HowTo: TDA7492/CSR8635 – Build a DIY Bluetooth Retro Speaker

At some point last year I stumbled on Ebay on the very cheap TDA7492/CSR8635 Bluetooth amplifier boards.
These include a Bluetooth audio receiver and a stereo amplifier with which speakers up to 50 watts can be controlled. Well, at first I didn't know what to do with it. But a potential application should not be long in coming.

A few days later I accidentally stumbled in the basement over an old loudspeaker in real wood look (Actually not just a look.) It was actually built of real wood.: D). Since the associated amplifier had long given up, I came up with the idea to breathe new life into the old loudspeaker with the amplifier board.

How to do it and what you have to consider is described in the following article.

Hints for our lovely english readers: Basically, many of the articles on are translations from the original german articles. Therefore, it may happen here and there that some illustrations are not available in english and that some translations are weird/strange/full of mistakes or generally totaly wrong. So if you find some obvious (or also not obvious) mistakes don't hesitate to leave us a hint about that in the comment section. 
Also please don't get confused, that instead of a "dot" often a "comma" is used as decimal separator. 🙂

Safety instructions

I know the following hints are always a bit annoying and seem unnecessary. But unfortunately, many people who knew it "better" from carelessness lost their eyes, fingers or other things or hurt themselves. In comparison, a loss of data is almost not worth mentioning, but even these can be really annoying. Therefore, please take five minutes to read the safety instructions. Even the coolest project is worth no injury or other annoyance.

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Helpful Articles:
Before you start with this article you should have dealt with the basics of soldering. Information on this can be found in the following article.
Electronics—My friend the soldering iron

Required material:

In the following list you will find all the parts you need to implement this article.

Required tools:

In the following list you will find all the tools you need to implement this article.

Collect required parts

In the following picture you can see the parts you need to build your own retro Bluetooth speaker.

Print the required 3D parts

Download all needed STL files: TDA7492CSR8635-DIY Bluetooth Recorder Speaker STL FILES

button panel:

You can rotate the 3D view of the STL file by holding down the mouse button. You can zoom in and out with the mouse wheel.

Connection panel normal:

You can rotate the 3D view of the STL file by holding down the mouse button. You can zoom in and out with the mouse wheel.

Connection panel including external speaker connection:

You can rotate the 3D view of the STL file by holding down the mouse button. You can zoom in and out with the mouse wheel.

Open speakers and locate connections

Of course, before you can start with the conversion, you first have to penetrate into the interior of the loudspeaker.

To open the speaker, you usually have to loosen all the screws visible on the back.
Open speaker with visible crossover.
Detail view of the crossover. Here you can now separate all (hopefully) plugged connections and put the back side aside. You need this later. But beware: Take a picture of it before disconnecting the connectors. Thus, you can easily understand later, which plug belongs where.

Attach the sensor cover

After you have opened the back, it goes to the first cosmetic extension of the speaker. Because so you can later also adjust the volume and control the playback of the music, the corresponding keys must be led to the outside. This happens in the following steps.

