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Electronic Building Blocks By Julian Edgar Quick and easy construction Great results on a low budget Your new laptop speakers Make these fun-looking speakers for nearly no cost – and learn about speaker building at the same time. This is an ideal practical audio project for an electronics beginner or for someone with no experience of building speakers. T hese DIY amplified speakers sound incredibly good for their price. Based on a Banggood kit, most of the bits will cost you just £5 to £8, depending on whether they’re on special (search online for: ‘EQKIT Mini Speaker DIY Kit USB Power Amplifier Wire Control Small Speaker DIY Speaker Parts’ kit). And that price includes delivery! You get a tiny but effective audio amplifier (complete with volume control), a 5V USB power cable, 3.5mm stereo plug and two 50mm speaker drivers. Add some parts from the local craft shop and you have a fully fledged sound system that can be plugged into your laptop or phone. The kit The Banggood kit is great value for money – especially given the quality of the result. (Note that there is quite a range of ‘kits’ available online, so do ensure that yours matches the one shown on the next page.) Initially, it looks like the electronics comprises just a few cables with an inline volume control. But that ‘volume control’ is also an audio amplifier that uses the LTK 5206 chip. Connecting to the amplifier is a USB cable (providing power) and the jack to plug into the audio source. Coming out the other side of the amplifier are two leads with bared ends – the speaker cables. The data sheet for the LTK 5206 amplifier appears to be available only in Chinese, but the specification graphs and tables are shown with English annotation. On a 5V supply, peak power of the chip is 3.2W per channel into a 4Ω load – but that’s at 10% distortion. However, at 1W output power, distortion is only 0.05% and at 2W, only 0.2%. Driven from an iPhone headphone output set to full volume, and with the audio amplifier also at full volume, distortion was minimal – so with the amplifier driven in this way, a maximum output is likely only a very few watts – plenty loud enough when you’re as close to the speakers as a typical laptop user would be. The speaker drivers each have a nominal diameter of 50mm and use a generous roll surround. The cone comprises what appears to be an aluminium diaphragm. The magnet is large – these are nothing like cheap 2-inch transistor radio speakers of the past. Designing the speaker enclosures The finished system – the amplifier and volume control are contained in the small black ellipsodal object in front of the laptop. Practical Electronics | March | 2022 If you wire up the speaker cables to the bare (boxless) drivers, connect an audio input (eg, your phone or laptop) 63 The tiny audio amplifier uses the LTK 5206 chip and is capable of 2W per channel output at low distortion. Note the volume dial underneath the PCB. and power up the system, it will sound terrible – tinny and thin. But why? What happens with bare drivers is that when the speaker cone moves forward, rather than creating an effective pressure wave in the air, the air simply flows around the edges of the driver’s frame to fill the rarefaction (low pressure) created behind the driver. When the cone moves in the other direction, so does the airflow. To create good quality sound, especially at low frequencies The kit comprises two small speakers, an audio amplifier with (bass), the front and rear pressure waves need to be separated volume control, a USB power supply cable and a 3.5mm stereo plug to connect to the audio source. (Image courtesy of Banggood) so that this cancelling effect cannot occur. The easiest way to do this is to place the driver in a sealed box. (Note that if the phase of the rear sound waves can be reversed, they can then be used to reinforce the front sound waves. This phase change is effected by using a tuned port enclosure.) Testing of the speaker drivers was carried out using Smith and Larsen’s Woofer Tester 2. This combination of hardware and software allows the measurement of what are called the ‘Thiele-Small’ specifications. These are much more complex specifications than power handling and impedance – often the only two specs supplied with low-cost speakers like these. Having the measured Thiele-Small speaker parameters available then allows different enclosure designs to be modelled. I modelled ported and sealed enclosure designs. This modelling showed that a sealed enclosure was likely to give much better results than a ported design. Fortunately, this makes it easier to construct a suitable enclosure. In fact, a sealed enclosure with an internal volume anywhere from about 0.5 – 1 litre appeared to work well. The modelled frequency response showed a lift from about 800Hz downwards, peaking at about 10dB at just over 200Hz. Now, if you were designing Hi-Fi speakers, you’d never want this – you’d have boomy onenote bass. And not very deep bass at that. However, with such small Woofer Tester 2 was used to measure the driver specifications and then model different drivers, bass response was always enclosures. Here is the predicted frequency response of the 0.55-litre enclosure used in the going to be a struggle and so a lift final design. Note the rise in response around 200Hz, giving increased bass from what is a in output here is, in practical terms, likely to give a more natural sound. very small speaker system. 