PassLabs AlephL free download

File name PassLabs-AlephL preamp.pdf

Pass Laboratories Aleph L Service Manual 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. Maximum gain is internally adjustable via a dip switch. 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 in parallel with C3 and the regulated output is additionally filtered by C4. 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. The circuit is inverting. The circuit of Q102 provides a constant current source of about 40 ma for the gain stage. It is controlled by the circuit of Q103, which works to keep a .7 volt value across R111. 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. The maximum gain of the preamp is controlled by the input resistor values in conjunction with the 6 position dip switch on the main board. 0 dB 6 dB 12 dB 18 dB switches 1, 6 = On switches 2, 5 = On switches 3, 4 = On all internal switches = 0n (default) A +85V R1 3.3 3W 4 X 1N4004 C1 + B V+ REG V+ DEC V+UNREG C D INPUT 4 C103 15 R2 R111 4.75K 4.75K 1N4004 4V 1000 UF 221 Z8 Z1 R11 IRF9610 9.1V 9.1V 1000 UF R109 221 R110 G S S G INPUT 3 MPSA92 R4 - Q103 REGULATOR .7V 4.7K 3W 1 D 680 UF R10 R5 D5 C2 CURRENT SOURCE Q1 IRF610 D1-4 1 +65V TH1 D S1 C4 C3 9.1V C104 1000 UF 680 PF LED1 1000 UF C102 10UF R6 9.1V 2.21K Z3 100 R112 SELECT OUTPUT Z2 THERMISTOR INPUT 2 4.75K C 1 C 2 C 3 C 4 221 Q102 +60V AVEL 4007 A B C D R108 10K R106 S 1 R101 R102 22.1K R103 47.5K INPUT 1 10K 2 47.5K 2 1 1 5 A 0 A 1 1 5 B 0 B +30V R7 G C 2 C 2 C 2 IRF9610 S Z4 S2 Q101 R105 R8 G D 0