Repurposing Bose 401’s into MTM’s - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 13 Old 05-28-2018, 12:52 PM - Thread Starter
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Decided to make some new cabinets for some old bose speakers, here is what I did. First, I bought these when I was 17, 24 years ago and they have been actually a good rear 5.1 speaker. That being said.... I had no idea they could have been much better, go figure.
I made some ported 1.25cuft cabinets and tuned them at 40-45hz (3” port 5.25” long) kept the xover for now and WOW. Ok, not off the wall good but night and day better. The original cabinets were a muddy mess and had very little imaging and now it’s not a muddy mess. I’m pretty sure the MTM design is what they should have done to begin with but we are taking Bose after all. I do have a question for anybody who knows xover design, 1. What the hell is the little (coil?) on the tweeter on the third picture? It was wired.... coil - cap - bulb - tweeter and the 6 1/2’ were run full range. 2. My high end is a little subdued and could be a little brighter, any advice (make new xovers, new tweeter?). Tweeter is 3.4 ohm and 6 1/2’s are 5.4ohm each for a total cabinet of 4ohms.
Rating - 6 - original cabinet was a terrible front speaker and was a great/good rear/side instead, speakers drivers were not the worst I’ve seen.
New MTM rating -8- moving the speakers into this design helped dramatically, made it a great full range tower speaker with good on and off axis sound. Lacking in the brightness though (very subdued high end), making new xovers probably would help or possibly changing out tweeter.
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post #2 of 13 Old 05-28-2018, 02:15 PM
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Tricky stuff to parse . .

Got REW?

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post #3 of 13 Old 05-28-2018, 03:39 PM - Thread Starter
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No REW, I’m pretty sure that would look like a mess (insert manufacture) which is typical.

And to clarify, I am praising the MTM design more than the speakers themselfs, as I said high end is lacking also. It was either have some fun or trash them, hmmm. Today I learned........ Bose. Lol
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post #4 of 13 Old 05-28-2018, 05:30 PM
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How do you get a total cabinet of 4 ohms? If you woofers are 6 ohm nominal (5.4), they should be either, 6, 12 or 3 ohm depending on how you wired it. Since you didn't change the crossover it is most likely 12 or 3 ohms. The nominal ohms is based off the woofers. Did you wire them in parallel or series? One of the reasons your high end is off, is due to the fact you have changed the nominal impedance of the low end, without changing the crossover. This will change your crossover frequency while also increasing at least 3 db in that range.

Please forgive me, if this is coming across as anything more than helpful tips. That is all I am trying to give. I'm not sold that you need a new tweeter, you probably just need to change the resistor value on the tweeter. However, that will only raise the high end. There will still be a lot more issues.

P.S. I love that you are reusing drivers. I wish more people would. I also love the fact that you are learning as you go.

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post #5 of 13 Old 05-28-2018, 08:46 PM - Thread Starter
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Quote:
Originally Posted by Al Toid View Post
How do you get a total cabinet of 4 ohms? Did you wire them in parallel or series? you probably just need to change the resistor value
First off the total ohm if both drivers are 3.6 in parallel which the industry describes this as 4ohm.
Second, I changed out (today) the capisator from a 5uf (roufley 7k crossover) to a 22uf that I had laying around (1000hz) and that helped. I also decided to order new tweeters, caps and a coil for a 2nd order 2550hz low and high crossover, the 6-1/2s were being run full range by the manufacturer but soon to be changed.
The farther I dive into this the more I learn about what most people know about ........ I’m glad I don’t have rew and we’re not able to measure them (forehead slap). If nothing else I have some nice cabinets and I’ve also learned I really like the MTM design for full range cabinets, if I didn’t already have 5 HTM-10’s set to go for my theater I’m sure 5 MTM’s would be my next choice.
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post #6 of 13 Old 05-28-2018, 09:09 PM
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Quote:
Originally Posted by Audiophile75 View Post
First off the total ohm if both drivers are 3.6 in parallel which the industry describes this as 4ohm.
Second, I changed out (today) the capisator from a 5uf (roufley 7k crossover) to a 22uf that I had laying around (1000hz) and that helped. I also decided to order new tweeters, caps and a coil for a 2nd order 2550hz low and high crossover, the 6-1/2s were being run full range by the manufacturer but soon to be changed.
The farther I dive into this the more I learn about what most people know about ........ I’m glad I don’t have rew and we’re not able to measure them (forehead slap). If nothing else I have some nice cabinets and I’ve also learned I really like the MTM design for full range cabinets, if I didn’t already have 5 HTM-10’s set to go for my theater I’m sure 5 MTM’s would be my next choice.
5.4/2=2.7 Where do you get 3.6? Parallel you halve the impedance. A crossover crosses over the frequency and impedance. It is the reason you can use a 4 ohm tweeter with an 8 ohm woofer and still have a nominal 8 ohm speaker. Here's a visual representation of what happens.


