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Old 16th July 2008, 07:29 PM   #1
amo is offline amo  United States
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Default how many VA?

I know the following is the not most scientific way to ask this. I have been trying to estimate how many VA or watts a single driver will take from an amp, and used every imaginable tool out there (short of actual measurement), but because the drivers are prepietary no TS params are to be found.

Can someone give me a ROUPH ESTIMATE of hamy many watts about it will take to drive a 7 inch aluminum cone woofer with a huge magnet structure to about 90 dB at about 40 Hz (its Fs?) in a medium sealed enclosure. If thats too hard, then what about 103 dB at 120 Hz? The driver is 4 Ohm nominal I know that. What can I expect with a 50 W current-limited little chip amp, driving just one driver? Here is a pic:
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Old 16th July 2008, 07:36 PM   #2
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Old 16th July 2008, 07:42 PM   #3
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The amp is lm3875 configured for a 4 Ohm load. Active XO.
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Old 16th July 2008, 08:52 PM   #4
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What you have here is a severe lack of information. Just looking at the driver is not enough. You need at the very least T/S parameters including the driver sensitivity, then you can model the driver in your intended enclosure with something such as Unibox, and get a rough estimate of how much output you get with how much input at what frequency, how much excursion at what frequency with how much input power, etc.

T/S parameters can be measured, with a tool such as the Dayton WT3 available at Parts Express. I'd say that device is a bit of an investment though if you only intend to measure a single driver with it.
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Old 16th July 2008, 11:05 PM   #5
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I agree! just trying to get a general idea if a singele 3875 is sufficient to drive such a unit.

One detail I left out is that the manufacturer rates the entire tower as 90 dB sensetivity, thats using third order passive xo on the woofers. I suppose its safe to assume at least that much sensetivity using active xo. Does it make sense to say then that only about 20 watts will drive this thing to about 103 dB above FS?
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Old 17th July 2008, 01:46 AM   #6
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If you get the specs you can use WinISD.

But below ~ 100 Hz your going to be excursion limited not power limited.

At 120 Hz your probably pretty close to 103 dB based on a quick estimate using WinISD and the Adire AV8 which is an 8" woofer with 89.54 dB efficiency.
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Old 17th July 2008, 01:53 AM   #7
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I took a closer look at your picture. Looks like you have some excursion available. Adding a Linkwitz Transform with an F3 point at 4 shows about 10 mm of excursion with the AV8. Your driver off course will require more since it is a 7". The bad news is you need 6 dB of gain which means you need 80 watts (20*2*2).
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Old 17th July 2008, 02:44 AM   #8
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Rodger, thanks so much for your help! I am not trying to push these to their limit for this application. I am infact sealing up the enclosures and addiding dedicated bass up to 40 or 50 HZ to take the pressure of these woofers (2 per tower), since I tend to listen to loud bass.

My main concern is whether the woofers will be able to keep up with the mid and tweeter given all drivers are eating off of identical 3875 amps. It will be far easier to put together 8 identical amps if you know what i mean. And it seems that the 3875 should not clip at higher volumes, even though the woofers are only 4 ohm nominal, as each has its own "engine". I hope my assumptions are all correct here, cuz the trigger is about to be pulled thank you for your help!
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Old 17th July 2008, 02:46 AM   #9
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I am just worried because I made the mistake previously of running the bass section off of a very powerfull amp, however in bridged mode. With its 500 Watts in tow, the thing clipped and the woofers bottomed out, something I would hate to hear once again.
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Old 17th July 2008, 03:11 AM   #10
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An active setup with 8 chipamps sounds like a lot of fun.

If you like how the mid-bass of the 7" sounds, I say use it but maybe loss the eq and rely on your subs for below 80 Hz. Only testing will tell if they can keep up with the rest of your system (SPL specs sometimes aren't exactly "as advertised") but at least you won't have to worry about bottoming out and you won't need the extra power.
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