Trying to understand impedance on MTM

[DougyD19]

DAYTON RS150S-8 6" = 8 ohm
DAYTON DC28FS-8 1-1/8" = 8 ohm
CROSSOVER 2-WAY 3,000 Hz 100W 12 DB

I'm not sure how I should hook them up -- Questions
1. Do I need to look at impedance of the whole MTM at the crossover or exactly how does that work. I've been reading and I can't find any good info. It seems like as long as you use a crossover, it doesn't matter what the impedance of the individual drivers are?

Here is my thinking...I wire up the two Mids in Series to get them at 16 ohms and in parallel to the tweeter at 8 ohms. The total ohms for the MTM will be 12 ohms.

My satelite speakers MT should be at 4 ohms right? M = 8 ohms and T = 4 ohms in parrallel on the crossover.

I'm I going about this all wrong or does the ohms only matter in a certain frequency range? If this is so, should I just hook up the MTM for my Mids in Parrallel?

2. How will my crossovers be affected by the ohm change. It says "A 12 dB per octave high pass filter with your choice of either a 3,000 Hz or 5,000 Hz crossover point" -- Does this mean the 3K is for 4 ohm and the 5K is for 8 ohm? The crossover uses a 100V 4.7mfd NP capacitor and can't find info on the wire wound thingy.

[KeithM]

I hate to say it, but trash that design and look at the finished designs like the Modula MTM or something on the forums. The crossover frequency is too high for the RS150 (and 2nd order isn't too good for a metal cone). Also the prebuilt crossover speakers never sound too good because it isn't tailored for any specific drivers.
Modula MTM link

[Dennis H]

Ditto what Keith said. The crossover needs to be specifically designed for the drivers. Chris (CJD) has a good design for an MTM using the RS150.

http://www.eldamar.net/audio/rs150mtm/

[DougyD19]

This is not a metal dome...it is a silk dome tweeter.

Since I am using different speakers, I don't think using their setups is going to help me any more than what I currently have.

I need to know the answers to my original questions. I also don't have test equipment to test frequencies and build my own.

If nobody knows the answers, if you can point me to a link where I can read up.... Surely with as many people that are building these MTM or more speakers in a box, somebody has to know how the ohms works for the speakers and crossovers.

I've already read up on hooking up parrallel vs in series, but I can't find anything that says what the over speaker setup ohm is when hooked up with crossovers and how ohms affects crossovers.

[Hdale85]

They were not talking about the tweeters. They were talking about the woofers they have metal cones.

[cjd]

1. none of what you described. ASSUMING the crossover is 3kHz with 8ohm drivers, wire the W's in series and the crossover will see 16ohm for them and not work as it should. Wire in parallel and it'll see 4ohm and not work like it should.

It will work as it should for a tweeter that is 8ohm.

The mid-woofers get attached at one spot directly to the crossover, the tweeter to another spot directly to the crossover, and then your terminal posts go to yet another spot on the crossover. Not only that, 3kHz is too high for the RS150 at 12dB octave in particular. Someone already linked my MTM with this mid-woofer, and I find 2100Hz 24db/oct is a bit high, I prefer 1800Hz 24db/oct.

PE has some stock crossovers that will work with either 4ohm or 8ohm mid-woofers, this could be used in this case...

It still won't work as you hope - drivers don't hvae flat impedance, so none of 'em will really roll off as a stock crossover will expect.

And, the tweeter you have selected will be the weak link sound-quality wise if you were to work with a crossover tailored to those drivers.

I think the recommendations you've seen have been as they are because, well... it is clear you're *very* new to all this and would almost surely be very very disappointed if you were to spend your hard-earned dollar on the items you have listed and put it together... finding the information can be tough. It's out there but it doesn't necessarily make perfect sense to everyone.

C

[ThomasW]

I need to know the answers to my original questions. I also don't have test equipment to test frequencies and build my own.

If nobody knows the answers, if you can point me to a link where I can read up.... Surely with as many people that are building these MTM or more speakers in a box, somebody has to know how the ohms works for the speakers and crossovers.

