Silicon Chip – May 2019

siliconchip.com.au Australia’s electronics magazine May 2019 27

Features & specifications

• Low distortion and noise: ~0.001% THD+N


• One stereo input, two stereo outputs (low/high), weird optional channel inversion


• Each pair of outputs can be crossed over using first, second or fourth-order digital filters


• Additional parametric equalisers: four, common to all outputs


• Optional high-pass filter for low-frequency outputs, to cut out subsonic frequencies


• Configurable delay for each channel, to compensate for driver offsets (up to 6.2m; 18ms)


• Individually configurable output inversion and attenuation settings


• Built-in volume control – no need to use a preamp


• Load and save setups to EEPROM


• Software written in Microchip C; could be adapted for other DSP uses (open source)

Since the chip is already processing the
digital audio data, we’ve also provided
some parametric equalisation, so that you
can modify the frequency response of the
unit to compensate for any deficiencies in
your drivers, cabinet, placement, room etc.
Basically, you can tweak the sound
profile to be exactly the way you like
it, and without any further degradation
to the audio signal, since it’s only con-
verted from analog to digital and back
to analog once, no matter how much ad-
ditional processing is done in the digi-
tal domain.

What the Active Crossover does

Fig.1 shows what the unit does. This shows the spec-
trum of an audio signal, with the frequency increasing
left-to-right, from the lowest frequency that we can hear
to the highest. The level of each component of this signal
is shown in the vertical axis.
The blue area shows the signals which are extracted from
the input to be sent onto the tweeter, while the mauve area
shows those which go to the woofer. Signal components
which fall in the crossover zone in the middle go to both
outputs, although at reduced levels, so that they add up in
such a way to give the original signal levels.
Since this active crossover is adjustable, you can set the
crossover frequency to be at the ideal point for your loud-
speaker. You can also adjust the steepness of the roll-off,
as shown by the dotted lines, as different roll-off rates suit
different situations.

There’s also an optional subsonic filter, so that very low

(inaudible) frequencies, or those which are too low for

the woofer to reproduce, are eliminated and do not waste

your amplifier power or possibly damage your woofer. Its

frequency is also adjustable. (This is essential for vented,

horn loaded and infinite baffle speakers).

The relative levels of the woofer and tweeter can also

be adjusted, to compensate for differing driver efficiencies

or amplifier gains, and although it isn’t shown on the dia-

gram, you can also delay one channel slightly relative to

the other, to give proper ‘time alignment’.

The four parametric equalisation channels are not shown

in Fig.1, but essentially, each can be configured as either

a high-pass or low-pass filter with adjustable stopband at-

tenuation and corner frequency. This allows you to ‘shelve’

frequencies above or below a specific frequency, or between

or outside a pair of frequencies, to shape the overall fre-

quency response at all four outputs.

The Active Crossover is used as shown in Fig.2. It’s con-

nected between the stereo outputs of a preamp and four

power amplifiers which power the four loudspeaker driv-

ers independently.

Note that you don’t need to use a preamplifier as this

Active Crossover has a built-in volume control, so you can

use it as a basic preamp too. In that case, the signal source

is connected directly to the Active Crossover’s inputs.

Why use an active crossover?

There are a few reasons why you may want to use an

active crossover. Firstly, if you are building speakers from

scratch, it’s probably easier to use an active crossover than

Fig.2: here’s how the Active Crossover

forms part of a bi-amplified hifi system.

The preamplifier is optional in this case since this Crossover has a built-in volume control.

Fig.1: this two-way active crossover splits a signal with a
spectrum covering the entire audible frequency range into
two signals, one with the components above the crossover
frequency and the other, the components below it. The
optional woofer high-pass filter removes subsonic signals.