Theory Of Operation; Inputs; Signal Conditioning; Compression - Klipsch SW12-I Service Manual

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Table of Contents

Klipsch Subwoofer Amplifier Service Manual

INPUTS

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 rolloff.

SIGNAL CONDITIONING

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

U4.

COMPRESSION

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.

DRIVER CIRCUIT

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.

OUTPUT

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.

POWER SUPPLIES

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.