Electronic Building Blocks
By Julian Edgar
Quick and easy construction
Great results on a low budget
Playing with Bose
Bose gear seems to vary from very good to very bad – so what’s there to buy
on the second-hand market… and can you make one system sound better?
B
ose is a world-famous maker of electronics
products, primarily in the field of speakers and amplifiers.
They sell professional-level sound reinforcement gear that
can be seen in stadia all around the world. And, at the other end
of the spectrum, they also sell domestic sound gear.
But say the word ‘Bose’ to anyone interested in Hi-Fi and they’ll
likely look rather ill. ‘No highs, no lows – that’s Bose,’ they’ll
mumble. But I hasten to add (and not just to stop the publisher
being sued), I don’t agree with that at all.
Decades ago, I remember first hearing a Bose Wave Radio (Fig.1).
For such a diminutive thing – well, big for a clock radio, but small
for the sound – I was amazed. So amazed in fact that I went
away and did some research. It used just two tiny 50mm (2inch) speakers, but had a very thick enclosure design (a folded
waveguide – Fig.2) so clearly a huge amount of electronic
equalisation was going on. But hey, I am no purist, and if
that’s what it took to get such excellent bass, treble, midrange
and stereo imaging out of a clock radio – great!
Then I came across some Bose 301 bookshelf speakers (Fig.3),
going cheap. I grabbed them, but when I listened, I wasn’t much
impressed – they just sounded like basic two-way speakers.
(Yes, unlike many Bose products, they had tweeters.) But one
thing did amaze me – no matter how hard you drove them,
they seemed impossible to distort. I opened one up to find,
inside, a series lightbulb. Yes, a normal low voltage lightbulb!
It acted as a variable resistor: when it got hot, resistance rose
and so power was limited. Of course, it probably did all sorts
of horrible things to the sound, but oh well…
Fig.1. A first-generation Bose Wave Radio – note the vertical grille for
the waveguide output (far right). (Courtesy Antique Radio Forums)
Fig.2. US Patent 6,278,789 for the waveguide in the Bose Wave
Radio. Only the speaker on the left acts as a bass driver. (Courtesy
US Patents Office)
Practical Electronics | September | 2023
Fig.3. A Bose 301 speaker – open it up and inside you’ll find a
lightbulb power limiter! I wasn’t much impressed with the sound.
(Courtesy kulthifi.de)
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music and video. And that’s what you want with PC speakers.
(And no subwoofer needed on the floor.)
Hmm, I thought – there seems to be a pattern here. Small Bose
stuff is pretty good, larger speakers are not very impressive.
(Or what are meant to be larger speakers – more on this in a
moment.) So, when a Bose Soundock portable system (Fig.6)
was on sale for £5, I bought it. And, as a portable amplified
speaker for my phone playing MP3s, it’s ideal. Great, roomfilling sound from something I can easily carry around and it
has its own rechargeable battery.
Bose cubed
Fig.4. A Bose 191 in-wall speaker – I gained much better sound
from my own homemade in-wall speakers. (Courtesy Bose)
Time passed, then I needed some in-wall speakers. Up popped
some Bose 191 speakers (Fig.4) that – yes, you guessed it – I
grabbed. But they weren’t much good, so I built my own in-wall
speakers using Wharfedale drivers and crossovers, salvaged from
tower speakers. I kept the internal volume and port size the
same as the original Wharfedale enclosures, and they sounded
great – much better than the Bose in-wall speakers.
Then – I know, you guessed it – some more Bose stuff crossed
my path. But this time it was free – no cost, nothing, nada. (No,
I don’t know why they were free – the ad appeared on social
media; my wife said, ‘Do you want these?’ I said ‘Yes!’, and
within 30 minutes they were on my desk.)
Bose companion
The product was a Companion 20 multi-media system (Fig.5)
– basically, two powered desk speakers for a PC. They’re
apparently still a current model, retailing at £200. (Like all
Bose stuff, very expensive new.). But, like the Wave radio, the
acoustic design of these speakers is not at all pedestrian. In this
case, a very long folded reflex port is used. And again, I’m sure,
a lot of equalisation (ie, bass and treble boost) from the internal
amplifier. I think they’re outstanding – excellent at low or high
volume, good on speech for video conferencing but also good on
Fig.5. The excellent-quality Bose Companion 20 PC speakers.
