Adding bass woofer to floorstanding speakers

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I'm wondering whether it would be possible to put another woofer in the bottom half of floorstanding speakers to improve bass response.

I had no hifi equipment previously, only Creative PC speakers, and just got a pair of Tannoy MX3s floorstanders very cheap. I plan to build a LM3886 amp using salvaged parts from a faulty Marantz PM66 to drive them. They sound good but the bass is a bit lacking for me. They are being driven at seriously low power at the moment (<10W i think) so i'm sure sound will improve, but I was wondering about adding more bass using a side mounted woofer. I've seen this in some floorstanding speakers.

When I opened up the cabinet of my speakers I found a horizontal MDF plate halfway up the speaker, about 45cm off the floor, below the bass port woofer and tweeter. I wondered if I could put a 6.5/8" driver in that ~ 17l enclosure. I'd plan to amp use a dedicated bass amplifier to drive the two new 'subwoofers' so that I could control the bass. Does this have potential or should I stop before I start?

For context even after this project my total for amp and speakers would still be under £100. I'm not expecting too much and would also be doing it for the enjoyment of the project, but I enjoy good sound! Cheers
So the part of the cabinet below the port is actually just a sealed-off empty space? Go for it. Keep the "subwoofer" amp/amps outside so you can change your mind on subwoofer plate amp, DIY line level crossover with regular amp, etc., for getting a lowpass and level that works.
I have also done this before when I started out. The tannoy 5 inch driver on top can run as per the original crossover and if you get some square framed 6 inch Audax drivers for the bottom half it will be a big improvement. I would not leave the bottom part sealed though and once you get the matching shaped 6 inch driver from audax or focal etc then you can work out the response from the thiel parametres and what size your port should be.
Actually looking at the speaker a single peerless nomex 7 inch driver will blend into the asthetics nicely and run it on a 3.9mh inductor with a sperate binding post on the back should do the trick without too much fuss.
depending on where you are eithe GB or USA there are Falcon acoustics in UK and then madisound, parts express should be your best bet.
I agree with chefsinc that a Peerless 830875 could work well to make it a 2.5 way like the MX4 but with the added woofer placed much lower in the enclosure. A lot of these type of speakers do not have BSC included in the design so benefit from a 0.5 woofer. When working out the working volume, the volume of the driver and port have to be subtracted from the 17 litres.

There's an easy way to test this out by using 1 channel only. Wire one speaker normally and place the second speaker upside down near that speaker but not against it. Add a 5.6mH - 3.9mH inductor at the speaker terminal +ve input as that will make it roll off like a 0.5 woofer. Run the first speaker and listen and then do the same by running the second speaker as well. This will give you an idea of the sound difference by doubling the driver Sd and adding BSC.

Usually in this arrangement when running in stereo, I use a separate amp for the 0.5 woofer as the impedance can drop too low when running in parallel to the TM section but depends on the speaker impedance. If it is an actual 8 ohm (not nominal) then you end up with 4 ohm.

Other drivers that may suit are Scan Speak 18W8434 or SB Acoustics SB17NRXC35-8.
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Another possibility is to try and add some extra BSC which will attenuate the mids and top end but leave the bass as is. The overall speaker SPL does drop however but with better bass output.

Add a LR network before the +ve terminal (or xo) by adding 2R2 + 0.56mH in parallel as shown below. That will give just under 3dB BSC. Other values are shown in the table for various dB changes. Note: an 8 ohm driver has an re around 6R. Ignore the zobel (CR) network across the driver in this instance.


  • bsc_calc.jpg
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