DIY Bluetooth Boombox - Repurpose Your Old Hi-Fi

Intro: DIY Bluetooth Boombox - Repurpose Your Old Hi-Fi

For this project I will be making a bluetooth speaker using secondhand bookshelf speakers. I will go through the design process and all the different stages of construction, this will be a comprehensive guide to building a your very own Hi-Fi quality boombox.

You will need:

- A pair of bookshelf speakers

- TDA7492P Wireless Bluetooth 4.0 Audio Receiver Digital Amplifier Board

- A latching switch (optional)

- A power supply between 9v and 24v (higher the better)

- 2 part automotive body filler

- Primer

- Top coat

- A small piece of perforated steel or ally (optional)

- pcb stand-offs

Tools:

- Soldering iron (can possibly use chocolate boxes for connections instead if no switch is used)

- Electric Drill

- Screwdriver

- chop saw or a table saw or even a bandsaw ( could also be done by hand if careful)

- Router (optional)

- Sander (optional)

- Jig saw (could also use a coping saw or hole cutter on a drill)

STEP 1: What Speakers?

In short you can use any bookshelf speakers to make this project work, I purchased three pairs. One for £5, one for £8 and the last for £20.

I started by looking at the quality of speakers I purchased, I noticed a broad variety of construction methods.

The most expensive speaker cabinet was made from a variety of materials. It has rigid composite supports that tie the baffle board to the rest of the enclosure. The moulds required to make this speaker would have been costly to make. The mid range speaker is made from chipboard and is relatively strong. It has a thick, strong baffle board. Recessed into it are the tweeter and bass driver. It is a simple but tried and tested method. The last cabinet (cheapest) is thin, flimsy chipboard. It is joined in a basic fashion using glue and small nails. It is cheap, mass produced and is the weakest of the three.

STEP 2: An In-depth Look

I decided to take a closer look at most expensive of the three, the Mission 731. It uses three different seals to keep the enclosure as airtight as possible.

Seal one: rubber gasket running around the edge of the baffle board. This is compressed to form a tight seal as the enclosure is put together.

Seal two: a foam pad around the front of the bass driver. This is squeezed shut when the driver is mounted to the baffle board.

Seal three: there’s a relatively thin gasket around the tweeter; however this is all that is needed. This is due to the large surface area the gasket has. Unlike the bass driver, the tweeter is able to have large mounting flange due to its compact size.

STEP 3: Calculating Volume

To measure the volume of the enclosure I needed to think outside the box. I knew I could not calculate the volume like I would a normal box, using maths. This was due to the complicated bracing system used inside the enclosure.

While 3D scanning and photo-mapping could have been used to get an accurate volume measurement it would’ve been expensive and hard to calibrate.

I also considered using callipers to take height and depth measurements. This could’ve made a rough plotting guide I could use to make a model in CAD. All of these ideas where unnecessarily complicated.

In the end I decided to use a measuring jug full of water to fill up the cabinet. This allowed me to accurately measure the internal volume of the cabinet in litres.

Before I could start pouring water I first needed to plug the binding post terminal holes. I used the original binding posts and silicon to do this.

I also sprayed to inside walls of the enclosure to stop the distorting when filled with water, thus giving an inaccurate reading.

We need to calculate the volume so as to replicate the tuning of the original enclosure, keeping the speakers sounding clean and refined.

STEP 4: Designing an Enclosure

I used an online speaker box calculator (diyaudioandvideo.com/Calculator/SpeakerBoxVolume) to make a box the same volume as two enclosures and then made it from birch ply.

I used a jigsaw for the speaker holes, but a router and a circle guide would be neater.

The corners are mitred at 45 and the front panel is sitting in 5mm groove 10mm inset. The groove adds a lot of strength and is a great help when gluing up those tricky mitres (use a tacky glue).

The baffle on the front driver is doubled up for a bit of strength and rigidity. 9mm ply was cut for the back, it was hand planed to a tight push-fit (boxes never end up as square as you hope!)

I used a 1/4 round-over bit on a handheld router to round the edges but sandpaper will work fine too.

STEP 5: Finishing

I painted the front panel with some cheap black paint and mocked everything up to make sure it fit.

After this I filled all the holes with a 2 part car body filler as it works a lot better than nasty wood filler.

The four holes you can see in the second picture are for the machine screws that attach the pcb stand-offs to the enclosure.

After filling and sanding I sprayed the enclosure with two coats of white primer, then sanded to 240 and sprayed my top colour coat. I then cleaned up the front and back edges with a block of wood and some sandpaper to show the birch ply.

Walnut offcuts where screwed to the bottom (countersunk) to make feet. This allows space for the driver, as well as preventing scratches to the finish.

STEP 6: Electronics

I ran my speakers bridged from the amp as I wanted more power, however it does distort slightly at it's highest volume. Use the amp normally - left speaker output to left speaker and so on - if you want to avoid this.

