Sunday, February 08, 2015



            There are two types of inputs and passive outputs. Four RCA jacks for LINE LEVEL left and right inputs and outputs and a panel of SPEAKER LEVEL inputs and outputs for left and right stereo channels. The SPEAKER LEVEL inputs have a high pass filter on each channel that allows frequencies above 120 Hz to go directly to the main speakers of the system. Frequencies below 120 Hz go through the subwoofer amplifier and on to the woofer. The LINE LEVEL inputs also use a high pass filter that allows frequencies above 80 Hz to go back out to a system amplifier. Frequencies below 80 Hz go through the subwoofer amplifier and on to the woofer. Both filters have a 6dB per octave roll off.
         The first op-amp of U1, a quad op-amp IC, is a summing amp for the RCA and SPEAKER level inputs. The LEVEL adjustment is located between the first and second op-amp stages of U1. The second op-amp of U1 is a buffer stage with a gain of 1. The third op-amp of U1 is a High pass filter configuration. The fourth op-amp of U1 is used to adjust phase from 0° to 180°. The next circuit is an adjustable RC filter for low pass filtering from 40Hz to 120Hz consisting of U2 and
The compression circuit consists of U5, U6, and Q28 and is adjusted via R30. If adjusted correctly, this circuit will limit the amplitude of the signal so the minimum amount of distortion in the form of clipping will occur at the output of the amplifier. R34 is used to set the maximum output level for the amplifier so it doesn’t underamplify or clip. Next, the signal goes through a buffer amplifier U2, which provides a gain of 4. Then the signal goes on to the driver circuits.
          The driver circuit consists of Q1 through Q12 and is located on the Filter/Driver board. The driver circuit has a DC offset adjustment, R50, which should be set at 0 Vdc +/- 50mVdc. This can be measured across the + and - output wires (white & black respectively) from the amp with power on and no signal applied. The driver circuit is a Class AB amplifier.
           The output section of this amplifier uses discrete components Q15 through Q26 to obtain the high power output required. The output amplifier circuit is a Class AB complementary symmetry configuration with three sets of four transistors cascaded. This provides high power output into a low impedance load. The components for the output circuit is located on the Power Board.
           The main power supply circuit is unregulated and supplies +/- 81 Vdc for the output and driver circuits. The +/- 81 Vdc is then divided and regulated by 34V zener diodes to supply +/- 34 Vdc to bias the driver circuit. There is a second power supply which is fed by a second set of taps off of the transformer. This is the regulated +/- 15 Vdc for the op-amps on the filter board.
                This amplifier uses a control circuit to control the operation of the output relay. When power is first applied the circuit provides a delay before activating the relay. This is a protection feature to protect the speaker and amplifier from initial current surges. The circuit also provides protection from over-current conditions by de-activating the relay when excessive output current is sensed. The relay and it’s control circuitry is located on the Filter/Driver Board and consists of U11, U12, RLY 1 and R131. R131 is used to set the threshold level for over-current sensing.

Align sub woofer amplifiers.
Equipment required:
A signal source capable of supplying a 30Hz sine wave at 300mVrms.
A true RMS Voltmeter such as the Fluke 8060B.
A 16 ohm load rated for at least 200 Watts.
An oscilloscope (optional).
To totally align SW12 series 1 amplifiers, follow this procedure:
1. Disconnect power from the UUT (unit under test).
2. Connect the UUT (unit under test) to a 16 ohm load.
3. Connect a signal generator to the RCA input of the amp.
4. Set all controls on the UUT to their full clockwise position.
5. Set the signal generator for 30Hz and 60mVrms output. (be sure and measure the output of the generator for 60mVrms.)
6. Connect the voltmeter leads to the output of the UUT.
7. Apply power to the UUT.
8. Adjust R34 for 33Vrms. Range is from 32.1 Vrms to 34 Vrms.
9. Change signal level to 1.5mVrms @ 30Hz .
10. Measure the output voltage. Should be between .94 Vrms and 1.06Vrms. Adjust R30 if necessary.
NOTE: Some interaction between adjustments is common. Recheck steps 8 and 10 for proper voltages.
11. Alignment of the UUT is now complete. Disconnect power and other connections from the UUT

A. Blows fuse with nothing connected to the output.
1. Check output transistors on power board.
Replace transistors that are shorted.
2. Check driver transistors on filter/driver board. Replace transistors that are shorted.
3. Check U7 and U9 for shorts.
4. Check bridge rectifier in power supply circuit.
5. Check transformer.
6. Check filter capacitors. C21,C22, C23, C24, C44, C45, C46, and C47.
7. Check for pinched wires going to and coming from
the transformer.
B. Low output level using speaker level inputs.
1 Check inputs for correct polarity from source. If polarity is not correct, the signals will be 180° out of phase and will cancel each other out causing a low output.
2 Check R79 and R80.Should read 91K:.
3 Check U13.
4 Check for proper signal flow using schematic.
C. Causing popping sound.
1 Connect amplifier output to a 16 ohm load. Apply a 60mVrms 30Hz signal to one RCA jack. The voltage across the load should be between
32.1 Vrms and 34 Vrms for SW12.
29.1 Vrms and 30 Vrms for SW15. For the THX amp., apply a 3Vrms 70Hz signal to one RCA jack. The voltage across the load should be between 35.1 Vrms and 36 Vrms. If this is correct, connect an oscilloscope to the
output signal and look for clipping. If the voltage was too high, re-align amplifier per procedure.
If the signal was clipped, re-adjust the amplifier per alignment procedure on page 5, 6, or 7.
D. Excessive hum.
1 Check all ground connections.
2 Check all filter capacitors. C21, C22, C23, C24, C44, C45, C46, and C47.
E. Amplifier quits working after a short period of time.
1 Check for excessive heat on heat sink. This indicates a a bad output device or a bad thermal switch.
2 Check output relay (RLY1).
3 Check U11 and U12.