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The Bose Lifestyle Music System Last month we previewed the new Bose system and we promised to tell you more about it this month. Having lived with it for about 5 weeks now, we are convinced that our first impressions were right - that it could be the end of conventional hifi systems, as we presently know them. By LEO SIMPSON Let's say at the outset that not everybody will like the new Bose system. Diehard hifi enthusiasts who have traditionally liked to mix and match their own systems, upgrading the system when new products were introduced and constantly "tweaking" the system to improve the performance - won't like it. It presents too much of an "end solution" to the problem of hifi sound. 38 SILICON CHIP Nor will those people who revel in the complexity of conventional hifi like the Bose system. It has no rows of knobs and dials to impress or confuse - it ain't a knobtwiddler's delight, that's for certain. Nor will the "high end" hifi purists like the Bose, for reasons that will become clear as we go along. But the great majority of people who like good music and the good sound that comes from conventional hifi will love the concept of the Bose Lifestyle Music System at least, that's what we think. Technical features Now let's have a closer look at the Bose System, starting with the compact unit which Bose refer to as the Music Centre. Finished in satin aluminium, it measures 420mm wide, 65mm high and 245mm from front to back. It combines a compact disc player and an AM/FM stereo tuner, along with switching for external program sources. There is a window on top of the unit which shows the source selected, the zone (1 or 2) and the station frequency, when the tuner is in use. In the same cutout of the lid are 7 pushbuttons which can be used to control some of the unit's functions ◄ Pictured at left are the normally visible components of the new Bose Lifestyle Music System. Not shown is the powered Acoustimass loudspeaker which provides the low bass output. but as we said last month, this is really best done from the remote control handpiece. Pushing a latch at the front of the case releases the lid so that it gently opens to reveal the ''well'' for the CD player. There is no fancy motorised disc drawer; it is simplicity itself. You just place the compact disc into tµe well, close the lid and the machine does the rest, by remote control. Also concealed by the lid are 7 buttons which are used for programming the AM/FM tuner and for selecting external program sources. Up to 30 stations can be programmed into memory and they can be any combination of AM or FM stations. Stations can be programmed in several times so that more than one person can have a preferred list, which can be pencilled into a panel on the back of the remote control handpiece. At the rear of the unit are two rows of RCA sockets, for a tape monitor loop (same as in any hifi system), for connection of two external line sources (such as a hifi video recorder and perhaps another tape deck), and for two pairs of stereo outputs, for zone 1 and for zone 2. Also on the back panel are the power switch, two screw terminals for connection of a loop antenna for the AM receiver and a 75 ohm coax socket for the FM antenna connection. Finally, there is a 3.5mm socket labelled "serial data output" which we'll explain later. On the righthand side of the unit are two 3.5mm stereo sockets. One is for connection of a W alkman style cassette player while the other is for connection of stereo headphones. If stereo headphones are plugged in, they mute the main amplifier and loudspeakers. Heard but not usually seen, the powe_red Acoustimass has a large heatsink on one side to provide cooling for the transistors in the power amplifiers. Also visible are the two ports which tune the enclosure and the bass and treble controls (near the lower port). Single microprocessor While we did not pull the Music Centre apart, it is evident that there are a number of clever things going on inside. For a start, there appears to be only one microprocessor instead of the two that would normally be used for a synthesised AM/FM stereo tuner and for a compact disc player. Secondly, the tuner and CD player must have been very carefully designed so that they don't interfere with each other. Remember that contrary to most hifi systems, it is possible to use both the CD player and the tuner simultaneously. Normally, CD players radiate a significant amount of radio "hash" which can cause reception problems for a tuner (particularly on AM) in the near vicinity. Not only that, but crosstalk between the two internal sources (and external sources) is very low. That has to the case otherwise someone listening to CDs would hear a radio station in the background when the tuner was selected for the other zone. Try this test with headphones and with the volume wound up on a conventional