Fosi Audio BT20A Pro Review: In Australia, the Supply in the Box Sets the Wattage

What the 300 W figure is a measurement of

The specification reads 'Output Power: 300Wx2 @4Ω' and names no supply. The manual prints the same row with 'Power Adapter 32V 5A' a few lines below it, so one table holds two different things: the chip's ceiling and the brick in the box.

Fosi's 'Power Variability' chart splits rated and maximum output by supply and impedance. A 24 V 4.5 A supply gives 56 W rated into 4 ohms; the 32 V 5 A adapter in the Australian bundle gives 100 W into 4 ohms and 50 W into 8; a 48 V 5 A supply gives 220 W and 110 W. The support portal's copy reads 230 W for that last row and adds the one the headline needs: 48 V 10 A, 280 W rated and 300 W maximum into 4 ohms.

The DC input range is 24–48 V, so a 48 V 10 A brick is electrically in range, but it is never part of a BT20A Pro bundle. Read the row that names the supply you are actually buying.

What the Australian listing is, and what it is not

Fosi's own store sells the same chassis in Blue or Gray with a 32 V 5 A, 48 V 5 A or 48 V GaN supply, plus a Blue version with no supply at all. The Amazon Australia listing offers none of those choices: it is the 32 V 5 A bundle, at A$149.99, sold by Fosi Audio AU and shipped from Amazon.

The adapter accepts 110–240 V on the mains side, and Fosi says the plug matches the delivery country, so the bundle suits Australia's 230 V. Anyone planning to fit a bigger supply later needs one that delivers 24–48 V, enough current for the row they want and a 5.5 × 2.5 mm barrel.

Wiring, pairing and the one caution worth following

Before the supply goes in, the manual asks for bass and treble at the centre and the volume at zero to avoid a sudden blast. The terminals take 4 mm banana plugs, and the manual warns that some large banana plugs can touch each other and short.

Bluetooth comes first: a remembered phone overrides the RCA input, and switching to RCA means holding the volume knob for 5 seconds or forgetting the amp on the phone. Bluetooth cannot be switched off completely; the support portal suggests removing the antenna to cut the range. Formats are SBC and AAC, and the manual answers aptX, aptX HD, aptX Low Latency and Dolby with a no.

The 3.5 mm PRE OUT follows the main volume knob but not the tone controls, and it carries the full range, so the low-pass filtering is left to whatever subwoofer is plugged in. Fosi documents two systems: passive speakers plus an active sub on the pre-out, or the Pro as a preamp into a second power amp.

Two tone knobs and a pair of sockets

The knobs are labelled '− TREBLE +', '− BASS +' and '− VOL +' with a white index line, and the centre position of bass and treble is the documented flat setting. Fosi prints no decibel range for either control.

Inside, two NE5532 op-amps sit in sockets. Fosi describes the swap with a hex key and lists compatible replacements, on condition that both are changed to the same model at once. It is still a user-side modification on a product whose warranty excludes 'unauthorized product modification'.

What the box leaves out, and what covers it

The box holds the amplifier, the antenna, the adapter, a mains lead and the manual — no speaker cable, no RCA lead and no banana plugs, all of which the terminals assume you already own. Budget for them before comparing the A$149.99 with anything that ships with cables.

Fosi contradicts itself on the warranty: the warranty page says 24 months from receipt, the manual 18 months from purchase, and repairs ask you to pay shipping to Fosi. Separate from either, the Australian Consumer Law gives you consumer guarantees automatically, and the seller here is Fosi Audio AU. Change-of-mind returns go through Amazon, which ships the order.

Where this amp stops being the right call

Three needs point elsewhere. A second zone or second pair of speakers: there is one RCA input plus Bluetooth. Better Bluetooth than SBC and AAC: in Fosi's range the T20X MKII adds aptX HD and aptX Low Latency, and the BT20A MAX adds LDAC. And a subwoofer without its own crossover: the PRE OUT is full range, while the BT20A MAX has a dedicated sub output.

A 48 V supply is the upgrade with a published number: 220 W into 4 ohms and 110 W into 8 on the product page's chart, against 100 W and 50 W for the bundled adapter. Neither chart ties that to speaker sensitivity or room size, so it is worth buying only when your speakers' recommended amplifier power runs past 100 W into 4 ohms or 50 W into 8.