Need help with small passive sub for motorcoach theater system - diyAudio
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Old 14th July 2010, 02:26 AM   #1
rotts4u is offline rotts4u  United States
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Default Need help with small passive sub for motorcoach theater system

I am a journalist working on a magazine article where I install a 5.1 home theater system in a diesel motorcoach. Since space is limited and wiring nearly impossible to pull, I am forced to live with the existing wires and space. The coach came originally with a cheap home theater in a box deal and it had a small passive Panasonic sub in a storage cabinet.

I have already upgraded all the other speakers, Blueray DVD, TV 40" LED LCD and a Samsung reveiver. Now I have a Samsung 5.1 AV receiver because it was one of the only ones I could find with a sub speaker level output. I am NOT able to run any type of preamp RCA wire to the sub location so I am forced to only use a passive sub and of course it has to fit. The current sub is about 8" wide and 13" tall and 15" wide. It has to go through a cabinet door so I need to stay really close to these dimensions.

So can someone help me design a DIY passive sub using an 8"' driver of the size above that will operate on 100 watts which is all the receiver puts out. The receiver has adjustable crossover and the front and surround speakers are Polk M10s is that matters. They claim -3db at 65hz if that matters and a 89db sensitivity. I will build and finish the box myself because it is hidden in a cabinet anyway so looks are not too critical.

Thanks in advance for your help

Don
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Old 14th July 2010, 03:48 AM   #2
woof is offline woof  United States
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This is an excellent small footprint design.
Cerberus
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Old 19th July 2010, 03:01 AM   #3
rotts4u is offline rotts4u  United States
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Nice link woof !! That one would fit perfectly too and the price is right.
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Old 19th July 2010, 04:05 AM   #4
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I think you'll like that driver. I enjoy mine.
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Old 21st July 2010, 12:42 AM   #5
rotts4u is offline rotts4u  United States
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Ok Woof, I ordered the parts today. Of course the Tang Band was out of stock and is on back order so I have to wait a week or so for it to arrive. Then I can start building the box. I thought about ordering the plate amp and feeding it with the receiver subwoofer speaker outputs but for now I am going to run it passive. If it seems starved for power Ill try the plate amp. With the low efficiency of the sub it may need it.

Thanks
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Old 21st July 2010, 02:43 PM   #6
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Yes, you will need the plate amp, no question there.
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Old 21st July 2010, 03:24 PM   #7
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Quote:
Originally Posted by woof View Post
This is an excellent small footprint design.
Cerberus
The woofer in that design is great, and the tiny enclosure is nice too, but personally I like it loud and I hate port chuffing (noise caused by excessive air velocity). In that respect, the Cerberus is a terrible design, the port is way too small. In my experience, the absolute minimum port diameter for that woofer is 3 inches with a healthy flare on both ends but a port that size won't fit in a Cerberus sized box.

There are a lot of good designs using that woofer, but for my personal needs, the Cerberus is the worst I've seen and I wouldn't recommend it.

Last edited by just a guy; 21st July 2010 at 03:31 PM.
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Old 21st July 2010, 04:49 PM   #8
woof is offline woof  United States
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Quote:
Originally Posted by just a guy View Post
The woofer in that design is great, and the tiny enclosure is nice too, but personally I like it loud and I hate port chuffing (noise caused by excessive air velocity). In that respect, the Cerberus is a terrible design, the port is way too small. In my experience, the absolute minimum port diameter for that woofer is 3 inches with a healthy flare on both ends but a port that size won't fit in a Cerberus sized box.

There are a lot of good designs using that woofer, but for my personal needs, the Cerberus is the worst I've seen and I wouldn't recommend it.
Down firing plus inside of another cabinet, I doubt chuffing will be audible. In the OP's case, size was of concern & I don't think he will be disappointed. If chuffing is audible inside of another cabinet, you can always increase the diameter & run the port external. I do agree there are many designs for this sub. My favorite is 2 cu ft with a 3" port - 24" long.
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Old 21st July 2010, 08:49 PM   #9
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Quote:
Originally Posted by rotts4u View Post
I am a journalist working on a magazine article where I install a 5.1 home theater system in a diesel motorcoach. Since space is limited and wiring nearly impossible to pull, I am forced to live with the existing wires and space. The coach came originally with a cheap home theater in a box deal and it had a small passive Panasonic sub in a storage cabinet.

I have already upgraded all the other speakers, Blueray DVD, TV 40" LED LCD and a Samsung reveiver. Now I have a Samsung 5.1 AV receiver because it was one of the only ones I could find with a sub speaker level output. I am NOT able to run any type of preamp RCA wire to the sub location so I am forced to only use a passive sub and of course it has to fit. The current sub is about 8" wide and 13" tall and 15" wide. It has to go through a cabinet door so I need to stay really close to these dimensions.

So can someone help me design a DIY passive sub using an 8"' driver of the size above that will operate on 100 watts which is all the receiver puts out. The receiver has adjustable crossover and the front and surround speakers are Polk M10s is that matters. They claim -3db at 65hz if that matters and a 89db sensitivity. I will build and finish the box myself because it is hidden in a cabinet anyway so looks are not too critical.

Thanks in advance for your help

Don
Quote:
Originally Posted by rotts4u View Post
Ok Woof, I ordered the parts today. Of course the Tang Band was out of stock and is on back order so I have to wait a week or so for it to arrive. Then I can start building the box. I thought about ordering the plate amp and feeding it with the receiver subwoofer speaker outputs but for now I am going to run it passive. If it seems starved for power Ill try the plate amp. With the low efficiency of the sub it may need it.

Thanks
You can always use an amp with speaker level inputs or add a speaker level to line level converter or use an amp with balanced connections. You can probably get away with just sending pre level down the speaker wire without noise too. All of these options are possible. Don't negate the amp or you will not really improve the performance from the original setup.

An RV or bus is a large mass and plenty of power is needed for desirable operation, especially when the system is used while the bus is in motion to overcome road noise.

Also, the sub does not necessarily need to be in the same spot as the original. You could mount it in another location in the bus too. It may even sound better further away from the listening area and you can use a more substantial sub with more power.
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Old 22nd July 2010, 02:19 AM   #10
rotts4u is offline rotts4u  United States
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I do plan on slightly changing the dimensions of the box while still yielding the same basic 1331 cubic inches or even more so I may be able to get away with a larger port. I have to fit the sub in the existing cabinet so the dimensions have to fit the door to get it in the cabinet.

I do not have the option of moving the sub as I could not get wires to it easily or at all. The whole coach is prewired and I dont have access to the AV cabinet to pull new wires without a lot of work. Not to mention the existing wires are TINY. I wish I could at least upgrade the wire diameter but no go.

If I can squeeze 1500-1600 cubic inches would it be worth the effort? some people warned that two cubic feet is better so I assume a little bigger is better than just 1 cubic feet.

I will try it with the receiver only as it has to be better than the molded plastic cabinet panasonic sub that came with it. If its not enough I will add the plate amp. I dont use it ever while going down the road so its only a home theater system when parked.

Thanks again for the ideas
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