hifi system with a separate tuner - you'll see what I mean. A hidden feature is the remote control receiver and decoder circuitry. Why do we say it is hidden? Well it operates at 27MHz and yet has no apparent antenna - this function is performed by the shield of the audio output cables to the powered loudspeaker. The compact disc player itself is a fairly conventional unit, with dual 16-bit digital to analog converters, two times oversampling (88.2kHz), and digital and Bessel analog filtering. Remote control We're not going to list all the functions of the remote control handpiece - they were covered pretty well last month. A few points should be made though. One very worthwhile feature is that when the I A closer encounter with ·a radical new hifi system I OCT0BER1990 39 Powered Acoustimass These are the twiddlers which handle virtually the whole audio spectrum down to just below 200Hz. Note the tiny size of the drivers with their plastic domes and miniature foam roll surrounds. Just how do they work so well over such a wide frequency range? system is first turned on and starts to play, the volume setting is fixed at - 40dB below maximum output - quite a modest level. This is a good idea and much better than having the system come on at the level at which it was last used. If that happened at night ·(as it can with conventional systems), it could wake everybody in the household. Security coding One of the potential problems with an RF control such as this is that there could be interference, because the unit has quite a long range - 20 metres at least. It is easy enough to imagine this situation where two neighbours, either in a block of home units or two houses close together, both have Bose Music Systems. Could one owner control his neighbour's system - inadvertently or otherwise? 40 SILICON CHIP The answer is yes but Bose have already thought of this situation and have the solution. Inside the battery compartment is a 4-way DIP switch (four tiny slide switches close together). Three of these switches set a security code for the remote control so that if you have an interference problem you can change the security coding. A similar DIP switch on the underside of the Music System needs to be set to match the remote control. Those three switches (out of four) give 8 possible codes - not many, but probably enough The fourth switch is there to set the volume mode for listening zone 2. The normal factory setting makes the signal level adjustable by the remote control in the normal way. The alternative setting makes the signal level fixed - to suit the situation where Bose powered Music Monitors are used. These have their own volume controls. As indicated in last month's preview, the loudspeakers are a powered version of the Bose Acoustimass 5 - a satellite cum superwoofer system. The Acoustimass is a pretty radical system in itself and quite unlike conventional loudspeaker systems. The superwoofer is perhaps the· most unconventional part. It measures 464mm wide, 321mm high and 250mm deep. Inside, there is a partition dividing the enclosure into two compartments and also carrying two 15cm (6-inch) drivers connected in parallel. Each compartment is vented by a tuned port. In effect, the Acoustimass enclosure is a double-tuned resonant system with the two resonances about an octave apart (actually around 55Hz and 130Hz). The system is then equalised electronically to cover a range of about two octaves. The two system resonances actually create a tuned bandpass system. This effectively stops the system from producing much output below about 45Hz (just like any conventional tuned system) but it also has a beneficial effect in attenuating any harmonics above about 200Hz. Thus, even if the bass drivers produce quite significant distortion, it is effectively filtered out by the physical bandpass system of the tuned ports. No other system around has any such system of distortion filtering. This filtering effect has two very interesting effects, which we'll come to later. This powered version of the Acoustimass 5 dispenses with some of the passive overload protection of the previous model and does it electronically instead. Most people, when they see the powered Acoustimass system for the first time, don't twig to the fact that it is a speaker, or an amplifier for that matter. The clue that it is an amplifier comes when you look at one side and see that more than half of it is a black anodised aluminium heatsink. The other clues are that it has a power cord and signal lead, and two pairs of speaker terminals for powering the satellite "twiddler" speakers. Mounted on the steel backplate of the powered Acoustimass is this large printed circuit board which carries the three class-G power amplifiers and all the associated small signal circuitry for electronic crossovers, automatic loudness compensation and overload protection. At one end of the box are the two ports