Stealthy Stereo Boom-box for Digital Projectors and House-parties

In early 2019 I bought a very nice HD digital projector as an alternative to a big TV. The 100" viewing experience is fantastic but I felt that the sound needed improvement, particularly for the kinds of films I like watching. This got me looking at soundbars, 5.1 systems and all the other things usually used for this job. Importantly, my projector is normally hidden away and only gets put out on Friday and Saturday evenings as a rule, so I didn't fancy the idea of needing to install loads of speakers with cables trailing everywhere. I also wasn't so keen on the idea of buying into all the modern DSP trickery used to generate respectable bass from small drivers.

I finally settled on the idea of some form of active speaker / boom-box, which would double as a 'coffee-table'-style support for the projector when in use. Both of these requirements meant that the finished item would need to be a significant size and weight, thus creating a potential storage problem. I decided therefore to make it serve as a bench seat for the hallway (which I needed anyway) when not in use. Finally, I had some good quality audio equipment left over from my 'boy racer' days many years ago, which I thought it would be nice to recycle. I should point out that nothing is particularly original in this build (there are loads of smaller versions on Instructables), but there might be aspects of this embodiment which are of interest to readers.

So this Instructable describes the design and build of a fantastic-sounding active loudspeaker system with the following features:

• 6x9" Alpine Reference car stereo speakers (circa 1994) mounted at +/- 15 degree angles from listener for modest stereo enhancement, whilst retaining a path for higher frequency content direct to the listener.
• 50 W class D amplifier with Bluetooth, 3.5 mm auxiliary input and 12 V DC power
• Electronic volume, bass and treble controls via LM4610 IC
• 'National 3D Sound' stereo wide function
• Substantial 80 cm x 44 cm x 30 cm braced infinite baffle enclosure which supports a short-throw projector at the correct height for my screen
• Thick padded foam cushion for bench seat, attached by velcro
• Fabricated from half-sheet of 18 mm MDF with little waste using only screws and battens
• Casters on each corner for easy positioning
• Easy to build over a weekend with common hand tools and jig saw

DISCLAIMER : It really should go without saying that jigsaws (especially mains-powered ones), cordless drills and many other tools can be very dangerous in careless hands - do not attempt to reproduce my work if you are not are competent in the use of the required tools or you are unsure of any aspects of the description. Naturally if you attempt to reproduce this work and/or make use of the result you do so entirely at your own risk.

Supplies

• 620 mm x 2440 mm sheet of 18 mm MDF
• 25 mm x 25 mm (or slightly bigger) batten, approximately 6 m total length - I used some left-over timbers from some roofing work I'd had done, which were a variety of sizes and finishes. It doesn't matter much as they aren't visible from the exterior.
• Box of wood screws (1.5"x8 CSK or equivalent)
• Pair of good quality car audio loudspeakers (~4 ohm impedance) with long blackened chipboard fixing screws (usually supplied with the speakers) and twin-core speaker leads
• Generic 50 W class D Bluetooth amplifier module (mine is a Sanwu). Make sure you get one with an Auxiliary 3.5 mm socket and 12 VDC power input, and that it can drive the impedance of your selected speakers.
• LM4610 tone board module (which should come with potentiometer knobs and ideally all pots on wired pigtails)
• Hook up wire (red and black for power - check current handling, Arduino-style ribbon wire with push-fit connectors for wiring up tone-board controls if needed)
• Miniature toggle switch and 10nF capacitor (unpolarized) for 3D sound function (optional)
• Heat-shrink tubing
• 2 off Stereo 3.5 mm plug to plug cable ~0.5 m long
• Either a 3.5 mm stereo socket (if you like soldering) or a 3.5 mm stereo extension lead with a threaded socket on one end
• Brick-type switched mode 12 V 3 A DC PSU with 2.5 mm plug
• Four casters which can collectively handle the weight of the heaviest person likely to sit on the bench seat
• Recessed steel panel for controls with round-headed blackened fixing screws
• Decorators' caulk
• Heavy-duty broom handle
• White one-coat gloss
• White screw-head covers
• Upholstered bench-seat cushion - I don't cover the fabrication of this because I had it made for me - you can probably get a suitable custom cushion from a motor-caravan supplier or someone like that.
• 2 or 3 ferrite snap-fit cores with ~10 mm inside diameter

Step 1: Planning Out the MDF Usage

A key aspect of this project was to ensure that I wasn't wasting money on materials and that I could transport them easily. So I didn't want to buy an 8'x4' sheet of 18 mm MDF that I didn't need. Careful planning was therefore essential.

