PassLabs X2 free download

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PASS X2 SERVICE MANUAL REV 0,1 PASS LABORATORIES 24449 FORESTHILL RD. FORESTHILL, CA 95631 TEL (530) 367-3690 FAX (530) 367-2193 www.passlabs.com The Pass Labs X2 is a high performance audio preamplifier, intended for maximum performance in reproduction of music. It is a simple design, having only a single balanced gain stages consisting of balanced pairs of power Mosfets biased by a constant current sources. Figure 1 shows the block diagram of one channel of the preamplifier. There are three unbalanced inputs and one balanced input. The balanced input is converted to single-ended via a differential amplifier. The signal from all four inputs is presented to the Select and Tape Mon switches. The wiper of the Select switch attaches to the input volume control, and its wiper feeds one input of a balanced gain stage. The output of the balanced gain stage is the output of the preamp. Fig. 2 shows the schematic of the balanced input circuit. Mosfets Q101 and Q102 form a differential pair operated without feedback. They are biased by a constant current source Q3. Balanced input signals are presented to the Gates of Q101 and Q102 and the output signal appears at the Drain of Q102. The gain of this circuit is approximately unity. Z101-104 protect the inputs from input voltages greater than 9 volts. Fig 2 also shows the various voltages at different points in the circuit which are helpful in confirming proper operation. These voltages are not critical, and if the value measured is similar to the specified, the circuit will operate properly. When using the preamplifier with a single-ended input, the negative input should be shorted to ground for best performance. This is accomplished by using shorting plug between pins 1 and 3 on the XLR input connector. Fig 3 shows the schematic of the balanced output stage. It is similar to the input stage, except that it is biased slightly higher and has only a single input but dual outputs. It also has approximately 10 dB of gain when loaded by the 1.5 Kohm output resistors R316 and R317. The preamp is shipped with the Gain jumpers shorted. With the output Gain jumpers open, the gain is approximately 17 dB. The Relay switches shown in this schematic are controlled by the power supply and are used to mute the output on turn-on and when the supply voltage goes to low to regulate. A common problem which occurs in the X2 is when the preamp does not operate due to low line voltage. To fix this, either get a higher line voltage or short across one of the Zener diodes in the supply stack Z9 ­ Z17 to lower the regulated supply by 9.1 volts. Fig 4 shows the detail of the wiring of the volume control. The volume control consists of a two pole selector switch with 23 positions which is wired with precision resistors to form an attenuating network for the input to the gain circuit. Fig 5 shows the power supply schematic to Rev 0, and Fig 6 shows the schematic to Rev 1. The dual power supply transformers each have dual primary w