mentioned earlier and the bass and treble controls which are intended mainly as presets - you set them at installation and then forget them. Internal electronics We took the liberty (curiosity got the better of us) and had a look inside the Acoustimass system to see what made it tick. As you can see from the photos, there is not much to the speaker enclosure but the electronics is another matter entirely. Again, it is very cleverly thought out. As can be seen, all the electronics is laid out on a large printed circuit board which accommodates a 100 watt amplifier to drive the internal woofer system and two 50 watt amplifiers to drive the satellite "twiddlers". Notice that the power transformer, propped up on a compliant suspension system, and the filter capacitors, are not very big. Yet the whole system is rated at 200 watts total. It delivers it too and a little more besides, as we shall see. Our first surprise came when we realised that the woofer impedance was around 4 ohms and not 8 ohms as in a conventional hifi speaker. But then again, why not. After all, since it is a powered speaker, speaker lead resistance is not a problem (they're only about 20cm long!). Designing for a 40 load means that the supply voltages can be lower, and so the overall power dissipation is lower. Going into it a little deeper, there is little need to derate the output transistors for second breakdown effects and long term reliability should be better. The twiddler speakers are a nominal 80 and so their 50 watt amplifiers are more conventional, at least in terms of their intended load. But there is more to come. Class G amplifiers All three power amplifiers operate in class G. This may be a puzzle to many hifi enthusiasts but it is an amplifier configuration pioneered by Hitachi in the early 1970s. Basically, class G is a conventional class AB amplifier in which the supply rails are varied according to the amplitude of the input signal. When the signal amplitude is low, the supply rails are low and so the quiescent power dissipation in the amplifier(s) is low. When a bigger signal comes along, requiring a lot more output power, additional transistors switch the output transistors to a higher supply voltage. Thus, the amplifier can deliver a lot more power for a short period. It is an effective way of increasing the amplifier "headroom" or peak power output. Amplifiers made by Carver, NAD and Proton also effectively vary their supply rails and get a similar result - · more bang for your buck. Back to the main plot. Remember how we said that the Acoustimass system was like a tuned bandpass filter - removing harmonic distortion caused by the woofers. Well, it also effectively removes harmonics which are caused when the amplifier is badly over-driven into clipping. Normally, when an amplifier is OCTOBER 1990 41 Bose lifestyle Music System driven into clipping it is only too apparent from the dreadful noises that emanate from the speakers. But with the Acoustimass, that "safety margin" is removed and it would be possible to badly overdrive the system without the listener being readily aware of it (apart from being deafened). The Bose people admitted, at the launch of the Bose Music System in July this year, that this presented a problem. They got over this problem with their earlier unpowered Acoustimass 5 system by a pretty fancy system of protection. They told us that they had a completely different method of protection with this powered version of the Acoustimass system but they weren't saying how it worked. Signal limiting We found out how it worked by accident, when testing the system for power output. We're getting a little ahead of ourselves here but basically it works by signal compression. Once the amplifier is near the point of clipping and therefore serious overload, the compression circuitry cuts in to limit the signal from increasing any further. It is very effective. Bose has borrowed from its experience in the public address field and has incorporated a limiter which is commendably distortion free. So what we have is a trio of power amplifiers which deliver quite a lot of power in their normal mode, can deliver more on signal peaks due to class G operation, and then are prevented from being over-driven by signal limiting (or compression). The result is a system which can be driven to very loud levels without being distressed. Perhaps we should rephrase that - the result is a system which can deliver far more sound than the average listener will ever want. Electronic crossovers As well as the three power amplifiers and signal compression circuitry, the large printed board also accommodates the electronic crossovers to separate the signal 42 SILICON CHIP bands for the satellite twiddlers and the Acoustimass woofers. In addition, there is circuitry for automatic loudness compensation. Now this is a feature which is going to cause a lot of debate, especially among