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Salvage It! BY JULIAN EDGAR Improving the sound of salvaged speakers Looking to buy – or scrounge – a secondhand speaker system? There are plenty of bargains around and you can often improve their performance for very little outlay. One area of consumer electronics that hasn’t fundamentally changed over the last 30 years is the design and manufacture of speaker systems. Whether they were originally connected to a record player, tuner, cassette deck or CD player, all boxed speakers use much the same technology. This means that the speakers you can now pick up at garage sales, the tip or secondhand are still very useful, no matter what music source you’re using. But nothing sounds worse than a really horrible speaker, so why bother sourcing cheap or no-cost discards? There are two main reasons: first, there are some very good speakers out there just waiting to be found and second, if you have a half-reasonable starting point, it’s not hard to make some major improvements for very little extra money. This pair of speakers was picked up at a local Salvation Army thrift shop for $10. Buying speakers In most cases, you won’t have a chance to listen to a speaker that you’re collecting, so how do you make any judgements as to how good it will Inside each box was a decent small woofer and cone-type tweeter with a single capacitor crossover. 68  Silicon Chip sound? Here are some buying points: (1) Pick them up and feel their weight. In nearly every case, heavier means better. (2) Detach the grille and inspect the cones. The roll suspensions should be intact and you should be able to manually move the bass driver back and forth without any binding (or interference) between the voice coil and the dust cap. Be wary if you cannot detach the grille. (3) Either a ported or non-ported design is fine but in the case of ported speakers, the port diameter should be large enough to ensure that whistling or “chuffing” noises do not occur. In other words, a tiny port diameter with a large diameter woofer isn’t a good sign. Very large diameter (but short) ports are also unlikely to be indicative of a good design, as they’ll be tuned to a high box resonant frequency. (4) Check the brand and any labelled specifications (eg, impedance and power handling). Often the specifications aren’t very trustworthy but the siliconchip.com.au better the brand, the more the figures can be believed. (5) Make sure that you will be able to later open up the enclosure, either by unscrewing the drivers or by detaching the back. (6) Assess the condition of the boxes. Making improvements Once home, the first step is to listen to your newly acquired purchases. Hmm, sound pretty bad? But what specifically is bad? Is the treble overbright? Is the treble dull? Is the bass lacking, or perhaps all one-note? Try the speakers on voice as well as on different sorts of music. In fact, listening to the human voice is surprisingly good way of assessing the mid-range response. In addition, PC frequency generator software is freely available on the web and it’s well worth downloading a suitable program. This can then be used to drive your amplifier and newly-acquired speakers across a range of input frequencies. If the speakers sound absolutely awful, just chalk the episode down to experience and go find some more! But if they have potential, there’s plenty you can do to improve their performance without much outlay. Here are some of the problems you might find and what you can do about them: (1) Problem: Over-Bright Treble Cure: install a resistor in the feed to the tweeter. Just try some different value resistors and you’ll soon get a feel for the changes that can be made. An 8.2W 1-watt resistor is a good place to start. (2) Problem: Poor Treble Cure: replace the tweeter. Unless you fluke a direct drop-in replacement, this is often most easily achieved by cutting another hole in the baffle and installing the tweeter in a new spot. The old tweeter can then just be electrically bypassed. If the grille cloth is dense and the treble improves with the grilles off, replace the cloth with a design that is more open-weave. (Just go to a dressmaking shop and buy black open-weave scrim fabric that’s easy to see through when stretched.) (3) Problem: Coloured Midrange Cure: in non-ported designs, place a loose fold of quilt wadding (or fibreglass insulation) inside the box. Aim to fill about 75% of the volume. Alternatively, in ported designs, staple a thin layer of quilt wadding to the internal panels, making sure you don’t block the port. As with grille cloth, quilt wadding is available very cheaply at dressmaking supply shops. (4) Problem: Poor Bass Cure: in non-ported designs, fill threequarters of the box with quilt wadding, as described above. Also, when the