an enclosure, both potentiometers can be pro-
vided with a scale to indicate the amplifica-
tion and indicator signal level respectively.
The enclosure is preferably made from
metal, because this minimises possible dis-
turbances from any potential external noise
sources. The metal of the enclosure needs to
be connected to ground of the circuit. For
example, a suitable enclosure is type
Box1590N1 from the company H.O.D. How-
ever, the space inside this enclosure is just a
little too small to fit the battery as well — in
this case the next larger size will have to be
used. For completeness’ sake, we will men-
tion that it was naturally the intention that D6
is mounted in such a way that it is visible
from the outside.
As already mentioned, the current con-
sumption of the circuit is so low (about 7 mA)
that a power supply consisting of a 9-V bat-
tery can suffice. However, if you will be using
the circuit in a more permanent application
than a mains adapter is certainly more appro-
priate. A good adapter with regulated output
is preferred. It is particularly important that
the no-load voltage of the adapter does not
exceed 12 V, because that is the maximum
operating voltage of IC1.
(020189-1)
the layout and component overlay of
which is shown in
Figure 2
. To
make sure that the adjustment and
indicator parts of the circuit don’t
‘bite’ each other, there is a deliberate
separation between the grounds of
both sub-circuits. The circuit board
is quite densely populated with
parts, so construction should not be
hurried.
The terminals for the inputs and
outputs are nicely grouped on one
side of the PCB, while on the oppo-
site side two SIL headers can be
found for the connections to P1 and
P2. These components have deliber-
ately not been fitted directly on the
PCB so that everyone can determine
for themselves what type of input
and output connectors (e.g. mini-
jack, cinch) and potentiometers to
use. In addition, this also affords
greater freedom in the selection of a
suitable enclosure.
The wiring between the input
and output connectors needs to be
done with shielded cable. Standard
hook-up wire can be used for the
potentiometers, but with the chan-
nel separation in mind, it is best to
keep the wiring to P1 as short as is
possible. It is also recommended to
use a potentiometer with a metal
housing for this, the case of which is
connected to ground with a short
wire. Make sure that when wiring P1
and P2 that they are connected in
such a way that the level increases
when they are turned clockwise. The
numbers of the connections shown
on the schematic correspond with
those of the SIL headers on the PCB.
Once the circuit has been built into
MINI
-PROJECT
45
11/2002
Elektor Electronics
Some measurement results
Input impedance
47 k
Ω
Input voltage max.
(THD = 0.1%)
3.2 V
RMS
Output voltage max. (THD = 0.1%)
3.1 V
RMS
THD+N (B=80 kHz) 200 mV in, 20 mV out
0.027 % (20 Hz - 20 kHz)
200 mV in, 200 mV out
0.005 % (20 Hz - 20 kHz)
200 mV in, 2 V out
0.002 % (20Hz - 1 kHz)
0.028 % (20 kHz)
THD+N (B=80kHz) 2 V in, 200 mV out
0.0026 % (20 Hz - 1 kHz)
0.007 % (20 kHz)
2 V in, 2 V out
0.002 % (20 Hz - 1 kHz)
0.01 %
(20 kHz)
Channel separation
> 66 dB (1 kHz)
> 42 dB (20 kHz)
Current consumption
LED off
6.7 mA
LED on
7.8 mA
Summary of Contents for EPROM
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