It turned out that I was able to do the whole job in a 620 x 2440 mm sheet of MDF (2' x 8' roughly) which were available pre-cut from my local Wickes. I played around with a few ideas before settling on a shallow 15 degree angle for each speaker - this was to give a modest enhancement of stereo separation through reflection off walls without losing the direct treble frequencies, whilst also keeping the speakers recessed to prevent damage to the grilles when the box was pushed against a wall for storage.

The plan is attached here. (Disregard the trapezoidal layout on the bottom left - this was an also-ran. I wanted to maximise stereo separation in a relatively small enclosure but worried that the more directional treble frequencies would not have a direct path to a listener located on the centre-line). There is a divider shown in the plan, to separate the enclosure into two halves to make two isolated infinite baffles. I didn't use this in the end as I was happy with the sound and it would have complicated the installation of the control panel.

The MDF was cut to the measurements shown in the drawing. I used a hand-saw to create all the basic panels. When using a tee-square (recommended), I made sure to reference it to one of the supplier-cut edges to ensure squareness during marking out.

Step 2: Fitting the Battens and Joining the Panels

Firstly, I loosely assembled the MDF into the finished shape just to check overall appearance. It is worth marking the various pieces with pencil to identify them and where they go (but this is done on the inside surfaces where possible!). The oblique panels which will hold the drivers didn't fit properly at this point, since two of the edges needed 15 degree chamfers applied which we'll discuss in the next step.

Then I started fixing the panels properly. I used softwood batten and screws for this job (1.5" x 8 CSK worked for me). The batten was typically about 10 cm shorter than the panel being fixing to it, as space was needed at each end.

There are several ways to go about positioning and fixing the batten. I don't think it's possible to stick with one technique throughout the build unless you are happy to keep disassembling panels. Unless you've done this sort of thing before, the first one will be scary as you start chewing through your precious MDF, but trust me - you'll be a pro when you've finished this job!

Make sure you know exactly where the batten is going to go so you can mark and drill your holes correctly. Relying purely on measurement is a bit risky so don't rush any of this, and be prepared to dismantle the join frequently if necessary.

First assemble the two panels to be joined as accurately as possible, then place the batten by eye where you'd like it to go, leaving clearance at each end. Then clamp the batten using large spring clips or G-clamps so it won't move. Flip the board over to mark out the holes, using a ruler or tape measure to get the position right. Note that this will be on the external side so don't draw long lines or press too hard with your pencil!

Ideally drill through the MDF first and straight into the batten using a 'pilot' drill, that is, a drill small enough to guide the chosen screw but still allow it to bite strongly. It can go all the way through the batten if you want. Then separate the batten from the MDF and drill through the MDF with a clearance hole (e.g. 6 mm diameter). When you get really cocky towards the end of the job, you won't need to separate the batten as you will know roughly when to stop pushing the clearance drill through the work.

Alternatively, draw a line round the clamped batten prior to drilling, then remove it so you can mark out in the right place. Then drill the clearance holes first in the MDF, replace the batten and use the clearance holes as guides for the pilot holes. If drilling from the inside of a panel outwards, make sure you use a block of wood pressed flush against the external face where the drill will emerge - this will prevent unsightly spalling of the material as the drill bursts through.

I countersunk all external screw-holes in the MDF using a countersink bit. I used a second drill for this (an old Stanley hand-drill). It saves a lot of bit-changing.

I put a fixing in every 15 cm or so, using a tape measure or ruler as a guide. It's worth developing a consistent system for this so that all the screws match up on the different interfaces, otherwise the finished item will look a bit 'thrown together'.

