I’ve decided that “loud bass practice amp” and “low bass extension with force cancellation” are not really ideal from the same cabinet. So the 2.1 sub will be a different project.
That leaves the pipe woofer as only a bass guitar case & practice amplifier.
Easiest would be to have a single driver permanently attached on one end, with amplifier and knobs and connectors on that side too, and some sort of screw cap on the other side for accessing the bass guitar. That cap could house a reflex port.
Currently looking at the 8″ SB20PFCR30-4. In a 120cm pipe, with two 60mm ports 20cm long, the response is nicely flat down to F3@46Hz, F6@40Hz, F12@31Hz. Up to 50W input power (4 ohm) and with a 2nd order high pass, displacement stays nicely under xmax. Without the high pass and at 46W input, xmax is reached below 35Hz but there should not be much signal there from a 4 string bass. At 46W, output is about 108dB (half space) so might be loud enough to play with a drummer.
Amplifier: TPA3221 can do about 46W into 4 ohm at 20V, or for some more headroom can do about 100W at 30V (but don’t go over 32V). That seems like a good fit.
Could be powered from an 8S “24V” LiFePo4 pack (25.6-26.6V from 20-80% SoC, 27.2V for 100% SoC) or even 9S (28.8-29.9V from 20-80% SoC, 30.6V for 100%). Or a 7S “24V” Li-ion pack (25.9-27.3V from 20-80% SoC, 29.4V for 100%). Would only pull 4A max, so any 21700 will do – a 7x5000mAh pack (€14 in cells) would run the thing at rated power for 100 minutes. Or I could do a 9x4000mAh 26700 LiFePo4 pack (€14 in cells). A built-in battery also means that I could get away with a smaller power supply and still be able to play loud, perhaps a 30V CV+CC power supply like Meanwell HLG-40H-30A (€36.31) or HLG-60H-30A (€40.63). Or perhaps a little CC+CV DC-DC coverter module – but be careful that they are good quality and the output does not drift. Or just a small CC+CV lab supply. Anyway, a small CC+CV power supply would keep the battery topped up to 80%, powers amplifier during use at low levels, at high levels the battery takes over some of the load. LiFePo4 can be float charged at 3.3-3.4V so 30V for 9S would be perfect. Power supply would need to be good quality with max 2% voltage tolerance, preferably less. Maybe 8S would be safer with the 32V amplifier limit. Then also RSP-75-27 (€33.53) is possible (2.8A current limiting, adjustable voltage).
Maybe a current limiting power supply is not such a bad idea anyway, to protect the amp from burning itself. Battery could be external/optional to save weight. Anyway, datasheet says 80-85% efficiency at 50W into 4 ohm, so up to 10W dissipation – that looks to be about the maximum that the passive heatsink could dissipate. 75-100W current limiting would give some headroom but prevent prolonged high power use. On the other hand, maybe I want more headroom to be able to push the driver into distortion – at the driver’s rated program power of 100W, excursion is gently over xmax between 50-95Hz. RSP-150-27 (€35.02) set to its maximum voltage would (according to the TPA3221 datasheet) allow for 100W into 4 ohm at 1% THD, with power to spare. But then the amplifier definitely would need a fan.
Maybe go with the TPA3255 module instead and 48V 200W PSU for about 170W into 4 ohm – to really drive the woofer into distortion. And probably break it. Ok maybe 100W is plenty and having the amp clip before damaging the driver is not so bad.
Parts list:
- Cabinet: 20cm PVC pipe (have), ~120cm long
- Baffle & end cap: 3D printed (€?)
- Port: 2x 6cm PVC pipe ~20cm long (~€6.00/scavenge)
- Driver: SB20PFCR30-4 (€37.05)
- Amplifier: TPA3221 module (have, €7.52)
- PSU: RSP-150-27 (€35.02)
- Preamp: ?
- Connectors & knobs
- Foam to prevent pipe resonance but mostly to protect bass guitar