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CAR SOUND ON THE CHEAP Pt.2: By JULIAN EDGAR In Pt.1, we showed you how to buy a quality AM/FM/CD head unit plus some wide-range speakers that can be mounted in the doors or rear parcel shelf. This month, we look at buying an amplifier and subwoofer and describe the wiring options. I F YOU’RE RUNNING a subwoofer, you’ll need an amplifier. That’s because subwoofers require plenty of amplifier power – certainly more than a standard head unit can provide. A budget subwoofer amplifier should meet these criteria: (1) The amplifier should have a bridging facility – ie, an option to allow the outputs of both stereo channels to be summed (or bridged) to provide more power into a single channel. Most amplifiers are two-channel (ie, stereo) but a subwoofer amplifier need only be mono – bridging a stereo amplifier makes use of both channels to give a higher-power mono output (theoretically four times the power output of a single channel). Not all car stereo amplifiers are designed for bridging, so you need to find an amplifier that’s capable of this function. This is usually indicated by labelling adjacent to the speaker terminals, which indicates that the output of two channels can be used to drive one speaker (see photo). Don’t attempt to bridge the output of an amplifier that’s not designed for bridging, as this will destroy the output stages. This amplifier allows its outputs to be bridged (for much higher power into a single channel) and runs a 20A fuse (which is indicative of decent audio output capability). It’s important that you look for these features when selecting a secondhand amplifier. 22  Silicon Chip siliconchip.com.au This amplifier is grubby and still has some of its old wiring attached. However, with 4 x 20W RMS outputs and both line level and speaker level inputs, it’s ideal for running four wide-range (or woofer/tweeter combinations) if the head-unit’s amplifiers aren’t up to the job. (2) The amplifier should preferably feature an in-built crossover. You don’t want to feed high frequencies to a subwoofer and if the amplifier has an inbuilt low-pass filter, this saves you the extra cost of buying other bits. In addition, it’s preferable that the filter crossover frequency be adjustable, so that you can choose the optimum point at which the subwoofer begins to do its stuff. (3) A variable gain control is important as it allows you to adjust the loudness of the subwoofer relative to the rest of the system. (4) If the amplifier has the ability to sum the left and right signal inputs (ie, L + R), you’ll be able to easily develop a dual-channel bass output. If the two channels of the amplifier can be bridged, this summing will occur automatically. (5) Another control that’s useful is an amplifier bass-boost switch. Because this boost occurs only in the signal that’s fed to the subwoofer, the rest of the sound remains unaffected. This prevents the wide-range speakers from being overloaded, as would occur if you simply cranked up the bass control on the head unit. These criteria might look pretty extensive but there are plenty of secondhand amplifiers that meet all of them – and plenty that don’t! So siliconchip.com.au check carefully before plonking down your cash. Meaningless figures Don’t be sucked in by dubious pow­ er figures. Car sound amplifiers are notorious for having misinformation printed on them in large letters – things like “330 watts total power”. Quite often these figures are meaningless – even if they’re correct. For example, the power output quoted may be the Peak Music Power (rather than the RMS power) and the distortion at the advertised power output might make the music almost unrecognisable. It’s also possible that the claimed output could not be sustained for longer than a few seconds before the amplifier started overheating. Car sound amplifiers that have high outputs generate a lot of heat in what’s Table 1: Sample System Costs Equipment Origin Cost Philips RC604 CD AM/FM head unit Original Equipment $100 – Peugeot? Cash Converters, Runaway Bay, Queensland US Audio 552X amplifier Aftermarket Cash Converters, Runaway Bay, Queensland Mitsubishi 5-inch single cone speakers Original equipment $2 pair Shop at municipal tip, Coolangatta, Queensland 5-inch single cone speakers with 2-inch cone tweeters, including crossover capacitors Stereo TV Shop at municipal tip, Coolangatta, Queensland Dome tweeters, including crossover capacitors Original Equipment $10 pair Jaycar Electronics, Gold Coast, Queensland Toyota Soarer 10-inch subwoofer Original Equipment $40 Toyota wreck, Thomastown, Victoria $100 $2 pair Source Here’s the breakdown in the costs of the system that we assembled for this story. Each of the parts was bought for the prices shown at the outlets shown. As you can see, the cost of the parts is only $254, well below our $300 limit. Throw in some wire, perhaps some grilles if required, and you’ll still be under budget December 2005  23 should reflect the advertised power. An amplifier capable of developing high output power will normally have two 25A fuses – an amplifier with a 10A fuse simply won’t be able to develop the audio power you need. As with head units and speakers, always look for