your knowledge with me. Itīs nice to find people like you and others, in the Net.
Tomorrow Iīll show you my measurements results.
I used SW, but Iīm prefer the hard way. Anyway, when I measured the woofer I used SW and I found pretty similar results.
In this days, Iīll probe how Itīs the tweeterīs frequency response with the SW. I donīt remember exactly, but I believe that SW had 30 degress response.
If you want, You can check "QTS value?" post. In this post I attached the measurements that I did with the woofer.
This weekend I designed the Xover. I donīt know if Itīs necessary use a trap for the Fs of the tweeter īcause Itīs 1K5 below the Xover frequency (2K5). Iīll follow your advise and Iīll post a new topic.
See you soon.
Marcelo
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