Marking out the oblique speaker panels is a bit tricky because you haven't yet cut the chamfers on them. Here there is no alternative but to use the measurements from the drawing to help. I used an offcut of 18 mm MDF as a proxy for the actual panel and clamped it in the correct place, then pressed the batton against it.

Once a batten has been fixed to one panel, the second panel can just be positioned against it in the correct position and drill 4 mm holes through MDF and batten (you have remembered to mark out neatly haven't you?). Then remove the MDF panel and open up the holes in it with the 6 mm clearance drill. Countersink the holes then position the panel and secure with screws.

I didn't use any PVA glue in this build, as it needed to come apart several times to get the amplifier and tone board fitted properly. The screws are solid enough for the intended use.

Step 3: Chamfering the 15 Degree Speaker Panels and Side Panels

The MDF panels which will carry the loudspeakers will currently not fit with the other parts of the build. They are mounted at a 15 degree angle to the front of the box, so the vertical edges will require a 15 degree chamfer. The same is true of the front edges of the side panels, which butt against the rear of the speaker panels. Chamfering is relatively easily achieved using a standard jigsaw, most of which have an adjustable 'shoe' which allows you to change the cutting angle of the blade in 15 degree increments. Also required is the 'fence' which holds the blade at a fixed distance from the edge of the work. This makes it possible to cut a good chamfer the whole length of the panel.

Careful pre-cut fitting and marking out are critical at this stage. It is easy to cut the chamfer in the wrong direction. As there is little spare material in the original MDF panel, there is no scope for mistakes. Take things slowly and think about your cuts before you make them. Offer up the jigsaw to the work while it's in situ and think about how you will make the cut and what the result will be.

I originally only chamfered the verticals closest to the centre of the speaker box, thinking I would clean up the external edges later once I was sure of the fit. This operation actually proved quite tricky, because I didn't want to damage the adjacent surface of the mating side panel. I ended up using a combination of a hand saw and plane. I think unless I had access to machine tools to cut the MDF perfectly into panels, I would probably do the same thing again.

Step 4: Fit the Lid, Speakers and Internal Bracing

The original packaging for the 6"x9" loudspeakers was long gone, so I had to scour the web for something else I could use as a cutting-out template. There is plenty of stuff available but make sure that you scale it properly. In practice you may be using different speakers anyway. You don't want to cut out a hole then have the speaker fall through it!

I printed a correctly scaled template onto a piece of paper, which I then glued to a piece of thin card (e.g. cereal box) before cutting out. This was then used to mark the MDF with a pencil line. It's worth marking out the desired position of the speaker on the MDF panel before positioning the template. The key is to make both panels the same, as the eye is very good at discerning a mismatch in position and it might bug you if you get it wrong!

Once the speaker apertures were marked, I simply drilled out a large 10 mm hole in the central scrap portion and used this to insert the jigsaw to cut out the entire shape.

I offered up the speakers and marked the fixing holes onto the MDF and drilled these through with undersized pilot holes. I then used the original screws that came with the speakers to fix them directly to the MDF. Chipboard screws would probably serve just as well but the heads should be blackened somehow so they don't show up through the grilles. Finally I attached the wires to each speaker for testing.

At this point I did a test run on the fully assembled speaker box to test for sound quality (I lashed up the amplifier etc.) and found that some of the panels were resonating and giving a 'muddy', boomy sound. So I fitted timber bracing to the centres of the large internal panels. This timber was much thicker than the battens used to hold the panels together, and I also braced the front centre panel to the rear panel. The brace for the front central panel was too small the first time round so I made a slightly oversized one with chamfers at 15 degrees to allow the panels holding the speakers to be secured to it with screws. The bracing was made with random timber from the scrap-bin so there was plenty of scope for modifying things as I went along.

Step 5: Fit the Electronics, Interface Panel and Wadding

As part of my drive to reuse some old car stereo bits, I intended to make use of an Aiwa 200W stereo linear amplifier in this build. I had always known that this was a little bit hissy (and very big and power-hungry). Whilst researching other options I discovered cheap Class D amplifier modules with integral Bluetooth. Electronics has certainly moved on a lot in 30 years! It turned out that a 50 W class D amplifier seemed to be more than adequate for my needs.