Note the length of the internal port. (Courtesy Bose)
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Now you might be wondering, where on earth is this going?
Well, there’s one more step before I get to the nitty gritty. And
that’s the Bose Lifestyle systems. These comprise very small
cube speakers, often with two stacked on top of each other,
and a subwoofer module – called by Bose an Acoustimass (see
Fig.7). The idea is that you tuck away the cubes, putting them
unobtrusively in the corners, for instance, and place the bass
module behind a piece of furniture. Then, hey presto, you have
a near invisible speaker system! Trouble is, even as long ago as
when I first heard the Wave radio, I’ve never been impressed
with the Bose Lifestyle systems. Indistinct, no deep bass – and
certainly no treble. The little cubes don’t have tweeters – just
two full range 50mm (2-inch) speakers. That’s it.
The Lifestyle Acoustimass bass modules pop up quite often
second-hand, and over the years I have collected three. Some
have one bass driver within them, some have two – and some
even three. These different configurations are for the different
front / rear / surround pairs of speakers that the various Lifestyle
systems boast. (You can get Lifestyle systems with anything
from two to five dual-cubes.) Basically, the systems use one bass
driver per pair of cubes. Some Acoustimass modules have an
in-built amplifier but the ones I’ve got are passive – they need
an external amplifier.
Bose has a patent on the acoustic design of some of these bass
modules. The driver is buried inside the module, mounted within
a partition that divides the interior into two. In parallel with
the driver is a port, and then a second, much larger flared port,
which connects the interior volumes to the room in which you’re
listening. (This design is called a bandpass enclosure.) On paper,
Fig.6. My Bose Soundock portable system – great sound and
pairs nicely with my phone.
Practical Electronics | September | 2023
such an enclosure is efficient within only a
narrow frequency range – and in practice,
my frequency testing shows a decent output
from about 40 – 110Hz.
So, if the bass module isn’t that bad, what
is it about the cubes that lets the Lifestyle
systems down? I needed to find out! (I know
40Hz isn’t the 20Hz that monster subwoofers
can generate, but in the real world, 40Hz isn’t
terrible for a very small enclosure.)
I kept a lookout for people selling
the cubes but weirdly, they don’t seem
anywhere as numerous as the bass modules
(it should be the other way around). Then –
and yes, we’ve finally arrived at the point of
this article – I found a system for sale with
five black dual-cube speakers and a white
bass module. At £25, worth a punt. (And,
when I pulled the bass module apart, it even
had Bose power-limiting lightbulbs in it!)
Lifestyle system
And what did it sound like? I’ll tell you in Fig.8. Inside the cubes – one 50mm (2-inch) speaker fot each sealed plastic enclosure
a moment, but first a word on the system. stuffed with wadding. There’s not a tweeter in sight!
The bass module is driven by your chosen
amplifier and then the bass module in turn drives the cube
So, was this a deal-breaker? Well, I was already £25 in, so was it
speakers. In addition to the power-limiting internal lightbulbs,
worth spending any more money? Maybe first have a closer look
the bass module has a reasonably complex crossover – I imagine
at these world-famous cubes? I opened-up a stacked pair (the
that as well as not feeding bass to the cubes, it also reduces their
grilles can be carefully prised off and then four screws undone
feed power to better balance their output with the bass module.
to allow each driver to be removed) and looked inside. The
I initially had the bass module propped on a chair, the two
50mm (2in) speakers are identical, with thin foam surrounds,
cubes (I wanted just a stereo system) located each side. And
large, shielded magnets and, well, not much else. The voice coil
when I played some music? Quite awful. There was no bass,
covers are shiny, hard plastic domes (perhaps they’re meant
strong but rather metallic mid-range, and zero treble.
to radiate treble?) but there’s not a dual cone (a ‘whizzer’ cone
So, first things first. The bass module should have been
as it used to be called) in sight. And are the enclosures hiding
performing better than that, so I acoustically loaded it by
something tricky in their porting? Nope, they’re just sealed
placing it on the floor near a wall. For a small system (ie,
plastic enclosures filled with wadding; each cube is around
one that could be hidden), the bass was now reasonable.
