First Build Questions - RS180 Modula MT

I'm designing a pair of bookshelf loudspeakers with a friend. My friend has decent experience in woodworking but little in crossover design. Prior to working on this project, has has completed a pair of full range and 3-way speakers (With a premanufactured crossover for the 3-way - It did not sound great). While his experience in electronics lack, his craftsmanship skills are above average (So I'm leaving the cabinet entirely to him).

The Cabinet Dimensions He Calculated:
Port ID: 2 Inches
Total Volume (Including driver & crossover displacement): 0.519 Feet Cubed
Total Volume without Displacement: 0.509 Feet Cubed
Port Length: 5.06 Inches

http://i.imgur.com/YijDPVm.png

Jon Marsh designed the crossover. After doing copious amounts of research (And dumping more money into this than I probably should have) I decided to go the safe route and implement the crossover from the build here: http://www.htguide.com/forum/showth...-asked-for-it-you-got-it!&p=173852#post173852 If you cannot login to view that thread: http://i.imgur.com/p1ub8Kv.png One thing I noticed was our cabinet dimensions differed. Could anyone verify that the cabinet dimensions calculated by my friend above are correct? He used BassBox 6 Pro [High Fidelity Suggestion] for his measurements.

In the image below, I have laid out a rough sketch of the crossover. The crossover is going to be placed at the bottom of the cabinet. Three potential concerns of mine are the following:
1) Will the inductors closer to the driver affect it's behavior? My reasoning was the inductors close to the woofer are set on a different axis than the voice coil inside the woofer... so there should be no issue, correct?
2) How far apart should resistors be spaced to ensure that heat is dissipated at an appropriate level?
3) Is it okay if dampening is not placed on the bottom panel (Where the crossover is mounted)? If I needed to shift the crossover to the wall directly behind the woofer, would no padding on the back wall be an issue then?

http://i.imgur.com/SQDeiAW.png
 
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eriksquires

Member
2013-05-10 4:11 pm
Hi

Many builders refuse to put their crossovers inside a speaker cabinet for a pair of reasons:

1 - Unpredictable EM interference with the large driver coils and magnets
2 - Unpredictable effects of the sound waves shaking the coils and caps

1. Possibly. But the issue is more than the coil, it's also the metal in the driver, the magnet et. all. It's crazy how far away coils have to be to stop interacting at all.

2. Depends on the construction and the real expected wattage. For instance, if you are using 10 watt resistors but expect each to handle 1 watt, it's probably OK to stack them. However if you expect them to get even to half of their wattage, and they are rectangular, I'd leave 1/4" to 1/2" of spacing. The rectangular resistors stack too well together and leave no space. Not that you want to spend the money, but Mills resistors are very small for the wattage. Just FYI.

3 - Speaker makers and designers often suggest a pad on top of the crossover, not underneath it.

Best,


Erik
 
Could anyone verify that the cabinet dimensions calculated by my friend above are correct?

0.5 cu ft is a good choice for the Dayton RS180-8 IMO. The port dimensions result in a tuning frequency of 50 Hz, that's fine as well. :)

The Modula MT is a highly regarded speaker. The crossover of Jon Marsh uses a unique topology which is confusing and hard to understand. No problem as long as you don't attempt to make a modification. :D

Crossover.tif


Speaker Builder: Modula MT - crossover and parts list
 
Hi,

Using the shielded version of the bass driver the x/o placement
should not be a problem. With unshielded drivers often the best
place for the x/o is on the rear, directly behind the bass unit.*

rgds, sreten.

* Often it is also built as part of the input terminal block.
 
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TMM

Member
2007-09-01 8:37 am
Australia
1) Will the inductors closer to the driver affect it's behavior? My reasoning was the inductors close to the woofer are set on a different axis than the voice coil inside the woofer... so there should be no issue, correct?
Basically yes. If the inductors are placed close enough and in the right orientation there will be coupling between the two coils like a transformer. Using appropriate spacing and/or orientation the coupling is negligible. Remember that the fields are not only in a single axis, but are a donut shape. This sheds a little more light:
Placement of coils in crossover networks

There is however another issue, and that is the effect on the magnetic permeability of the speaker driver materials. If you place an inductor too close to the driver it's inductance will go up as parts of the driver become a metal core for the inductor. From my tests if you keep the inductors further than 2 inches away from the driver the effect is negligible.
 
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