I was just typing up a reply and realized Chris already said what needed to be said.

You have the wrong crossovers...and are wasting a really good set of midwoofers with a poor tweeter.

BTW, the people posting in this thread have DECADES and DECADES of experience designing and building speakers. So you might consider paying attention to what they're saying.

Return the tweeters and the crossovers. Build the design in the link Dennis provided.

http://www.eldamar.net/audio/rs150mtm/

[DougyD19]

Ok, the crossover is crap. I'll return it.

My other question was about the Ohms of a MTM. If I setup the Mids in Parallel to get 4 ohms and with my 8 ohm tweeter....does this mean the overall speaker is 6 ohms. The reason I'm asking is my Onkyo says to not go below 6 ohms or it might trigger an overload.

[ThomasW]

The crossover isn't crap, it's just the wrong crossover and you've got the wrong tweeter.

The impedance of the tweeter doesn't figure into the impedance of any speaker. The impedance of the speaker is how the woofers are wired.

[DougyD19]

Question about the tweeter in a MT setup then. Going by the PE sample response curves...

DAYTON DC28FS-8 1-1/8"

http://www.partsexpress.com/pdf/275-075g.pdf

DAYTON RS28AS-4 SHIELDED 1-1/8"

http://www.partsexpress.com/pdf/275-135g.pdf

DAYTON RS150S-8 6"

http://www.partsexpress.com/pdf/295-362g.pdf

Using my existing 2nd order 3K cutoff on the tweeters for a MT setup. Is the tweeter still crap and is crossover still wrong. Does it not look like 3K is a good point for the tweeter and the Mid.

[Dennis H]

the tweeter still crap and is crossover still wrong.

Yes. Return the tweeters and crossovers like Thomas said. Buy Seas 27TDFC tweeters from Madisound. Buy the crossover components from either PE or Madisound and build the design on Chris's page. Your proposed design will sound like ass. Don't expect any help building something that sounds like ass. Chris's design won a best of show award at a recent DIY get together so build that one.

[joecarrow]

It's about more than just the response curve, and it's about more than the impedance. What the manufacturers of most home drivers don't tell you is about the speaker's linear and nonlinear distortion. What this means is that while the DC28FS looks like it has decent response (based on the frequency response curve), it might have a lot of distortion at some frequency within the range that you'd like to use it.

Here's a primer on linear and nonlinear distortion: http://www.markk.claub.net/primers.htm

Beyond distortion, frequency response, and impedance, the best designs also take into consideration things like directivity, diffraction, and interaction with the listening room. You would do well to build a design with an established crossover using exactly the drivers intended.

You can definitely get into speaker design as a hobby- I hope it doesn't sound too discouraging here- we just don't want you to get a bad taste in your mouth from building something that you won't be happy with.

[Doug Lockwood]

I have a couple of questions. First, what is your goal? Are you trying to learn something, to create a set of speakers on a very tight budget, or to make something better than you can buy?

If you are just trying to get some tunes, definitely check out the Modula MT here on this forum. Many of the projects here are better than commercial designs at many times the cost. For more of a budget orientation, check out Zaph @ http://www.zaphaudio.com/
I have heard a sample of Zaph's speakers as well, and it was very impressive, especially because of its humble driver compliment.

If you want to learn to design your own, you need to buy some books.
I see Speaker design cookbook recommended on another of the forums I post on.
A measurement microphone is really necessary to design anything useful.
Trust me on that. I was having sound card troubles, so my measurement setup was down. I tried to tune it by ear. Laughable.

The woofer you selected is a complete pain to design with, and really requires exotic Elliptical crossovers. If you get it right, its great, otherwise, oh well.
The tweeter is a nice budget tweeter, but has consistency issues, and for $30, you can get something that will be easier to work with.
I wouldn't use a stock crossover in a PA system to call my dog.

Let us know what you are trying to do and maybe we can help.

