Last weekend I stopped by my local Goodwill hoping to find some audio-related goodies I could play around with. Lately it’s been slim pickins in the used stereo department with mostly junky old Blu-ray players and bits of pieces of all-in-one home theater systems (and those were never any good when they were new). This time I noticed a pretty nice Vizio subwoofer that looked like it had potential. It was at least in good shape. The woofer was hidden behind a grill cover that at the time I couldn’t tell if it was going to be easy to remove. But the nice flared port on the front and the compact size, and decent heft, made me think this just might be a worthwhile buy. It was listed for $12.99, but since it had a blue tag, it just so happened to be a blue tag weekend, which meant it was 50% off. So for a whopping $6.50, I decided it’d be worth taking a shot. If nothing else, the enclosure alone was worth at least that much and I could buy a new driver and amp if I absolutely had to.
Once I got this little subwoofer home I found a model number and looked it up. Turns out it is a Vizio S4251W-B4 Subwoofer that was part of a Vizio S3851W-D4 Soundbar Surround Sound combo back in 2014. I found the old Amazon listing for it and it got surprisingly good reviews. There were no specifications for the sub itself, other that a completely needless output rating of 100 dB. Thanks for that Vizio! So after tearing this little subwoofer down we find a pretty hefty 6″ paper cone subwoofer with a decently fat foam surround. A stamped steal basket and a good-sized magnet make up the motor structure with a label indicating a 4 ohm rating and a 90W power capability. Pressing on the cone revealed it moved just fine with no voice coil rubbing, so at least it didn’t appear to be blown. But man that spider and surround sound were stiff, like this sub has zero compliance. I should have set up REW to measure the T/S parameters (I may still do this later when I measure the amp) but for now I was just checking things out and so far it was looking pretty good.
The enclosure looked decent, made from just 1/2″ MDF, but I really like the long flared port that fired out the front of the cabinet. It reminded me of the old Bose Acoustimass subs from back in the day. Of course this sub just has a regular side-firing driver in a basic 4th-order vented design, but size-wise and visually looked comparable. The amplifier module is mounted in its own smaller enclosure, since they made no attempt to seal off this part of the box, so the subwoofer is in its own enclosure which was really handy for providing options for how we were going to power this thing. So being that it’s part of a soundbar, even though it has an internal amplifier, it doesn’t have any way to connect to it with a regular RCA cable. It’s supposed to connect to the soundbar which most likely uses some kind of proprietary connection. I tried to see if the sub would show up as a Bluetooth device on my phone by using the PAIRING button on the back but to no avail. It wouldn’t show up. Doesn’t really matter, I couldn’t have used it as a Bluetooth device anyway, it just wouldn’t have worked. I needed a direct connection via an RCA to be driven with a regular line-level signal like a normal subwoofer.
I thought there might be a way to bypass the wireless connection portion and get straight to the amp. After poking around this sub actually contains two small chip amps, as the sub was supposed to provide power to a pair of surround sound speakers too. I was able to make out the part number to be a TSA5713 which is a 25W stereo class D chip amp from TI. So let’s take a look at the datasheet and see what we have to work with here. Since there are two amps on this board, one is clearly used in stereo mode (BTL) to drive 25W to each surround sound speaker (assuming they were 4 ohm speakers), which would be connected to the sub via the RCA jacks on the back. The second chip to probably was wired in mono mode (PBTL) to drive the single 4 ohm sub to an unspecified power level. The graph that shows this configuration stops at 40W, just before the amp distortion curve spikes. I’m guessing it can provide around 50W which is respectable, but honestly not great. These little amps are typical of what you find in cheap sound bars and/or TVs and aren’t really anything to write home about. So with that I figured I wouldn’t waste any more time trying to figure out how to tap into a line-level signal and decided just to replace the amp entirely.
My first thought was to check out the cheapest subwoofer plate amp I could get from Parts Express. Parts Express has a great line-up of subwoofer plate amps, but even their cheapest amp seemed like too much investment into this cheap Goodwill find. Also, they were just too big to fit onto the back of enclosure. So I turned to Amazon in hopes of finding a cheap single-channel amp board, preferably one with a crossover that could be used in a subwoofer-only configuration. That’s when I ran across this little gem for about $18 containing the famed TPA3116D2 chip amp from TI in addition to a low-pass crossover with adjustable crossover frequency. This looked like a perfect fit. The TPA3316D2 can produce about 80W into 3 ohms with a 24V power supply. This seemed like plenty to drive this small 6″ subwoofer with a purported rating of 90W. Though at 4 ohms, we’re probably getting only about 70 useable watts but still that should be plenty. Now in order to get that kind of power we need a good-sized power supply. I came across this no-name branded one on Amazon for only $15 that had very few reviews, but all seemed positive. This power supply is rated at 24V/6A, or 144W. So this little guy should provide plenty of juice to keep a single TPA3116D2 chip amp plenty happy, and won’t be the limiting factor in this setup. I bought both the amp board and the power supply that same day. Now we are a total of $40 into this little subwoofer makeover, which isn’t too bad and probably about the limit for what I’d want to spend.
