Bose PACKLITE A1 Manual page 48

DC Offsets
Because of the DC offsets involved (the RING of the input jack is connected to a DC voltage
and will momentarily shift when a plug is inserted into the jack), there can be a clicking sound
output from the amp when plugs are inserted or removed from the input jacks, as well as when
the gain changes. This is normal operation for the circuit, and while not ideal, is usually avoided
by the user by keeping the power off while inserting or removing plugs.
Limiter
The limiter circuit's limiting rate is dependent on the input signal level, that is, a larger input will
cause the gain to lower faster.
U305 [C4] is an Operational Transconductance Amplifier configured as a voltage-controlled
resistor in the feedback loop of U302B [D5]. As the AC signal level at U302A [D6], pin 1 in-
creases past one base-emitter junction voltage, it will turn on either Q303 or Q302 [C6], both of
which will discharge the base of Q304, increasing the current into the control pin of U305
through Q304's collector. This has the effect of lowering the gain once the signal exceeds a
threshold. The limiting threshold can be set by adjusting the gain of
U302, and thereby the amount of level fed into Q302 and Q303. The
unlimited gain is about unity.
R378, R333 and N300 [C5] are chosen to keep Q304 just off until the
threshold is increased. If the quiescent voltage at the base of Q304 is
too high, there will be a delay when an input signal reaches the limiting
threshold, as C331 [C5] discharges enough to turn on Q304. N300
compensates for temperature differences Since Vbe gets smaller at
higher temperatures, as N300 goes low in resistance at high tempera-
tures, it dominates and compensates the smaller Vbe on, but as it goes
very high in resistance (at low temperatures) , R-new keeps the resis-
tance from going too high.
The attack time of the limiter is set by the C331/R323 and R331[C6], and
is on the order of single miliseconds. The release time of the limiter is
set by C331 with R333, and is set to be on the order of 100 msec.
Hard Limiting
If the input signal exceeds the input voltage limit of the input buffer amp, about +18 dBV steady
state, the signal will clip in the input amp, and the limiter will bring the overall level down appro-
priately. However, because transient signals can pass through the limiter before it has time to
lower the gain, it is possible for transients above the limiting level to appear on the input of U303
[D7]. These transients could put too large an instantaneous demand on the amplifier converter
current, causing it to enter a protection mode. To avoid this, the output of U303 is limited to less
than +5VDC by zener diodes D319 [D6] and D320 [D7]. The 3V zener voltage was chosen
empirically to provide clipping without adding too much distortion from partial turn-on in the
useful audio range.
THEORY OF OPERATION
48
V+
R-new R378
39k 2.2k
N300
R333
750K
F