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This stereo 3-way active crossover is for those enthusiasts who
want the very best from their 3-way loudspeaker systems. It
avoids the disadvantages of passive crossover networks and
allows the power levels to the speakers to be optimised.
W
hat is an active crossover
and why would you want
one?
Most hifi enthusiasts are aware that
2-way and 3-way loudspeaker systems
contain passive networks to split
up the audio spectrum into two
frequency bands in the case of
2-way systems and three bands in
the case of 3-way systems.
Passive crossover networks use
inductors, capacitors and resistors
to split the audio into the various
bands and set the signal levels to
the various speaker drivers.
For example, the woofer is often
less sensitive than the midrange
driver and tweeter and so the signals
58 Silicon Chip
to the latter drivers have to be reduced
so that the overall output from the
three drivers is equal.
In the higher performance speakers,
the crossover networks are often very
complex and they can be very difficult to design and optimise. And because they usually do attenuate
the midrange and high signals,
that means they do waste ames
ur
at
Fe
plifier power.
1-unit rack case
They also interpose a com Single PC board
plex
network between the
r
transforme
speakers and the amplifier
15V+15V 20VA toroidal
which means a loss of damping
Stereo module
stages
t
tpu
ou
d
factor, particularly for the lowan
ut
inp
Buffered
impots)
(tr
ts
er frequencies where it is most
tpu
ou
ble
ria
va
Individually
needed, if you are to achieve
ly
pp
su
r
On-board powe
tight, clean bass and midrange
tors
On-board RCA connec onents
reproduction.
mp
OK, so that’s the passive ap Made from standard co
proach. It involves just one stewww.siliconchip.com.au
An active
3-way
crossover
for
loudspeaker
systems
Design by Mick Gergos
reo amplifier to drive the two speaker
boxes in a stereo system.
In an “active” system, we eliminate the passive crossover networks
and electronically split each of left
and right channel signals into three
frequency bands: low, midrange and
high. This is the job of the “active
crossover”.
Its output signals are fed to six (yep,
six) separate amplifiers to drive the
woofer, midrange and tweeter units
in each loudspeaker enclosure. The
overall system layout is shown in the
block diagram of Fig.1.
So you end up with a lot more amplifiers than in a conventional system
but it gives you a lot more flexibility.
And ultimately, you can end up with
a system with higher performance,
including much higher power levels.
This shot inside the box reveals the simplicity of construction. Everything except the transformer is on one PC board!
www.siliconchip.com.au
January 2003 59
Fig.1: the block diagram shows the overall system
layout. It replaces the crossover currently in the
speaker enclosure.
The active crossover approach also
means you can mix 4Ω and 8Ω drivers in the same system and match the
levels easily, without power wastage.
Active crossover
The Active Crossover presented
here is housed in a 1-unit high rack
case with just the power switch on
the front panel.
There are no user controls for the
crossover; no switches to alter the
crossover frequencies nor external
level controls for the output signals.
To alter the drive to the loudspeakers,
you will need to adjust the volume
controls of the driver amplifiers.
On the rear panel there are four pairs
of RCA sockets, one pair for the stereo
input signals while the other three are
for the stereo low (woofer), midrange
and high (tweeter) signals.
Also on the rear panel is the IEC
mains power socket and a fuseholder
for the primary circuit of the power
transformer.
Inside the case, all the circuitry is
on a PC board measuring 219 x 99mm
and this includes the dual RCA input
and output sockets. The only external
wiring to the board are the secondary
connections to the toroidal power
transformer.
Circuit description
Now let’s have a look at the circuit
of Fig.2. Since both channels are identical, this shows only the left channel.
While the power supply is also on the
PC board, it is shown in Fig.3.
In total, the left channel uses 12 op
amps, in three TL074 quad FET-input
op amp packages.
Four op amps, IC1a, IC1b, IC5a &
IC5b, act as input or output buffers
while the remaining eight op amps
are Linkwitz-Riley active filter stages
with 12dB/octave filter slopes.