Once the amp board and power supply came, it was time to figure out how to mount this thing into the enclosure. There was a convenient 95mm square opening on the back where the old amp connections were located and a long slotted opening on the side where the old amp slid into place. The bigger opening would have worked better, since it was deeper, but it was covered by the grill cover, so it was not accessible. So I knew we’d have to figure out how to mount the amp into the shallower opening on the back. Unfortunately the new amp board was too long to fit easily into this location. So that’s when turned to the trusty 3D printer to save the day. My son whipped up a design in Fusion 360 that basically looked like a square box of sorts, that mounted into the opening in back of the enclosure which increased the total depth of the enclosure by 14mm allowing the amp to install normally, with the volume knob and crossover knob facing outwards. This seemed way easier than other options we had considered, and it really didn’t turn out that bad. The back of most subs aren’t really flat anyway, once you hook up an RCA cable and power cable, so having this little box with a 1/2″ protrusion on the back seemed like no big deal. He added holes for the 1/8″ mini stereo jack, the two potentiometers, a DC power jack and an RCA jack. He had recently purchased a Bambu Lab P1S 3D printer so he was excited to get to print something new that he had designed. Quick plug for Bambu Lab, this 3D printer is awesome. Everything he’s printed has turned out absolutely flawless. This has got to be one of the best 3D printers out there at this price point and for what it does, and how freaking fast it prints, it is completely mind-blowing. He gave me his old Ender3 that I have just begun to tinker with. 3D printing is pretty fun. I’m hoping to incorporate more 3D prints in my audio projects in the future (can you say, waveguides anyone?). So this print took all of about 1-1/2 hours to complete and turned out perfectly. It needed supports due to having the flange around the box and little screw pockets hanging off midway up the walls, but they worked out just fine, we could have sanded it down to make it look a little nicer, but we were really not super concerned about some leftover support lines for this thing. Dimensionally it came out perfectly and did the job.
I had the RCA jack leftover from another project as well as the DC power plug, but I bought both from Amazon for just a few dollars. Amazon doesn’t sell the exact ones I bought but you can find them by searching. You don’t need the RCA jack, you can use the 1/8″ mini jack on the amp board for the input, but this just seemed easier as I have hundreds of RCA cables laying around and fewer 1/8″ stereo to RCA cables. Which is what we would have needed to connect this amp to sub-out of the Loxjie A30 amplifier. So yes, my son ended up with my Loxjie A30 amp along with my old Dayton Audio 6″ reference series bookshelf speakers. He has a pair of Audio Technica ATH-M40 headphones so the A30 makes it easy to bounce back and forth between speakers and headphones when he’s gaming or just listening to music. The Loxjie A30 is a great little DAC/AMP combo and he loves it. The only thing that was really missing from his setup was a subwoofer. The little Dayton bookshelf speakers definitely lack any significant bass below about 70 Hz, as they were always meant to be used with a subwoofer. So once we got everything connected, we set this new little subwoofer under the desk in his room, connected it to the sub-out on the A30 and cranked up some jams. And this thing bumps! I was seriously surprised at how much output this thing can produce. The crossover setting we set all the way down, as it doesn’t appear to do a great job of filtering the higher frequencies. The A30 also has a crossover, so between the two acting together, we get a decent amount of bass-only to the sub. I need to measure it still in REW to see what it can do. But I’ll save all the measurements for a follow-up post. But this sub can crank definitely louder than he needs to be playing in his room when the rest of us are all home. It shakes the whole house and when you’re not in his room, all you hear is just bass emanating from his room. To get it somewhat balanced with the main speakers, the volume knob is set to about 10 o’clock, any more than that it’s just too much bass. And while it’s deeper bass than the bookshelf speakers, it’s not digging super deep. I’ll get some measurements of it up soon but I’d say if nothing else this thing at least has output down to about 40Hz before rolling off. It’s mostly got that great mid-bass punch, that 50Hz-60Hz type of bass that really sells subs, because it’s easy to produce without too much power, doesn’t require big speakers to get there, and just kind of hits you like wow, that’s a lot of bass. Even if it’s not really deep bass. I can see why soundbars with these types of subwoofers sell really well. They don’t sound bad, if you cross them over correctly, drive them correctly, and put them it a good spot in your room.
So all in all, I was able to take this $6 Goodwill soundbar sub and with a few extra bucks, a sub amp board, and a 3D printed part, turn it into a useable subwoofer in a regular 2.1 channel stereo setup. Would I recommend this exact setup if you were to do it from scratch? Honestly no. I noticed that you can buy this sub on eBay from a variety of people, but realistically just for the box and driver, it’s not really worth what most people are selling it for. If you have this sub already collecting dust somewhere and want to re-purpose it outside of its original soundbar home, then yes I think it’s totally worth it. However if you were to start from scratch, I’d say the best (cheapest) driver-amp combo you could come up that would be comparable or maybe even better would be to buy a cheap 6.5″ or 8″ subwoofer from Parts Express and pair it with this TPA3116-based 100W amplifier board from Amazon. Build a simple ported box, slap it all together and Bob’s your uncle. Cheap bass that’s ready to add to any stereo or home theater receiver with a dedicated sub-out port. Anyway, this was a fun little project and stay tuned for a follow-up post where I’m going to measure the total power and frequency response of the amp as well as the subwoofer itself. Until then, thanks for reading!