Doug

[cjd]

Ok, a little more time now that I'm at work and paid for it. :P

At a high level, a crossover manipulates impedance. A capacitor has an increasing impedance as frequency goes down - so, throw a cap in series with a tweeter and no other drivers, and your impedance below cutoff will approach infinity - above cutoff it'll be the raw tweeter response (which by itself isn't flat still). On the flip side, an inductor has a rising impedance as frequency increases (note that a voice-coil is similar, which is why impedance rises with frequency...), so above cutoff it'll approach infinity, below it'll be raw mid-woofer impedance. Sum the two, and you may find a peak at or near the crossover point, but nothing near infinity - when you add in the impedance peak caused by mid-woofer resonance you get the classic double-peak impedance. More complex crossovers produce different results, but that's the basics.

Now, from this you can learn that tweeter and mid-woofer impedances *never* sum. Thus, impedance low is dictated by the driver with the lowest impedance. Pick a 4ohm tweeter and an 8ohm mid-woofer and you'll have a 4ohm nominal speaker, generically speaking. Impedance is (almost) never flat though, it's just a 'this is about where it bottoms out' rating, which is what your amplifier cares about. Thomas was over-simplifying when he said that the mid-woofers dictate impedance - that's the usual in an MTM but not the rule.

I'm not as pessimistic about the RS150 as Doug - 4th order standard works fine, no exotic elliptic filters required (but perhaps helpful).

If you want to stick with the tweeter you have and a bit more of a budget, check out Curt C's Triune design(s) - well regarded, good sound, and a great starter project. My MTM was designed in part as a follow-up project to the Triune - if you wanted you could actually add a new baffle to the Triune, new drivers and crossover, and have my MTM. I'm working on some page updates to make it more clear as it's kind-of all over the place, but you can always ask questions here - there's already a thread out there for it even.

The crossover you have in your hands is always going to be a compromise - you could add zobels to bring your drivers closer to the linear impedance it expects, but you still would need different mid-woofers for that 3k crossover point. A 5" poly cone driver perhaps. A 12db slope at 3k on the RS150 will leave your ears crying - very harsh breakup.

C

[DougyD19]

I’ll be building a sonotube for my sub and chop off all the other speakers lows at 100 HZ using my Onkyo. Because of the chop, will I still need to use a port on the MTM or can I stay sealed?

DAYTON TWC-0.25BK .25 CU FT TWO-WAY CABINET PR GLOSS BLACK
DAYTON MTM-0.75BK .75 CU FT MTM CABINET GLOSS BLACK

I bought 4 satellite boxes and one for the center channel in an MTM setup. Since the center channel is the most important…I’ll just get a different tweeter and use the following crossover setup --

I was reading about the overall ohms of the MTM speaker. On the Triune he has his rigged and goes into the 4 ohms for a while, but mentioned that it would be sold commercially as a 6 ohm speaker. In looking at your setup, it would appear yours would be the same. Thanks Chris for the info on crossovers and impedance…it clears things up.

My goal is to have a nice sounding HT setup, but not pay an arm and leg for it. I’ve seen one speaker use $250 just for crossovers to obtain the nth degree of perfection – I’m not looking/can’t afford to do that.

Question on my sealed MT setup – not the MTM. I have already built one MT using the 3K crossover. I am by no means an audio expert, but it sounds good. I played some Tina Turner and the movie Chronicles of Riddic using one of my old 3 way speakers I built a decade ago for comparison. They sound very similar, with the MT sounding a little better at higher volume in the vocals. Is there a sound file that goes through and plays all the frequencies so that I can listen for areas that might be left out by the crossover or are too high. I will listen to some more music tonight to help determine problem areas.

[cjd]

Tuning by ear is an iffy proposition at best. There are programs out there you can use to play a sin sweep though. Frequency response only tells you one part of the story.

You should be fine building sealed versions.

Also, do you need the shielded center? If so, that's definitely one option, or you can build one of the Seas 27TDFC versions as well. I *would* try to keep the same mid-woofer/tweeter combination across the front stage. I have too many variants - which is why I'm working on an update to the web-page. Gotta keep *me* clear, let alone all the rest of ya.

C