In each case, two 12dB/octave filters
are cascaded to give an overall filter
slope of 24dB/octave. This is far steeper than is normally used in passive
crossover networks.
The voltage gain of all these filter
stages in the passband is unity.
Low pass, high pass
Before we go any further we should
explain some terms which often confuse beginners: low pass, high pass
and bandpass.
A low pass filter is one that allows
low frequencies to pass through and it
blocks the higher frequencies. Hence,
a circuit to drive a subwoofer would
be called a low pass filter since it only
delivers frequencies below 200Hz or
thereabouts.
Similarly, a high pass filter is one
that allows high frequencies to pass
through and it blocks low frequencies.
Hence, the part of a crossover network
which feeds a tweeter is said to be a
high pass filter, even though it may
consist of only one capacitor.
If we cascade (ie, connect in series)
a high pass filter with a low pass filter,
the combination will pass a band of
Specifications
Voltage gain: Unity
Frequency response
Within ±1dB from 10Hz to 20kHz (see Fig.5)
Filter attenuation slope 24dB/octave
Total harmonic distortion
Typically .003% at 1V RMS
Signal to noise ratio
-94dB unweighted (22Hz to 22kHz) with respect to 1V RMS
Separation between channels Typically better than -100dB from 10Hz to 20kHz
Input impedance 47kΩ
Output impedance
60 Silicon Chip
less than 200Ω
www.siliconchip.com.au
www.siliconchip.com.au
January 2003 61
Fig.2: just 12 op amps and a few other components make up each channel of the active crossover. The six
outputs (three only shown here; three more in the right channel) each drive separate power amplifiers for the
tweeter, midrange and bass drivers in your loudspeakers.
Fig.3: the power supply is entirely conventional, using positive and negative 15V regulators to give ±15V rails. Everything
from the bridge on is mounted on the PC board. The seven 100nF capacitors are bypasses distributed around the PC board.
frequencies and we then refer to it as
a bandpass filter. We use a band-pass
filter for the midrange output in this
active crossover circuit.
The other points you need to know
about high and low pass filters are the
so-called cut-off frequency and the
filter slope.
The filters used in this circuit have
an attenuation of 12dB/octave; this
is the filter slope and it applies for
frequencies after the cut-off frequency.
The cut-off frequency is where the
signal output is -3dB down on the
normal level.
For example, in a low pass filter we
might have a cut-off frequency of 1kHz
(ie, -3dB point) and from there on the
filter slope could be 12dB/octave. In
theory, this means that the response at
2kHz (ie, one octave above 1kHz) will
be -15dB although in practice it might
not be quite that good.
The filters used in our circuit are
of the Linkwitz-Riley configuration
and we use eight of these filters, four
high pass and four low pass, in each
channel. Each filter consists of an op
amp connected as a voltage follower,
preceded by two RC networks.
As already noted, for each high pass
and low pass filter we are using two
12dB/octave filters cascaded, to make
the total roll off 24dB/octave (4th order) per filter stage.
The basic filter configurations are
shown in Fig.4, together with the
formula for calculating the crossover
frequency. In this particular case, the
crossover frequency is at the -6dB
point and the reason for this is that
we are cascading two filters for each
section (2 x 3dB = 6dB).
Note that the capacitors in the low
pass filter are shown with values of C
and 2C while in the high filter we have
resistors with values of R and 2R.
In the main circuit of Fig.2 you will
note two capacitors of equal values
have been used for the 2C component,
as it is difficult to obtain capacitor
values exactly double that of another.
On the other hand, resistors are much
easier and so we have values of 10kΩ
for R and 20kΩ for 2R.
Now after that little diversion, let’s
refer back to the circuit of Fig.2.
The input to the left channel is fed
via an RC filter, to roll off frequencies
above 100kHz, and then to op amp
IC1a which is connected as a unity
gain buffer (or voltage follower).
It drives two high pass filter stages
associated with IC1d & IC1c, and two
low pass filters associated with IC3a
& IC3d. Both these low pass and high
pass filters have cutoff frequencies set
to 5.1kHz.
