Elektor EPROM, Manual

Page: 31 / 50

the maximum amplification of IC1a is equal
to (R5+P1)/R4 and the minimum is equal to
R5/(P1+R4). Because of small losses else-
where, this results in an adjustment range
from 10.5 to 0.09, which pretty much corre-
sponds to ±20 dB.
Resistor R6 (R15) was added to prevent
potential instability of the amplifier as a result
of parasitic capacitance at the input of the
opamp in case the wiring to P1 is a little bit
long. Should the wiper of P1 be sporadically
intermittent, R7 (R16) ensures that there is
always negative feedback around the opamp
and the output will always be nicely at half
the power supply voltage. For stability rea-
sons, C2 (C5) limits the bandwidth to 60 kHz
at maximum amplification.
Another two components to complete the
list: R8 (R17) ensures that the amplifier will
be unconditionally stable even with a capac-
itive load at the output (because of long output
cables, for example). R9 (R18) makes sure
that the output electrolytic capacitor is
always charged to prevent loud noises when
the output load is plugged in.
It is worth mentioning that for IC1 we
deliberately selected a rail-to-rail opamp, so
that despite the low power supply voltage,
reasonably high input voltages are still pos-
sible. The maximum is nominally more than
3 V
RMS
, while with a nearly exhausted bat-
tery 2 V
RMS
is still not a problem.
Indicator
The overdrive indicator is built around quad
comparator IC2. This IC is used to build a
window comparator for each channel. The
window comparator compares the output
signal of the adjustable amplifier stage with
a variable reference. The outputs of IC2 are
of the open-collector type and can therefore
be simply connected together. If one of the
comparator input signals exceeds the refer-
ence voltage, adjusted with P2, LED D8 will
light up.
A few additional details regarding this cir-
cuit. The reference is set symmetrically
around half the power supply voltage with
the aid of R19, D5 and R20, so that both the
negative as well as the positive half of the
signal voltage is monitored. Because the
zener current is set to a really small value of
only 1 mA, the voltage drop across D5 is only
about 5.3 V. R21, R22, R23 and P2 are dimen-
sioned such that the reference voltage,
adjustable with P2, corresponds to signal
voltages from 200 mV to 1 V
RMS
.
As can be seen in the schematic, LED D6
(and series resistor R24) is connected in par-
allel with an electrolytic capacitor (C7). This
has been added to provide a certain amount of
afterglow so that short duration sig-
nal peaks will become better visible.
R25 limits the maximum charging
current of C7 to a safe value for IC2.
Finally, R26 and C8 decouple the
power supply so that any potential
noise from the switching of the LED has the least possible impact on the
amplifiers.
Construction
An extremely compact circuit board
has been designed for this circuit,
MINI -PROJECT
44 Elektor Electronics 11/2002
020189-1
(C) ELEKTOR
C1
C2
C3 C4
C5
C6
C7 C8
C9
C10
C11
C12
D1
D2
D3
D4
D5
D6
H1
H2
H3
H4
IC1 IC2
OUT1 OUT2
P1 P2
R1
R2
R3
R4
R5
R6
R7
R8
R9 R10
R11
R12
R13
R14
R15
R16
R17
R18
R19
R20
R21
R22
R23
R24 R25 R26
R27
R28
020189-1
-
+
9V
S1
T T T T
R R L L
020189-1
(C) ELEKTOR
Figure 2. The dimensions of the PCB make an extremely compact construction of
the level adapter possible.
COMPONENTS LIST
Resistors:
R1,R10 = 270k
Ω
R2,R6,R11,R15 = 1k
Ω
R3,R12 = 56k
Ω
R4,R5,R13,R14 = 4k
Ω
7
R7,R16 = 1M
Ω
R8,R17,R26 = 100
Ω
R9,R18 = 100k
Ω
R19,R20 = 1k
Ω
8
R21,R22,R27,R28 = 47k
Ω
R23 = 10k
Ω
R24 = 6k
Ω
8
R25 = 820
Ω
P1 = 47k
Ω
stereo potentiometer,
linear
P2 = 100 k mono potentiometer,
linear
Capacitors:
C1,C3,C4,C6 = 4µF7 63V radial
C2,C5 = 47pF
C7 = 47µF 25V radial
C8,C12 = 220µF 25V radial
C9,C10 = 100nF
C11 = 100µF 10V radial
Semiconductors:
D1-D4 = 1N4148
D5 = zener diode 5.6V, 0.4W
D6 = LED, red, high-efficiency
IC1 = TS924IN ST (Farnell)
IC2 = LM339
Miscellaneous:
S1 = rocker switch, 1 make contact,
for chassis mounting
BT1 = 9 V battery with holder
Two 3.5 mm stereo jack sockets,
chassis mount
PCB, order code 020189-1 (see
Readers Service page). PCB layout
file also available from Free
Downloads at
www.elektor-electronics.co.uk