PassLabs-X2-pre-sm 维修电路原理图.pdf

PASSX2 SERVICE MANUALREV 2,3PASS LABORATORIES24449 FORESTHILL RD.FORESTHILL, CA 95631TEL (530) 367-3690FAX (530) 367-RadioFans.CN 收音机爱 好者资料库The Pass Labs X2 is a high performance audio preamplifier, intended formaximum performance in reproduction of music. It is a simple design, consistingof dual complementary input JFET transistors driving dual complementaryMOSFET output transistors connected in the patented Super-Symmetry topology.Figure 1 shows the block diagram of one channel of the preamplifier. There arethree unbalanced inputs and one balanced input. The balanced input isconverted to single-ended via a differential amplifier. The signal from all fourinputs is presented to the Select and Tape Mon switches. The wiper of theSelect switch attaches to the input volume control, and its wiper feeds one inputof a balanced gain stage. The output of the balanced gain stage is the output ofthe preamp.The balanced input and balanced output active stages of the circuitry are made ofcircuits UGS3 or UGS4. The UGS circuits are the subject of a different servicemanual, separate from this one.Fig. 2 shows the schematic of Rev 2 of the balanced input circuit. Resistors 101,102, 103, 105, 106 and 107 form the input and feedback circuits for the stage.R104 and 108 are used to control absolute DC offset of the circuit outputs.The gain of the circuit is approximately unity. It accepts balanced differentialinputs, and outputs a single-ended signal through the + output. The output isgrounded.When using the preamplifier with a single-ended input, the negative input shouldbe shorted to ground for best performance. This is accomplished by usingshorting plug between pins 1 and 3 on the XLR input connector.Fig 3 shows the schematic of Rev 3 of the balanced input circuit. R108 and R104have been removed, C103, C104, C107 have been added.Fig 4 shows the schematic of Rev 2 of the balanced output stage. It operates witha single-ended input, and the negative signal input is grounded. The gain of thecircuit is approximately 12 dB.Fig 4 shows the schematic of Rev 3 of the balanced output stage. It has addedbypass and compensation capacitor.RadioFans.CN 收音机爱 好者资料库The Relay switches shown in this schematic are controlled by the power supplyand are used to mute the output on turn-on and when the supply voltage goes tolow to regulate. A common problem which occurs in the X2 is when the preampdoes not operate due to low line voltage. To fix this, either get a higher linevoltage or remove R14 on the relay control circuit.For best performance with single-ended outputs, short across the SE OUTjumper terminals next to the balanced output stage. The preamplifier will workwell without this jumper, but will give better performance with single-endedoutputs with this shorted. The preamplifier is shipped with the jumper open.Fig 6 shows the power supply schematic to Rev 2 and 3.The supply transformer each has dual primary windings which can be set up for115 or 230 volts AC. These primary windings are shown in Fig 5 as parallel for115 volts, but they may be wired in series on each transformer for 230 volts.The power supply is rated down to approximately 100 volts AC and 200 volts ACbefore the supply unregulates.The positve supply is regulated to +33 volts by Zener diodes 1-4 and N channelMOSFET Q1. The negative supply is regulated to 33 volts by Zener diodes 5-8and P channel MOSFET Q2.The output muting relay is controlled by the circuit of Q3 and the surroundingcomponents. Q3 conducts only when there is approximately 8 volts moreunregulated voltage than unregulated output, insuring that the output relays willopen only when adequate rail voltage exists for proper regulation.Fig 7 shows the detail of the wiring of the volume control. The volume controlconsists of a two pole selector switch with 23 positions which is wired withprecision resistors to form an attenuating network for the input to the gain circuit.Fig 8 shows the board layout for Rev 2. Components 1-99 are power supplycomponents. Components in the 100-199 range are the left channel circuits,200-299 are the right channel circuits.Fig 9 shows the board layout for Rev 3, where the same numbering schemeapplies.