The output of the second high pass
filter, IC1c, is fed to the level setting
trimpot VR1 and then to op amp IC1b
which is connected as a non-inverting
amplifier with a gain of two. It drives
the left treble output (tweeter). Hence
the tweeter only gets frequencies above
5kHz.
Midrange band-pass
Fig.4: the basic arrangements for the low pass and high pass filters.
62 Silicon Chip
The output of low pass filter IC3d
www.siliconchip.com.au
feeds high pass filters based on IC3c &
IC3b, both with cut-off frequencies of
239Hz. The output of high pass filter,
IC3b, is fed to trimpot VR2 and then to
op amp IC5a which has a gain of two.
This drives the left midrange output
which gets the band of frequencies
between 239Hz and 5.1kHz.
As well as driving high pass filters
IC3c & IC3b, op amp IC3d also drives
the cascaded low pass filters based on
IC5d & IC5c, again with a cut-off frequency of 239Hz. IC5c drives trimpot
VR3 and then op amp IC5b which has
a gain of two. It drives the left bass
output which only gets signals below
239Hz.
All the outputs from each stage
are in phase at the crossover points.
Voltage gain at the crossover frequency
for each section is -6dB (ie, half the
reference level).
Thus when the response curves of
all three sections are added together,
the result is an extremely flat frequency response with an overall gain of
unity.
Just how well this works is shown
in the response curves of Fig.5. We’ve
plotted the three filter responses and
then the resultant curve is plotted
along the top. The adder circuit we
used to do this is shown (for interest
only) at the end of this article in Fig.8.
Power supply
The power supply circuit is shown
in Fig.3. It uses a 20VA toroidal power
Parts List – 3-Way Active Crossover
1 1RU rack-mounting case, Altronics H-5011 or equivalent
1 PC board, code 01101031, 219 x 99mm
1 IEC power socket
1 chassis-mount safety fuseholder (3AG or M205 type)
1 0.5A fuse (3AG or M205 type to suit fuseholder)
1 DPST rocker switch with inbuilt neon (S1)
1 20VA toroidal transformer with 2 15V secondaries
1 3-way insulated terminal block
4 dual gold-plated RCA PC-mounting sockets, Altronics P-0212 or
equivalent
6 multi-turn 100kΩ trimpots (VR1-VR6), Altronics R-2382A or equivalent
Semiconductors
6 TL074 quad FET-input op amps (IC-IC6)
1 7815 positive 3-terminal regulator
1 7915 negative 3-terminal regulator
4 1N4004 silicon diodes (D1-D4)
Capacitors
2 1000µF 25V PC electrolytic
2 100µF 25V PC electrolytic
2 1µF 50V bipolar electrolytic
14 100nF (0.1µF) multi-layer ceramic
(code 100n or 104)
20 47nF (.047µF) metallised polyester (code 47n or 473)
20 2.2nF (.0022µF) metallised polyester (code 2n2 or 222)
2 220pF ceramic
Resistors (1% metal film)
2 47kΩ
(yellow violet orange brown or yellow violet black red brown)
8 20kΩ
(red black orange brown or red black black red brown)
38 10kΩ
(brown black orange brown or brown black black red brown)
4 100Ω
(brown black brown brown or brown black black black brown)
transformer with two 15V secondaries
driving a bridge rectifier (diodes D1 D4) and two 1000µF 25V capacitors
to derive unregulated DC supplies
of around ±22V DC. These are fed to
3-terminal regulators REG1 and REG2
to produce supplies of ±15V DC. These
are each bypassed by a 100µF 25V capacitor and seven 100nF multi-layer
ceramic capacitors distributed around
the PC board.
Construction
Fig.5: this graph shows the three filter response curves which were plotted
separately. The overall response curve at top (red) was plotted using the mixer
circuit in Fig.8. The overall response curve is extremely smooth.
www.siliconchip.com.au
As already noted, all the circuitry
is on a single PC board measuring
219 x 99mm, so construction is very
straightforward.