well-known, highquality brand names when purchasing an amplifier. Subwoofers This factory Toyota Soarer subwoofer is perfect for budget applications – it’s efficient, designed to run in a free-air configuration and cost just $40 from a wrecking yard. However, it has a 2-ohm impedance, which means that the amplifier used with it must be capable of driving 2-ohm loads. really a very small package. As a result, they need extensive heatsinking. All current amplifiers use a cast-alloy extrusion as the main body of the amplifier and this becomes the heatsink. Physically heavy amplifiers with multiple fins and perhaps fan cooling generally indicate a rugged, reliable design in which overheating is unlike- ly to be a problem. As with speakers, a heavy amplifier is likely to be a better design than a light amplifier. High-power amplifiers draw a lot of current. The positive and negative supply terminals should be suitablysized to take this into account and heavy-duty screw-down terminals are often used. In addition, the fuse rating Designing a high-quality subwoofer is an involved process. Two car subwoofer designs have been described in SILICON CHIP, both of which work very well and are far below the cost of many commercial equivalents. They were the Little Dynamite Subwoofer in March 2003 and the Big Blaster Subwoofer in May 2003. However, despite the effectiveness of those two designs, in the context of our budget system, they’re way too expensive (and in any case, the specified subwoofer is no longer available). These designs both used drivers with extensive specifications that were computer modelled over many hours in a variety of enclosures. However, if you’ve sourced a no-name (or even branded) woofer without specifications (resonant frequencies, Qts and all that stuff), how are you going to go about designing an enclosure? The answer is that you don’t. Instead, you simply place the woofer in the largest sealed box that you can. In a sedan, you use the whole boot (ie, the woofer is mounted behind the rear seat or in the rear deck), while in a When selecting a subwoofer amplifier, pay particular attention to the features. Here you can see that this unit has (from left) adjustable gain, line level (RCA) inputs, an adjustable low-pass filter and a bass-boost switch. 24  Silicon Chip siliconchip.com.au Only One Amplifier? It usually doesn’t make a lot of economic sense to run an extra amplifier just for four full-range speakers. As indicated last month, head units with good-quality inbuilt amplifiers are available very cheaply and if you pick efficient speakers, plenty of distortion-free high-level audio can be developed, without the need for an external amplifier. Consider also that a 4-channel amplifier with specifications that are better than the head unit’s inbuilt amplifiers may well cost twice as much as the head unit itself. Our recommendation is that you drive the wide-range speakers directly from the head-unit and spend the extra money on the subwoofer amplifier. hatchback, you make the largest box you’re happy to have occupying the luggage space. That way, the volume of air trapped behind the cone (adding to its springiness and so increasing its resonant frequency) is as large as possible and you don’t need to worry about tuning the response of a port. All quality original-equipment sound systems in sedans use a “freeair” subwoofer design – although, they use a driver that’s specifically designed for the application. It therefore makes sense to get hold of an original-equipment free-air subwoofer if you can. Japanese-importing wreckers often carry the 10-inch and 8-inch subwoofers from the Toyota Soarer and Lexus LS400 respectively. These subwoofer are usually quite cheap to buy – $40 being a common price. However, be careful as these drivers are 2-ohm impedance. Some car sound amplifiers will happily drive these very low impedance speakers but others will blow their output transistors. Almost certainly, you will not be able to run an amplifier in bridged mode into such a low speaker impedance. However, you can probably use just one of the two channels to gain an effective outcome (provided that channel can handle a 2-ohm load). Note that a 2-channel amplifier rated for 4-ohm speakers actually sees a 2-ohm load when operating in siliconchip.com.au Fig.1: the simplest system is one where just two front speakers are used – one for the right channel and one for the left. Fig.2: using front “splits”, where separate woofers and tweeters are used for each channel, will provide a substantial improvement in sound quality – especially if the tweeters are mounted high in the doors. Fig.3: a 4-speaker system adds rear speakers. If you pick speakers with high efficiencies, you will be able to run all four straight from the head unit’s amplifier stages. bridged mode and driving a 4-ohm speaker. Therefore, each channel must be individually capable of driving 2-ohm loads. Operating an amplifier in bridged mode into a 2-ohm loudspeaker is out of the question, since each channel would see an impedance of just one ohm. If you can’t get hold of an originalequipment subwoofer, buy aftermarket. As with full-range speakers, look for a heavy magnet, long-throw suspension and good brand name. If