I use 3mm dowel to push the surface mount switches on the PCB.

At this point in time there are no batteries. When I find a suitable charging system I may well consider them.

The electronics in this circuit are quite basic, all you need is a soldering iron and a bit of patience. There are many tutorials on soldering here on Instructables should you need any help.

STEP 7: Grille

For the grille I used a 1.5mm plate of perforated steel, I bent it using a mallet a bit of 2x3. I used a chop saw to cut the corners.

The grille will eventually be powder-coated in an off-white (when I have the time!).

STEP 8: Finished!

I now use this everyday to play music while I'm in the shower and getting ready for work. It's not crazy loud, but is clear, crisp and clean. I occasionally use it for garden parties as background music, it always gets compliments on the sound and aesthetics, so overall I'm very pleased.

Comments

[ricocamus]

Question on using the speaker box calculator, am little confused since it requires keying in the driver diameter and the dimensions of the box. Assuming I am making a sealed box (no port),

1. do I get the volume of the box (using the method you describe above); then

2.drag the dimensions around in the illustration, until the calculated volume matches what I measured in no. 1;

3. then get the calculated box dimensions on the lower half of the page?

Or is there another way to do it?

[diggoryrush]

Sorry for the late reply, just seen this message. That is the way I did it, you can also go in a CAD program like Rhino or Sketchup and draw a solid the volume you need. Then design an enclosure around that, you would also need to minus the driver/drivers displacement from you overall volume.

Some manufacturers list the SD (The effective projected area of the cone or diaphragm) in there specifications but if you can't find them you can just take some of a similar speaker the same diameter. For example the SD of a B&cC 15ndl756 has na SD of 855.0cm2. So from this I know that a 15" speaker with a small neo-magnet will displace about 855.0cm2.

If any other questions I'll do my best to help.

[ricocamus]

Thanks very much.

That clears up the question I had about using the volume calculator.

[AimeeMichaels]

Do you (or anyone else) know how to judge if a BlueTooth audio receiver is any good? I haven't yet found any data on the quality of the D/A converter which I think might make a difference. Any thoughts?

[NarenS4]

Maybe easier to make a list of ones that have not been good. I got one recently and it has a lot of hum. It was ok for my application, but probably not good for this. So I am also on the lookout for some thing that will fit. I have just ordered a second one (not bluetooth for this application, but if the sound quality is good, I can always add on a bluetooth when needed).

[diggoryrush]

Unfortunately not, I would be very interested if someone did.

[kartik negandhi]

awesome job working out wid the wood..I like your wood working!??

[diggoryrush]

Thanks very much! I love working with wood, such an interesting medium!

[bandaboy]

Great Job!

[diggoryrush]

Thanks!

[AndreyG1]

can you be more spesific on the build schematic ? and what kind of speaker are you using, any advice?

[diggoryrush]

What part were you unsure of ?

I used a pair of mission 731 speakers I bought from a junk market. The idea is that you can use any speakers and make an enclosure to suit, using the box calculator.

[AndreyG1]

thank you. do wood enclosure matter, i mean there are softwood, plywood, mdf etc, is that all matter? i am using 150w 2-way pioneer stereo. but cant make the boom sound. my enclosure is MDF 1" thick. any advice mate?

[diggoryrush]

The enclosure material does matter a lot! The shape, well not so much. MDF is a good material for speaker building as it has very little impact on the sound. If it was hardwood for example it would have lots of resonant frequencies. People use this to their advantage when making guitars (tonewood).

Birch ply is commonly used in PA loudspeaker construction as it is very strong and holds screws better than MDF.

The most important thing for your enclosure will be its volume and also port size. This is the reason I made this instructable so as to help people make decent sounding boomboxes without needing large amounts of speaker design knowledge.

If you match your original enclosure volume and port size your speaker should so no different from when you first heard it. The only differences in sound would be caused by the electronics used to power it i.e. amplifier, receiver even cables to a point...

If your speakers are car speakers try making you enclosure smaller and see what you think ?

Car speakers are low efficiency but high Xmax (the amount the cone moves in and out) so they can subjected to quite powerful bass frequencies before the cone becomes unloaded. The low efficiency might be the reason they're not getting going (boom sound). They may just need more power.

Look at the tpa3116 with bluetooth module if you need more power.

also no harm in experimenting with ports sizes, lengths and enclosure volumes!

[AndreyG1]

i made my self using 2 150w pioneer 2-way speaker. and it sounds bad :( too much treble and less bass. how to improve it folk?

[LuisC188]

Good job! i think your creation is great, especially from the receiver-amplifier. I am converting old radios to blutetooth receivers and actually I use receivers who have little power and quality, I will learn from your experience, thank you very much, Diggory

[diggoryrush]

No problem, glad I could help.

[gauravsanu]

Great project..

[diggoryrush]

thanks!

[Majtrix]

Nice!