Pass Laboratories Aleph L User manual

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 switches 1, 6 = On
6 dB switches 2, 5 = On (default)
12 dB switches 3, 4 = On
18 dB all internal switches = 0n

2.21K
R6
100
R112
TH1
THERMISTOR
1 1 5 A
0 A1
1 5 B
0 B
A B C D
D1-4
4 X 1N4004
- +
1000 UF
C2
R1
3.3 3W
C1
1000 UF
IRF9610
Q3
S D
G
6.8K 3W
R3
750
RV1
562
RV2
475
RV3
392
RV4
331
RV5
274
RV6
221
RV7
182
RV8
150
RV9
124
RV10
100
RV11
82.5
RV12
68.1
RV13
56.2
RV14
47.5
RV15
39.2
RV16
33.1
RV17
27.4
RV18
22.1
RV19
18.2
RV20
15
RV21
C 2 C 1
10
RV22
RELAY NC
L 2
L 1
SELECT
S1
S 1
C 3
C 2C
4
C 1
10UF
C102
R111
15
IRF9610
Q102
S D
G
R110
221
R109
221
MPSA92 Q103
R108
10K
C103
680 UF
R106
47.5K
C104
680 PF
IRF610
Q101
S D
G
R105
221
C101
10UF
R107
6.81K
S2
0 DB
C 2 C 1
+6 DB
C 2 C 1
+12 DB
C 2 C 1
R104
221K 10K
R101
22.1K
R102
47.5K
R103
9.1V
Z101
9.1V
Z102
OUTPUT 2
INPUT 1 INPUT 2 INPUT 3 INPUT 4
9.1V
Z7
9.1V
Z6
9.1V
Z5
9.1V
Z4
9.1V
Z3
OUTPUT 1
R2
4.7K 3W
F1
.25A SLO
221K
R8
2.21K
R7
Z1
9.1V
Z2
9.1V
D5
1N4004
1000 UF
C4
1000 UF
C3
9.1V
Z8
IRF610 Q1
S D
G
221
R4
4.75K
R5
C5
220UF
R9
221
Q2
IRF610
G
D S
D6
1N4004 R10
4.75K
R11
4.75K
LED1
D7
1N4004
4V .7V
+25V
+4V
+30V
V+UNREG
OUTPUT
INPUT GAIN
CURRENT SOURCE
POSITIVE RELAY
GAIN TRANSISTOR
LEVEL CONTROL
AVEL 4007
2/1/96 PASS
PL20.S01
ALEPH L PREAMP
PL20
100-120V
200-240V
V+ REG
+60V
V+ DEC
+65V
+85V
REGULATOR
NEGATIVE RELAY
+4V
AB C D
4
3 2 1
DCBA
1
2
3
4
BRevNumber
Title Size Date Filename
Drawn by
ofSheet

SPECIFICATIONS
Gain 17 dB (all internal switches = 0n)
12 dB (switches 3, 4 = On)
6 dB (switches 2, 5 = On)
0 dB (switches 1, 6 = On)
Freq. Response +0, -3 dB @ 10 Hz and 100 kHz
Distortion < .2 % THD typically .02% @ 1 volt @ 1KHZ
Maximum Output 10 volts rms.
Output Impedance 0 - 750 ohms
Input Impedance 20 Kohm, 6.5 dB gain setting (default)
10 Kohm, 13.4 dB gain setting
Output Noise less than 10 microvolts, unweighted 20-20khz
Power Consumption 15 watts
Dimensions 12 " W x 11.5 “ D x 4" H
Shipping Weight 22 lb.
Pass Laboratories
Foresthill CA 95631
tel [phone]
fax [phone]

Pass Laboratories
Aleph L Service Manual
Rev 1.2 9/26/96

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.

R106
221
10UF
C102
R11
4.75K
1000 UF
C4
1000 UF
C3 10
R22
10
R21
2.21K
R6
221
R112
TH1
THERMISTOR
1 1 5 A
0 A1
1 5 B
0 B
A B C D
D1-4
4 X 1N4004
- +
1000 UF
C2
R1
3.3 3W
C1
1000 UF
IRF9610
Q3
S D
G
6.8K 3W
R3
750
RV1
562
RV2
475
RV3
392
RV4
331
RV5
274
RV6
221
RV7
182
RV8
150
RV9
124
RV10
100
RV11
82.5
RV12
68.1
RV13
56.2
RV14
47.5
RV15
39.2
RV16
33.1
RV17
27.4
RV18
22.1
RV19
18.2
RV20
15
RV21
C 2 C 1
10
RV22
RELAY NC
L 2
L 1
SELECT
S1
S 1
C 3
C 2C
4
C 1
R111
10
IRF9610
Q102
S D
G
R109
221K
C104
220uF
IRF610
Q101
S D
G
R104
221
C101
10UF
R107
221K
9.1V
Z101
9.1V
Z102
OUTPUT 2
INPUT 1 INPUT 2 INPUT 3 INPUT 4
9.1V
Z7
9.1V
Z6
9.1V
Z5
9.1V
Z4
9.1V
Z3
OUTPUT 1
F1
.25A SLO
221K
R8
2.21K
R7
Z1
9.1V
Z2
9.1V
IRF610 Q1
S D
G
221
R4
4.75K
R5
C5
220UF
R9
221
Q2
IRF610
G
D S
D6
1N4004 R10
4.75K
LED1
D7
1N4004
R120
6.81K
R113
1K
C104
10pF
R115
470
R114
221
R110
22.1K
9/24/96
LEVEL CONTROL
L
L R
+25V
+4V
+30V
V+UNREG
POSITIVE RELAY
AVEL 4007
PASS
ALEPH L PREAMP
100-120V
200-240V
V+ REG
+60V
V+ DEC
+65V
+85V
REGULATOR
NEGATIVE RELAY
+4V
PL20MB12
AB C D
4
3 2 1
DCBA
1
2
3
4
BRevNumber
Title Size Date Filename
Drawn by
ofSheet

ALEPH L Update
Rev 1.0 to 1.1
The kit contains one new main board with the correct value for R212, but please
verify before doing the conversion.
To update the Aleph L from rev 1.0 to either rev 1.1 is a simple matter of
replacing the main board, with two minor exceptions.
1. R212 must be 562 ( see attached drawing for location).
2. When keeping the old volume control board the white wire must be
attached as shown in the drawing of the old volume board.

SPECIFICATIONS
Gain 17 dB
Freq. Response +0, -3 dB @ 10 Hz and 100 kHz
Distortion < .2 % THD typically .02% @ 1 volt @ 1KHZ
Maximum Output 10 volts rms.
Output Impedance 0 - 750 ohms
Input Impedance 20 Kohm
Output Noise less than 10 microvolts, unweighted 20-20khz
Power Consumption 15 watts
Dimensions 12 " W x 11.5 “ D x 4" H
Shipping Weight 22 lb.
Pass Laboratories
24449 Foresthill Rd.
Foresthill CA 95631
tel [phone]
fax [phone]