30 Silicon chip Australia’s electronics magazine siliconchip.com.au
the CS5381, which offers even lower distortion.
The stereo line-level audio signals are fed in via RCA
sockets CON1a & CON1b. They pass through ferrite beads
with 100pF capacitors to ground, both intended to remove
any RF signals, either from the signal source or picked up
in the connecting leads. As the two channels are processed
identically before they reach the inputs of IC1, we’ll just
describe the left channel path.
The audio signal is then AC-coupled to non-inverting in-
put pin 3 of op amp IC2a, an NE5532 low-noise, low-distor-
tion device. Schottky diodes D1 and D2 prevent excessive
voltages from being applied to this op amp, eg, inductive
spikes generated by lightning or from incorrectly connected
equipment. A 100kresistor to ground provides a path for
direct current to flow out of that input pin.
IC2a buffers the signal, providing a low-impedance
source for the following filters. The signal is then fed to
op amp IC2b, an inverting amplifier with a gain of -1, due
to the use of two resistors of the same value in the feed-
back network.
A 33pF capacitor across the resistor between pins 7 (out-
put) and 6 (inverting input) rolls off the ultrasonic frequen-
cy response to provide stability.
The reason for this inverting stage is that the ADC chip
(IC1) is a differential design, so for both the left and right
channel inputs, it expects two signals, one 180º out of phase
with the other. The in-phase signal comes from the output
(pin 7) of IC2b, while the out-of-phase signal is taken di-Fig.4: the circuit of the ADC board. The two single-ended
input signals are filtered and converted into balanced signals,
then fed into analog-to-digital converter chip IC1.
Its digital output signal is fed to a ribbon cable via CON2 and onto the microcontroller DSP board.