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A
LOW DISTORTION AUDIO PREAMPLIFIER
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In
an audio amplifier the quality of sound depends upon a
number of factors, e.g. quality of active and passive
components, circuit configuration, and layout. To an extent,
the selection of components depends on the constructor’s
budget. The discrete active components like transistors
have been increasingly replaced by linear ICs, making
the task of designer easier. With the passage of time,
the general-purpose op-amps like LM741, which were being
used in audio/hi-fi circuits, have become The preamplifier
circuit presented here is based on a dual precision op-amp
for the construction of a low distortion, high quality
audio preamplifier.
A dual op-amp OPA2604 from Burr-Brown is used for all
the stages. The FET input stage op-amp was chosen in this
context it is worthwile to mention another popular bi-polar
architecture op-amp, the NE5534A. It has, no doubt, an
exceptionally low noise figure of 4nV/ÖHz but rest of
the specifications compared to OPA2604 are virtually absent
in this IC. Also This IC is also capable of operating
at higher voltage rails of ± 24V (max.). Also its input
bias current (100 pA) is many orders lower than its bipolar
counterpart’s. This ensures a multifold reduction
in noise.
A channel seperation of 142 dB exists between In the circuit,
buffer is essential for the proper working of the subsequent
blocks. A nominal input impedance of 47k is offered by
this stage which prevents overloading of the preamplifier.
The tone control is a baxandall type filter circuit.The
bandwidth limiter is basically a low-pass filter with
an upper cut-off ceiling at the end of the useful audio
spectrum. The gain at 10 kHz is approximately 17 dB. The
design is essentially 3-pole type and the upper frequency
is set at 25 kHz. This lSetting the unit is fairly simple.
Check the power leads feeding the IC for symmetrical voltages.
High quality audio output from the line output socket
is to be fed as the input signal to this preamplifier.
Output of the preamplifier is fed to the power a The whole
circuit consumes about 10 mA when the above-mentioned
ICs are used. Power supply requirements are not critical
as the circuit works on 7.5V to 15V DC.. |
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