64 Practical Electronics | March | 2022 Given that the modelled response varied little with enclosure volumes from 0.5 to 1 litres, the smaller enclosure volume was selected, so giving a reduced desk footprint. a) d) b) e) Choosing the enclosures If you’ve ever tried to make speaker enclosures, you’ll know it’s a lot of work – and very small enclosures are even trickier because they’re viewed in closeup most of the time. So, in this project I didn’t make the speaker enclosures. Instead, I went to the local craft and cheapo stores (aka dollar/pound stores, penny shops) to find something prebuilt that would be suitable. The criteria were: n Internal volume approx 0.5 litres (~50% bigger than a soft drinks can) n A flat face that’s easy to cut, allowing a hole for the speaker driver n Resonantly ‘dead’ (when tapped it didn’t ring) n Surface/texture that’s easily sealed n Cheap! The most suitable item proved to be a bamboo open-ended wooden box – a bit like a large square drinking mug. Sold as a cutlery holder, it had external dimensions of 90 x 90 x 120mm. Internal volume was 0.55 litres and the cost was just £2 each. The bottom panel was recessed by 5mm, making it the ideal location to mount the driver. The other (open) end could be easily sealed with a square piece of plywood cut to size and glued in place. Other items I looked at that would be suitable included toothbrush mugs made from resin, the large cups in which toilet brushes sit, unpainted ‘craft’ boxes and wooden pencil holders. c) Building the speaker enclosures The hole in the base panel was cut first. The hole needs to be exactly 50mm in diameter – this is important as too small a hole and the driver won’t sit flush, and too large a hole and there will be gaps around the frame and it will be hard to screw the driver into place. I cut the hole with a powered holesaw but if you don’t have one of these, take the following approach. First, accurately mark the circle, then drill many closely spaced small holes around the inside of the circle until the middle piece can be pushed out. Use a file and sandpaper on the jagged edge to fine tune the final shape of the hole. The new rear panel can be cut from any acoustically dead, stiff material. I used plywood that was 7mm thick. This panel should fit within the box, sitting flush with the end. It can be hard to precisely cut such a piece, but in fact it doesn’t have to be accurately sized – any small gaps can be filled by glue when you permanently attach it. (But at this stage don’t glue the Practical Electronics | March | 2022 a) The speaker enclosures were made from these bamboo boxes. They are sold as cutlery holders and cost only £2 each. Any similar box with an internal volume of about half a litre can be used. b) The boxes featured a recessed bottom panel – the speaker driver was placed here. c) The hole for the speaker was cut with a holesaw. Note the pilot holes for the speaker securing screws. d) A new bottom panel was made from 7mm-thick plywood. The small hole is for the speaker cable. e) Filling the enclosures with fluffy polyester material noticeably improves the quality of the sound. panel into place). Drill a small hole in this panel for the speaker cable. Initial tests The next step is to do some initial tests. Feed the speaker cables through the bottom panel holes and then the main speaker openings, soldering the leads to the speakers. Note the required polarity – in each case, the black wires are negative. Screw the speakers into place – you’ll need to supply some small self-tapping screws and you should first drill pilot holes to avoid the wood splintering. Just push the rear panel into place for this testing. Provide USB power and an audio source and listen very carefully to how the system now sounds. The audio quality should be radically better than 65 much greater audio output. You will hear a resonant peak around 200Hz (as predicted in the modelling) but listen for other odd peaks. For example, when I did this test, I found the speaker frame vibrating fiercely at one frequency. Slightly loosening the nearest mounting screw fixed that. If your enclosure’s rear panel is too loose, you may well hear air leaks past the gaps – a good demonstration of why in the final build, a sealed enclosure does need to be completely sealed. Finishing off and tweaks The speaker drivers are 50mm in diameter. They use roll surrounds and what ‘appear’ to be aluminium cones. Driver impedance is 4Ω impedance, and they’re rated at 3W. when you listened with the speaker drivers bare on the