Pass Laboratories Aleph L, Service Manual

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The Aleph L is a simple Mosfet stereo preamplifier having four single-ended inputs and
two outputs. Its controls consist of an input selector and a level control. The volume
control is passive up to the 3:00 o’clock position where the gain is unity. At 3:00
o’clock, the connection from input to output is direct, with no components in the signal
path, and past the 3:00 o’clock position the unit becomes an active preamp.
The gain stage of the preamp is a single Mosfet operated common source and biased
by a Mosfet constant current source.
Figure 1 shows the schematic of the preamp, indicating the component designations
and values and various voltages.
The power supply and the left channel are shown. Power supply and components
common to both channels have reference designators less than 100. The left channel
is 100 to 199, and the right channel is 200 to 299.
The power supply transformer delivers approximately 84 volts (for 120 or 240 VAC
operation) unregulated DC into C1, which is then passively filtered and then actively
regulated down to about 60 volts at the output of Q1. Q1 is driven by a stack of Zener
diodes and the regulated output is additionally filtered by C3 and C4 to form the left
and right supplies.
The output muting relay is controlled by the circuits of Q2 and Q3. For the relay to
activate, both Q2 and Q3 must be conducting. Q3 turns on slowly via the charge on
C5, while Q2 will only conduct when there is at least 8 volts difference between
regulated and unregulated supplies. Thus Q3 delays relay turn on, and Q2 shuts it off
quickly.
The audio gain stage consists of Q101. The input goes to the gate, and the output
appears at the drain. Q102 inverts the signal, so the circuit as a whole is non inverting.
The output of the gain stage goes to the level control which is a 23 position stepped
attenuator whose wiper is attached to the preamp output.
Fig 2 shows the circuit board layout, with the components identified by designators.
Fig 3 shows the circuit board layout, with the components identified by values.
Fig 4 shows the circuit board layout of the volume control.
Fig 5 shows the Aleph L upgrade.