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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 sens itive 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.
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Arcam P-1000 Amplifier Boar d 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 lo cal or global mute circuits are activated.
In the un-muted state
CLA-MUTE is -“12V. When activated this line goes high to +12V
which removes Q14s base voltage when Q25 is turned on.
The bias temperature tracking is from diode D8 and the initial setting is made by
adjusting RP1 for a voltage reading of .5 to 1.0 mV across either R51-R53 or R44-R45
after the unit has been on and running for a minute or two. A better method of setting
the bias is to use a distortion analyzer and adjust the amplifier output for 1 volt at 2 kHz
into an 8 Ohm load. After the amplifier has been on and allowed to warm up for a few
minutes, adjust RP1 until the crossover notch just starts to disappear.
The output section is a complimentary feedback pair topology with Q9 and Q7,Q8 in the
positive leg and Q12 and Q10,Q11 in the negative leg. The advantage of this
configuration is higher peak output voltage and, because the emitters of the driver
transistors become the effective output of the amplifier, crossover discontinuities are
very fast and almost negligible without any bias setting. With the bias correctly adjusted
the transition through the crossover region is seamless and the very low bias current
holds the output stage dissipation to approximately 1 watt.
Output stage V-I limiting is through Q5 and Q6. The short circuit current limit is
approximately 10 Amps and is set to this high value in order to handle the out of phase
currents in highly reactive loads. At high output voltages, however, R37 increases this
limit to 20 Amps. For short circuit loads where the current is very high but the output
voltage is close to zero, Q4 is turned on and the
PROT line activates the local signal mute
circuit. This mute removes the output signal momentarily and then releases, cycling
on/off until the fault is removed. To verify the short circuit protection, drive the amplifier
to an output of 5 volts or more and short the output terminals together. The shorted
channel output should cycle on and off and the front panel status indicator should toggle
between green and amber.
To increase the efficiency and reliability of the amplifier, multiple voltage rails are made
available to the output transistors. This addition of variable power supply voltages to the
amplifier circuit creates what is known as a cl ass H amplifier. In conventional amplifiers
the output devices are simultaneously exposed to high voltage and high current. The
product of this current and voltage is dissipated in the form of heat. To make matters
worse, the efficiency of the amplifier is the poorest at lower power output levels. To side
step this problem the class H amplifier greatly improves the efficiency by running at low
power supply voltages when the signal level is low. The operating voltages increase
only as required by the program material. Another benefit is in the form of reliability
under demanding conditions. Because the output transistors are never exposed to the
maximum positive and negative supply voltages at the same time, the amplifier is able to
withstand both very high current, under short circuit conditions, as well as highly reactive
currents presented by some speakers. With the overall efficiency gain, amplifier
heatsink requirements are reduced by half.