I ordered two of these amplifier modules from separate suppliers from a well-known auction site. They both seemed to exhibit the same problem of a loud buzzing noise on one of the stereo channels. I found I could get rid of this completely by placing a wet finger on the Bluetooth antenna. This was obviously not a practical long-term solution, so I reasoned that if I hard-wired an 'equivalent circuit' for a human body to the Bluetooth antenna, it should get rid of the problem. In EMC work, a human is modelled as a simple series combination of a resistor and a capacitor. I found that a 1 kohm resistor between the antenna and the ground rail on the PCB was sufficient to eliminate the noise. This was carefully soldered in place and secured with blobs of hot-melt glue.

The amplifier module was connected to the tone board via a short lead with a 3.5 mm stereo plug at the amp end and a pig-tail at the other, which was spliced onto the output lead provided with the tone board. The easiest way to do this is to buy a short stereo 3.5 mm plug-to-plug cable and cut it, then strip back the wires. The tone boards are available from everyone's favourite auction site but can be quite expensive. I opted for the cheaper PCB with potentiometers fitted to the board, which I then needed to remove manually. I used Arduino-type hook-up leads and 2.54 mm header strips to remake the connections between the pots and the PCB, covering the soldered ends with heat-shrink tubing for insulation. If I were doing this again I'd get the version with the pots attached by flying leads and save all the hassle.

In my build, I decided not to bother with the balance and loudness functions as I couldn't see them being necessary. So I replaced the balance potentiometer with a fixed potential divider made from two 1kohm resistors. The loudness function is accessed by a jumper on a three-way header. I just left this in the off position.

The switched potentiometer that controlled the 'National 3D sound' feature had lots of connections to the PCB and I managed to damage it during removal. I referred to the datasheet for the LM4610 and discovered that I didn't really need a pot. So I replaced it with a simple on-off switch. The datasheet also advises the use of a 10 nF capacitor in series with the switch, so I had to break a track on the PCB and soldered an appropriate capacitor across the gap.

I used a recessed steel connector plate to hold the sockets and controls for the electronics. Six centres were marked in two rows of three then punched to locate a drill bit. The correct drill size will depend on the sort of connectors / controls being using. I don't try to cut the full diameter straight away, instead starting off with a small pilot drill of say 3-4 mm then open out the holes gradually. Securing the work is really important as steel sheet can snag on the drill bit and start spinning round (and slicing up soft things like nearby fingers). I used a block of wood that fitted neatly into the recess to 'catch' the drill as it breaks through onto the front side of the plate, otherwise the finish would have looked poor. I deburred the holds and mounted the connectors etc. in some sort of order which made sense to me. The connector plate was fitted on the centre panel between the two loudspeakers (another jigsaw job to cut out the hole) and secured with four colour-matched screws.

The 12 VDC power input socket was a standard 2.5 mm panel mount socket. These often have a special contact and a third terminal to cut battery power when an appliance is plugged in. It's quite hard to figure out which one is which. The best way is to connect a 12 VDC plug, power up the PSU and use a voltmeter to determine which pins are needed. The Aux in was a standard 3.5 mm stereo socket.

To mount the electronics inside the enclosure I used four off self-adhesive pcb standoffs per PCB. These were just perfect for the job, very quick and easy to use. I fitted the amplifier module to a side-wall close to the top of the enclosure for easy access and for air circulation.

Once the electronics units were fitted and wired in, In order to minimise any cabinet resonances I stuffed the interior with a couple of old, thin duvets that weren't used any more. It was very important to make sure that the stuffing did not come close to the amplifier module, as in principle this could get hot.

Step 6: Finishing

Finally I added a pair of round 'columns' to the vacant corners next to the speakers. These were made from heavy duty broom handle cut to length. These columns are handy as additional speaker protection and as grab-handles to make fine adjustments to the position of the projector when setting up. A single screw into end-grain (yuk!) at each end was sufficient to secure these.

Any small gaps in the joins between panels were filled using decorators' caulk from a mastic gun.

