Backup power for F&D T-60X speakers

My F&D T-60X speakers have one annoying flaw: after any power loss they come back in standby, and when turned on, the volume resets to the default 15, while I almost always use 3. With frequent power outages I switch the power source of my apartment often, and every time I had to find the remote and set everything again.

My TV power station sits right next to the speakers, so I added a DC input to them. Now they run from mains as before, and switch to the power station when mains goes out, without turning off.

Inside the speaker

I opened the main speaker and traced the wiring. It is built from three separate boards:

• PSU: AC to 24 V DC (ER2421 board).
• Main board: amplifier and everything else.
• Control board: buttons, switches and so on.

The PSU is connected to the main board with a simple 24 V cable, which makes it easy to insert a switch in between.

How it works

    flowchart LR
  MAINS["Mains"] --> PSU["Original PSU<br/>24 V"]
  PS["TV power station<br/>19.5 V"] --> JACK["New DC jack"] --> BOOST["Boost converter<br/>19.5 → 24 V"]
  PSU -- main --> SW["YX850<br/>auto switch"]
  BOOST -- backup --> SW
  SW -- 24 V --> MB["Main board"]
  

• YX850 is a relay module that switches the load between two sources. The original PSU is on the main input, so it has priority. When mains goes out, the relay switches to the backup input fast enough that the speakers don’t notice.
• Boost converter is needed because the DC output of the power station is 19.5 V (that’s what the TV needs). It raises it to 24 V, the same as the original PSU.
• New DC jack on the rear panel connects the power station.

Why not run the speakers from the power station all the time? Mains stays the main source to save battery capacity and cycles. Also, the power station is charged by a TV charger module, which isn’t powerful enough to run all connected devices and charge the battery at the same time.

Parts

Build

Both modules are mounted inside the main speaker, next to the original PSU, with hot glue. The PSU output cable is cut and spliced into the YX850 main input with butt connectors; the YX850 output goes to the main board. The wire from the DC jack comes in through a terminal block to the boost converter.

Result

It works: the speakers switch between mains and the power station without turning off, and the volume stays where I set it. During long outages I don’t even bother plugging the speakers into the wall outlet; they just run from the power station.

Caveats

• The wire from the DC jack to the boost converter has no fuse of its own. It relies on the protection of the power station and the fuse on the boost board.
• The boost converter is powered all the time, so it draws a small idle current from the power station even when mains is on.
• The original power switch only cuts mains. With the power station connected, switching it off doesn’t turn the speakers off; they just move to the power station. To turn them off completely, unplug the DC jack too.