Elektor EPROM, Manual

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MINI -PROJECT
43 11/2002 Elektor Electronics
ply voltage. Because the circuit is powered
from an asymmetric power supply, a coupling
capacitor is present at the input. This capac-
itor is always charged by R1 and R10 to pre-
vent any unwanted sound effects when the
signal source is plugged in. The coupling
capacitor is followed by an additional protec-
tion network, guarding against high peak
voltages. This network consists of two diodes
and a resistor.
For practical reasons, an inverting topol-
ogy was purposely selected for the adjustable
amplifier stages IC1a and IC1d. By using the
inverting inputs, the load on the potential
divider for the virtual ground (R27/R28) is
smaller, allowing higher resistance values
and consequently a lower total current con-
sumption.
If, for the moment, we limit the description
to the left channel only, we can establish that
few mA, the level adapter can sim-
ply be powered from a 9-V battery;
that makes it pleasantly universally
deployable.
Control circuit
If the circuit were only required to
attenuate, then we could have made
it easy for ourselves and a simple
passive voltage divider would have
sufficed. But in this case we also
wanted to amplify and, in addition,
we would also like a tidy circuit in
which the input and output-imped-
ance are not affected by the position
of the potentiometers.
That is why we decided to go for a
relatively ‘mature’ design. Each
channel consists of an input buffer
and an adjustable amplifier stage.
There is a single indicator common
to both channels. In order to power
the opamps symmetrically from a 9-
V battery, a virtual ground point has
been created with voltage divider
R27/R28/C11.
Figure 1 shows the
complete schematic.
The indicator will be described
later, we will first take a look at the
buffer and amplifier stage.
The function of the input buffers
IC1c and IC1b is to provide a con-
stant load for the attached signal
source. The values are such that the
input impedance is fixed at the well-
known standard value of 47 k
Ω
. The
opamps are connected as voltage fol-
lowers, while resistors R3 and R12
bias them to half of the power sup-
6
7
1
IC2.B
4
5
2
IC2.A
10
11
13
IC2.D
8
9
14
IC2.C
R23
10k
R19
1k8
R20
1k8
P2
100k
3
1
2
R21
47k
R22
47k
D5
5V6
0W4
+9V
R25
820
Ω
R24
6k8
R26
100
Ω
D6
C7
47
µ
25V
C8
220
µ
25V
+9V
2
3
1
IC1.A
9
10
8
IC1.C
R7
1 M
R8
100
Ω
R5
4k7
R4
4k7
R2
1k
R1
270k
R3
56k
R9
100k
R27
47k
R28
47k
R6
1k
C1
4
µ
7
63V
C2
47p
47k
1 3
2
C3
4
µ
7
63V
D2
1N4148
D1
1N4148
+9V
13
12
14
IC1.D
6
5
7
IC1.B
R16
1 M
R17
100
Ω
R14
4k7
R13
4k7
R11
1k
R10
270k
R12
56k
R18
100k
R15
1k
C4
4
µ
7
63V
C5
47p
47k
4 6
5
C6
4
µ
7
63V
D4
1N4148
D3
1N4148
+9V
C11
100
µ
10V
C12
220
µ
25V
BT1
9V
S1
C10
100n
C9
100n
IC2
12
3
IC1
11
4
+9V
020189 - 11
L
L
R
R
IC1 = TS924IN
IC2 = LM339
lin.
lin.
P1.B
P1.A
P2: 200mV... 1V(RMS)
Figure 1. The level adapter actually consists of two separate circuits: the adjustment section and an overdrive indicator.
1
2
3
4
5
6
P1