impedance and wattage rating....

[jimbob212005]

Ok so here is my dilemma... I am building a pair of 2 way towers, and I am wondering how much power I am going to be able to shoot them. I have (1) 4Ù sonus faber tweeter that is rated at 150 watts and (4) 6.5" VIFA woofers rated at 70 RMS/100 watts peak @8Ù. I believe that the correct thing to do would be to parallel wire the (4) 8Ù drivers into a 2Ù load that way the overall impedance of the tower is 6Ù. (4Ù+2Ù=6Ù) Is this the correct approach? Also if this is the case how much power am I limited to when pushing this speaker? Am I in fact limited to the 100 watt rating of the 6.5" drivers or does the load draw evenly across all of the units? I am not familiar with paralleling inductive loads, so I didn't know if they will divide the current like a resistive load or not. I assume that it would since that is the case with the impedance. However since my amplifier is 150 watts/channel at 6Ù so this could be a problem. Just trying to see how much power i am looking at being able to put into these. Thanks in advance for your help.

[Cal Weldon]

Each driver carries it's own portion of the spectrum so you do not add the 4 to the 2 to get 6. The impedance of each portion shouldn't drop below 4 ohms so you would wire the mid bass unit as series/parallel to achieve 8 ohm. The XO will have to take into account the impedance (among a number of other factors) of each driver or set of drivers. If you are asking questions like this, it may be a bit soon to start designing XO's.

The mid bass units theoretically will handle 280 watts RMS, the tweeter rating is a bit more difficult to peg but usually comes with a rating according to a certain XO point and slope.

An amplifier with a power rating higher than the speakers is usually safer than one with a lower rating. It's all about control. A lower powered amp can clip causing a rather quick tweeter meltdown when pushed beyond it capability.

[BlueWizard]

Impedance for two speakers in parallel -

If the speakers are all the same, it is the value of one speaker (R1) divided by the number of speakers (Ns) -

Assume FOUR 8 ohms speakers -

Rt = R1/Ns = 8/4 = 2 ohms

for TWO speakers of different values -

Rt = (R1xR2) / (R1+ACs-R2)

for more than two speakers in parallel -

Rt = 1 / (R1 +ACs- R2 +ACs- R3 +ACs- R +ACs- ...Rn)


Now, you can't wire your speaker into a two ohm load unless you have an amp that is 2 ohm stable, and that means very very few consumer Stereo or AV amps.

The best way to wire four speakers is either put -

Two speakers in parallel with each other, the put the other two speaker in parallel, then wire the two gangs together in series.

Alternately, you wire two speaker in series, the other two speakers in series, and then those two gangs of speakers in parallel.

In either case the result will be equal to the impedance value of one speaker. So if you have FOUR 8 ohms speakers, the result is one 8 ohms gang of four.

Now, if you do as I suggested, you will have an ohm gang of four speaker that you can treat as a single unit. In your case unless I misread, the total will be 8 ohms.

You are going to combine that with a 4 ohms tweeter.

When you design your crossovers, you simple need to indicate to whatever method your are using (formulas, on-line design guide, etc...) that the woofers are 8 ohm and the tweeter is 4 ohms.

http://www.apicsllc.com/apics/Misc/filter2.html

To make this work, I suspect 2nd order Linkwitz-Riley would be best. But then I don't know the exact frequency response of the speakers you mentioned.

Next, you have to decide where to crossover; at what frequency. For this to work, the frequency range of your woofers has to overlap with your tweeters, as in the woofer being 30hz to 3000hz, and the tweeter being 2000hz to 20,000hz. In this random illustration, you could then crossover in the middle of the overlap, or at 2,500hz.

The problem is, you can't believe the rated frequency response given, especially for woofer. It may claim response up to 3,000 or 4,000 hz, but if you look at a sample frequency response chart, you will see that that rated range is not really the usable area.

Most woofers get a little squirrelly above 1,000hz. Though many can be functionally stretched up into the roughly 2,000 to 3,000hs range with a little work.

Relative to the crossover and the overall speaker impedance, it is going to be 4 ohms. At bass frequencies only the woofer is playing. At high frequencies only the tweeter is playing. So, we have to rate the package as the lowest speaker. You've got 4 ohm tweeters, so the overall package is a 4 ohms speaker.

Now, we are not quite done yet. You need to take the Sensitivity or SPL rating of your woofer, and assuming you use the Series/Parallel arrangement I described, add 6 db to whatever the SPL rating is.

(Can someone confirm that 6 db is the correct woofer gain from this series/parallel arrangement?)

Now, likely the tweeter is louder than the woofer. So, subtract the woofer SPL+ACs-6 from the tweeter SPL, and assuming you still have a positive number, you need to reduce the tweeter by that amount.

Go to the bottom of the link I provided to where it says L-pad (Speaker Attenuation), enter the impedance of your tweeter and how much you want to lower the sound level, and it will tell you what resistors you need.

Next at bare minimum you need to add a Zobel network to your woofers. Though I'm not sure on this matter, I would guess you can treat the entire gang as a single speaker and put Zobel across it. Though some might suggest putting a Zobel across every woofer, which is probably the safer route. I'm not sure on that aspect

But down near the bottom of the link I provided, second from the bottom on that webpage, is a calculator for a Zobel network. You need speaker DC resistance (Re) and voice coil inductance (Le) both should be available on the spec sheet for your speakers.

