Hi, After several months of non-stop work on my Liquid Extasy waterblocks, I needed a short break, so I spent three days building Bluetooth speaker number four. The goal was simple: get down to 50 Hz in a small cabinet, keep the cost as low as possible, and mainly use parts I already had staying around. Hardware: Aiyima 4" subwoofer, 2× Aiyima 2" full-range drivers, 2× 4" Chinese passive radiators, ZK-MT21 Class-D 2.1 amp, Tinyshine TSA1701 DSP and 6× Samsung INR18650-35E cells in a 3S2P configuration. The 3–3.5-litre enclosure was 3D-printed in PETG, with a 22 mm Ebony Fineline front panel and PMMA GS for the top and handle. Dimensions: 400 × 206 × 199 mm, weighing around 5 kg. The biggest surprise was the 2" full-range drivers at €20 / pair. Quality is typical budget Chinese stuff, but even without DSP they sound surprisingly neutral. The 4" Aiyima subwoofer is rubbish, with limited excursion and low levels. The passive radiators weren't particularly impressive as well, but adding 15 g of mass to each radiator made a huge difference. For around €20, the ZK-MT21 is a great little amp with separate controls for bass, treble and the subwoofer. The TSA1701 DSP, also cheaper than €20, is another excellent choice for projects like this. Measurements & DSP Measurements were taken using a UMIK-1 and REW. The current signal chain is: Input → 50 Hz High-Pass → DC Blocker → Parametric EQ → Limiter → Gain → DAC Everything below 50 Hz is cut aggressively: 52 Hz @ 0 dB and 48 Hz @ −8 dB. The speaker can play comfortably above normal room volume. The subwoofer is clearly the limiting factor, while the little full-range drivers can still go considerably louder. The 3S2P battery pack uses six Samsung 35E cells and provides around 6,800 mAh. A push button and segment display finally give me a battery-level indicator that actually works. At normal listening levels, I get 24+ hours of runtime. Conclusion For €20 / pair, the full-range drivers are excellent: smooth, unobtrusive and easy to listen to for hours. They're not harsh at all. After six weeks at my desk, I like the character so much that this little speaker has replaced my much more expensive LIA AMTs for everyday listening at work. Obviously, the LIAs are in a completely different league technically, but the tonal balance is surprisingly similar. Check the comparison The Aiyima subwoofer remains the weak point. Next time I'd spend an extra €20–30 for a branded woofer. The DSP and amplifier, however, I'd happily use again. The basic audio hardware comes to just €90: €20 for the full-range drivers, €30 for the sub, €20 for the passive radiators and €20 for the amplifier. For that kind of money, it has nothing to loose compared to mainstream Bluetooth speakers. Add a better woofer and there would be considerably more output and lower frequencies. And now it's time to get back to my Liquid Extasy waterblocks. I've got some new waterblocks to release. Best regards, Marc
