Bose Acoustimass 3 Series Manual - Page 10

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1. Specifications

7. Low Frequency (LF) Equalization

Low frequency bass channel equalization and crossover is accomplished in two active filter

stages. The first stage consists of op-amp U3 (pin 5, 6, 7), capacitors C9 and C10 and resistors

R19, R20, and R21. It creates the sharp band-reject attenuation below 50 Hz. The second

stage consists of op-amp U5 (pin 1, 2, 3) capacitors C13 and C14, and resistors R24, R25, R26

and R27. It acts as a combination 2

order 50 Hz high-pass and 200 Hz low-pass filter. The

combined response of these two stages is shown in the bass channel frequency response

graph in Figure 1 on page 5.

8. Auto Turn-On/Off Mute

An improved version of auto turn-on/off is used in the AM-3P II. A separate 3.5 mm stereo mini-

jack (J7) is used to receive the turn-on signal from the serial data output of the Lifestyle

music

center. When the 3.5 mm plug is inserted, the audio sense circuit is disabled and the amplifier

outputs will only unmute if the DC control signal is present at J7 pin 3. This action occurs

through the built-in DPDT switch in J7 which connects J7 pin 3 directly to the Q9 mute circuit.

Therefore, the system is not subject to random false turn-on (from electrical noise, etc.) when

used with the Lifestyle

music center.

When the AM-3P II is used with a source other than the Lifestyle

music center, the stereo mini-

plug is not inserted into the jack, and therefore mute transistor Q9 is directed to the output of

the audio detect comparator U6 (pin 5, 6, 7). This comparator uses hysteresis to achieve a

positive turn-on of at least 90 seconds before the system will revert to stand-by (mute) mode

after removal of the audio signal.

9. Compressor Detect

The output of each left/right amplifier is half-wave detected through diodes D108 and 208. This

voltage is divided down by R53 and R54 and presented at the emitter of Q12. Power supply

voltage is similarly divided down by R50 and R51 and presented at the base of Q12. Diodes

D13 and D14 simulate the saturation voltage drop that occurs in the power amp stage.

Therefore, when the audio output of any amplifier comes within 2 volts of the instantaneous

power supply voltage, Q12 will conduct. This signals the compressor (limiter) to act. Current

from Q12 charges hold capacitor C35 and releases capacitor C36. Voltage at C35 is buffered

by transistor U1 (pins 5, 6, 7) and decreased down to a logarithmic voltage by resistor R67 and

diode U1 (pins 12,13,14). This voltage is then presented to the base of compressor transistors

U1 (pins 1, 2, 3) and U1 (pins 8, 9, 10) which are described in the compressor section.

The DC control voltage present across capacitor C35 is derived through the use of an improved

hold and release characteristic, resulting in lower compressor distortion. In conventional

limiters, this DC control voltage is derived by a simple series diode and parallel RC network to

ground. Unfortunately, for low frequency overload, this results in substantial ripple voltage

present on the DC gain control voltage. This results in the all too common problem of mid-range

voices or instruments being badly "modulated" by low frequency overload. This problem is

solved by having two separate capacitors, one that "holds" the smooth DC voltage just long

enough to prevent low frequency ripple (approximately 40ms) and the other (C36) which quickly

releases the hold capacitor (through diode D17) at the proper time to maintain overall 100ms

release time.