Bose PACKLITE A1 Manual page 47

THEORY OF OPERATION
Input/Limiter PCB Assembly (continued)
Power Regulation
+/-18 VDC is brought to the preamp pcb from the amplifier power converter. A total of 500 mA
can be drawn from these two supplies. In this application the supplies are linearly regulated to
+/-15 V to use for audio amplifier rails, and to drive LEDs. The linear regulators are U310 and
U311 [B/C7].
Balanced/Unbalanced Detection
The PackLite A1 amplifier is primarily used with the bass line output of a PS1 Power Stand,
TM
which is a balanced output. We require 24 dB of gain in that case. If a user connects to a PS1
using an unbalanced cable, they will lose 6 dB of signal going into the PackLite A1. In that
case, U304 [C2] will detect that RING is connected to SHIELD, which is grounded, and, after a
few seconds, will increase the gain of U301A [D4] by 6 dB in order to compensate. This way the
user will always have the correct gain whether they use a balanced or an unbalanced cable.
The RING connector is tied to V+ through R310 [C2], then low-pass filtered by R319 and C328
[C2] with a time constant on the order of a second. This voltage is compared to a reference
(R324 and R328 [C2]), and if it goes below 700 mV, U304 output goes high to turn on Q301
[C3].
Gain Switch
After a 6 dB pad consisting of R305 and R307 [D3], the gain of U301A [D4] is +12 dB for unbal-
anced signals, and +6 dB for balanced signals. It is derived as follows:
Assuming the open loop gain of the opamp is large, the voltage gain is expressed approxi-
mately as Gain = 1/K, where K is the voltage divider
created by two impedances, Z1 and Z2:
Gain=G(open loop)/1 + (G(open loop) K),
approximately = 1/K
+
-6 dB
U301A
-
K = Z2/Z1 + Z2
Z1 = R318//C318
Z2 = Z(C318) + R325 +R327//R329 Z1
C313 [D4] is chosen to provide a low-pass corner at 100
kHz, well above the audio band, and C318 [C3] is chosen
large enough to provide DC blocking with a corner below
Z2
10 Hz. So, for mid-band calcualtions, Z1 = R318, and Z2
= R325+R327//R329, providing a gain of +6 dB when
Q301
Q301 is off (balanced input case).
When Q301 [C3] is on, it essentially shorts the R327//
R329 [C3/4] combination to provide an impedance of 0,
so the overall gain becomes +12 dB. Although R329
connect to positive DC voltage in order to bias Q1's emitter, and R327 connects to ground, both
of those connections are small-signal (AC) ground, and are shown in parallel.
47