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Old 11th March 2009, 01:41 AM   #1
photon is offline photon  United Kingdom
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Default First time build - looking for advice.

I decided to build my own subwoofer after seeing one that a freind knocked up at college and thinking 'I could do that!'. After some research I think I'm close to arriving at a final design but thought that it would be prudent to run it past the knowledgeable people in this forum, especially seeing as I've already read quite a few of the posts here prior to signing up.

It's going to be used in a home hifi setting. I've already ordered the Peerless 830667 and I have decided to construct a power amp to run it. I took advice from a sound-engineer friend on this and we decided on a Velman 200W amp connected to a low pass filter. I have a crudely-drawn schematic that I can post if anybody is interested. We decided on this route as my source is a (DJ) mixer that has 1/4 jack outputs that can send the same signal to the sub as is going to my integrated amp. It seemed like this would be more straightforward than a plate amp built into the sub enclosure (and, more importantly produce a decent sound for a lower price).

So my main question about the amp side of things is whether anybody thinks this is a reasonable way to approach things?

As far as the sub goes, I took some good advice from the (really helpful) man at Audio Components and he suggested a closed-box enclosure. I'm probably going to go for 18 or 22 mm MDF and make the thing very solid. I'm having some trouble with working out how to get the internal volume of my enclosure right though. I've read a pretty detailed guide on how to use WinISD, so I think I'm using it correctly (I couldn't face the maths involved in working through the equations myself - I'm an arts graduate :s ) and it's telling me that I need an internal volume of around 44 litres. However, the data sheet on this site suggests an encolsure of 15-25 litres. So now I'm confused and I'm not sure who to believe. Any thoughts on where I might be going wrong here?

And finally, is there a straightforward way to predict the internal volume of my enclosure, including bracing (which will have holes in it) and driver to within a tolerance of 5% (as recommended by the man at Audio Components)? I have a feeling that I'm being a bit thick here but at least stupid questions are easy to answer.

Thanks for reading, any advice will be greatfully recieved.
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Old 11th March 2009, 06:59 AM   #2
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By the factory spec, a sealed box should be 46 litres or larger (butterworth Q or less)

I'd not bother with MDF but use plywood (good void free many layers -- the 18mm Baltic Birch we use has 13). Well worth searching out (and paying the premium for -- the price of good ply insn't going to go down until the demand goes up and decent quantities are imported)

dave
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Old 11th March 2009, 09:19 PM   #3
photon is offline photon  United Kingdom
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Thanks for the reply, I've checked out the plywood and I'm going to have the timber merchant order some in as long as it isn't too costly.

I tried to find out what the butterworth q is but didn't quite manage it. It's a term I've come across in my reading but not really understood. Also, is the 'factory spec' you mentioned the factory that built the driver?

Could you point me to a some material that would explain this? Or explain it yourself if you have time.

Thanks again.
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Old 12th March 2009, 08:57 AM   #4
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Butterworth Q = 0.707 or a maximally flat alignment. It is as high a Q as you want to make a sub. My preference is lower, but the Qt of the driver of 0.54 (IIRC) means box size goes up dramatically for incremental lowering of the system Q. A sealed box is pretty forgiving... my suggestion is make the box as big as you can withing reason and then stuffing can be added to fine tune it.

Factory T/S parameters are those on the spec sheet. Rarely are they what you'd measure (plus unit-to-unit variation can be over a range of 20% or more)

dave
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