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THX Powered Subwoofer
HT PS300
BEFORE THIS AMPLIFIER IS PLUGGED IN, make sure its rated voltage corresponds to the voltage of the AC power source to be employed. Failure to use the correct voltage could cause damage to the amplifier when the AC power cable is plugged in. Do not exceed the rated voltage by more than 10%; operation below 90% will degrade performance or cause the unit to shut off.
1. PRINCIPLES OF OPERATION
Basic Bulk DC of ±50V nominal is rectified by bridge D1 from the 76AVC center-tapped winding of T1. The primary of T1 is protected by an embedded, non resettable thermostat, a 4A, 250V fuse (2A for 230V input) F1 (in clips on the PCB), and panel thermostat TM1. Resistors R1-R6 and zener diode D2 and D5 provide auxiliary ±15V DC for the low level signal processing circuits. In non-THX mode, the pair of low level signal inputs are summed at U1b together with attenuated and inverted (U1A) high level inputs. The feedback gain control, R95, closely approximates a logarithmic characteristic. The adjustable low pass section U2B combines with the fixed section U3A to attenuate high frequencies at an ultimate slope of -36dB/octave to minimize voice content in the sub-woofer. Selector switch SW2 chooses either the above (processed) inputs or a fixed-again direct "THX" input to pass on though the first stage of speaker EQ (U5A) to phase selection (U5B) and then to the fixed two EQ stages (U4A & U4B). R144 is factory preset to compensate for small variations in woofer efficiencies to meet THX calibration and should only be readjusted if the drivers are changed and the shop has proper equipment and experience to carry out the procedure. U8B and U8A continually monitor +V and -V respectively for their minimum values which automatically adjust the threshold for the signal peak detectors U9B (+) and U9A (-) which in turn control the onset of AGC compression by J-FET (voltage - controlled resistor) Q20. U10A provides linearizing feedback to Q20, while U7B and U10B generate out-of-phase signals for the two power amps with optional negative output impedance if components associated with speaker current sense resistors R160 & R154. Each power amplifier includes U1 feedback amplifier, class A level shifter Q1-Q4 and complementary Darlington buffer Q5-Q6. FET Q7 in series with Q6 sustains half of the rail-to-output voltages, as does Q8 for Q5 (with quasi-complementary driver Q9). The amplifier also contain an "auto mute" circuit which silences the amplifier after a signal absence of about 6 minutes. U6B amplifies level inputs by 40dB for threshold
comparator U6A. U11 is a retriggerable one-shot formed from a 16 stage counter and an oscillator of about 93 HZ which times out about 6 minutes after the last signal input is detected by U6A. Some models also contain an LED indicator DS1B (showing that the amplifier is enabled) driven by current source Q19. The versions, a second LED DS1C is illuminated in "stand by" by Q21, and on others, a dual color DS1A/DS1B combines the modes. The balanced bridges R205, R206, R207 & R211 and R208, R212 & R213, inverter U3B and Schmitt trigger Q23-24 detect the blowing of any of the amplifier fuses (F2-F5) and force counter U11 to flash the LED�s at about 0.5 Hz to alert the user to the need for service.
2. TROUBLE SHOOTING BEFORE OPENING
Check connections, control settings, driver and other possible external problems. If there is Output, determine if all controls and Inputs function properly. Rotate Pots over full range while applying lateral and vertical oscillating forces to locate possible intermittent function. High Level Inputs should be tested individually both differentially (signal from "-" to "+" with normal output) and in common mode (signal from low level ground to both "+" and "-" shorted together, giving virtually no output). While passing a signal, corner drop the enclosure a few inches to expose possible intermittent problems. Check woofer for rubbing of voice coil or tears in cone or surround. Check cabinet for loose extraneous articles which may have been pushed into rear port. If LED(s) are flashing at about 0.5Hz, then one or more of the internal fuses are blown.
3. REMOVING THE AMPLIFIER
This amplifier has a power switch. Hazardous voltages are present within the unit whenever it is plugged in. This still applies when the over-temperature thermostat opens.
