Download bass extension for surround sound

Contents
bass extension for surround sound
AMPL(dBr) vs FREQ(Hz)
10.000
5.0000
0.0
-5.000
-10.00
-15.00
-20.00
-25.00
-30.00
-35.00
-40.00
10
100
1k
974038 - 12
The extension is intended primarily
for surround-sound installations
that need some boosting of the
bass frequencies but where an additional subwoofer cannot be afforded. It is based on a disused
mono a.f. amplifier and loudspeaker. If these provide reasonable
bass performance, they can be conK4
verted into a fairly good subwoofer
with the aid of an active low-pass
filter—see Figure 1.
The input signals for the lefthand and right-hand channels are
applied to audio sockets K1 and K2
respectively. They are output via
audio sockets K3 and K4 to which
the surround-sound decoder is
connected.
The signals of the two channels
are summed in IC1a, which also
functions as input amplifier. The
amplification, and therefore the
sensitivity of the ‘subwoofer’, can
be adjusted with P1.
The output of IC1a is applied to
a 2nd-order Butterworth low-pass
filter. The cut-off frequency of this
active filter can be set between
40 Hz and 120 Hz with stereo potentiometer P2. The response char-
C1
R
22p
K3
P1
C2
C3
220n
180n
L
47k
K1
L
2
R2
47k
IC1a
P2a
R3
1
P2b
4k7
IC1b
R4
10k
K6
R6
10k
100Ω
C4
C5
100n
100n
IC1 = TL072
Tr1
C15
K7
IC2
47n
15V
7815
B1
C14
C13
47n
47n
Resistors:
R1, R2 = 47 kΩ
R3, R4 = 4.7 kΩ
R5, R6 = 100 Ω
R7 = 8.2 kΩ
P1 = 47 kΩ logarithmic potentiometer
P2 = 10 kΩ, linear stereo potentiometer
Capacitors:
C1 = 22 pF
C2 = 220 nF
C3 = 180 nF
C4–C7 = 100 nF
C8, C9 = 4.7 µF, 63 V, radial
C10, C11 = 22 µF, 40 V, radial
C12–C15 = 47 nF ceramic
Semiconductors:
D1 = LED, high efficiency
C12
47n
Parts list
7
5
4k7
3
8k2
R
100Ω
6
47k
K2
K5
R5
R1
acteristic of the filter at both these
frequencies is shown in Figure 2.
The actual cut-off point depends
on individual taste.
The said mono amplifier is connected to audio output sockets K5
and K6.
The power supply for the circuit is simple and consists of a
small mains transformer, Tr1, a
R7
C10
C8
C6
22µ
40V
4µ7
63V
100n
C11
C9
C7
4µ7
63V
100n
Integrated circuits:
IC1 = TL072CP
IC2 = 7815
IC3 = 7915
D1
2x 15V
1W5
B80C1500
POWER
Miscellaneous:
K1–K6, K8–K9 = audio socket for
board mounting
K7 = 2-way terminal block, pitch 7.5
mm
B1 = B80C1500
Tr1 = mains transformer, 2 × 15 V
secondaries, 1.5 VA
8
IC1
22µ
40V
IC3
7915
62
4
974038 - 11
15V
Elektor Electronics
12/97
Contents
H2
bridge rectifier, B1, antihunt capacitors C12–C15, a number of
smoothing and decoupling capacitors, and two integrated voltage
regulators, IC2 and IC3.
The filter circuit is best built on
the printed-circuit board shown in
Figure 3, which is, however, not
available ready made.
The filter should be housed in
a metal case. Moreover, P1 and P2
should preferably be types with a
metal enclosure. Hum is prevented
by earthing the case and the enclosures.
The harmonic distortion, with
two input signals of 200 mV and a
bandwidth of 22 kHz, is 0.0016%
at 30 Hz.
Although not of prime importance at low frequencies, the polarity of the ‘subwoofer’ should be
the reverse of that of the remainder
of the system since the present circuit inverts the signals.
L
K1
K2
R
K4
IN2
K3
R
R2
C6
K6
IN4
974038-1
1-830479
IN5
R6
C8 C15
C10
R4
IC1
R1
IC2
C4
C5
C12
H7
B1
K7
C1
C3
C11
P2
H1
C14
IC3
H5
TR1
C9
C13
R7
H6
H4
C7
P1
D1
[974038]
ELEKTOR
240V
~
50Hz
No. 974038
974038-1
P = 1VA5
Elektor Electronics
H3
L
R5
C2
R3
K5
IN3
12/97
63