there’s a choice of several drivers, go for the one with the highest sensitivity (ie, the highest dB<at>1W/1m rating) – all else being equal, of course. Pretty well any subwoofer that satisfies the above criteria will have plenty of power handling capability. Getting it going So you have the components – now what? A lot will depend on exactly what components you’ve bought, so we can only cover the installation and December 2005  25 Do You Need Really Thick Cables? Very thick hook-up wire and cables are beloved by car audio enthusiasts. However, look in a $200,000 car with a fantastic sounding system and you’ll be surprised to find barely any thick wires in sight! Normal heavy-duty figure-8 cable is fine for nearly everything – but perhaps doubled up in the case of the amplifier power leads. This amplifier was bought secondhand for $100. It has “330 watts total power” written on it but that’s not continuous RMS power. Decent sound pressure levels will be available without resorting to a really high-power (and expensive) amplifier if you choose an efficient subwoofer. set-up details in general terms. As explained in the “Only One Amplifier” panel, we suggest that you drive the full-range speakers directly from the head unit. In fact, the simplest arrangement is to use the two front amplifier outputs of the head unit to drive the left and right full-range speakers – see Fig.1. A somewhat more sophisticated arrangement is to drive front “splits” – ie, two woofer/tweeter combinations (Fig.2). If you’ve salvaged matched woofer/tweeter pairs from a TV set (or some other source) or you are using commercially bought “splits”, the crossover capacitors will already be present. However, if there isn’t a crossover capacitor, you’ll need to 26  Silicon Chip install one to prevent bass frequencies from being fed to the tweeter. Typically, a 3.3mF or 4.7mF non-polarised capacitor can be used and these are available for about $1.20 each from electronics stores. Be careful to keep the phasing of the tweeters and woofers correct – ie, the positive terminal of each woofer goes to the positive terminal of its matching tweeter via the crossover capacitor. The next step up the ladder is to add rear speakers. Wide-range speakers are commonly used for rear-deck mounting (see Fig.3), while “splits” are again often used for rear-door mounting. Adding a subwoofer To add a subwoofer, you’ll need to interface its amplifier with the existing system. Just how that is done will depend on whether the head unit boasts line level (RCA) outputs in addition to its amplifier outputs. If the head unit has RCA outputs, these can be connected directly to the line inputs of the subwoofer amplifier – see Fig.4. The single subwoofer can then be driven by bridging the two output channels of its amplifier. This will result in a subwoofer output that includes the bass component of both channels. Both the crossover frequency and the output level of the subwoofer can be set by adjusting the appropriate controls on the amplifier. But what if you don’t have line-level outputs on the head unit? In that case, you’ll need to use a speaker-level to line-level converter box. These cost about $25. As shown in Fig.5, the converter box is wired in parallel with a pair of left and right speakers (either at the back or the front, depending on access) and provides line level outputs to the amplifier. The rest of the system is then the same as before. Setting up There are plenty of car sound systems around that sound awful, not because of the basic components used but because of the way the system has been adjusted. In most systems, you’ll have these adjustable settings: left/ right balance; front/rear fade; bass and treble; loudness; subwoofer level; and subwoofer crossover point. The key to getting them optimised is to adjust them one at a time, in the right sequence. Follow these steps: (1.) Select a well-recorded CD that siliconchip.com.au Select your microcontroller kit and get started... From $295* RCM3400 Fax a copy of this ad and receive a 5% discount on your order! Feature rich, compiler, editor & debugger with royalty free TCP/IP stack • Prices exclude GST and delivery charges. Tel: + 61 2 9906 6988 Fax: + 61 2 9906 7145 www.dominion.net.au 4007 A NOTE TO SILICON CHIP Fig.4: if the head unit has line level (RCA) outputs, it can be plugged straight into the subwoofer amplifier. If the amplifier’s two channels are bridged to drive the subwoofer, the subwoofer will reproduce the bass from both channels. SUBSCRIBERS Your magazine address sheet shows when your current subscription expires. Check it out to see how many you still have. If your magazine has not turned up by the first week of the month, contact us at silchip<at>siliconchip.com.au Silicon Chip Binders REAL VALUE AT $12.95 PLUS P & P Fig.5: if the head-unit doesn’t have line level outputs, a speaker level to line level adaptor will have to be used. This takes the signals from the left and right speaker outputs of the head-unit, attenuates the signals and then drives the subwoofer amplifier. H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A12.95 plus $A7.00 p&p per order. Available only in Australia. contains a variety of music styles, preferably including tracks