Theory Of Operation; 502C Controller; Differential Input Stage; Equalizer For High Frequency Outputs - Bose Panaray 502A Service Manual

502C Controller

The following provides an overview of 502C electrical operation. Equalization curves are refer-
enced and are located at the end of this section. Additionally, the 502C block diagram and
schematic should be referenced as required.

1. Differential Input Stage

The input stage features protection against RFI (radio frequency interference), ESD (electro-
static discharge), and overvoltage. Referring to the Channel 1 circuit, R2/C1, R1/C120, R3/C2,
and R9,10/C7 provide low pass filtering to reduce the possibility of RFI. Diodes D1-D4, D25,
and D26 provide input clamping to protect against overvoltage and ESD.
Op-amp U1 amplifies the difference between the + and - input pins, and converts it to a single
ended signal that drives the equalizers. Gain switch S3 selects between two gains, a high gain
setting for -10dB sources and a low gain setting for +4db sources.

2. Equalizer for High Frequency Outputs

Op-amps U1, U2 and U3 constitute the principal equalization for the high frequency output.
Each stage makes a contribution to the total low or high frequency contouring required by the
loudspeaker. Op-amp U4 (pins 12, 13 and 14) implements a high pass filter suitable for the Full
Range mode, while op-amp U4 (pins 8, 9, and 10) is selected for a high pass filter with a higher
corner frequency for use in the Bi-amp mode. The circuitry is duplicated for both channels.
Refer to the high frequency equalization curves for more information about the stage by stage
response of the equalizer.

3. Output mode switch, Low frequency level control, and Normal/Sum mode stage

Output mode switch S2 (on the rear panel) controls op-amp summing amplifier U6 (pins 5, 6,
and 7). In the normal mode, the stage is not used, and the Channel 1 and Channel 2 low fre-
quency outputs pass independent, 2-channel bass to 2 bass cabinets. In the sum mode, Chan-
nels 1 and 2 are combined, and this "mono" bass signal appears at the Channel 1 low fre-
quency output only. Potentiometer R126 (on the rear panel), Low Frequency Level, provides for
adjustment of the signal sent into the low frequency equalizer.

4. Equalizer for Low Frequency Outputs

After the normal/sum stage, there is op-amp stage U7 (pins 12, 13, and 14) and op-amp equal-
izer stages U7 (pins 8, 9, and 10), U8 (pins 12, 13, and 14), and U8 (pins 8, 9, and 10). The
equalizer provides high pass, low pass, and contouring equalization. See the low frequency
equalization curves for more information about the stage by stage response of the equalizer.

5. Mode switch

Rotary switch S1 is located on the rear panel. It selects the four operating modes for the
®
Panaray system. Currently, only Modes 1 (Full range) and 2 (Bi-Amp) are supported.

THEORY OF OPERATION

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