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6. High Frequency Equalization

High frequency equalization is achieved through four cascaded active EQ stages consisting of

U101, U102, U103 and U104. The purpose of equalization is two-fold. (1) It provides very sharp

low frequency roll-off below 180 Hz (the crossover frequency of the acoustics) to prevent

overload of the L/R speakers; and (2) It provides finely tailored correction in the L/R speaker

pass band so that overall frequency response of the system is correct.

7. Automatic Dynamic Loudness

In order to compensate for the ear's loss of bass response at low listening levels, a Bose

patented automatic loudness circuit is employed. This circuit automatically senses the volume

level of the incoming audio signal, and properly adjusts the amount of low frequency bass boost.

When the volume level is high, the frequency response of the circuit is flat. When the volume

level is decreased (at the customer's audio source or through the electronic volume control),

the low frequency gain is increased in the region between 50 and 150 Hz.

Circuit operation is as follows: After passing through the Tone Control PCB, audio at U201 and

U202 (pin 7) is summed by resistors R101 and R131. The AC signal at pin 5 of level detect op-

amp U203 is negative -peak detected, and is presented across emitter resistor R236. The peak

detected signal has a 4 second hold time, and a 4 sec / 10 dB release time constant which is

developed within the feedback loop of U203 (pins 5,6 and 7). The voltage across R236 is

converted to a current source through transistor Q203, and flows into pin 5 of transconductance

amp U206. U206 and U207 (pins 1,2 and 3) form a variable gain stage (delta-G). Action is

such that increased current into pin 5 of U206 causes gain of the stage to decrease.

The variable gain stage is followed by a 55 Hz, Q=2.3 band pass filter (U207 pins 5,6 and 7).

The output of this stage is summed through resistor R247 with the flat response signal present

at resistor R248. This creates a composite frequency response curve with varying levels of 55

Hz boost. The overall desired loudness contour response is visible at U208 pin 1.

8. Low Frequency Equalization

A single stage Sallen-Key 2nd order high-pass / low pass network (U208 pins 5,6 and 7)

provides bass channel equalization and low frequency roll-off to prevent subsonic low

frequency overload.

9. Power Amplifier

All three power amplifiers are of identical topology and use high efficiency Class G power amp

stages. The L/R amplifier channels are both rated at 50W. The bass channel is 100W, and

because of its higher power, uses higher current output transistors and slightly different

component values. Otherwise,operation is the same.