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Old 16th January 2007, 10:39 PM   #1
jonz is offline jonz  Canada
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Default Some questions on crossovers.

I put together some MTM's for HT... which consists of 2x 6 woofers and a 1 inch vifa tweeter in a 42 liter (1.5 cu.ft.) vented enclosures.. My first mistake was I bought ready 2 way crossovers... which made this set up sound super bright....

The tweeter has a 91 db effiecency to the woofers 90db...so I'ts not that apart...

My question is can I use an online crossover program like..this one to build a better crossover... ?

The woofers have an fs. of 49 and a range to 5000 and the tweeter has a 1500 fs..also it's 6 ohm compared to the 8 ohms of the woofer... (wired in paralell)

What would be a good crossover point?

I notice many MTM designs often use a third order Butterworth..

Would something like this...be okay?

Click the image to open in full size.


Thanks guys..any answers would be greatly appreciated..
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Old 16th January 2007, 10:47 PM   #2
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Hi,

Programs and spreadsheets are great for getting you in the ballpark, but some tweeking by ear or using a program will probably needed to get the sound you're after.

Cheers!
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Old 16th January 2007, 10:53 PM   #3
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What are the impedance ratings for the store bought XO?

EDIT: Is it 2nd order?
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Old 16th January 2007, 11:05 PM   #4
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A few points.

MTMs are better crossed over as low as the tweeter allows. Your tweeter's Fs of 1500 Hz is too high for an MTM with 6.5 inch woofers. Each driver arrangement has its own pros / cons and issues that need to be factored into the overall design. Way over my head at this stage... but the more I read, the more tricky it becomes!

If your woofers have a nice natural acoustic rolloff - then a simple crossover (whatever order you like) may be all you require. If however the woofers exhibit nasty breakup nodes (the stiffer the cone - the more likely this is - look at a manufacturers freq response chart to get an idea) - then you'll need to put a notch filter of some sort in place in addition to crossover components. Since your Tweeter has a high Fs, you'll want a higher crossover frequency or a sharp rolloff (= higher order highpass filter) in the crossover.

Baffle step compensation may also need to be factored into the design, given your room environment, placement, intended baffle dimensions

I don't know whether you will end up with fair, good, great or horrible sound. Problem is - just picking some drivers then trying to make a design work is a bit like buying a small car then expecting it to tow a large heavy trailer, go fast round a corner or even fly.

Not trying to put you off - but set your expectations low and be prepared with lots of by ear tweaking / tuning, spare parts (= $). Without modeling software and measurement equipment - it will take longer.

The best thing you could probably do now is....
1. Try and find a design that uses the same drivers and copy it (enclosure, crossover etc...)
2. Assuming you cant find an exact design - find 2 designs that share a similar enclosure - and see if you can obtain the frequency response measurements (per driver - no crossover). Regardless of 1 or 2. - you will need to build the enclosure to the same dimensions as that used in the woofer project you got the measurements from. The tweeter will be placed on baffle in the place used for the tweeter design. Put woofers and tweeter as close together as possible
3. Download a crossover modeling program (ie. Speaker Workshop) and load the frequence and impedance plots you obtained in 2.
4. Use the textbook formulas (as you have done via the lalena site) to start with a crossover that suits the will achieve the target slopes you have intended for each driver
5. Tweak until get you get a modeled frequency and phase response that looks good. (Note - this will be good on-axis - who knows what off-axis response problems may be lurking).
6. Build a prototype - with your crossover... and ideally be able to measure its response
7. Repeat from 5. until happy

I'm sure there are gaping holes / errors in my approach that others can fill / correct.

Hope that helps. Not intending to put what you are doing down, but be prepared for things not sounding great, some frustration and a lot of time to get it right.

Cheers,
David.
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Old 16th January 2007, 11:11 PM   #5
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also... speakers don't present a constant impedance throughout the frequency range. the impedance rating is an "average". The speaker will have a higher impedance at some frequencies, and lower than 8 ohms at others. you might find the woofer's impedance dips to 6.1 ohms (or less). In parallel - you might find the load on your amp to be only 3 ohms. You'll need an AMP that's solid for 4 ohm nominal speakers.

David.
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Old 16th January 2007, 11:26 PM   #6
jonz is offline jonz  Canada
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Quote:
Originally posted by Cal Weldon
What are the impedance ratings for the store bought XO?

EDIT: Is it 2nd order?

Yes it is... now I removed it and I m experementing with an attentuation circuit of -3 db on the tweeters and the woofers are straight through...

Still I am not happy...


Is there anything I can do with these tweeters in short of chainging them...?
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Old 17th January 2007, 12:33 AM   #7
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Can you describe where it sounds loud or not loud enough in the frequency spectrum?

Chances are - the bottom end is lacking - meaning you'll need some baffle step compensation. Adding BSC will require further attenuation of the tweeter.

Is it still too bright? Chances are you'll need to bring the midrange down a bit. Do you hear sibilance - chances are you'll need to bring the 8KHz area down too.

Can you tell us driver models.... we might be able to identify at least the areas of frequency anomalies.
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Old 17th January 2007, 02:09 AM   #8
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Quote:
Originally posted by Dave Bullet
Can you describe where it sounds loud or not loud enough in the frequency spectrum?

Chances are - the bottom end is lacking - meaning you'll need some baffle step compensation. Adding BSC will require further attenuation of the tweeter.

Is it still too bright? Chances are you'll need to bring the midrange down a bit. Do you hear sibilance - chances are you'll need to bring the 8KHz area down too.

Can you tell us driver models.... we might be able to identify at least the areas of frequency anomalies.

If you don't have sophisticated measuring equipment, an excellent tool for determining the frequency location of crossover problems is a digital 31 band graphic eq like the Behringer DEQ2496, along with the ECM8000 mic. You can tweak the eq for a sound you are overall happy with, then adjust the crossover accordingly. To do this, you need a crossover modeling program which plots the electrical responses of the woofer/tweeter circuits, with the exact R and L of the drivers you are using. Then you can tweak part values to get the response adjustment you desire. As an engineer, I able to use Matlab (similar capability to Mathcad) for this. I'm sure there's freeware to do this, Speaker Workshop maybe.

The ECM8000 MIC can be used with the DEQ2496 noise generator to see the speaker acoustic response. This is an in-room response on-axis only so it is inherently flawed, but it will show large crossover problems, such as suckouts at the crossover point from phase cancellations between the woofer/tweeter.
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Old 17th January 2007, 02:09 AM   #9
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Another option is to have Madisound design the crossover using their LEAP software. At the very least, this will provide a good starting point for tweaking.
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Old 17th January 2007, 02:14 AM   #10
jonz is offline jonz  Canada
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I hear the tweeter just little bit too much..

The woofer specs are:

Fs:43
Qes: .038

Qms:7.1

Qts: .36

Sd: 139

Mms: 18g

Xmax:+/-3.5

Vas: 20L

F.Range: <5khz


They are called MaxPentivent PV-6520


The tweeter is a vifa D26TG0506
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