the hifi purists. Contrary to the loudness feature on most amplifiers, the Bose automatic loudness compensation cannot be switched out. In fact, I have argued in the past that loudness compensation on any amplifier was bad and the best point about it was being able to switch it out. One of my reasons for arguing this way was that the designer could not even guess what sort of loudspeakers the amplifier was going to be driving, let alone the size of room, the hearing acuity of the listeners and so on. But at least with the Bose system, the designers knew exactly what The System At A Glance Good points • Clever engineering • Remote control • Inbuilt overdrive protection • Nearly invisible speaker system • Simplicity of use • Sound quality Points for improvement • Noisy CD player • No provision for turntable • Switch-off 'plop'; see text. sort of speakers the amplifiers would be driving. So they could tailor the loudness compensation pretty closely to suit most circumstances - that's probably how they would argue anyway. To be frank, we're not keen on this feature. If we'd had our " rathers" we'd rather it was able to be switched out, if only to give the listener the option. Balanced inputs The audio inputs to the Acoustimass are balanced. This is desirable when the cables are very long as it greatly reduces the pickup of interference signals. However, the audio outputs from the Bose Music Centre are unbalanced (ie, as in conventional stereo preamplifiers) and are run in figure-8 shielded cable with two RCA sockets at one end and a conventional 5-pin DIN socket at the other. Just how balanced lines would help in this set-up we're not sure. (We did not have access to circuit information at the time of this review). And what other tricks does the system get up to? Well there is another and it's automatic switch off. If the circuitry does not get any audio signal for more than about 90 seconds, it switches itself off. This is another good feature which is desirable on a system like this which is just about invisible. Walk into a room and there is no way you'd know it was on, with no program playing. Without the automatic switch-off feature you might forget to switch it off for days or even weeks at a time. When the Music System is next used, the powered Acoustimass senses the presence of audio signal and switches on the amplifiers again. It's all very neat but this could have presented a problem when the CD was switched on. Compact disc players have such a small residual noise that it is not until the disc actually begins playing that the amplifiers would turn on, resulting in the first few notes being missed. To solve that problem, the Music Centre has its "serial data output", as mentioned earlier. This is an BV signal which is generated as soon as the compact disc function is selected, to "wake up" the power amplifiers. To make this automatic switchon/switch-off function possible, the Acoustimass has two power supplies, similar to a remote-controlled TV set. There is a small power supply which is always on and it controls the large power supply (which uses the big transformer) via a relay. Twiddler speakers While the powered Acoustimass system is very intriguing in its engineering features, so are the View inside the Acoustimass enclosure, showing the two parallel connected woofers and one of the tuned ports. There's not a lot to it, is there? Also visible is the small PC board for the tone controls. "twiddler" speakers. Each twiddler consists of two cubes about 90mm wide, each with a 60mm driver inside. These drivers combine the features of a tweeter and a woofer. They have a Mylar dome, a doped cone and a roll surround. They are magnetically shielded, so they can be used in close proximity to TV sets and video monitors. Note that two of these tiny speakers in each channel can handle program material in excess of 50 watts! But the most amazing thing to us is that they cover the whole frequency range down to below 200Hz, which must be well below the cone resonance of the drivers. Just how do Bose manage that? It has us stumped. There is another paradox with the Bose system and that involves the integrated circuits used in all the processing circuitry in the powered Acoustimass. The line-up involves some pretty ordinary ICs, such as 4558s (a dual op amp from Motorola) and a 3080 (a transconductance amplifier from RCA). But that hasn't stopped Bose from extracting very good performance from them. Again, it is a case of some very clever engineering. Performance Having read this far, you must be itching to know how well the system performs. We'll give you the objective results first. We tested the performance of the compact disc player, FM tuner and the Acoustimass amplifiers. We first tested the CD player. It can be summarised as having middle-of-the-road performance but with above average error correction and tracking. This means that it is very good at