This conducts the output signals from the power amplifiers on both L125 and
L129 to the speaker terminal blocks LS100 – LS103 inclusive, via 4 double
pole relays RLY100 – RLY103. The 24V dc supply from the relays is fed from
L129 via CON101 pin 18, returning to ground via pin 17. The relay switching
transistors are TR100 – TR 103 and the logic level signals to control these
also enter via CON101 pins 19 and 20. Note that the SBL and SBR channels
are switched independently from the other 5 channels, as these can be re-
assigned in software to support zone 2 in stereo if required.
Zobel networks, comprising 1 uH air cored chokes in parallel with 4R7 2 Watt
resistors, are in series with all outputs to isolate the power amplifiers from
capacitive cable loads which might otherwise provoke instability.
This contains all the power amplifier electronics for the Centre (C), Left
Surround (LS) and Right Surround (RS) channels. It also contains the power
supplies for all 7 channels, which are fed directly from the secondaries of the
power transformer L951TX as raw AC, via CON106, to the bridge rectifiers
DBR201 and DBR202 and two reservoir capacitor banks, C206/7 plus
C210/213 for Vcc/Vee, and C214/5 plus C220/1 for +Vlo and –Vlo.
The power amplifier design is Class G, which is why two sets of power rails
are employed; Vcc and Vee are approximately +/- 59V off load and +Vlo and
– Vlo are approximately +/- 30V off load.
The rectified and smoothed power rails are fed to the lower power amp PCB
L129, along with an AC power feed for checking the mains is present, via
CON100.
CON103 is used to bring in the C, LS and RS line level input signals from the
rest of the AVR via L129. It also imports +/-12V supplies for the front stage
op-amps and +5V for the top heatsink temperature sensor IC400. Outputs
comprise this sensor’s signal and two DC offset protection lines (ERR_POS
and ERR_NEG), which are returned to the system microprocessor via L129.
CON105 carries the C, LS and RS outputs to the Speaker PCB L124. It also
carries the speaker ground returns for all 7 channels back to the star ground
point on L125.
All 7 power amplifiers are identical, except for the Centre channel where only
one half of its set of the various dual driver ICs is used.
Each power amplifier is topologically split into two halves – the input stage IC,
LM4702 high voltage driver IC and DC servo IC comprise the driver stage
(e.g. U_driver_C); the power transistors plus their drivers and protection
circuitry comprise the output stage (e.g. U_PA_C), as shown in the L125
block diagram.
L124AY – Speaker board
L125AY – Power amp upper
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