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SMALL CIRCUITS COLLECTION
012
Speech Filter
with
a
skirt
steepness
of
100 dB/decade.
The first opamp (IC1) acts as a
buffer. The circuit can be powered
by a unipolar supply voltage
between 5 V and 18 V, which is a
broad enough range that it should
always be possible to find a suitable
voltage when building the filter into
existing equipment. The current
consumption of the filter is only a
few milliampères, which should
rarely pose a problem.
There is fairly wide selection of suitable candidates for the
opamps, since the circuit is not critical in this regard. In
addition to the indicated OP27A, you could consider using
a TL081N or even an old-fashioned 741.
Due to unavoidable spreads in component values, the pass-
band curve of the filter will never be completely perfect in
actual practice. However, the deviations will be very small
and in any case inaudible. In the passband region, the gain
is approximately unity.
The printed circuit board design shown here allows the
speech filter to be built in a very compact form, which can
G.Baars
In communications receivers and microphone amplifiers
for transmitting equipment, there is frequently a need for a
narrow, low-frequency bandpass filter that lets only the
voice band through. This band is usually defined to be the
portion of the audio frequency spectrum between approx-
imately 300 Hz and 3300 Hz.
In order to implement such a filter, we have calculated
the values for two fifth-order Butterworth filters having
these corner frequencies and connected them in series.
The result is a bandpass filter for the desired passband
COMPONENTS LIST
Resistors:
R1.R2 = 22k
C16
Ω
R3,R11,R12,R18,R19 = 100k
C10
C12
10 µ
16V
IC3
18n
10n
R4 = 470
1
R13
3k3
R14
3k3
R15
3k3
R16
3k3
R17
3k3
7
3
R1
C15
R5 = 150
8
6
OP27A
R6 = 10k
2
100n
Bt1
4
R7 = 18k
9V
100k
R8 = 15k
C9
C11
C13
R18
R2
R19
R9 = 33k
Ω
R10 = 82k Ω
R13-R17 = 3k Ω
33n
22n
4n7
C14
3
100n
Capacitors:
C1,C8,C14,C15 = 100nF
C2 = 1µF MKT
C3-C7,C11 = 22nF
C9 = 33nF
C10 = 18nF
C12 = 10nF
C13 = 4nF7
C16,C17 = 10µF 16V
R3
C17
10 µ
16V
R7
R9
R10
IC1
1
C1
IC2
7
3
1
C2
C3
C4
C5
C6
C7
R4
470
8
7
6
3
OP27A
100n
Ω
8
2
6
OP27A
1 µ
22n
22n
22n
22n
22n
4
2
4
R5
R6
R8
100k
Semiconductors:
IC1,IC2,IC3 = OP27A, TL081CN
R11
R12
C8
Miscellaneous:
Bt1 = 9-V battery
100n
014050 - 11
38
Elektor Electronics
12/2001
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SMALL CIRCUITS COLLECTION
be an important factor if it must be fitted into existing equip-
ment. You can quickly check the fully assembled circuit by
momentarily measuring the voltages at the inputs and out-
puts of the three opamps. Half of the supply voltage should
be present at all of these locations.
(014050-1)
014050-1
C15
014050-1
C16
H1
+
R18
R19
(C) ELEKTOR
0
C2
C10
C12
C17
C14
(C) ELEKTOR
C1
H3
C8 C9
C11
C13
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