300cm3 in volume. (Fig.8)
The midrange (probably the lower midrange) now sounded
a little better (although I wasn’t entirely convinced) but the
Adding tweeters
treble – still nothing. I ran a frequency generator app through
Hmmm… how about simply adding some tweeters, each
my phone and at 9.5kHz, treble just stopped. I honestly don’t
mounted on top of the dual cubes? The price would have to be
remember ever testing a speaker system where treble fell off
low, the tweeters would need to suit a ~4Ω system (6Ω measured)
a cliff at such a low frequency – the same app easily exceeds
my limits of hearing on other speakers.
Fig.7. An Acoustimass 5 System – my starting point for
modifications. I got what is pictured here plus another three doublecube speakers for £25. ‘My’ bass model was white.
Practical Electronics | September | 2023
Fig.9. The modified cubes – an Alpine tweeter fitted to each,
complete with a 3.3µF capacitor crossover inside. Not visible here
is the much smaller mass of fluffy wadding being used in each
enclosure. The addition of the tweeters and change in wadding
made a major improvement in sound.
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But there was a lot still wrong. A
frequency test now showed treble well
above my ability to hear (you could now
hear it fade slowly away as the frequency
rose, rather than abruptly stopping as
before) but there was a distinct hole in
the upper bass. Hmm, what about simply
reducing the power to the cubes, while
leaving the bass module untouched? I
experimented with some series resistors
and settled on 10Ω, 10W ceramic resistors
in the feeds to the cubes. With the bass
module positioned on the floor against a
wall, the bass was now over-strong and
muddy, but moving the module about
300mm from the wall reduced it nicely.
But now what about the midrange? It
wasn’t good. I couldn’t quite put my finger
on it at first, but if anything, it sounded
muffled. When I’d previously had the cubes
apart, I’d noticed the little enclosures had a
lot of rather high density wadding in them
– far more than I’d have expected. Was this
Fig.10. High-density original cube enclosure wadding (right) and new wadding (left).
wadding simply too dense, making the
enclosures so dead that any midrange brightness was lost? Time
and they’d need to have their own mounts – and I didn’t want
to find out! I opened-up all four enclosures (two cubes in each
to spend very much at all! A quick browse on eBay found just
assembly) and removed the dense wadding, replacing it with the
what I wanted – a pair of new Alpine DDT-S30car dome tweeters
much less dense ex-quilt fluffy wadding that I like to use inside
with crossovers for £12, post included.
speaker enclosures. (Fig.10) The removed wadding from each cube
The Alpine ‘crossovers’ ended up being merely 3.3µF
weighed 10g; the replacement just 2g. So, what was the sound
capacitors, but all else looked fine. Mounting the tweeters on
like now? Better – but if anything, the midrange was now a bit too
top of the cubes, drilling a small hole for the wiring and then
bright! Open up the speakers again, add a little more wadding,
connecting the tweeters to the internal wiring of the top cube
and now all was fine. And the final sound – quite acceptable!
speaker via the supplied capacitor took very little time (Fig.9).
I must admit that Bose stuff intrigues me. It is plentiful on
So how did they now sound? Vastly better. (Don’t let anyone
the second-hand market, with prices varying from very high to
tell you that, just because you’re an old geezer, anything above
literally giveaway. Would I recommend a used Bose Acoustimass
10,000Hz is irrelevant – it isn’t.) The treble was actually now
system with cube speakers? Yes, I would – if you want roompresent and the stereo imaging was also now clearer – interesting
filling sound with almost no footprint and you’re prepared to
how much of the ‘position’ of instruments is carried within the
make some simple modifications.
treble frequencies.
Fig.11. Crossover within the bass module – note the two lightbulb
power limiters. An extra two 10Ω, 10W resistors were added in
series to the two ‘cube speaker’ outputs.
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Fig.12. Interior of bandpass bass enclosure – note the port that
connects the two internal chambers.
Practical Electronics | September | 2023