Now, if you have the ability to test the speaker on your computer, we can go farther. If you don't, then that is about the best you can do with what you've got.

Any chance these are Ported Cabinets? If so, you need to calculate the cabinet resonance based on volume to get the proper port size and depth. I've got a link for that too.

But before you even go that far, you need to decide how big the cabinets are going to be? Four 6.5" woofer is going to require a sizable cabinet, something on the order of 3 cubic feet to 5 cubic feet (just a guess on my part). Check the Vas spec on your speakers for a rough estimate of the size needed for one speaker.

There is free software available to help you there too.

That is about the best you can do unless you want to spend money on test equipment and software.

As for power rating, that is extremely dicey and to some extent depends on where your crossover. The higher you are up from the low end frequency rating of the tweeter, the more power it will handle. Assuming you chose a reasonable crossover, I think 100watts is a safe rating for the speakers.

Again, you can't rate the complete cabinet at the highest possible rating, you have to rate it for the weakest link, and that would be the tweeter, and the tweeter power handling is related to the crossover point.

Does that help? Or just complicate things?

Steve/bluewizard

[BlueWizard]

I'm not sure why my formulas were changed, but as they stand now, they are incorrect. There is no ACs. And even if there is, it has not been defined.

The standard accepted Ohm's Law formula for resistance is -

Two speakers of unequal size in parallel -

Rt = (R1 x R2) / (R1 + R2)

For multiple resistances in parallel -

Rt = 1 / (1/R1 + 1/R2 + 1/R3 + .... + 1/Rn)

Steve/bluewizard

[jimbob212005]

First i just want to say thank you for your help thusfar, it has been very informing. ok now to plug some information into the equation so that there are less question marks(bear with me its late and i typed this in once but closed the tab before submitting) the woofer specs are as follows = Impedance: 8 ohms *Re: 5.6 ohms *Frequency response: 38-5,000 Hz *Fs: 38 Hz *SPL: 87 dB 1W/1m *Vas: 1.05 cu. ft. ---and the tweeter = 91.1db sensitivity and *Re: 2.9 ohm *power rating: 150 W (12 dB/oct. cross-over at 4 kHz). The pwer isgoing to be supplied fom my (please don't laugh) Denon AVR-4802. It is rated at 125w/channel @8 ohms and 150 w/channel at 6 ohms. It is supposed to handle down to 3.2 ohm load but no specifics on what it does down there(how convenient). The box is going to be a ported design and I have a pair of 2way x-overs at 4500hz. I have had many concerns though about the woofer extending that high. If i must i will scrap out the x-overs for a 3 way design and add opt to add a dedicated midrange. I hope that some of this information helps make the picture a little less fuzzy. Will this even be practical in a 2way design? I do like crossing the tweeters at or near this range because that seems to be the range in which sonus faber always utilizes them. The problem is they also use a dedicated midrange on their more highend models. again i appreciate all of the help.

[BlueWizard]

Make this easy on us, give us the exact brand and model numbers of your components.

Once again though, just because the specs say the woofer has a response of 38 to 5,000hz doesn't mean you can work in the full range. I think, in reality, you will have a hard time getting that woofer to work smoothly above 3,000hz.

But then, I don't actually know what woofer you have, and if you don't know the exact brand and model, then we are dead in the water.

Also, with the tweeter, as I've already pointed out, it handles more power at a higher crossover point. But to know if and which lower crossover point is workable for you, we need to know what tweeter you have.

The devil is in the details.

Also, you might want to check out this website.

http://www.audiocomponents.nl/speake...lassic_eng.htm

Steve/bluewizard

[jimbob212005]

EDIT: magnet weight on tweeter was a misprint and that is the net weight of the tweeter

[jimbob212005]

sorry I didn't post this sooner, here are the models

mid bass - Vifa TP16WJ-06-08

http://www.parts-express.com/pe/show...number=299-277

tweeter- is a sonus faber, but after some research i found that they are manufatured in denmark by Vifa. the closest match i saw was this - Vifa XT25TG30-04 (i believe many of you joined in on a group buy a while back) mine has a different plastic baffle for mounting and a larger shield than the sheilded model of this tweeter. and the magnet is 14 oz rather than the 8.5 oz magnet on the vifa. those are about the only difference between the two. the rest of the data is nearly identical.

http://cgi.ebay.com/ws/eBayISAPI.dll...m=230321131676

X-over- Dayton = Dayton XO2W-4.5K 2-Way Crossover 4,500 Hz

http://www.parts-express.com/pe/show...number=260-148

would it then be adviable to go with a midrange between the tweeter and woofer (switch to a 3 way design)? I would also like to know your thoughts on dome midranges, I have never peronally had the chance to listen to one and was curious as to their performance.

[critofur]

Vifa TP16WJ-06-08 - I've got 8 of those on the way.

I'm confused though, you said you have 4 woofers (total) but you want to make a PAIR of speakers, but, you were going to wire all four woofers together? Is that right? That would make one speaker then, not two.

How many woofers will you have per speaker?

I'm going to make a thread for this woofer after I finish this post.
(here it is: Vifa TP16WJ 6-1/2" Woofer (PE Buyout) )

After I measure one of my woofers (Impedance and Frequency Response) we'll be able to better assist you with designing a crossover.

[jimbob212005]

i have 8 of them as well.... my speakers will be 4 woofers and 1 tweeter in each. I hope you like them yours, did you order them from partsexpress?