Position the button panel in the center of the top of the speaker. Check with a ruler (or similar) that the distance to all sides is the same.
If you have aligned the stylus cover, you can drill the first mounting hole. Before drilling, check twice, that the panel is really in the right place.
Did you drill the hole...
… then align panel and the holes and put a first provisional screw through the panel in the drilled hole. So you've already created a "reference", so that the remaining holes are all in the correct position.
Before you drill the next mounting hole you should check once again that the button panel is parallel to all outer edges of the loudspeaker. This goes, for example, with a large triangle.
If the button panel is aligned correctly you can drill the next hole …
… and fix it again provisionally with an inserted screw. Now the panel is already secured against slipping in all directions.
Now you can drill the remaining mounting holes without slipping the panel.
Now the holes for the actual buttons have to be drilled. To do this, drill a hole in the top of the loudspeaker with a small drill, as shown in the middle. Here a drill diameter of 3mm is sufficient. The holes are "drilled" to larger diameters in the following steps.
Have you drilled all the holes you can …
…remove the panel again.
Now it's time to widen the pre-drilled holes with a "flat drill bit" - to bring it to the actual size needed. Of course, the drill should be slightly larger than the diameter of the buttons used later.
Drill the holes with the "flat drill bit" rather with a slightly lower speed. So you can react faster if the drill should get stuck. Also, before drilling, check that you do not damage any components on the inside of the speaker when drilling through.
Did you drill all the holes...
… you can use the button panel again and check if all holes are big enough and in general everything fits.
Now that you know that the speaker is ready for the fully populated button panel, you can start populating it completely. To do this push the buttons from the top of the stylus panel through the holes for the buttons …
… and secure them with on the back with nuts.
So that the buttons can later be connected to the amplifier board you now have to prepare six double lines. (Here you can – if available – also recycle the disconnected speaker cable) These should be so long that you can easily solder the line to the amplifier board later. If you are uncertain about the length, let it be more lengthy than too little. With a length of 30cm, you should actually get along very well in most cases.
Then remove about one cm of the insulation at all ends of the double cable and tin them with solder. This ensures that the strand of the individual line does not "break up" later.
Now tin the contacts of the buttons …
… and repeat that for all other buttons. So soldering with the previously tinned double line later is a breeze.
Before you solder the double line with the buttons, you can shorten the stripped part of the double line a bit. Five mm are enough here completely.
Then try to heat the solder on the line and the button contact simultaneously with the soldering iron. As soon as the solder on the line and the button contact is liquid, you bring the line and the button contact together and wait until the solder gets cold and the button contact and the line is soldered. At this point you can once tenderly pull on the soldered line and check if this really holds. A correct solder joint should then not disconnect anymore.
This repeats for the second button contact and …
… the contacts of the remaining buttons.
Did you finally soldered all wires the whole look something like this.
Unfortunately not to be seen in the photo: Label the individual cables so that you know later which cable belongs to which button!
The leads of each button can then be passed through the holes in the lid of the speaker and the panel can be placed.
If the panel is in the right position it can …
… so be fixed directly with screws.
It looks like that from above.
If you have the aperture attached, you can begin to summarize a line of the double line of each button.
Since you have to solder the combined cable later also to the amplifier board, it is advisable to solder a small thin wire here. This makes soldering easier later.
After that everything should be done with shrink tubing …
… to be isolated.
The ready-prepared pushbutton connections then look something like this.

Attach the connection panel on the back

Next up is the preparations on the back of the speaker.

If the loudspeaker cable is firmly attached to your loudspeaker you can simply "clip it off" on the back. An electronic side cutter is perfectly suited for this purpose because it separates the cable flat with the housing wall. Of course, every other side cutter does it too.
So it is almost impossible to see that there was a cable in the housing previously.
You repeat all this on the inside.
It's best to leave as much speaker cable as possible, as this will need to be connected to the amplifier board later.
Now it is the attachment of the aperture for the rear ports. This will later accommodate an on/off switch, power connector, and a jumper connector for an optional second speaker. At this connection for a second speaker, the second channel of the stereo signal received via Bluetooth is output.
After you have aligned the connection panel correctly you can – as with the stylus previously – start with the first mounting hole.
Then you go just as with the button panel. First drill a mounting hole and temporarily secure the panel from slipping with an inserted screw. Then the other holes are drilled. Every time you drill a new hole, check once more whether the panel is still aligned correctly. So in the end everything is straight and fits right together.
Once you have drilled all the holes and inserted the fixing screws (provisionally) it should look like this.
Now you drill again with a 3mm drill the rough position of the openings for on/off switch, power connection and jumper connection.
After removing the connection panel the pre-drilled holes can..
… be widened again with the flat drill bit. Make sure that you do not drill too much with the flat-drill bit - so the hole will look out at the side of the connection panel.
Once all the holes have been drilled, you can check if the drilled holes are big enough by hanging up the connection panel. In this picture you can see that something has to be reworked because parts of the wooden back wall can still be seen in the hole for the terminal connection (rectangular hole on the right).
Here, then, some manual work is required. So get to the file and file …
… until the cut-outs in the back of the speaker are big enough.
Before you can use the terminal connection in the connection panel this must still …
… be “pimped”. The protruding "collar" must be stripped, because the clamp would not otherwise fit through the hole in the back of the speaker. Separate the "collar" of the terminal connection with a side cutter. Here you should definitely use a "right"(stong) and no electronic side cutter. In the latter case, the "cutting jaws" can quickly break off or be damaged if the force is applied excessively.
After processing, the collar is removed around it.
So you can, next to the power socket for the power supply and the on/off switch, …
... also insert the terminal connection.
So that you can also solder the connections to the amplifier board later you have to prepare a few double lines. To do this, make three three-inch pieces of the double line. You can also use the separate speaker line.
Then strip the ends of the wires about 5-10mm.
To make soldering with a little easier...
… tin the contacts – as with the buttons before – with solder. At this point, you can also tin ends of the wires.
As a result, the soldering of the individual lines is quickly done again.
If you use one-color cable at the switch and the hollow socket, please mark which cable is soldered to which connector. You must be able to understand later, which line corresponds to the plus or minus pole.
Have you soldered the wires you can insert the connection panel in the back …
...and fix it with...
As a small safety measure, it is convenient to fix the contacts at this point with some hot glue. This ensures that they cannot bend (due to the soldering work, etc.) and thus possibly cause a short circuit.

