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Open Baffle Bluetooth Speaker


Open baffle loudspeakers intrigue me but I haven't really experimented much with them in case the results were disappointing, yet something in me still wanted to make one anyway. I had also been wanting to make a Bluetooth speaker at some point too, so thought why not combine the two ideas and build an open baffle Bluetooth speaker! 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 on a visit to Bletchley Park. It was in the post office there and it 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





Back then a 'Wireless' set was the focal point of a room... your go-to source of entertainment. I wanted to recreate that concept but in a modern way. So this speaker kind of mimics the old Murphy with its upright stance and top mounted control panel, but as the source is now your phone (or any other Bluetooth device), the control panel is simply a push button and a Blue (of course) LED. That's progress! I tried to make it look a bit Art Deco'ish too but that's a matter of opinion

The Bluetooth


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 think it sounds pretty good and so would still recommend it if 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 Amplifier


For the amplifier I used the trusty old TDA2030A audio IC which can provide over 10 watts of power (depending on its voltage supply) allowing quite high volume levels to be reached... and it sounds great too! The n47 capacitor and 100K resistor in series at the input were added to give a bit of high frequency boost because I prefer some sparkle to the sound but are optional and can be omitted. 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 speaker side of things. The schematic and veroboard layout are shown below along with the finished module








The module is powered 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. The 'Power On' and 'Bluetooth Paired' LEDs are removed from the Bluetooth 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 Speaker


The structure itself is made from 12 x 607 x 1220mm MDF sheet (Wickes), the parts being cut out with a jigsaw. I should have used 18mm thick sheet but when I started, everything was experimental and I didn't want to spend too much money in case it turned out to be a 'dead' project! As it happened I'm pleased with the result so 12mm was actually OK. The dimensions of the large baffle are 607mm (the width of the sheet) x 580mm, with the top corners rounded off. The White board that holds the drivers is a 250mm x 600mm '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 capacitor sounded the best. 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


            


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. Well if you have a jigsaw you may as well use it!




The finished Bluetooth open baffle loudspeaker

So how does it sound?

Open baffle speakers 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 needed to go out!