CIRCUIT OPERATION
trans•ana Implementation
The transistor Q1 is configured to operate as a switch which controls the constant current source, Q6, of the input
differential amp, Q4 and Q5. When Q1 is off the emitter voltage is low turning off Q6. Timing of the Soft Start function
is controlled by the charging time of C29 through R13. The THERMAL Protection circuit uses Q1 to shut down the
channel when excessive heat is detected.
U1A and U1B are buffer amps configured as unity gain, non-inverting voltage followers. The output of these buffers is
fed to the attenuator network controlled by R24. The buffer amp U2A stabilizes the impedance at the non-inverting
differential input Q4. U2B is configured as a DC servo-integrator to null the output offset voltage.
Transistors Q19 and Q20 are the second gain stage and are the pre-drivers of the output stage drivers Q21 and Q22.
The driver stage level-shifts the signal from the low voltage operation of the input circuit to the high voltage operation
of the output stage. Drive current for the positive output MOSFETs is supplied by the current amplifier Q23 and Q24.
Drive current for the negative output MOSFETs is supplied by the current amplifier Q25 and Q28.
Class AB bias current is established by Q18 and adjusted by RV1. Local feedback is supplied by the network C16, R82
and R102, and global feedback by C10 and R35.
CALIBRATION
WARNING: Only a competent technician should attempt the following procedure.
Adjusting Bias:
The bias control establishes the quiescent Class AB output current of the amplifier. The bias should not need
readjustment from the factory setting; however, if the amplifier is repaired and output devices have been changed, or
if the two channels of the amplifier do not run at the same temperature, recalibrating the bias is necessary.
Disconnect
the power to the amplifier before removing the cover.
To adjust the bias, disconnect the input and speakers and remove
the jumper J4. Connect an amp meter across the exposed pins. The correct polarity is marked adjacent to the jumper.
Adjust RV1 to get a current reading of 100mA.
WARNING: Only a competent technician should attempt the following procedure.
Calibrating Common Mode Rejection:
The input common mode null is adjusted by the trim pot R8. The CMRR should be greater than 70dB below rated output.
If the CMRR requires adjustment, feed the amplifier input with a common mode signal and adjust R8.
Disconnect the
power to the amplifier before removing the cover.
Use a sinewave generator set to 1 volt output at 1kHz. Connect the
generator signal output to the tip and ring of a 1/4" plug and ground to the sleeve. Plug this into the amplifier input.
Connect an AC voltmeter to the amplifier output binding posts. Adjust R8 to give the lowest voltage output from the
amplifier. For a temporary adjustment when a signal generator and voltmeter are not available, use an FM tuner and
tune it to an unused station as your signal source, and connect the signal to the amplifier as described above. Connect
the amplifier output to a small full range speaker. Turn the amplifier level controls full down and turn the amplifier on.
Turn up the level control until you hear a signal through the speaker. Alternate between adjusting R8 for the lowest output
signal and increasing the input control until you have the level control full. There should be very low output from the
amplifier if any is detected at all.
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Содержание trans-nova P1000
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