MTM Impedance & Crossover Design

[dmaclaughlin]

Hi folks,

I'm a newbie, so thank you in advance for your patience. I searched but didn't find - if you know of a thread that covers this topic, I'd be happyt to be directed there.

I've been reading Vance Dickensen's Loudspeaker Cookbook (much of it beyond my current level of understanding) and looking at his home theater loudspeaker design.

My question is related to the sum impedance of his cross-over setup. With a nominal impedance of 6 0hms on the two woofers and 8 ohms on the tweeter, when set up in a series, I get a sum impedance of 2.17 ohms? (1/r=1/8+1/6+1/6). He indicates that he doesn't want the impedance to drop below 3 ohms. The impedance plot indicates that with his setup, it does not do so.

I think I know:
1. Speaker impdedance is not a static value - it changes with frequency
2. A zobel network will typically flatten the impedance out to the level of the DC resistance.
3. When speakers are connected in series, impedance values are doubled
4. When speakers are connected in parallel, impedance values are halved.

I'd like to build something simliar to Mr. Dickenson's MTMs, using the AUDAX AP100ZO 4" & MOREL MDT-22 1" NEODYMIUM TWEETERS. They have similar impedance values to the speakers he uses.

My questions are:
What impact does the crossover network have on the sum impedance of a loudspeaker?
If I want to design a set of speakers that can be driven by a av receiver with a suggested 6ohm-8ohm load, would it be better to look at an MTM setup as a series (e.g. Darren Kuzma "Lytle 5" project at partsexpress.com http://www.partsexpress.com/projects...ive/index.html)?
Or given the power handling of the 4" Audax Drivers is 45W MAX, would a parallel setup not produce an excessive on such a receiver, rated at around 100 Watts @ 8 ohms.

If the questions are too big for the purposes of this forum, I'd be happy with any suggested reading tips. Many thanks for your time and advice.

Dan

[ttan98]

While I cannot answer your questions directly, I would like to suggest you build a an MTM based on differently drivers and tweeters.

1. Based on Usher 8945A and Usher tweeter. You will get good results at a very competitive price.
see this site:
http://www.zalytron.com/usherkits.htm
they no longer offer the kits but you can get the components from Parts express

or

2. The Eros II based on Vifa and Scanspeak,
see this site
http://www.speakerbuilder.net/web_fi...ct/erosmk2.htm
You may want to swap the scanspeak tweeter with a pair tweeter which is less bright eg Seas or Vifa.

I believe both are excellent speakers and at a good price.

I am thinking seriously of building a pair myself.

cheers.

[Cal Weldon]

Originally posted by dmaclaughlin
My question is related to the sum impedance of his cross-over setup. With a nominal impedance of 6 0hms on the two woofers and 8 ohms on the tweeter, when set up in a series, I get a sum impedance of 2.17 ohms? (1/r=1/8+1/6+1/6). He indicates that he doesn't want the impedance to drop below 3 ohms. The impedance plot indicates that with his setup, it does not do so.

Hi and welcome to the world of speaker building.

The impedance shown to the amp is frequency dependant. You already know that, but what you are about to learn is that when you cross over drivers so that each is receiving a different portion of the spectrum, you do not sum the impedance. The woofer is handling it's portion as is the tweeter. The amplifier does not see a parallel as such but a single driver in that each is looking after a different area and are not combined.

ie: If you take two 3" 8 ohm drivers for example and parallel them, you will see a 4 ohm impedance. If you take those same two drivers and XO them at 2khz, the amp will see 8 ohms.

Summing the two 6 ohm with a tweeter will give you 3 ohms. As you know, this figure is nominal.

That's one of your questions.

Another is about MTM. I cannot give you accurate info about MTM as I have not had good luck with them. The XO point must be kept low and that sometimes goes against my thinking. I much prefer the TMM configuration. Although I must confess, I was making MTM's before they had a name.

Most recently I have had good luck with a TM set up and the second M on the side. Very nice and much easier to XO. See here:

Scottmoose and planet10, this is for us.

[Svante]

I have not looked at this specisfic design, but crossover filters in general "connects" the tweeter at high frequencies, but "disconnects" it at low frequencies. The opposite holds for the woofers. This means that the impedance of the tweeter has no effect on the system impedance at low frequencies and that the woofer has no effect on the impedance at high frequencies.

Connecting two 6 ohm woofers in parallel results in 3 ohms (nominal), and this will be the (nominal) impedance for low frequencies. For high frequencies, the (nominal) impedance will be 8 ohms (same as the tweeter).

In real life the impedance varies and is not at the nominal impedance for all frequencies, but I think you get the point.

[dmaclaughlin]

Thank you all for your help and suggestions. The impedance picture is much clearer now.

So many options to tinker with, and now you've given me more to think about. I guess that's what drew me into this project in the first place.

I've tried the "Big speakers are better than little speakers" line on my wife, with little luck. Funny how we both agree that little speakers are better than no speakers. ;-)

Thanks again!

[Cal Weldon]

You've given me an idea.

[augerpro]

Do tell Cal <------hey that rhymes

[sreten]

Hi,

If you want to build some good small AV speakers look no further than this design IMO :

http://www.zaphaudio.com/audio-speaker19.html

/sreten.