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.