speakers are working hard, use a moistened finger to check for air leaks, especially around the terminal block and the edges of the woofer. In ported designs, try changing the length of the port. Place a rolled-up cylinder of thin cardboard in the port and move it back and forth within the port to effectively lengthen the port by different amounts. Use the frequency generator software and your PC and make lots of listening tests. The aim here is to reduce any bass resonant peaks – say, over the range from 30–150Hz. In most cases, the port will be too short rather than too long. When you have found the right length, glue the cardboard in place. The woofer and cone-type tweeter are mounted on a front baffle which is easily removed. Note the rather odd port design and the large gap around the tweeter! Rat It Before You Chuck It! Whenever you throw away an old TV (or VCR or washing machine or dishwasher or printer) do you always think that surely there must be some good salvageable components inside? Well, this column is for you! (And it’s also for people without a lot of dough.) Each month we’ll use bits and pieces sourced from discards, sometimes in mini-projects and other times as an ideas smorgasbord. And you can contribute as well. If you have a use for specific parts which can siliconchip.com.au easily be salvaged from goods commonly being thrown away, we’d love to hear from you. Perhaps you use the pressure switch from a washing machine to control a pump. Or maybe you have a use for the highquality bearings from VCR heads. Or perhaps you’ve found how the guts of a cassette player can be easily turned into a metal detector. (Well, we made the last one up but you get the idea . . .) If you have some practical ideas, write in and tell us! Another oddity was the internal box fill, which was rolled into a cylinder and placed at one end of the box (in front of the port?). May 2006  69 A piece of scrap chipboard was used to close off the opening around the tweeter and the port. This was simply was screwed and glued into place. The tweeter was then re-installed from the front and the gap around its rear magnet assembly closed off with sealant. Some black spray paint concealed the blanking plate and the changed tweeter mounting. It’s easy to use a spray can to paint the insides of the new port black, so that no-one would ever know! Note that it’s no big deal if the port is lengthened so that it protrudes through the front grille – after all, several very well known speakers come like this as standard! (5) Problem: Speaker Overloads Cure: if the speaker is easily driven into bass distortion, fit a 200mF nonpolarised capacitor in series with it. This will reduce the amount of bass being fed to the speaker and is an ideal approach if you have other speakers in the system (eg, a subwoofer) to provide the required “bottom end”. This also works well if you’re using the newly-acquired speakers as extension speakers but still want the main speakers to be powered at high levels. Check out http://www.jaycar.com.au/ images_uploaded/crossovr.pdf for the crossover frequencies that various values capacitors give in systems with different impedances. (6) Problem: Cabinet Finish Cure: unless you’ve got yourself a really high-quality design, it’s usually not worthwhile spending hours improving The grille cloth was reinstalled and the baffle glued back into place. And the results? Comparing the modified and unmodified speakers showed a much more natural sound. All that remains is to paint the boxes and then these will be great for the garage or for the kids. the finish of dilapidated boxes. However, one quick and easy approach is to give the box a quick rub back (or if it’s a plastic finish, a wipe over) and then spray-paint the box flat black. It won’t come up with that famed “piano” finish but the poor surface will no longer stand out and the boxes will look quite neat! Finally, note that the sound that the speaker makes can be dramatically altered by its room placement. If they lack bass response, put them in the corners of the room. If the bass is strong and muddy, bring them out from the corners or even try raising them off the floor them on stands. Similarly, if the treble is muted, raise the speakers so that the tweeters are at ear level when you’re seated. Always try moving speakers around – if you haven’t done this before, you’ll be amazed at how much you can vary their sound. Conclusion More internal fill (based on old quilt wadding) was added to supplement the original fill, which was replaced more loosely in the enclosure. 70  Silicon Chip You don’t have to spend a fortune to get good sound from low-cost secondhand speakers, In fact, with just a little work, you can often get them to outperform many mini and midi-sized SC off-the-shelf systems. siliconchip.com.au