Definitive Technology BP7004

Bipolar SuperTower Subwoffer Bypass/Repair.

I've had these speakers for a good many years and have really, really enjoyed them.
Fantastic clarity of sound with some really nice staging.

No idea when, as the past 2+ years of my audio life have been very chaotic, but the subwoofers in the bottom of both speakers stopped working.
My shop is large enough to need the extra bottom end to fill things out.
I have two standalone wooffers, one 10-inch and one 12-inch, scattered along the length of the shop, but the two in these towers really add a lot to the overall sound from memory, so I really wanted to get them back up and wooffing.

Did a Google on the brand and part number, and it quickly seems like it's a very common problem.
I took a look at both amps on the workbench, and it's pretty tricky with no schematic diagram and a whole lot of hot glue that they have dished out over a lot of the parts to seal the speaker box.

After spending about an hour probing around and coming to the same conclusion as the Internet, they are not worth the time.
Sure, they have a massive, heavy toroidal transformer, but everything after that seems weak and poorly spec'ed out.
For just under 50 bucks US, you can buy a 50W class D amp that should kick it back into life.
So that's what we are going to run through here....

Remove the amp

Carefully lay the speaker on its face.
Remove all the screws around the outside of the plate. They are all the same length, so just throw them in a pile, no need to keep track.

Get a good flathead screwdriver and lever up the plate from the speaker box.

There is a tacky rubber gasket that might rip in places, and it might lift a few of the staples that hold it down, so just keep all that in mind.
Don't go too crazy in lifting away the amp, it has two speaker wires going into the tower.

I cut both very close to their amplifier connectors.
Both? I hear you say. Yeah, the speaker wire connectors are super loose on the back of the speaker wire binding posts, so I did not care for those connectors either, as they will be soldered back on at the end.

Repair the gasket

Here is the key to the bypass. You really (really) need to keep that speaker box as air-tight as Defnitive Technology set out from the get-go. Air-tight bass is critical to good sound.
So, what we are going to do is keep that tightness intact by using the mains power cable as our subwoofer speaker wire.

Prep the subwoofer wires

Dig into the ziptie, hot glue, and heat shrink to get the mains power lead out of all that crap inside the amp and dress the ends ready for soldering.

The 2 sets of speaker wires are now soldered.
Make sure you keep track of the color code. You want your speakers to be in phase (both sets moving in and out at the same time).
Black to black and orange to red.

Hook up the new amp

If you go with the Amazon amp I got, it needs a LOT more than line level to drive it. I tapped on the back of the speaker output from the amp to the main tower, and it's about the perfect level for the little amp.
The speaker post cable goes up to an RCA plug to go into the little class D amp.

Cut the mains plug off and dress the ends to bind to the output of the little amp.

The little amp comes with pretty much the bear minium. Just the power supply and not much else, but that's about fine, as your needs to drive and connect it will vary.

PBTL is full-range output and 'sub' switches in a low-pass filter and is the mode you need for the SuperTower application.
The sub freq is the upper cut-off for the low-pass and is very effective. I'm glad to see it included.

It's a mono amp, so you don't need to think too hard about whether you feed the drive to the Left or Right channel.

I chose to sit the amplifier on top of the speaker, but of course, you can place it anywhere within range of the cables you use.

I'm very happy with the result.
They thump very much in line with the original amplifier.