MINI
-PROJECT
42
Elektor Electronics
11/2002
Standardisation is a long way off in many
areas. This is certainly true for audio signal
levels — a phenomenon that many of you will
have encountered at one time or another, usu-
ally at the most inopportune moment. With
equipment such as cassette decks, tuners,
CD/DVD-players, one can never be quite sure
what signal level to expect. Any-
thing is possible from around 200
mV to 2 V, or so.
If you wish to connect a signal
source to an input that does not have
a good input level adjustment, then
this can easily lead to unpleasant sit-
uations. A PC sound card, for exam-
ple, is generally not provided with a
level control. In addition, the maxi-
mum allowable input voltage of such
a card is limited by the 5-V power
supply of the chipset. This limits the
input to signals of less than 1 to 1.5
V, before clipping occurs.
The drawbacks that are caused in
practice by such fixed input sensi-
tivities and limited output signal lev-
els can be mitigated by preceding
the relevant line input with the con-
trol circuit that is described here.
The signal levels of 2 V that modern
CD- or DVD-players often provide
can be reduced to an acceptable
maximum. On the other hand, the
output level of 200 mV that older
equipment typically source, can be
amplified, something that will
improve the signal to noise ratio.
The adjustment range of our cir-
cuit amounts to no less than ±20
dB, so every conceivable signal
adaptation should be possible. In
order to avoid being unpleasantly
surprised by excessive signal levels,
an LED indicator has been added
which will indicate immediately
when the output signal exceeds a
certain (adjustable) value. Because
the current consumption is a mere
Audio Level Check
for Line Input
for the PC sound card
Design by T. Giesberts
This simple control circuit allows the level of an external stereo audio
source to be adapted to the sensitivity of, for example, a sound card. In
addition, an LED indicates if any signal levels are present that can cause
the input to be overdriven.
Summary of Contents for EPROM
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