JBL A6000GTi, Service Manual, Page: 16 / 62

137159-1 Rev. 1 A6000GTi Service Manual

2003 Crown Audio, Inc.
Theory Of Operations 3-5
3.2. Protection and Control Circuitry
3.2.1. Current Monitor
Audio output current levels are monitored by the use of transformer T801. A small primary
winding is in series with the output current and the secondary develops a voltage across
the input of U512-C. This op amp provides a current monitor signal that is scaled to
approximately 5Amps/1Volt. This output current is used for the following purposes:
1. High Frequency Feedback
2. Low Frequency Feedback
3. Current Limit LED
3.2.2. Current Limiter
To prevent excessive output current, the A6000GTi incorporates an active current limit
circuit for each channel. Resistors (R809-R811, R812-R814) in series with the Lp and Ln
output coils are used to sense the output current. The voltage created by the output
current thru the resistors is compared to a reference voltage set by R801 & R802. These
voltages are compared on U801. If either the Lp or Ln current reaches the maximum
permitted output current, the output pin of the comparator goes low, which pulls the I LIMIT
signal low thru an optocoupler (U800). This signal goes to the front-end board and is used
to turn the channel off. When the current out of the channel fails below the limit, the
channel is enabled. This method of current limit allows the amplifier to play thru current
limit situations without severely degrading the sound.
3.2.3. Display Circuitry
After the input signal leaves the input board, it is routed to the signal detection circuit. With
a signal approximately 45 dB below that required for full output, the signal; LED will
illuminate. Note that this LED will indicate whether signal is present at the amplifier but not
whether it is being output from the amplifier.
The compressor circuit is initiated by either the Input-Output Comparator (IOC) circuit or by
the Thermal Limit Control (TLC) circuit. The IOC circuit is on the front-end board, and
monitors the error amp signal. If the error amp signal becomes too large, indicating too
much distortion on the output, the IOC signal will drop to
–15 volts. This will cause the
compressor drive circuit on the input board to compress the inputs of the amplifier. U102
on the input board turns on Q100, which causes U103 to become active. U103 shunts the
input signals together, thereby limiting the input to the amplifier. IOC will cause the CLIP
LED to light only if it occurs long enough to be audible. The TLC circuit lights the
THERMAL LED when the audio section is beginning to overheat. The LED can light before
the TLC circuit begins to compress. The TLC circuit will be discussed in another section.
The FAULT LED indicates that something catastrophic has happened to the amplifier.
When the amplifier senses a fault condition, the audio section and power supply are both