Woofer Crossover
The input signal at U4A pin 1 connects to the 2500Hz 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
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 acco4dB, +2dB, –2dB, or –4dB of gain shelving below 200Hz. The last stage
is another 2500Hz 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.
Temp
TS1, R25,
Comparator
U5B
Soft Start Switches
Q8, Q24
CLIP/THERMAL
Indicators
CR3, CR4
Thermal Protection
The thermal protection is activated, and shuts down audio operation, when the amplifier heatsink reaches an
excessively 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 in parallel with TS2, and R25 and R162. TS1 and TS2
are NTC (Negative Temperature Coefficient) thermistors mounted on the heatsink. As TS1 or TS2 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 both CLIP/THERMAL indicators red.
– 18 –
Unbalanced
Input Signal
U4A
Low Frequency
Shelf
U112A
DS9 - DS12,
C85
2nd Order LP
2500Hz
U112B, R85,
R98, C64, C48
Sensitivity Trim
R136, R137
2nd Order HP
30Hz
U112A, R164,
R130, C45, C46
2nd Order LP
2500Hz
U2A, R82, R44,
C36, C42
!
Qualified Service Personnel Only
