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).

Bad product design: Wharfedale DX-1

When I moved to a new apartment a few years ago I decided to get a 5.1 home theater system. Wharfedale Diamond speakers are quite nice so I decided to get Wharfedale DX-1 home theater. I have been pretty happy with it except for one major annoyance – the amplifier inside the subwoofer tends to break when I move the subwoofer for cleaning.

Broken Wharfedale DX-1 amplifier module, based on TDA7294.

I changed the TDA7294 chip the first time this happened, but now I think that I will get a class D amplifier module from China and use it instead. I will post some pictures when I complete the mod.

Making an usable solid state relay

Using a microcontroller to control a device which uses mains power can be dangerous. There are products which try to overcome this problem, but I think they still have a lot to improve. If we take a look at the Sparkfun catalog, the Beefcake Relay requires you to find a safe enclosure for your project. On the other hand, the PowerSwitch Tail  comes in a nice enclosure but supports only American power plugs. Moreover, the low voltage connector is a screw terminal which is slow to mate and de-mate. Industrial solid state relays, like eBay Foteks, also require a proper enclosure.

Finding an enclosure which has suitable dimensions is quite easy, but there is quite a lot of boring work if you want to use connectors instead of just running a cable through the enclosure wall.

DIY solid state relay used to turn the heat bed of 3D printer on and off.

Recently I upgraded the heatbed of my 3D printer so I had to figure out a solution to this. I decided to use Schurter 4300.0301 for the mains connection and a 5.5mm DC plug for the low voltage connection. The circuit board, which implements the functionality from the application circuit of the MOC3043M datasheet, is soldered directly to the terminals of the mains connector. The whole thing fits nicely to a Kradex Z56 enclosure.

I hope decent commercial MCU controlled mains switches emerge soon.

Boot selector for Amiga

Over the holidays I had once again the pleasure of tinkering with my friend’s Amiga 500. This time helped him with flashing the HxC firmware to his Gotek floppy drive emulator which he had mounted in an external drive case. The flashing went smoothly, but in order to be able to boot from the Gotek device the Amiga needed a hardware modification.

This came as no surprise as we had done our homework. There are multiple products in the market but I did not really like them as they connected the IC pins to a DPDT switch with a long wire in order to swap the signals in pins 13 and 14. The CIA ICs are getting rare, and although unlikely, killing one with ESD would ruin the day. Moreover, the required switch type greatly limits the selection of switches and mounting methods one can use to make the solution as neat as possible. The prices of the existing solutions are also quite high if you consider the shipping costs as well.

For those reasons I created my own design. It is a really simple solution which uses a 74HC153 multiplexer to do the switching. The top side of the board has a precision DIP socket for the even CIA. The bottom part has the turned male pins for plugging the whole thing to the original IC socket. The circuit will draw its power (approx. 2 mA) from voltage supply of the CIA chip.

You can build your own Amiga Boot Selector by using these Gerber files The project is licensed with a CC BY 4.0 license.

Transmitting audio using light

I’ve wanted to try transmitting audio using light for a long time, and this afternoon I finally tried it. To modulate the beam I shifted the MP3 player output using a MCP602 op amp. 3 V +/- 0.3 V seemed quite ideal. I generated the 3V reference using a zener diode (the current limiting resistor doubled as the R4 of the aforementioned level shifter circuit). This level shifter is in the left side of the picture.

Audio transmission setup

The receiver circuit (top right) consisted of a phototransistor with a few passive components for setting the current through the transistor and a high pass filter. I amplified the signal using a TBA820M amplifier chip (in the middle) and used an 8 ohm speaker for output.