OP470
SQUELCH AMPLIFIER
FIVE-BAND LOW-NOISE STEREO GRAPHIC EQUALIZER
The graphic equalizer circuit shown in Figure 18 provides 15 dB
of boost or cut over a 5-band range. Signal-to-noise ratio over a
20 kHz bandwidth is better than 100 dB referred to a 3 V rms
input. Larger inductors can be replaced by active inductors but
this reduces the signal-to-noise ratio.
The circuit of Figure 17 is a simple squelch amplifier that uses a
FET switch to cut off the output when the input signal falls
below a preset limit.
The input signal is sampled by a peak detector with a time
constant set by C1 and R6. When the output of the peak detector
(Vp), falls below the threshold voltage, (VTH), set by R8, the
comparator formed by op amp C switches from V– to V+. This
drives the gate of the N-channel FET high, turning it ON, re-
ducing the gain of the inverting amplifier formed by op amp A
to zero.
C1
0.47ꢀF
V
IN
R2
1/4
3.3kꢅ
R1
R14
OP470E
47kꢅ
1/4
100ꢅ
V
OUT
OP470E
R4
R13
1kꢅ
C2
3.3kꢅ
L1
1H
R3
6.8ꢀF
D2
680ꢅ
+
R5
60Hz
1N4148
100kꢅ
TANTALUM
C3
R4
2N5434
1kꢅ
L2
1H
R5
1ꢀF
680ꢅ
+
R2
200Hz
10kꢅ
TANTALUM
C4
R1
R4
2kꢅ
1kꢅ
L3
1H
V
R7
IN
0.22ꢀF
1/4
680ꢅ
+
800Hz
V
– –5V
OP470E
A
OUT
IN
TANTALUM
C5
R4
1kꢅ
L4
1H
R4
R9
0.047ꢀF
10kꢅ
680ꢅ
+
3kHz
R3
2kꢅ
TANTALUM
C6
D1
1N4148
R4
1/4
1kꢅ
L5
1H
R11
OP470E
B
0.022ꢀF
680ꢅ
1/4
+
C1
R6
10kHz
1ꢀF
1Mꢅ
OP470E
C
TANTALUM
R4
10Mꢅ
Figure 18. Five-Band Low Noise Graphic Equalizer
ꢇ
= 1 SECOND
R7
10kꢅ
C2
10ꢀF
+
V+
R6
10kꢅ
Figure 17. Squelch Amplifier
–14–
REV. B
ADI [ ADI ]