Once I was happy with the audio performance (and confident that the enclosure no longer needed to be opened up), I painted the entire enclosure (except underneath, where no-one looks) with one-coat white gloss. Ha ha.... three coats were needed for a decent finish, applied once every 24 hours or so. I wrapped the brush up tightly in a plastic bag between coats to save on cleaning effort. On the 'end-grain' of the MDF, quite a bit more paint is needed as this is highly absorbent. The loudspeakers and recessed connector panel were removed prior to painting to avoid unsightly brush-marks.

Once the paint was sufficiently dry, the loudspeakers, grilles and connector panel were refitted. Screw-head covers (of the type used with flat-pack furniture) were used to cover up the countersunk screw heads. Where these were recessed too deeply to get a purchase, I used a drop of hot-melt glue to secure the covers.

Four casters were fitted to the underside of the box. I chose the smallest caster I could find which could support at least around 25 kg, giving a load capacity of 100 kg. This felt like a reasonable number, as the box is relatively narrow so it's hard to imagine more than one adult sitting on it at once.

For the final touch, I acquired a foam-filled rectangular seat cushion with velcro fixtures on the underside. I won't describe how I made this as I didn't - it was custom made for the job by a third part. The seat is similar to those you find in motor-caravans etc. so this may be a potential source of this item. I used mating self-adhesive velcro strips on the top of the speaker box, fitting them first to the cushion before removing the protective film. Finally I flipped the cushion over and pressed it into place on the top of the box, thereby locating the velcro in the correct position on the box.

Step 7: In Operation

When setting up the system for operation, I ensure I make all the connections before applying power.

For use with the projector I routed the audio output on the projector to the aux input on the speaker box, using a 0.5 m stereo lead with 3.5 mm plugs on both ends. To minimise amplifier noise I found I needed a ferrite core at each end of the cable with four turns passing through it. It might be possible to get the same effect by installing these inside the enclosure immediately behind the Aux-in socket.

In order to power the speaker box I needed a 12 VDC power supply with low noise. Very low-cost PSUs generate a lot of ripple which comes through on the amplifier as a buzzing noise. Fortunately I had a large selection of PSUs lying around (I never throw these out) and just found the best one. It may be possible to reduce the noise further by winding several turns of the power cable through a ferrite core. I'm not sure of the average power draw of the amplifier module but I found that a 3 A supply seems to operate it without any distortion. I probably wouldn't want to go below this rating.

Powering up the speaker box results in an excessively loud pair of beeps in rapid succession as the amplifier board starts up. I don't know why they program them like this...

It should be clear from the schematic shown previously that the tone board and thus the controls only operate on the Aux-in signal (from e.g. the projector). Connections direct from Bluetooth devices bypass this completely. I didn't consider this a problem as the Bluetooth was always a secondary feature and most Bluetooth speakers lack this kind of control anyway. The amplifer board also has an array of buttons (volume, next track etc.) which I haven't bothered to break out at all, but to do so shouldn't be difficult if desired.

I found very quickly that I had to change the audio settings of my video content to 'PCM Stereo'. Most stuff now seems to default to 5.1 Surround, which sounds really dire from a pair of stereo speakers. I also selected flat equalizer settings. This system has very solid bass response so driving it with a 'cinema' or 'game' type equalization gives an overly boomy, indistinct sound in which it's hard to hear dialogue. With completely flat settings and standard stereo signal the sound quality is to my mind excellent, with good frequency response and clear dialogue on soundtracks.

Flicking the 'National 3D sound' (aka stereo wide) switch considerably enhances the perceived width of the sound field, but seems to accentuate the higher frequencies (e.g. tinkling glass) at the expense of mid-range such as dialogue. I found it works very well on some content and not so well on other content - take your pick.

If I want to connect a Bluetooth source, I just search for a 'Sanwu' device (the make of my amplifier). I noticed that the ampifier makes a faint buzzing noise with periodic pips when no Bluetooth or auxiliary source is connected. These are very quiet and don't detract from the overall enjoyment of the system.

Anyway, that's it. I hope you found it interesting and useful. If you have a go at building one, please post a picture on 'I made it'.