Page 1
If you examine the vast majority of cur-
rent - and past - hi-fi amplifiers and
speakers, you will find the
same story: a single channel of amplifi-
cation handles the full audible frequency
range of the system. A single pair of
cables carries this signal to the loud-
speaker cabinet, and inside the enclo-
sure the high-level audio is split into
multiple bands and fed to appropriate
drivers. The circuit that handles this
splitting is the crossover, and it consists
of a number of filters that separate out
the different bands to suit the require-
ments of the different drivers.
The simplest example of this traditional
approach is the two-way speaker shown
diagrammatically in Fig. 1, where the
full-bandwidth output from the amplifi-
er is fed into a passive crossover that
derives signals to drive the tweeter and
woofer.
This, it turns out, is one of the worst
things you can do, as processing high-
level analogue signals requires compo-
nents to be chosen primarily for their
power-handling capability and not for
their audio quality. The filters require
inductance, capacitance and resistance,
and to operate at high levels and low
impedances - in the order of a few
ohms - without losing efficiency, these
components are often far from perfect.
Inductors are iron or ferrite cored and
capacitors are non-polar electrolytics,introducing distortion. In fact, every-
thing is more difficult to manage at high
power levels. Suddenly the cables that
connect the amplifier outputs to the
loudspeakers can impact the sound of
the system, for example - something
that benefits only the makers of expen-
sive cables.
Even if it is practical, at great expense,
to use air-cored inductors and film
capacitors, it is still difficult for the
designer to avoid making compromises
in the frequency characteristic of the
crossover without presenting unpleasant
loads to the power amplifier in terms of
impedance or phase angle. In addition,
the relative efficiencies of the drivers
have to be well matched to avoid wast-
ing power and damping - this limits the
designers choices of which units to use.
Look at it another way: In a passive sys-
tem, the only power available to drive
the crossover components is the signal
itself.
A solution, long known in the profes-
sional field, is to operate the crossover
at line level, ie before amplification
takes place. The amplification then fol-
lows the crossover instead of preceding
it. In modern professional live sound
installations it is extremely common to
pass the line-level signal to an active,
electronic crossover, then on to the
amplification and finally to the actual
drivers of the loudspeakers.
page 2 Meridian Loudspeakers: The DSP Path The Meridian Papers - 1
Hotrod or Hi-Fi?
Consider your car for a moment. Did you
buy the engine from one manufacturer,
the suspension from another and the
transmission from someone else?
Probably not. Yet this is the way that
high-priced hi-fi systems are often assem-
bled. It is generally not known - and
impossible to know - how such a compos-
ite system will perform.
Components are chosen independently of
one another and out of context with the
sound of the system as a whole, often on
the basis of irrelevant, anecdotal or sim-
ply erroneous information.
Hardly surprising, then, that the elusive
Grail of audio - musicality - is difficult,
if not impossible, to find in the traditional
audiophile arena. At best, hi-fis like this
are not integrated systems but hot-rods:
they do one thing well. This is why expen-
sive systems often only sound their best
playing back one type of music.
Meridian believes in the complete, inte-
grated system, and that a system should
be judged on how well the entire package
performs in the real world. This is why all
our components explicitly speak the same
electric and acoustic language. While their
performance with other manufacturers
equipment is exemplary, they positively
sing when placed in chorus with equip-
ment of their own pedigree.
In almost thirty years of existence,
Meridian has learned not only the param-
eters upon which a superb-quality total
system is based: we have also refined our
capability to design the individual compo-
nents that comprise such a system.
Dont forget, a Meridian system can be as
simple as a CD player and a pair of DSP
speakers. Because the amps and control
are in the speakers, thats all you need.
Fig. 2: An alternative arrangement to that
shown in Figure 1, in which an electronic, line-
level crossover drives a pair of amplifiers feed-
ing woofer and tweeter. LINE LEVEL IN LOUDSPEAKER
ENCLOSURELF DRIVER HF DRIVER HF POWER
AMPLIFIER
LF POWER
AMPLIFIER ELECTRONIC
CROSSOVER
Page 2
However, such an approach - bi-amp-
ing or tri-amping the system - is far
too complex and prone to error to be a
very practical approach in the consumer
field.
