Arcam P1000 pwr sm
This is the 32 pages manual for Arcam P1000 pwr sm.
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Page 1
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.
Page 2
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.