Does the Fosi BT30D Pro SUB FREQ Knob Control a Powered Subwoofer?
Short answer
Do not assume the BT30D Pro SUB FREQ knob controls a powered subwoofer connected to PRE OUT.
Fosi’s current product page says the 3.5mm PRE OUT can feed a powered subwoofer, powered speaker or another amplifier. But its current user-instruction page describes SUB FREQ specifically as adjusting the frequency output of the passive subwoofer, over a published 0-300Hz range.
- Passive subwoofer on the amplified sub output: use the BT30D Pro’s SUB FREQ control as documented.
- Powered subwoofer on PRE OUT: use the powered subwoofer’s own low-pass/crossover control unless Fosi documents different behavior for your exact hardware revision.
View the current BT30D Pro at Fosi Audio
Why This Is Confusing
The BT30D Pro can connect to a passive subwoofer through an amplified speaker-level output or to a powered subwoofer through line-level PRE OUT. Both are called subwoofers, but Fosi documents the SUB FREQ control for the passive path.
ELI5
Think of the amplifier as having two roads to a subwoofer. The passive road carries amplified speaker power and has the documented SUB FREQ control. The PRE OUT road carries a line-level signal to a powered subwoofer with its own amplifier. Use the powered subwoofer’s own crossover unless the manufacturer says otherwise.
What Cable Does a Powered Subwoofer Need?
The BT30D Pro PRE OUT is 3.5mm. Depending on the subwoofer, that may mean 3.5mm-to-dual-RCA, 3.5mm-to-single-RCA when supported, or another line-level cable specified by the subwoofer maker.
Related
For the full 32V/48V, speaker, turntable and passive-vs-powered-subwoofer explanation, see the Fosi Audio BT30D Pro Data Dossier.