tracking and playing dirty or damaged discs - it will load and play some discs which other top quality players will reject as unplayable. It also has fast track access times which is important for a remote controlled player such as this. Bose rate the frequency response as being within ± 0.5dB from 5Hz to 20kHz; signal to noise ratio, 95dB; dynamic range, 90dB; harmonic distortion, 0.08 % and channel separation, 70dB (from lkHz to 20kHz). We measured the frequency response as flat, across the range from 20Hz to 2kHz, and then slightly rising in response to a slight peak of + 1.3dB at 18kHz and then - 0.2dB at 20kHz. We would guess that this slight peak is caused by the Bessel analog filtering. The signal-to-noise ratio was - 102.5dB unweighted or - 109dB A-weighted, which probably means that the player output is muted, for this test. (Most players are effectively muted for this test). However, a look at the residual distortion products indicate that the dynamic range is close to 90dB, as claimed. Harmonic distortion was generally around .05% which is better than claimed but still fairly average. Linearity was pretty average too, with an error of + ldB at - 70dB, + 3dB at - B0dB and + 8.5dB at - 90dB. For the record, the best result we have ever seen for a CD linearity test was + 0.4dB at - 80dB and ldB at - 90dB. The measured channel separation was - 91.2dB at 100Hz, - 86dB at lkHz, - 65dB at 10kHz and - 60dB at 20kHz. Control unit As a stereo control unit, the Music Centre measures quite a lot better, with frequency response flat from 20Hz to just - 0.6dB at OCT0BER1990 43 20kHz and - 3dB at 50kHz. The sensitivity is 525mV for 1V out. Distortion is generally less than .005 % while the signal to noise ratio is - 104dB unweighted and - 108dB A weighted. Separation between channels was better than - 96dB at lkHz and lower frequencies; - 82dB at lOkHz; and better than - 76dB at 20kHz. These are good figures. More important is the figure for crosstalk between the different sources, such as when the tuner is selected for Zone 1 and CD for Zone 2. We measured this for the Aux and Video inputs and achieved results of - 97dB at lkHz and - 81dB at lbkHz. This is an excellent result and far better than could be expected from a conventional stereo control unit. Of course, as we remarked previously, it has to be. FM tuner Bose's tuner specs are as follows: 50dB quieting sensitivity, mono 18dBf, stereo 38dBf; distortion at 65dBf, mono 0.15 %, stereo 0.25%; signal to noise ratio, mono 75dB, stereo 72dB; frequency response, 30Hz to 15kHz ± ldB; separation 30dB; capture ratio 1.7dB; alternate channel selectivity, 65dB; image response 95dB; AM suppresion 60dB; and subcarrier product rejection, 40dB. Unfortunately, we are not set up to test some of these specs but we were able to confirm the frequency response as better than ± ldB from 20Hz to 15kHz and the separation at lkHz as 40dB (better than claimed). We measured the signal to noise ratio at 65dB in both stereo and mono modes but the limiting factor there would be our Meguro FM generator. Amplifier performance Measuring the amplifier performance for the Powered Acoustimass system is bit of an obstacle course. You can measure the performanc e of the satellite driver amplifiers readily because their outputs are accessible. But even then you have to be aware of the equalisation and automatic loudness compensation. To measure the bass driver though, you have to gain access inside the enclosure, disconnect the woofers and connect a dummy load. Having done that, we can report that the amplifiers perform very well, as straight amplifiers. We'll clarify that in a moment. For the satellite amplifiers, we verified their power output at 50 watts per channel into 8-ohm loads for a total harmonic distortion of around .01% (dependent on the signal frequency). At lower power output, the distortion was much lower, around .005%. The frequency response was not flat (nor is it designed to be, with equalisation and so on). Interestingly, Bose appear to use an external RLC network at the amplifier outputs to roll off the high frequency response quite heavily: - 7.5dB at 20kHz. We're not sure why - they just do it. This oscilloscope waveform shows the effectiveness of the overdrive limiting circuitry. It shows the amplifier output with a + 4.6dB input overdrive. At this input level, conventional amplifiers would be clipping severely and yet the Bose amplifiers are giving a good waveform (5% harmonic distortion). 