JBL A6000GTi, Service Manual

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137159-1 Rev. 1 A6000GTi Service Manual

2003 Crown Audio, Inc.
Theory Of Operations 3-1
3. Theory of Operations
3.1. Audio Signal Path
3.1.1. Input Stage
Signal is presented to the A6000GTi through one of two RCA connectors. Each RCA
connector is a balanced input. Each input is immediately treated with a ferrite, to block any
high frequency noise. A TVS (Transient Voltage Suppressor) and DC blocking cap are
also included to protect the amplifier against high voltage and DC voltage levels. The
optoisolator (U103, U203) acts to compress the input level if needed to protect the
amplifier from dangerous conditions such as over temperature, clip conditions or DCLF
(DC or low frequency). The signal is then converted from balanced to unbalanced in the
Balanced Input Stage (U100).
After the two channels have been converted to unbalanced, they are summed together to
form a single mono signal that will continue through the amplifier. At this same point in the
circuit, the channel one and two signals are also sent to the op amp buffer stage of U104,
where some gain is added. The signals are then passed thru the photomos relay K100.
When the amplifier is powered on, the relay closes and routes the signal to the pass thru
RCA connectors on the input panel.
The signal is then passed through the low pass Butterworth filter. The pot (R131) is used
to adjust the signal phase from 0

to 180

. The switch (S100) is used to switch in a
second filter stage, making a fourth order filter. After the filter stage, the signal is passed
thru the DBO (Dynamic Bass Optimization) stage. In this stage S101 is used to turn DBO
on and off, R141 is used to adjust the DBO frequency between 20Hz and 80Hz, and R143
is used to adjust the DBO boost (gain) b1dB and +12dB.
Following the DBO circuitry, the signal passes thru the VCA control (U105). This is where
the remote level control is used to adjust the gain. The VCA is followed by U100, which
acts as a buffer/driver and sends the signal to both the channel 1 and channel 2 circuitry
on the input card.
For the sake of simplicity, from this point on only channel one of the audio signal path is
described.
Inherent in all PWM amplifiers is a rise in gain at higher frequencies. Because of this, a
800Hz 6
th
order Gaussian low pass filter has been included in the input stage of the
A6000GTi (on the input PWA). The Gaussian filter-type is unique in that it has minimal
ringing and excellent phase response so even a high-order filter such as this one does not
adversely affect the sonic excellence of the product. U201-A, -B, -C and
–D comprise this
filter.