Now when you halve or double the impedance you change your woofers crossover frequency by using the same crossover. You do not change your tweeter crossover frequency at all. In the crossover you need to change the inductor and if it is a 2 way, you need to also change it's capacitor value. You should not touch the capacitor on the tweeter, unless you plan to change the crossover frequency, as changing the woofer impedance will have no affect on the tweeter.

You should consider getting rew and a testing microphone. So you can see what these changes you are making is actually doing.

Last edited by Al Toid; 05-28-2018 at 09:39 PM.
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post #7 of 13 Old 05-29-2018, 03:42 AM - Thread Starter
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Maybe I didn’t express myself right, after building my cabinets all I did was transfer the speakers and the parts. I did not change anything or retire anything from the factory at first but I did meter everything.
Yes, rew would help but this experiment was to also see how far off they really were just by ear and I think we all knew that answer, I just confirmed it that’s all.
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post #8 of 13 Old 05-29-2018, 04:13 AM
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one of the bigger reasons you notice rolled off highs now, is because you've doubled the number of woofers next to the tweeter. the original enclosure did that Bose multi-directional thing, which if sitting on axis, would lead to just listening to one woofer and one tweeter with some "fill" happening. you have options on cheap changes, but seems you are going to swap components and play. In your situation I would do a quick and dirty filter similar to what is used for compression driver compensation, basically attenuating frequencies BELOW a given point. (kind of a reverse BSC) would cost you a cap and a resistor, those values would need to be derived from measuring or dead reckoning, perhaps others could assist.
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post #9 of 13 Old 05-29-2018, 04:59 AM
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Quote:
Originally Posted by Al Toid View Post
5.4/2=2.7 Where do you get 3.6? Parallel you halve the impedance.
The original speaker already had two woofers. He just moved the one from the back to the front.
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post #10 of 13 Old 05-29-2018, 06:42 AM
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Quote:
Originally Posted by Audiophile75 View Post
Maybe I didn’t express myself right, after building my cabinets all I did was transfer the speakers and the parts. I did not change anything or retire anything from the factory at first but I did meter everything.
Yes, rew would help but this experiment was to also see how far off they really were just by ear and I think we all knew that answer, I just confirmed it that’s all.
That makes more sense. In this case, assuming you hooked up the speakers the same way to the crossover you didn't change the impedance of the speaker at all. Originally you had said it went from a 4 to 8 ohm. It is still good to know how to measure the impedance.
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post #11 of 13 Old 05-29-2018, 03:04 PM
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I would also be interested in some REW measurements. Kinda curious if, and how Bose treated baffle step compensation with the crossover, and how the FR was affected by the new cabs and baffle. I would guess the XO is designed specifically for the bipole arrange, but not sure exactly what would happen by just changing cabs. I also have a hunch that the “new and improved” performance has something to do with the new monopole arrangement as well. Have you tried these in the side surround position since the new build?
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post #12 of 13 Old 05-30-2018, 04:26 AM - Thread Starter
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Originally Posted by Polyrythm1k View Post
I would also be interested in some REW measurements. Kinda curious if, and how Bose treated baffle step compensation with the crossover.
Have you tried these in the side surround position since the new build?
That’s the funny part, Bose only put a cap on the paper tweeter and and ran both 6-1/2’s fullrange without anything (see third photo, actually the tweeter had a ??? — cap —bulb in-line) and now I just designed a 2nd order xover and it’s in the mail.
No, have not placed them back in the theater, only 2.0 on my main tv living area and can tell you that the main drivers sound pretty damn well but the tweeter must go..... I expect after a new tweeter and adding a “real” xover they will transform into “impressive”. Crossing fingers.
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post #13 of 13 Old 05-30-2018, 04:46 AM
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Originally Posted by Polyrythm1k View Post
Kinda curious if, and how Bose treated baffle step compensation with the crossover, and how the FR was affected by the new cabs and baffle.
they didn't address BSC at all. they used a second woofer firing away from the listener, which would net a low frequency boost if next to a wall or in a corner and basically doing the same thing as a compensation circuit, a la 2.5 way for a direct radiating design. OP could do similar by rear firing one of the woofers... a lot of companies played with the process eons ago, Cerwin-Vega comes to mind in the late 70s era, I believe it was their S-series cabs with rear firing tweeters.
OP has new parts coming, but I really would be interested to see the effects of say a 2.2uf or 4.7uf cap with a 4 or 8ohm resistor that would attenuate the mid and low frequencies, with the cost being like $2
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