28 Silicon chip Australia’s electronics magazine siliconchip.com.au
design a passive one, since you can
easily experiment with it and change
the crossover frequency/frequencies,
relative amplitudes and so on until it
sounds ‘right’.
Also, if you’re building a seriously
powerful system with big amplifiers
and big speakers, it’s difficult to de-
sign a passive crossover to handle all
that power.
Since an active crossover is con-
nected before the amplifiers, and the
amplifiers can then power the drivers
with nothing in between, efficiency
is maximised and you can deliver as
much power as your amplifiers and
drivers can handle.
Depending on the speaker design,
you may also wind up with better
overall sound quality using an active
crossover than a passive one. Partly
this is because it’s hard to create a
very ‘steep’ passive crossover, which
crosses over across a small frequency
range, but this is relatively easy to do
with an active crossover.
Also, when using an active crosso-
ver, especially a digital one, because
you have separate line-level signals for
the tweeters and woofers, it is possible
to compensate for the slightly different
distance from each diaphragm to the
listener by delaying one of the signals.
The exact delay required depends
on the driver and cabinet design; it’s
tough to achieve perfect ‘time align-
ment’ mechanically, so being able to
adjust this electronically is a boon.
Another advantage of an active
crossover is that if you drive the sys-
tem into clipping, usually this will be
due to a huge bass signal. With a sin-
gle amplifier for each of the left and
right channels, that means that the tre-
ble signal will be clipped off entirely
each time the bass signal hits one of
the rails. That can sound really bad.
But with bi-amplification, even if
you’re clipping the bass signal, since
most of the treble is going through a
separate amplifier, it won’t be affected.
The result will still not be ideal, but
won’t sound anywhere near as bad; be
thankful for small mercies!
Basically, except for the extra com-
plexity that comes with the use of an
active crossover, there are only benefits
to this arrangement. It’s much easier
to adjust and tweak to give near-ideal
sound quality, has minimal effect on
signal quality or speaker power han-
dling and can be adapted to any two-
way loudspeaker system, as long as
you can wire up each driver separately.Modular design
This DSP Crossover is built by
combining several different modules,
each with a specific function. It was
designed this way so that it could be
reconfigured to do many different au-
dio DSP tasks. In fact, with the same
hardware but different software, it
could be used for a variety of audio
processing tasks such as echo/reverb/
effects, equalisation, delay and so on.The basic configuration is shown
in Fig.3. It uses seven main boards:
one stereo analog-to-digital convert-
er (ADC) board, two stereo digital-to-
analog converter (DAC) boards, a mi-
croprocessor board, a power supply/
signal routing board and a front pan-
el interface board. These are rounded
out with a graphical LCD module for
display, and a mains transformer to
power it.
Interconnections are made between
the boards with ribbon cables fitted
with standard insulation displacement
(IDC) connectors. This is a conveni-
ent and easy way to join boards where
multiple signals and power need to be
routed between them.
Audio signals are fed into the unit
via the ADC board where they are con-
verted to digital data. This data pass-
es through the power supply/routing
board and onto the microcontroller,
which stores it in RAM before doing
whatever processing is necessary.
It then feeds this data back out
through a different set of pins, again
as serial digital audio data, where it
passes back through the routing board
and onto one (or both) of the DAC mod-
ules. The DAC modules then convert
these digital signals back into line-
level analog signals which are avail-
able from two RCA connectors on the
rear panel.
The microcontroller board is wired
directly to the graphical LCD, so it can
show the current status and provide
the user interface, while the separate
front panel control board connects to
the micro via the routing board, allow-
ing the user control over that interface.
The whole thing is powered from a
9V transformer, which could be a plug-
pack or mains type. If a mains trans-
former is used, it would generally be
an 18V centre-tapped (9-0-9V) type, to
give full-wave rectification.
But half-wave rectification, as
would be the case with most plugpacks
(as they usually have a single second-
ary winding), is good enough.Circuit description
Let’s start with the place where
the audio signals enter the unit, the
ADC board. The circuit diagram for
this board is shown in Fig.4. It’s built
around an ultra high-performance
ADC, the CS5361 (IC1), which has a
dynamic range of 111dB and a typical
THD+N figure of 0.001%.
There is a compatible alternative,Fig.3: the Active Crossover is built from a modular DSP system. It uses seven
boards: one stereo ADC, two stereo DACs, a CPU board, LCD, power supply/
routing module and front panel control board.