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Open Baffle Bluetooth Speaker






I'm fascinated by open baffle loudspeakers but until now hadn't really experimented much with them in case I was disappointed with the results. Inspiration can come from the most unlikely places and the design of this speaker is a modern take on an old radio set that I came across while visiting Bletchley Park. Displayed in the post office was a Murphy A146C (also known as the Batwing) which I thought was simply stunning, although it would probably look out of place in most homes today. Years ago a 'Wireless' set was the focal point of a room and I wanted to recreate that concept by kind of mimicking the old Murphy with its upright stance and Art Deco'ish look, but I suppose it's a matter of opinion whether I succeeded or not! There are two versions described here, Bluetooth and non Bluetooth. The non Bluetooth version takes its input from a standard line level jack socket, which could of course be an external Bluetooth receiver if desired. The 'body' of the loudspeaker itself remains exactly the same for both versions


The Amplifier




For the amplifier I used the well proven TDA2030A audio IC which can provide over 10 watts of power (depending on its voltage supply) allowing quite high volume levels to be achieved with excellent quality. The 100K resistor and 470p capacitor in series at the input were added to give some high frequency lift for a more detailed sound. Power is provided by a 24 volt 3 amp 'brick' type mains adapter purchased from eBay. Although the circuit will run from 12 volts, the audio power will be significantly reduced. The upper limit is about 30 volts but I never feel happy running things too near their maximum and so 24 volts it is. Before I even made the baffle or purchased any drive units, the amplifier was auditioned on conventional stand mounted speakers (Q Acoustics) which I am very familiar with. I wanted to make sure the electronics was up to scratch before experimenting with the actual speaker hardware. The schematic and veroboard layout are shown above and below respectively. You may notice that the veroboard for the Bluetooth version looks different to the active version. The active version is the later design of the two and has an improved layout. The schematic is the same for both





The Bluetooth Version


The Bluetooth receiver is a Bayan Streamport Universal, although this isn't critical so long as whatever used is of acceptable quality. I already had the Bayan but it was sitting in a drawer slightly unloved, so I decided to put it to good use here. It has varying reviews, mainly about connectivity, but I've had no issues myself and so would definately recommend this excellent sounding device if still available. The case is not required here and is discarded leaving just the PCB which needs to be modified as described below





Most receivers of this type are powered from 5 volts usually derived from a USB socket and are stereo. This project requires a receiver that can run from 24 volts (the amplifier supply) and is mono. All the sockets (USB power input, 3.5mm and phono audio outputs) are removed and replaced with 2 way PCB mounted terminal blocks for just power in and mono out. 24 volt operation is achieved by adding a 5 volt regulator at the power input and mono is achieved using two combining resistors at the audio output, all added to the underside of the PCB as shown in the photo above. Schematics of the regulator and mixer are shown below








The 'Power On' and 'Bluetooth Paired' LEDs are removed from the PCB along with the single 'Power/Pairing' tact switch and replaced with 2 way Molex KK connectors which feed up to the control panel (made from a double surface box). I decided not to use the power LED and have just the one Bluetooth LED on its own along with a momentary push button for power and pairing





The Non Bluetooth Version


As this speaker is mono and most audio sources are stereo, conversion to mono is required. A 3.5mm switched stereo jack socket is used for the input. Two 680Ω resistors are connected as shown in the image below to create a simple stereo to mono mixer for the single channel amplifier input. The earth tag is connected to one of the switch contacts so that when there is no jack plugged in the input is grounded. This effectively mutes the amplifier when it's unplugged. A screened cable is used for the audio feed but the screen is only connected at the socket end, the other end being left open. For neatness, the final assembly is covered in heatshrink sleeving


                   


The amplifier is mounted onto its aluminium heatsink as shown below. It's always a good idea to put a fuse in line with the supply just as a safety precaution in case of a short circuit, and a screw terminal block type is used here (F1A fuse fitted). For simplicity and neatness I wanted the least wiring possible, and so decided to use the tab of the IC for the ground connection as this is connected to the centre pin anyway. I pondered a lot whether this was a good idea but in the end decided to go with it as it is the perfect solution. A heavy duty 2.1mm DC socket was used for the power input which grounds to chassis through its body as does the audio input socket through its mounting nut. I have not experienced any issues with the layout and considering this is an open chassis amplifier with a high impedance input, there is no discernable hum whatsoever. Job done!




The Speaker


The structure itself is made from 12mm x 607mm x 1220mm MDF sheet (Wickes), the parts being cut out with a jigsaw. The dimensions of the large baffle are W607mm (the width of the sheet) x H580mm, with the top corners rounded off. The White board that holds the drivers is a W250mm x H600mm 'finished on all sides' shelf from B&Q. I used an 8" woofer from Maplin (pt.no. L74AW) and a Monacor DTM-104/8 tweeter. I had intended to use a ready made crossover but after listening tests, decided that a simple 4.7uF bipolar capacitor sounded better. I thought it would look good to have the Black drive units completely contrast with their White background and so made sure to use Black 40mm M4 bolts so they would blend in. After cutting out holes for the drivers, the White board can be used as a template to cut out corresponding holes in the main panel. Rather than making detailed drawings it's probably easier to refer to the photos below


            

The Bluetooth speaker




The active speaker


MDF absorbs paint like a sponge and so needs to be sealed with a primer (Wilkinsons). Using a roller, the main baffle can be painted any colour you like (I used Dulux 'Redcurrect Glory' to match my sofa) and when dry, the White speaker board can then be fitted. I didn't paint the rear but did make an effort to keep the edges tidy. Stability of the structure is achieved by a single centrally mounted rear stand with cut outs for the bass driver and a carry handle




The finished speaker

How does it sound?

Open baffle loudspeakers have a very different sound character to infinite baffle (boxed) speakers. They produce less bass considering they're generally larger, but have a very open and transparent presentation. Basically, the purpose of the baffle is to stop sound from the rear cancelling out sound from the front (especially the lower frequencies) by effectively creating an isolation barrier. The bigger it is the better it performs, but in a domestic environment there has to be limits! More bass can be achieved by positioning near a corner of the room and also by adding some equalisation to the music source. The speaker here is not an all rounder as indeed not many speakers are, but when it hits the 'sweet spot' it really does produce an exceptionally realistic performance, particularly with jazz, vocal and acoustic instruments. The speaker is quite efficient and the sound just seems to fill the room space. I've found myself wanting to stay and listen when I really should be doing other things!