The only complication will occur if
you you wish to set your own crossover frequencies. If so, you will need
to select values from Table 1.
For example, if you decide you
want a tweeter crossover frequency of
around 3kHz, go to Table 1, run your
finger down the righthand column
until you get to 3100 and the R and C
values are in columns 1 & 2.
In practice, the 2.2nF capacitors
in the high- pass and lowpass filters
associated with IC1 and IC3 now have
to be changed to 3.3nF, while the 10kΩ
January 2003 63
Fig.6: the component overlay, as viewed from above the PC board.
Note the polarity of electrolytic capacitors and ICs when soldering
them in!
64 Silicon Chip
resistors increase to 11kΩ and the
20kΩ values go to 22kΩ.
Note that it is essential that both the
high pass filters (ICd & IC1c) for the
tweeter and the low pass filters (IC3a
& IC3d) for the midrange must have
exactly the same cut-off frequencies
otherwise you will not get an overall
flat frequency response.
Similarly, if you want to change the
bass cut-off frequency to around 350Hz
(say), run down the righthand column
of Table 1 to 347Hz. The R values then
become 11kΩ and 22kΩ while the C
values become 27nF.
Alternatively, if you want to do
the calculations yourself, visit www.
sherlab.com/filter/filter.htm for a filter
calculator.
Lots more information regarding
Linkwitz-Riley crossovers can be
found at www.rane.com/note160.html
Here they discuss lobing errors,
driver alignment & phase correction,
phase shift vs frequency etc.
Having decided on your crossover
frequencies, you can start assembly
of the PC board by closely checking
it for shorts between tracks, open
circuits etc, against the pattern
opposite.
Then install all the resistors, followed by the capacitors and multi-turn trimpots. Make sure that the
electrolytic capacitors are installed
the right way around. The bipolar
electro-lytics are not polarised and
can go in either way.
Ideally, 1% capacitors should be
used in all of the filter circuitry. As
an alternative, purchase a bag of 100
capacitors of the value you require
and pick the 20 that are the closest
in value to each other, using a capacitance meter or DMM with capacitance
ranges.
Next, install the two regulators
which are laid flat on the PC board. Be
careful not to swap them over otherwise the circuit definitely won’t work
and you may have to replace quite a
few damaged semiconductors. Finally,
you can install the op amps and the
RCA sockets.
You will then need to wire up the
power transformer, using the diagram
of Fig.7. Temporarily install the PC
board into the chassis and you are
ready for some voltage checks.
Voltage check
Apply power and check the regulated supply rails with your digital
www.siliconchip.com.au
This photo of an early prototype PC board shows the general layout of components. It should be noted that there have
been substantial changes since this photo was taken, particularly along the bottom (rear) of the board. The component
overlay (Fig.6) shows the final version.
multimeter. They should be close to
±15V DC.
Then check that +15V is present at
pin 4 of each TL074 and that -15V is
present at pin 11 of each IC. Lightly
touch each IC to ensure that none of
them are getting hot – they should all
be cool.
The next step is to align the whole
circuit using the trimpots. This is
a simple matter of setting up each
output for unity gain in its passband.
This can be done at three frequencies,
say 100Hz for the bass, 1kHz for the
midrange and 12kHz for the treble.
You will need an audio oscillator
and a digital multimeter with an AC
frequency response to 20kHz or better.
Connect your audio oscillator to the
input RCA connector in one channel.
Set the frequency to 100Hz, 1kHz or
12kHz, depending on which section
you wish to align. Set the level of the
oscillator to 1V RMS.
Then measure the signal level at the
output of the stage that you are adjusting. For the treble output, use 10kHz
and adjust trimpot VR1 (left channel)
or VR4 (right channel) to obtain 1V
RMS at the output socket.
Similarly, for the midrange, use
1kHz and adjust VR2 (left channel) or
VR5 (right channel) to obtain 1V RMS
at the output sockets.
Finally, for the bass, use 100Hz
and adjust VR3 (left channel) or VR6
(right channel) to obtain 1V RMS at
the output. That done, it is now a
matter of finally completing the wiring
inside the case and checking it before
connecting the unit to your amplifiers.
