12
THEORY OF OPERATION
3.3 Surround Detector (Refer to schematic sheet 1)
Output from differential amplifier U13 is applied to a pair of 85Hz 1st order high pass filters,
one inverting, the other non-inverting. The inverting stage facilitates full wave detection in the
next stage. This output is sent to full wave negative peak detector stage U700. U700 is a quad
comparator with open collector output that can only sink current, and thus (with capacitor
C706) acts as a negative peak detector with fast attack and medium (30ms) decay through 3
k
Ω
discharge path R722. This voltage is further averaged by C707 at a slower time-constant of
332ms through resistor R715. A similar negative peak detector circuit is used for the L-R
signal and is comprised of 1/2 of U701 (pins 11, 12 and 13; 8, 9 and 14).
The voltage outputs of the three level detector stages (C705, C707 and C709 for L-R, L, and R
channels respectively) are converted to current sources through quad op-amp U702 and quad
transistor's Q700, Q701 and Q703. Referring to the most basic L-R current source circuit
(U702 pins 8, 9 and 10), the voltage at the inverting input of the op-amp is forced to be equal
to the non-inverting voltage by feedback through transistor Q700. This voltage appears across
resistor R710, and forces a current (V/2000) to flow through the emitter (and hence collector)
of Q700. Operation of the two other voltage to current converter stages is similar to the above
except that for the (L)-(R) current source the emitter resistors (R733 and R735) are not refer-
ence to ground, but instead are referenced across the left and right outputs. Therefore the
current developed through either one of the transistors (Q707 or Q703) is proportional to the
difference between the left and right voltages at the non-inverting inputs. If left is more positive
than right, then the current flows through Q703, pins 10 and 8. If right is more positive than left,
then the current flows through Q701, pins 12, and 14. In either case the current is directed out
through R736 in one direction only. Operation of the (L)+(R) current source is similar except
that emitter resistor R734 is reference to ground, and therefore its current is proportional to the
sum of left and right DC voltages. Finally, these three current sources are sent off to the
decoder’s current controlled gain cells, (see section 3.4 Decoder Gain Cells).