The Active Loudspeaker
Back in the mid-1970s, multi-amping
was almost unknown. Even more
unconventional was Meridians first
product, the M1 Active Speaker, which
placed both active crossover and ampli-
fication, plus the associated power unit,
in the same enclosure as the loudspeak-
er drive units. The principle is illustrated
in simple form in Fig. 2.
This method delivers a number of
important benefits. First, there is a sim-
ple line-level connection between the
preamplifier and the loudspeakers: a
large, heavy box and associated cabling
disappears at a stroke.
Second, the crossover is operating at
line level, so the considerations as far as
components are concerned are the
same as with, say, a preamplifier, and
the quality delivered by such a crossover
should be equivalent. The highest quali-
ty components can be employed, such
as metal-film resistors and plastic capaci-
tors, for example.
But there are not simply benefits on the
component side. The designer of an
active crossover can design each ele-
ment of the crossover - including inde-
pendent adjustment of phase and
amplitude, and filter curves as complex
as are required by the acoustic system -
without having to be concerned with
issues such as matching driver efficien-
cies or the impedance of the configura-
tion.
In addition, there is another major bene-
fit in that the amplifiers are connected
directly to the drivers: there is one
power amplifier per crossover band. The
directness of the connection means that
the amplifier can control the driver overits entire range. DC coupling between
amplifier and driver results in a high
damping factor.
In simple terms, this means that if the
speaker cone makes a movement other
than because of an input signal - as a
result of a resonance, for example, the
electrical energy generated by this
movement is fed back to the amplifier
and restrains the motion of the cone -
allowing the amplifier to control the
driver.
This electromagnetic damping reduces
resonance, cone effects and spurious
responses. Not only that: the direct con-
nection between amplifier and driver
means that the amp can control cone
movement beyond the range of the
crossover band assigned to it - impor-
tant, because a tweeters resonant fre-
quency, for example, can often be out-
side the frequency band supplied to it
by the crossover. In an active system, the
amplifier can deal with this; in a passive
system, it cant.
This tight control also allows a Meridian
to sound excellent at any level, from a
whisper to a surprisingly loud shout.
Its also important to consider the loud-
speaker as a complete system. For the
designer, this gives a great deal more
possibilities than simply multi-amping an
existing passive design.
Less power, more sound
On the face of it, there is a downside to
this approach: the system requires a
power amplifier per crossover band,
rather than just one.
The truth is, however, that the active
loudspeaker is much more efficient.
When a single amplifier is used in a pas-
sive system, apart from the power wast-
ed producing heat in the crossover,
there also have to be allowances in the
power amplifier for all manner of
extraordinary unknowns: strange imped-
ances at certain frequencies, a wide
page 3 Meridian Loudspeakers: The DSP Path The Meridian Papers - 1
Digital Audio:
Music to the Ears
Digital audio technology is now widely
regarded by industry experts as the best
means to hear music in the home.
Because music recorded digitally can be
transmitted, even over long distances,
and played back with no change from the
original, like Morse code over a telegraph
line.
Why is this important? Traditional ana-
logue systems behave much like the child-
hood game in which each player repeats a
whispered message into the next players
ear, and so on down a chain. Each ana-
logue link - turntable, amplifier, cable,
speaker - whispers an analogy of what it
hears to the next, but something is
always lost or added. What emerges at
the end, while charming, may not resem-
ble the original message.
By contrast, digital audio first encodes
music as digits - ones and zeros - in pat-
terns describing specific sound waves.
This is the binary language of computers,
and its very difficult to mistake a one for
a zero. No matter how long the chain, at
its end digital equipment listens only for
patterns of those two digits, which it
reassembles into music, ignoring all other
whispered information as noise.-¦ -€