bench. In fact, I’d be surprised if you weren’t rather amazed at how good this incredibly cheap system already sounds. But – and this also depends on the enclosure materials – you may hear a ‘hollow ringing’, a bit like the speaker is working in a concrete pipe, but not that bad. This is due to the sound waves inside the enclosure bouncing back and forth and even coming back out through the cone of the speaker. To absorb these reflecting vibrations, we need to place some fluffy material inside the enclosure. Polyester ‘wool’ is cheap and readily available – it’s sold for use inside stuffed toys, quilts and even for use in aquarium filters. Some people may have access to real sheep’s wool – you can definitely use that too. Basically, anything that resembles this material can be used – even a small piece of fibreglass insulation. Place this material inside the enclosures – just enough to loosely fill them, but don’t stuff the enclosures tightly. Listen again and you should hear an immediate improvement in the quality of the sound. Tone generation I’ll now assume that you are using your PC or phone as the audio source. Access a web page or download an app that gives you an audio frequency generator – there are plenty available. For example, the Tone app for the iPhone, or use these more general web-based ones on your PC: https://onlinetonegenerator.com or www.szynalski.com/tone-generator These will allow you to generate tones from 20Hz – 20kHz, sweeping through them in a sound that varies from the deepest bass to the highest treble. Now test the speakers with such a frequency sweep. Ensure that you keep the volume low – it’s easy to damage a speaker with over-loud sinewave signals (read and note the excellent advice on the szynalski.com web page). When doing the frequency sweep, listen for resonances. A resonance is a frequency at which the system, for a constant electrical power input, has a Once the resonances are fixed, you can now glue the rear panel into place and seal the hole through which the speaker cable passes. Some rubber feet stuck to the bottom of the enclosures will prevent scratching of furniture surfaces and give clearance for the cable. At this stage you may well be quite happy with the sound – and that’s fine. But I wondered if two further improvements couldn’t be made. The first was that the treble was just a bit too strong – often picked out as singer’s sibilance on some tracks. The other was that I thought it would be good to give the speakers less directionality – that is, make them more omni-directional. Back to the craft shop I went to pick up something I’d seen earlier – some low-cost polystyrene balls of varying diameters (£2 the lot). And, having a pretty good idea of what I wanted to do with them, some small diameter, coloured dowels (£1 a pack). I then oriented the speaker enclosures vertically and held the different diameter polystyrene balls above the drivers while listening to the differing frequency responses. Held too close, the treble and ‘presence’ decreased too much; held too far away there was no difference in the sound. But positioned at just the right distance (about 20mm from the cone to the underside of the ball) the 60mm diameter balls reduced the sibilance without degrading other aspects of the response. I then sharpened the dowels and pushed them into the balls until the Want to take your speaker designs to the next level? A free audio frequency generator app can be used to test the speakers. It’s particularly effective in finding unwanted resonances – for example, those caused by a loose speaker. This iPhone app is called Tone. 66 In our May 2017 issue, Julian Edgar reviewed the Smith and Larsen Audio Woofer Tester 2 hardware and software package mentioned in this article. If you want to design, tweak and experiment with all aspects of speaker design then this is a great product to consider. EPE product review Smith & Larson Audio Woofer Tester 2 by Julian Edgar Find speaker specs in minutes! Directly measures Thiele Small speaker specifications, and design effective speaker enclosures for drivers that don’t normally have proper specs available. I f you’re into sound systems, you’ll be well aware of the famed Thiele Small speaker parameters that are especially important when designing woofers and subwoofers. These parameters are the speaker specs that you plug into software (or an on-line calculator) to allow you to design the speaker box. That box design includes aspects such as internal volume and length, and the diameter of any ports. Without the Thiele Small (abbreviated to TS) specs of the driver, you’re just guessing the box design – and the chances are overwhelming that your guess will be less than optimal. So to design a good speaker enclosure, the TS specs are needed. That’s fine if you’re buying a new driver. But what if you’ve sourced a speaker that is literally an unknown? – for example, a secondhand driver? In that case, to get best results, the driver’s Thiele Small specs need to be measured. Woofer