with strong bass components, treble components and acoustic components. If you can siliconchip.com.au get one, the test CDs used by professional installers are excellent. (2.) Turn the subwoofer off (eg, by turning down its output on the ampli- Just fill in the handy order form in this issue; or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. December 2005  27 Getting The Correct Speaker Phasing For best results, the speakers should be connected so that they all “move” in the same direction simultaneously – rather than some “pushing” in one direction and some “pulling” in the other direction at any one instant. If all speakers move forwards and backwards together (ie, at the same time), they are said to be “in phase” and this give much better bass and imaging. It’s easy to use a 1.5V battery to check phasing. Let’s say, for example, that you are connecting a new head unit to some existing speakers and you’re unsure of their phasing. All you have to do is connect the 1.5V battery across each set of speaker terminals in turn (before connecting them to the head-unit). When the positive lead of the battery is connected to the positive terminal of a speaker, the speaker cone will move forward. fier), turn off the Loudness and set the Bass and Treble controls to their “flat” positions. (3.) Set the Balance and Fader controls to their central positions. (4.) Listen to some music and adjust the Fader control until the music appears to be coming slightly more from the front than from the back (this takes into account the differing efficiencies of the front and rear speakers). That done, adjust the Bass and Treble controls and decide whether the Loudness button improves or degrades things. Make sure that the speakers are not overloaded, even with the volume up Conversely, if you connect the positive lead to the speaker’s negative terminal, the cone will move backwards. If the speaker is hidden, a sheet of paper over the grille will usually indicate the direction of cone movement. You can also generally figure out the phasing using a simple listening test. First, set the fader so that you can hear only the front speakers, listen closely, then swap the wiring polarity at one speaker and listen again. You will get a much better bass response when the speakers are in phase. Wire the speakers for maximum bass response then do the rear speakers. That done, check that the front/rear phasing is correct. The trick is to check that the bass doesn’t suddenly improve when you move the fader or balance controls to their extremes – ie, listening to either a single pair or even a single (you may need to reduce the bass setting to achieve this). (5.) Set the crossover point of the subwoofer amplifier to about 100Hz and turn up the amplifier’s output. Think about what the subwoofer is there for – to fill the low frequency “hole” in the music that’s caused by the inability of the other speakers to reproduce those low frequencies. Listen to the music and make a judgement as to whether the bass is all one note, or has a range of deep frequencies. If you can hear only one-note bass, turn the crossover control to a lower frequency. This will immediately reduce the ap- Overloading Your Wide-Range Speakers? If you find that you can easily overload the wide-range speakers when you turn up the volume, the first step is to reduce the bass output of the head unit and let the subwoofer do more of the low-frequency work. However, in some systems, this can result in a “hole” in the lower midrange. In this case, you might want to install crossover capacitors to stop the very low frequencies from reaching the wide-range speakers. In a 4-ohm 28  Silicon Chip A 1.5V battery makes it easy to check speaker phasing. speaker should not improve the bass response. Note that whatever the ‘battery test’ tells you, always configure the phasing so that it sounds best. Some funny phasing effects can occur in cars and your ears are the best judge. Speaker phasing can make an immense difference to how the system sounds. Don’t overlook it when setting up a car audio system. parent amount of bass but the test is whether or not the bass sounds better than before. When you think you have the level and crossover settings optimised, listen to some speech – eg, on the radio. If the voices are artificially deep, you might need to make some more adjustments. (Note that with some head units, you can adjust the bass and treble separately for different sources. In this case, you might turn down the bass control on the head unit for the AM/FM radio output). Above all, remember that a welladjusted subwoofer isn’t at all obvious in its action! Instead, the music simply has more depth and energy – you don’t want “boom, boom, boom” (unless you are a complete idiot)! Conclusion system, a non-polarised 400mF capacitor will roll-off bass frequencies below about 100Hz. No doubt, they’ll be some who’ll pour scorn on many of the points that have been made in this series. Speakers out of TVs? Using speaker level to line level converters? Only one amplifier? What’s this guy on? However, provided you follow the basic rules, you will end up with a really good in-car sound – certainly far better than its modest cost would SC suggest. siliconchip.com.au