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DLS SOUND HANDBOOK - PASSIVE X-OVER FILTERS
PASSIVE 12 dB LOW-PASS
A 12 dB x-over filter has a 12 dB slope / octave.
A 12 dB filter is a combination of a coil and a
capacitor.
It is also called 2:nd order filter. 12 dB
low-pass filters are often used for subwoofers in or-
der to stop frequencies over the x-over frequency,
for example 100 Hz. A combination of a low- and high-
pass filter is called a band-pass filter.
2:nd order 12 dB low-pass filter:
1 2 3 4 5 6
1 2 3 4 5 6
L
CALCULATION FORMULA:
L (mH) = 225 x Z
Fc
Z = speaker impedance in Ω
Fc = x-over frequency in Hz
L = coil inductance in mH
C = capacitor capacitance in μF
When connecting coils in series the values are added.
Use this formula when connecting in parallel:
1
1 + 1 + 1
=
L
L
L
L
COMPONENT VALUES FOR 12 dB PASSIVE CROSSOVERS
X-over freq.
C (μ μ μ μ μ F)
in Hz
62,5
900
95
600
140
400
190
300
375
150
520
108
800
70
3500
16
5000
11
The same component values are used for both high-
and low-pass filters, but they change place.
Use coils with low resistance, air coils are the best.
Coils with iron core must be able to handle high
current or the iron core magnetic saturation becomes
to high causing distortion.
Capacitors must be of bipolar type, 50 - 100 Volt.
C
C (μF) = 112500
Fc x Z
2 Ω Ω Ω Ω Ω
C (μ μ μ μ μ F)
L (mH)
7,2
450
5
300
3,2
200
2,35
150
1,2
75
0,87
54
0,56
35
0,12
8
0,09
5,6
PASSIVE 12 dB HIGH-PASS
The difference between the passive 12 dB high-pass
x-over filter and the low-pass filter is that the coil and
capacitor change place. For a certain x-over
frequency the component values are the same for
both high- and low-pass filters. A high-pass filter let
high frequencies pass, and stops the lower.
2:nd order 12 dB high-pass filter:
C
CALCULATION FORMULA:
L (mH) = 225 x Z
Fc
Z = speaker impedance in Ω
Fc = x-over frequency in Hz
L = coil inductance in mH
C = capacitor capacitance in μF
When connecting capacitors in parallel the values are
added. Use this formula when connecting in series.
1
1 + 1 + 1
=
C
C
C
C
4 Ω Ω Ω Ω Ω
C (μ μ μ μ μ F)
L (mH)
14,4
225
10
150
6,4
100
4,7
75
2,4
38
1,75
27
1,12
18
0,25
4
0,18
2,8
IMPORTANT WHEN CONNECTING FILTERS!
When connecting a 12 dB low-pass x-over to a
subwoofer it´s suitable to solder the capacitor directly
on the sub terminals between + and -.
If the sub is disconnected without disconnecting the
capacitor at the same time the amplifier can be
damaged.
A 12 dB filter connected without a speaker will
overload the amplier (if it´s turned on) and damage
the output circuits.
8
1 2 3
1 2 3
1 2 3
L
1 2 3
C (μF) = 112500
Fc x Z
8Ω Ω Ω Ω Ω
L (mH)
28,8
20
12,8
9,5
4,8
3,5
2,25
0,5
0,36
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