Filter
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Woofer Crossover
The input signal at U4A pin 1 connects to the 3200Hz 2nd order low-pass filter at U112B. Approximately 2dB of
additional gain can be added to this stage by adjusting R136 against the divider resistor, R137. The next stage is a
30Hz–60Hz switchable sub-sonic high-pass filter. This is followed by an adjustable low-frequency shelving filter
which defaults at +6dB gain, according to the feedback resistors R129 and R134, and dividers R131 and R128.
DS9–DS12 will switch in various reactive feedback networks to accomplish +4dB, +2dB, -2dB, or -4dB of gain
shelving below 200Hz. The last stage is another 3200Hz 2nd order low-pass filter at U2A, which cascades with the
previous filter at U112B to produce an overall 4th order slope of 24dB/octave for the woofer amplifier.
Thermal Protection
The thermal protection is activated, and shuts down audio operation, when the amplifier heatsink reaches an exces-
sively high temperature. The voltage divider R22 and R23 establishes the reference voltage on pin 5 of U5B. The
control voltage on pin 4 is established by the voltage divider TS1 and R25. TS1 is a NTC (Negative Temperature
Coefficient) thermistor mounted on the heatsink. As TS1 warms and the resistance falls, the voltage on pin 4 rises.
When the voltage on pin 4 exceeds the voltage on pin 5, the output on pin 2 goes low, shutting down the Soft Start
switches Q8 and Q24, and lighting the CLIP/THERMAL indicator red.
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