# Quick question: which ohm value to use in xover

Discussion in 'Archived Threads 2001-2004' started by Chris Carswell, Jun 29, 2002.

1. ### Chris Carswell Supporting Actor

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When using 2 8ohm mids in a MTM configuration wired in parallel, it gives a 4ohm load. So, when I check the charts
to see what value inductor & capacitor to use I would read from the 4 ohm side not the 8ohm correct? If wired in series I would use the 16ohm chart.

***Also, if the woofers are now 4ohms and the tweeter is 8 ohm, what problems am I going to run into? Do I need to put a resistor in there /some where to make it 4 ohms? I now know how to make L-R 1st and 2nd order xovers but have no idea how to do something like this. What is this Zobel thing/impenance matcher?
Thanks again all.

2. ### Mark_E_Smith Second Unit

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Treat them as separate circuits, however the resistance changes with the frequency so you need to find a resistance plot for your drivers and check the resistance at the frequency you want to target. I bought X-over Pro and learned alot just playing around with designs.

3. ### VinhT Second Unit

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As Mark indicated, the resistance of a driver changes depending on the frequency (a peak at the resonance frequency, the Fs, and a parabolic rise as the frequency increases). A Zobel filter, which is a capacitor and resistor in parallel with a driver, minimizes the rise in the resistance, presenting a more stable impedance to the crossover.

But to answer your first question, yes, you calculate the crossover part values for the overall impedance of the driver or drivers. Hope this helps.

4. ### Chris Carswell Supporting Actor

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Ok, 1st part done. It is 4ohms.

Now to the second. I understand now after reading lots of stuff what a Zobel does. It makes it a constant impedance but it does not lower it, say from 8ohms to 4ohms.
With the 8 ohm tweeter and 4 ohm woofers is there going to be a problem? Can I lower it? Do I need to? I understand if I use a Zobel on the 8ohm tweeter with a DC resistance of 5.2 ohms it can only smooth it out to 5.2? Yes? NO? I'm not sure. Basically, should I just not worry about it since it is my 1st DIY crossover/speaker build................

5. ### Patrick Sun Moderator Moderator

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You simply can't use nominal impedance values for the midwoofers and assume you won't overdrive your receiver/amp because the crossover components contribute to the overall impedance profile (impedance vs. frequency), and can cause that profile to dip into dangerous territory for your receiver/amp. Unortunately, you need to have some test gear to find out the impedance curve for the overall speaker to determine if you have impedance dips that might cause some problems.

The receiver/amp won't have much problem driving a higher impedance (like the tweeter), so it's not as problematic, nor an issue to worry about, though you seem adamant to lower the impedance of the tweeter for no good reason.

It's very rare that a tweeter needs a zobel because zobels help stabilize the impedance, but people use this stabilization to design the crossover filter around the crossover point. The reason a zobel is used mainly on woofers/midwoofers is because in the crossover region, these drivers will feature a rising impedance profile due to the voice coil, and it's much easier to design a filter for a "constant" impedance load in the crossover region, than it is to design a filter with a rising impedance and hoping (guessing) the filter does what it's supposed to do. Most tweeters don't feature much rising impedances in their intended crossover regions (but most will show a rise after 8KHz, but that's not anywhere near most crossover points, usually around 1.5Khz - 6Khz depending on used with 2-ways or 3-ways or higher).

6. ### Chris Carswell Supporting Actor

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Thanks Pat.
It's not that I WANT to lower the Impedance but more of ignorance on my part. I always thought that you need to have everything the same. 8 ohm tweeter, 8 ohm mid, 8 ohm woofer. I thought if you had an 8 ohm tweeter with a 4 ohm woofer the tweeter wouldn't play as loud. This based on the principle that less resistance = more power. (Ohm's or Watt's law, I'm not sure) I'm afraid I didn't learn much about electricity at the Fisher School of Business here in UF. What little bit I did pick up was from this great place This weekend I plan on hitting the book store. Its been noted that a good read is found in the Vance Dickason's Cookbook.
Also, how is your center coming? In my project I am going to use the MDT-20 (due to \$\$). How does it compare to the MDT-30. It looks about as good on paper but that's just what it is, paper. The only thing for sure about that is the fact that you can flip it over and try again

7. ### Patrick Sun Moderator Moderator

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I went with the MDT30's because I wanted the ferrofluid cooling and the increase in power handling for this tweeter, plus I heard it was a proven performer, and I got them for a decent price (@ \$40/tweeter last year from Zalytron). The center channel project is shaping up, just waiting on a new XO to try from my friend with the gear/XO simulation setup. The current XO isn't too bad, maybe a little bandpass gain, but nothing horrendous, probably about 85% "there".

8. ### Brian Bunge Producer

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Good luck finding the LDC at a bookstore (in stock anyway). You'd be better off ordering it online through Amazon or maybe Parts Express.

Brian

9. ### Chris Carswell Supporting Actor

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I called around and no luck it's too late now but I will check the Library tomorrow. Should have looked there 1st, duh.
I hope you guys have your calculators ready. Next post is for xover ideas and pics......mmmuuuahahaha

10. ### Mark_E_Smith Second Unit

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Vance's book is a pretty heavy read, you might also try "Great Sound Stereo Speaker Manual" by David B Weems a little more basic. He has another book "testing Loudspeakers" that is a near repeat of the Great Sound. I bought both of them from PE along with BassBox Pro and X-over Pro. If you give me your drivers and your inclosure design I will try to help BUT I am a beginer at this, though with little fear.