Tester 2 Woofer Tester 2 is a complete speaker test unit. This incredible piece of hardware plugs into the USB port of a PC or laptop and connects straight to the speaker under test. Open the software, press a button and within minutes many of the TS specs are measured. Do some more testing (eg, weighting the cone a known amount) and the rest of the important specs are there in front of you – it’s that easy! Furthermore, the Woofer Tester 2 also includes a box design tool so you can develop the enclosure without Fig.2. Directly measuring Thiele Small parameters allows you to exiting the package. use high-quality drivers (like these Alpine car sound speakers) And here’s the in custom-designed Hi-Fi enclosures. cherry on the cake – once the box is built, piece (ie vent hole in the magnet) this you can use the Woofer Tester 2 to test the is not blocked. speaker mounted in its enclosure, seeing In the software, the ‘WT control’ is if the measured performance matches opened from the View tab. The ‘Q, Fs’ test the prediction. (Woofer Tester does not button is then pressed, and the speaker test include a microphone, so you cannot starts automatically. The impedance (Re) directly measure frequency response – of the speaker is measured and displayed, but indirectly you can get a good idea of then the resonant frequency (Fs) and total what is happening, for example through Q (Qts) are ascertained. (Other factors are the impedance plot.) also measured, but I am trying to keep this At US$160, Woofer Tester 2 is not dirt simple.) This element of the test procedure cheap – but if you buy second-hand takes a few minutes, during which you can speakers, you need to use it only a few watch the impedance plot for the speaker times to save that amount. (Of course, developing on the screen in front of you. if you are using new drivers, you can So, for example, the measured specs of a also check to see if a manufacturer’s good 5-inch woofer might be: specification for an individual speaker l Re = 5.35Ω is correct – there is often some variation l Fs = 68.2Hz in the build from driver to driver.) l Qts = 0.48 At its most complex, the Woofer Tester 2 has plenty of in-depth capability. But The next step is to press the ‘Vas’ test at its simplest (as we’ll look at here), it’s button. A dialog box pops up that asks easy to get up and running. you to add a weight to the upwards-facing cone. I live in Australia, so I use two Testing a driver Australian $2 coins that have a mass of Woofer Tester 2 is used in the following 6.6g each (a current UK 10p coin weighs manner. The software is installed and then 6.5g and is a little wider and thinner). the module calibrated using the provided The added weight is therefore 13.2g. The test resistor. The provided alligatoreffective diameter of the speaker cone clip leads are then used to connect the is also manually measured (callipers or hardware to the speaker under test. a ruler) and then this figure is entered. The speaker is placed on its back, The compliance (Vas) is measured by Practical Electronics | March | 2022 Fig.1. Woofer Tester 2 comprises a hardware and software solution for quickly and easily measuring the Thiele Small parameters of loudspeaker drivers. The software also The speakers use a foam sphere positioned a critical distance above the speaker driver. This gives better sound dispersal and helps absorb some of the treble that can otherewise be peaky. balls were supported at the right height. And, with the balls at this height, the directionality of the speakers was also much reduced – the sound now better fills the room, rather than apparently coming from two tiny speakers. I don’t think the balls make a dramatic difference – it’s perhaps 10-15% of the final sound – so they’re very much an option, but a very cheap option at that (And the look kinda cool!) Note that I chose to leave the speakers in the colours the parts came in, but you could of course paint each component – identical colours or a variety. The results No one who listened to the final system could believe the results for the amount of money that I spent – and that includes people who have worked on audiophile-level, exotic speakers. For a desk-top system there’s plenty of volume and the speakers work surprisingly well in reproducing intricate details. If you want lots of bass, add a subwoofer (or even just a woofer) that works from 200Hz downwards. But if you just want quiet music while you’re working, or to be able to watch YouTube videos while still understanding poorly recorded audio that laptop speakers make unintelligible, this system will do it. And, if you’re venturing into home speaker building, this is the cheapest toe-in-the-water I’ve ever seen. It’s an excellent way to experiment and learn about the basics of speaker construction. You can tweak and experiment, safe in the knowledge that you’re not risking expensive parts or days of fine woodwork. ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. 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