Install the TDA7492 amplifier board

Now that you have prepared wires and connections for a long time, it is finally time to connect these ports to the buttons.

This is what the "virgin" amplifier board looks like.
So that the soldering of the individual connection wires will be easier now, all needed contacts will be tin-plated with solder. Apply some solder to all circled contact surfaces. Make sure that the solder does not touch the silver housing of the respective button.
Also on the power supply socket of the amplifier board two contacts must be prepared.
So that you have both hands free in the following work and the amplifier board does not have to be hold …
… you should attach the amplifier board to a free space on the inside of the back wall. Here it can simply be glued with some hot glue.
Before gluing, make sure that the cables that are led out reach everywhere as far as your amplifier board is off.
Before you solder the wires prepared to the amplifier board you have to solder the on/off switch to the hollow socket. To do this, solder the plus pole coming from the hollow socket to one of the lines of the on/off switch.
Insulates the soldered place with ….
… a small piece of heat shrink tubing.
The now free line coming from the on/off switch you can solder now as shown on the plus pole of the amplifier board.
Closeup of the soldered plus pole.
The remaining wire that comes from the power socket you have to solder with the negative pole of the power socket.
Close-up of soldered wires to power socket of amplifier board.
Now you can also connect the speaker cable of the terminal connection with the screw terminals of the amplifier board. Which screw connection you choose does not matter. You should, however, make sure that the cable which is attached to the red terminal connection is also attached to the positive (+) screw terminal. The black terminal connection is therefore connected to the negative screw terminal.
Close up of screw terminal connection.
Now you have to prepare the connecting wire of the loudspeaker. To do this, you isolate the wires again 5-10mm and …
… tin them with some solder.
Pay attention to the correct polarity of the speaker cable when connecting to the screw terminal. At the mass symbol next to the "1" you can see that this wire is connected to the negative pole.
Closeup of the fully connected screw terminals.
If you have all wires (except for the button) connected, the whole thing should look something like this. Now is the time for a first test. Connect the speakers to the crossover and test again before switching on for the first time, that you have not really messed with the polarity of the power socket.
If the first test was successful, you could…
… lash down protruding lines.

Solder probe leads to the amplifier board

As a final soldering task, the prepared wires must now be soldered from the buttons to the pushbuttons of the amplifier board.

Since you have already prepared the appropriate contacts, the soldering of the wires is done quickly.
Make sure that no contacts are short-circuited to the silver button housing during soldering. First, you should also solder the merged probe cable to the ground contact. This is the place where the black cable is soldered in the picture.
Its recommended to test the amplifier board each time after soldering a new push-button wire. If the amplifier board still starts, you can be sure that the new soldered button didn't cause a short circuit. If a short circuit occurs between the contact and the silver key housing during soldering, the amplifier board does not start anymore and you know directly which solder joint you have to rework.
If you have...
…then soldered all…
… button wires the whole thing should like this.
So that the freshly soldered push-button wires should be protected against mechanical stress during installation, you should coat the contacts/cables with some hot glue.
Have you soldered all wires and tested the amplifier board and all connected buttons, the housing can now be closed again.

Close the back

Have you tested everything, the back can now be closed of course. Make sure that you do not squeeze any cables and the insulation in the speaker is in the right position.

When screwing the back, make sure that you do not overtighten the screws. Otherwise, it can happen that they "suck in" very deep in the back.

Congratulations! You've brought your old speaker back to life and added a cool new retro Bluetooth speaker.

Before starting up, you should now follow the tips from the article Electronics—Commissioning a new circuit.

Have fun with the project

I hope everything worked as described. If not or you have any other questions or suggestions, please let me know in the comments. Also, ideas for new projects are always welcome. 🙂

P.S. Many of these projects - especially the hardware projects - cost a lot of time and money. Of course I do this because I enjoy it, but if you appreciate that I share this information with you, I would be happy about a small donation to the coffee box. 🙂

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