Bad product design: Wharfedale DX-1, part 2

A long time ago I wrote about a subwoofer I had that tended to break down whenever I moved it for cleaning. I have since sold the unit, but I was contacted by others who have the same problem and I would like to share my findings.

First of all, every time the TDA7294 blew up for me it broke nothing else on the PCB, so you can probably repair the device just by changing the chip. If you wish to prevent it from breaking again, read on.

The subwoofer has three modules inside; a switch-mode power supply, preamplifier board (with all the backpanel connectors and potentiometers) and the main amplifier. I looked for a suitable amplifier board on AliExpress and found one which uses TPA3116 which is one of my favourite chips. It cost five dollars at the time. I wanted to keep the preamp as it was as it worked and I didn’t want to redesign the back panel.

I designed a new power supply for the amplifier board out of a toroidal transformer I had at hand. I created a simple PCB which provides an unregulated 12-24 V line for the main amplifier, and a regulated +- 12V for the preamp.

 

After that it was just a matter of hooking up my custom power supply to both of the amplifier boards. There was some problem with the mute signal of the preamp so I eventually left it unconnected. The preamp has the output signal labeled “S+ GND S-” and you can connect those directly to the amplifier board (I left S- floating).