11. ### Chris Carswell Supporting Actor

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Ok Mark you asked for it
Let's see what you can do:
---------------------------
Driver Stats T/S parameters
---------------------------
Morel MDT-20 Tweeter:
Fs: 650 Hz
Frequency Response 1,600Hz - 25,000Hz
Z Impedance: 8ohm
Re DC Resistance: 5.2 ohm
Le: .05 mH
1w/1m: 90 dB
PE: 120W nominal
Voice Coil Diameter: 28mm
Voice Coil Height: 2.5mm
Mms: .5 grams
Piston Area: 6 cm^2
Flux Density: 1.45 T
SPL 1w/1m: 90 dB
PE: 120W nominal
Stryke SA071 Mid:
Fs: 45Hz
Qms: 2.6
Qes: .65
Qts: .53
Vas: .78 ft^3 or 22 liters
Z Impedance: 8ohm
Re DC Resistance: 7.2ohm
Le: .5 mH
BL: 7.2 Tm
Mms: 16.4gram
SPL 1w/1m: 87dB
Xmax: 4mm
PE: 60W nominal
SVS Sub:
FS- 21 hz
Qts- .38
Qes- .39
Qms- 6.6
Vas- 4.6 ft^3 or 130 liters
Z Impedance- 4ohms
Re- 3.2ohms
SPL- ~87dB
X-max- ~14mm
power handling- ~200w
Here is a pic. What do ya think?

12. ### Mark_E_Smith Second Unit

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Did you already build these? Is the sub powered? If not consider it strongly. It would make life MUCH simpler. Does stryke have a website to get a measured curve?(Audax had one) And how do you post a picture? So I can send you the graphs. You are building the system I was thinking about ending up with when I started my center channel project. Only with diferent drivers.
Ok I found the Stryke site and it has a measurement. Are you going to do the felt mod to the driver? The morell looks like an impressive tweeter. PE has a graph of it. It will take a little time as I have to input these graphs manualy into the software. Just on the surface it seems we will need to get the tweeter crossed over by 2.5 KHz.

13. ### Chris Carswell Supporting Actor

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No the subs won't be powered. I though about it but I had to keep costs down so no go. I will be powering them with a 350watt Adcom amp so I'm not too worried.
Stryke has a site HERE
Here is the graph though. I can also send you the tweeter graph by Morel in .PDF form if you need it.
To post a pic just click on the IMG tab in the Code tool bar. If you can't get it to work just shoot in to me via email.
Thanks

14. ### Chris Carswell Supporting Actor

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Wow Mark. You beet me to my post. Seems like you found the site as I was writing that post. It looks like the woofer starts to go crazy & break up after ~3000 Hz. I'm not going to bother with that felt ring mod. You have to do it to the front and it looks like PoPo on that cone. That actually is a common thing for drivers with a higher surround. Like the site said, many manufacturers will simply apply some smoothing to the response curves so the dip will not be noticeable.
One good thing about that tweeter is that is can go that low. so a 2,500Hz ain't bad. I was thinking 2000Hz or 2250Hz so that's darn close but now that I think about it, 2500Hz seems better
No I have not built these yet but they will look exactly like that. I took a pic of a tower and added my drivers. all done on good old Microsoft paint. Just a simple pic.
Thanks again Mark.

15. ### Brian Bunge Producer

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Chris,

The downside to not having the sub drivers self powered is that the large inductors required in the passive crossover will suck up a lot of power from your amp that will merely be dissipated as heat. This means you'll lose a lot of power from your amp that will never reach the sub drivers.

Brian

16. ### Mark_E_Smith Second Unit

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It also makes the crossover get really weird. I would rethink the non powerd sub. Even if you have to buy lower powered amps to save money it makes for some interesting posibilites for tuning to various rooms because the room response is most affected in the sub woofer range. Once a 3 way crossover is done thats it and you will not be able to adjust the bass output without redoing the crossover. I will be building my system with dual powerd subs for that reason and to make adjustments between HT and music. My sujestion is : power the subs, sealed box for the L-R so the roll off will match the sub.( kind of a non crossover crossover) Maybe a vented center to achieve sub 100 Hz F3 this is what I will be doing.

Ps having a power sub really makes a difference in the sound in both HT and music.

17. ### Chris Carswell Supporting Actor

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Mark,
Think of these more as a 2.5 way type design, not 3 way, since I will run the full load to the mids as well. That way all I have to do is make the 5 crossovers all the same for all the speakers but then run a inline 1st or 2nd order xover to the sub. Am I crazy or just thinking tooo far out of the box
Also, do you mean seal the sub part of the LR? Elaborate please.
I do have 2Ch's open on my other Adcom 5 ch amp I could use to power them with if it sounds like shi*. Problem is I just got a 7.1 ch processor and wanted to use it for that.

18. ### Mark_E_Smith Second Unit

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No sealed box for the mid/tweeter on the L/R,this gives a nice rolloff below 100 Hz so you can match the roll off with the active crossover in the sub amp.
As far as a sealed box, I dont know on the sub as I would have to run it through the design software. You can also buy 1 plate amp of 150w or more and run both subs off of it. I dont know what you mean by 2.5 or running the mids. If you are just going to run a parallel circuit that wont work as it will drop the load on the sub/mids to low. For every type of speaker you MUST have some kind of crossover or you will have a large peak during the overlap of the speakers and that over lap on a 12" driver and the mids might be from 50 Hz to 1500Hz. The plate amps come with a active -12 db/oct crossover usually adjustable with a pod between 40Hz and 125 Hz. Some have a tunable bass boost to give a little rise below 25 Hz for sealed inclosures.

19. ### Mark_E_Smith Second Unit

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Do you know the manufacture of the sub or a model number off the speaker? SVS is not listed in the speaker database and they have a really big DB. I think the QTs you listed is wrong. It is usually 3. something

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