Sylvania SP305 bluetooth portable speaker mod

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I bought one of these from Dollar General for $15. It's not a bad speaker for the price and I saw right away that a simple mod can dramatically improve the bass. I have not tested them but there are other similar models like the Craig CR4189BT and a Proscan from Big Lots. The lower speaker in the tower is not really a speaker but a passive radiator. Not only is it not larger than the driven speaker but it is poorly tuned resulting in hollow sounding boomy bass at best. All I did was add a little mass to the passive radiator to lower its resonance and the sound is much better. I glued 1 g of cardboard to the backside but this could be done many ways. I tried up to 3 g of additional mass but 1-2 g seems to work fairly well. 1 g extends the bass to 100 Hz while 2 g extends it to 80 Hz with a slight reduction in mid-bass. I also see that a second speaker could be added or the cabinet could be used for a DIY portable speaker project. I'd be interested to know if anyone else has hacked on one of these.
 

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After a little listening, I think it could use a little help on the high end. The original speaker has OK highs but only on-axis. It's a cheap speaker anyway so can anyone recommend a tiny tweeter or a replacement full-range driver with better highs. It's 4 ohms. I'm not sure how to measure a speaker this size but it's about 2 1/2 inches across the cone and 2 3/4 across the surround. I'm thinking an add-on tweeter might be better but it would have to fit between the 2 existing drivers.
 
I'll keep that in mind. I would also need a battery and charge circuit. I can see the Sylvania enclosure is too small for those speakers so definitely a scratch build.
On another note, Ive been wondering if I can access L and R channels separately on this thing but then again, the enclosure probably won't do well with 2 speakers with its volume. I figured it's about .15 cu ft. I'm all about keeping it simple on modifying such a cheap speaker.
 
I found some info based on the FCC ID of the SP305 at the FCC ID Search page. It is manufactured by Winsonic as model WM2899. Here is a list of some other speakers that are similar and may benefit from this mod.

Winsonic WM2899
Naxa NAS-3040
EMB Euro10
SUPERSONIC SC-1321
Craig CR4189BT, CR4197BT, CR4191, CR4190, CR4198
QFX CS-224
Axess SP1002
Pyle Home PSBU9
Tectron HT-985
SoundLogic CWBS-6-5948

If you have one of these, any info including FCC ID from the label would be appreciated.
 
After a few months of use, I started to hear a rattle. The foam surround on the passive radiator started coming loose. A little glue fixed that but I wanted a little more weight for lower tuning. The cardboard just wasn't going to work so I cut it loose and glued a penny in it's place. That made it easy to add another gram of tacky putty to the penny. Still not a bad speaker for the money and loader than the palm-size speakers I've heard.
 
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