Your amplifiers
We mentioned before that six amplifiers are required; one for each of the
bass, midrange and treble speakers,
times two (for stereo). But what amplifiers should you use?
The completed project showing the rear panel arrangement, power supply wiring and PC board placement. Use this in
conjunction with Fig.7 (opposite) during final assembly.
www.siliconchip.com.au
January 2003 65
Fig.7: follow this wiring
diagram and you should
have no problems with final
assembly. Be especially
careful with the mains
wiring – note the heatshrink
covering all the “bitey” bits!
66 Silicon Chip
www.siliconchip.com.au
amplifier module
featured in this
issue for the midrange and treble.
The Ultra LD
(Nov, Dec 2001,
Jan 2002) or even
the Plastic Power module (April
1996) would make
a superb bass amplifier.
Connection
It is simply a
matter of connecting the stereo outputs from
the 3-Way Active
Cross-over to the
appropriate bass,
Fig.8: here is the adder circuit we used to produce the
mid-range and trediagram shown in Fig.5. You don’t have to make one of
ble stereo amplithese unless you are interested in measuring your own
fier inputs, then
circuit.
connecting the
amplifiers’ outTypically, the woofer amplifier puts direct to the appropriate drivers
needs to be about double the power of
in each of the speaker enclosures.
the midrange and tweeter amplifiers,
Needless to say, the existing crossoto take into account the lower sensi- ver network in the speaker enclosures
tivity of the woofers.
is disconnected completely – and you
So if you have been running a 100W will need to put an extra two sets of
per channel stereo amplifier into your
terminals on the back of your enclo3-way speaker system, you will still
sures with each of the three connected
need two 100W amplifiers for the
directly to a driver (and appropriately
woofers (eg, your exisiting amplifier!) labelled).
but you can get away with two 50W
The tone controls should ideally be
amplifiers for each of the midrange
flat on all amplifiers (although that can
and tweeters (ie, four total).
be a matter of individual taste – but the
You may be able to put back into ser- treble control won’t do much on the
vice an amplifier that you pensioned
bass amplifier nor the bass control on
off as “underpowered”.
the treble amplifier!).
Or, if you want to go the whole hog
Volume controls can be individuand build new amplifiers to go with ally adjusted to get the best balance
your new active crossover, you could
between the bass, midrange and treble
SC
do a lot worse than the new SC480 speakers.
There are no screws holding the PC board in place. Instead it sits on selfadhesive holders (as used in many computers) and the RCA sockets on the back
panel are themselves held in by screws.
www.siliconchip.com.au
Table 1: Values for R & C
R
C
2R
Crossover
Frequency
(kΩ) (nF) (kΩ)
(Hz)
15
47
30
160
15
39
30
192
12
47
24
200
11
47
22
218
15
33
30
227
10
47
20
239
12
39
24
240
11
39
22
262
15
27
30
278
12
33
24
284
10
39
20
289
11
33
22
310
7.5
47
15
319
15
22
30
341
10
33
20
341
12
27
24
347
11
27
22
379
7.5
39
15
385
10
27
20
417
12
22
24
426
7.5
33
15
455
11
22
22
465
10
22
20
512
7.5
27
15
556
7.5
22
15
682
15
4.7
30
1596
15
3.9
30
1924
12
4.7
24
1995
11
4.7
22
2177
15
3.3
30
2274
10
4.7
20
2394
12
3.9
24
2405
11
3.9
22
2623
15
2.7
30
2779
12
3.3
24
2842
10
3.9
20
2886
11
3.3
22
3100
7.5 4.7
15
3193
15
2.2
30
3410
10
3.3
20
3410
12
2.7
24
3473
11
2.7
22
3789
7.5 3.9
15
3848
10
2.7
20
4168
12
2.2
24
4263
7.5 3.3
15
4547
11
2.2
22
4650
10
2.2
20
5115
7.5 2.7
15
5558
7.5 2.2
15
6821
January 2003 67
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