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DiVA Service
Manual
Issue 1.0
P1000 Amplifier
ARCAM
Bringing music & movies to life
Arcam P-1000 Amplifier Board functions and operation
The power amplifiers in the P-1000 are of class H design and use a three rail power
supply for high operating efficiency. These amplifier boards are field replaceable and
the unit can be safely operated with one or more boards out of the unit. The operating
descriptions below are divided into three groups. The input and signal control section.
The amplifier section and the power switching section. Signal net name references as
they appear in the schematic are designated here in bold type.
The line level audio path of the P-1000 very simple and short. Unbalanced audio
sources are buffered by U6-A and summed with the balanced input from U1-A. The
signal then passes through the gain set stage of U6-B and then on to the AMP-IN
amplifier drive. The AMP-IN signal is also contoured by U2-A and on to the power
switching section via the COMM output.
The gain switch S1 works by changing the local feedback around U6-B and has three
settings. The "A" gain position provides an overall gain of 31.5 dB and is compatible
with other Arcam amplifiers. The "B" setting is for use in THX compliant systems and the
low gain "C" position can be used to lower the overall system noise level in installations
where the speakers are very close to the listener.
Also at the input of the gain set stage are the mute transistor Q1 and the resistive
element of the clipping eliminator circuit. When the P-1000 is turned on, the global mute
signal at the MUTE input of U3-A is low. This forces its output to +12V which activates
the mute transistor Q1 as well as the clipping eliminator circuit through D15. The
FLT_OUT from the emitter follower Q24 and also goes high. Q1 then shunts the audio to
ground and R11 goes to a low resistance state. The FLT_OUT passes on out to the
channel status display board. When the global mute cycle finishes and the output from
U3-B goes low, Q1 turns off and passes the audio signal immediately but the resistive
element R11 has a slow release time which allows the audio output to ramp up in a
controlled manner.
The local mute circuit of U3-B also has two other inputs. The thermal shutdown circuit of
U2-B and R22 monitor the heatsink temperature of the amplifier. If the HS temperature
exceeds approximately 95 Deg C, the output of U2-B goes high and toggles the local
mute circuit on. Because of the hysteresis around U2-B, the thermal protection will
remain active until the amplifier has cooled down approximately 20 Deg below the trip
point. The second input is the PROT line from the amplifier. This is a fast acting input
which goes to a low impedance state if a short circuit is detected at the amplifier output.
The clipping eliminator circuit has two inputs. The AMPOUT monitors the amplifier output
signal and the OPA-OUT signal from the output of U1-B. The OPA-OUT line is inside the
overall amplifier feedback loop and is very sensitive to any differences between the input
and output signals in the audio path. If the unit is driven into clipping the difference
signal from these two lines is amplified and used to drive the CLM5000 LDR. This
causes the resistance of R11 to decrease and work as part of a voltage divider against
R85 or R5. The effect of this is to reduce the signal level going into the amplifier thereby
reducing the output clipping to a very low value. Typically this circuit will hold the THD to
less than 1% with 10dB of overdrive at the input.
Arcam P-1000 Amplifier Board functions and operation
In the amplifier portion of the board, Q2 acts as a level shifter and drives the class A
transistor Q13. The voltage source for the class A stage is from Q14 and is regulated by
Z1. This constant voltage causes Q13 to act as a constant current source and stabilizes
the output transistor bias regardless of changes in the AC mains. The class A drive
voltage is also removed from Q13 anytime the local or global mute circuits are activated.
In the un-muted state CLA-MUTE is