62
Power Amplifier PCB (continued)
Two circuits provide power to the digital amplifier driver. Linear regulator U602 [C2] supplies 5V
for internal processor power. Buck switching converter U600 [B1] operates between power
ground and the negative rail, generating VS
NEG
-referenced 12V power supply VN12REF, used by
the driver for low-side output FET gate drive. U600 operates at a nominal 150kHz switching
frequency. Input and output filter caps C606, C673 are low ESR types. Series-pass regulator
Q600 limits U600 input to a maximum of 60V referred to VS
NEG
.
Digital amplifier driver U605 enables or mutes the power amplifier depending on the logic level
applied to Mute pin U605p4. The amplifier is turned off when Mute is held at logic high. The
amplifier turns on roughly 200ms after the Mute pin is brought to logic low. HMute output pin
U605p35 indicates driver status. When driver is muted (Mute pin high), HMute output is driven
high. When driver is enabled (Mute pin low), HMute falls at time of driver turn on and remains low.
When the amplifier is turned on, voltage divider R464 [Signal Processor PCB sheet 2, E1], R439
[E2] pulls comparator output U425Bp1 [E1/2] and Mute input U605p4 [Amplifier PCB, D3] up to
5V (J602p5 Signal Processor Assy to J602p5 Power Amp Assy). When R442 [E1] charges C432
to a voltage greater than 3.2V, U425p1 pulls Mute low, comparator U425C lights “Ready” LED
D710 [Display PCB, D2], and the amplifier turns on after a brief delay. Feedback from HMute
U605p35 [Power Amplifier PCB, D3] (J602p3 [A4] Power Amp Assy to J602p3 [Signal Processor
PCB sheet2, B3]) is locked out by U428B until C432 charges above 6.1V.
Digital amplifier driver U605 monitors rail currents to protect itself and the output MOSFETs in the
case of short circuit or other overcurrent conditions. Differential inputs U605p15&16 monitor the
voltage drop across sense resistor R621 [F4] due to VS
POS
rail current. Similarly, differential
inputs U605p13&14 monitor the voltage drop across sense resistor R618 [F5] due to VS
NEG
rail
current. Overcurrent threshold for both rails is set at 28A by the value of R615 at U605p11.
When overcurrent threshold is exceeded, the driver turns the amplifier off and sets HMute high.
On the Signal Processor Assy, R456 [Signal Processor PCB sheet2, E2] pulls comparator
U428Ap2 output high, firing one-shot U425D and turning Q433 on, lighting “Protect” LED D713
[Display PCB, D3] and discharging C432 [E2] through D446. Discharge of C432 forces U425B
to set Mute high, U425C turns off the “Ready” LED, and U428B locks out buffered HMute, pulling
U428p2 low. One-shot U425D is allowed to time out, Q433 turns the “Protect” LED off and
allows C432 to charge. The amplifier powers up in the normal sequence. In the case of a con-
tinuous short on either of the outputs, this protection cycle will repeat until the short is removed.
Over- and under-voltage events also force an amplifier protection cycle. Under-voltage thresh-
olds are ±35V maximum, over voltage thresholds are ±60V minimum. These voltage extremes
are not encountered when the amplifier is operated within the normal AC mains voltage range.
Both the power amplifier and power transformer are thermally protected. A self-resetting thermal
cutout is installed in the power transformer, connected from the center tap to the violet trans-
former lead. PTC thermistor RT600 [Power Amplifier PCB, B3] is mounted to the power amplifier
heatsink between the two output stages. These devices are series connected (violet lead to
Rectifier Assy terminal WL402, through Rectifier Assy J300p18 to Signal Processor P300p18,
through Signal Processor Assy J601p4 to Power Amp Assy J601p4 to one lead of RT600, from
second lead of RT600 through Power Amp Assy J601p3 to Signal Processor J601p3). With
R431 [Signal Processor PCB sheet 2, C1], PTC thermistor RT600 forms a voltage divider
whose output rises as heatsink temperature increases.
THEORY OF OPERATION
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