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Open Baffle Active Speaker


This is the 'non-wireless' version of my Open Baffle Bluetooth Speaker project. It takes its input instead from a standard line level jack socket, which could of course be an external Bluetooth receiver if desired. The amplifier schematic remains unchanged but the veroboard layout has been improved





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, creating 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. The schematic and veroboard layout are shown below





Incidentally, the feedback resistors were chosen not just to give the amplifier an ideal gain of 22, but also to give it a high input impedance like an old valve (tube) amp. There's nothing quite like that old fashioned buzz when you touch the input!





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. I used a heavy duty metal 2.1mm DC power socket 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 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 photo 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




The finished open baffle active speaker

The design concept

Open baffle loudspeakers have always fascinated me but I never really experimented with them
much in case I found the results disappointing, yet at some point I still wanted to build one. 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 keep that concept by kind of mimicking the old Murphy with its upright stance and Art Deco'ish look but it's a matter of opinion whether I succeeded!

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!