44 SILICON CHIP Also the electronic crossover has a rapid rolloff to protect the twiddlers against signal frequencies below 200Hz. The class G amplifiers give a headroom of 1.5dB, meaning that the music power output is 70 watts - quite a useful increase. For the woofer amplifier, we confirmed the power output as 100 watts into a 4-ohm load for a total harmonic distortion of around .035%. Again, at low power, the distortion reduces. You have to "step around" the equalisation and the electronic crossover here too. This prevented us from doing a headroom test on the woofer amplifier, principally because it has a severely curtailed response at lkHz, the normal headroom pulse test frequency. But we have left the best till last - the limiter circuitry. This is in circuit all the time but has no effect on the distortion at normal signal levels. The maximum level of over-drive possible before clipping was + 12dB, amounting to four times. At + 4.6dB, the distortion was 5% while at + l0dB it was 10%. While a distortion level of 5 % may seem high, it consists mainly of low order harmonics which are not anywhere as audible as even a very modest degree of clipping. What we're saying is that this is a very effective system of protection, meaning that no matter how high the level is set via the remote control, the speakers will not be over-driven. Therefore, damage to the speakers due to careless use of the volume control (via the remote) is impossible. In effect, Bose have made the system idiot proof. What a good idea. How it sounds Enough of the objective results, how did it sound. In a word, dramatic. Everyone who came and heard the system went away very impressed. People just could not believe that those tiny speakers were producing all that beautiful sound. Even when the Acoustimass woofer box was brought to their attention, they still could not believe a:: UJ We also found that on constant tone testing, there were a couple of frequencies that produced audible rattling from the Acoustimass heatsink panel. This did not become audible on music programs though. -1 0 ~ 0... 5 0... -20 ~ ~a< ~ I I -30 ~3: < (I) ~ ... I a: co -40 ~"O a:-~ UJ 0 -50 I 0... 1:J 0... 1:J 0 - J :J .- 3: ......... -60 '\ Other comments ' \ \. ' I' j 100 10 ~ \. \" 1000 FREQUENCY IN HZ Essentially, the Acoustimass enclosure is a double-tuned system which covers a frequency range of just over two octaves. This gives a bandpass effect which filters out harmonic distortion produced by the woofers. SHORT PORT FRONT Al R VOLUME LONG PORT ENCLOSURE This diagram shows the basic layout of the powered Acoustimass superwoofer system. The cabinet is divided into two compartments and carries two 15cm drivers connected in parallel. The amplifier electronics sits in one half of the cabinet & is mounted directly on the heatsink. it. And neither could we, actually. Personally, while my brain told me that all the bass was coming from the Acoustimass enclosure, my ears told me that it was coming from the twiddlers. Why is it so convincing? We put it down to the fact that the Acoustimass system produces very little harmonic distortion and that it cuts in below 200Hz. This means there is nothing to "cue" your ears, to tell you that the bass speakers are working. It is incredibly effective. More objectively, the frequency response from the twiddlers is quite smooth and extended. The bass though, we found to be too heavy, in most listening situations. In fact, we. used it with the maximum bass cut setting and even then it was a little too strong, particularly on classical music. On a negative note, we think that the CD player is a little too noisy. This is a criticism that can be aimed at quite a few CD players. They are not at all silent, unlike phono turntables which are totally quiet. Another problem we noted is that if the Music Centre is turned off using its rear power switch (or at the wall power point), it gives a loud clunk from the speakers. This situation will not occur in normal use but its something Bose should look at. And finally, how would we sum up the system? Intriguing and exciting, would be the operative words. Clearly, the Bose Lifestyle Music System is the most innovative hifi product we have seen for many years. It borrows a number of ideas which have been seen before (such as in systems made by Bang & Olufs(;ln) but the way in which all the features have been brought together is quite outstanding. The team who designed this system really have thought long and hard about what should and should not be in a hifi system aimed at the widest number of consumers. Is it really hifi? This is the question that we posed at the end of last month's preview. Of course it is. By a number of standards, such as the overload protection, it represents a big advance. But no, it is not up to the standards set by "price is no object" high end hifi components. Nor could you expect it to be. The overall performance though is to a very high standard. We predict that the Bose Lifestyle Music System will be much imitated in the years to come. There must be many hifi companies around the world wringing their hands and saying, "Why didn't we think of it?" At around $4600 for the basic system, Bose are going to be hard put to satisfy the demand. And now, enough of this. Go out and have a listen at your nearest Bose retailer. ~ OCT0BER1990 45