LF Processing
The input signal of magnitude of .49Vpp from P1 at pin 1 from the input/output PCB enters
the first stage of the low frequency filtering circuits at R34. The two pole Butterworth filter
is used as the active filter configuration due to its flat frequency response. Unity gain to
prevent signal distortion with concise filtration characteristics begins at about the 3 kHz
cutoff frequency at a rate of -60dB/decade. U5B performs the first order filtering while
U5A continues with second order filtering to provide total low pass filtering of –
80dB/decade. The low noise NJM4580, U6A and B, provide a signal gain of two to increase
the signal level necessary to interface with the low frequency power amplifier without
adding distortion.
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