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

 A B C D

INPUT 4
V+ REG +85V V+ DEC R1 V+UNREG

REGULATOR
3.3 3W 4 X 1N4004

680 UF

C103

1N4004
4.7K 3W
CURRENT SOURCE

C2

C1
Q1 +

D5
4.75K

4.75K
R111

R10
R2

R5
15
1 D IRF610 1
D1-4
4V .7V

1000 UF

1000 UF
MPSA92 R4
G -
INPUT 3 R109
S
221
221
Q103
Z8 +65V

9.1V
S

THERMISTOR
R110

Z1
G

4.75K
221

R11
IRF9610

TH1
C C C C
1 2 3 4 9.1V
Q102 +60V
D

INPUT 2 AVEL 4007

S1

1000 UF

1000 UF

9.1V
Z2
OUTPUT

C4

C3
680 PF

C104
SELECT

LED1
C102 A B C D

47.5K
R108

R106
10K
S
1
10UF
R6
2 2

R112
100

9.1V
22.1K

47.5K
R101

R102

R103

Z3
10K

2.21K 1 0 1 0
INPUT 1 1 A 1 B
5 5
A B
+30V
C C C
R7 IRF9610 S
2 2 2 G

9.1V
+4V 2.21K

Z4
+12 DB

+6 DB

0 DB

S2

GAIN TRANSISTOR
Q3 D
NEGATIVE RELAY
9.1V
Z101

D
Q101
100-120V
C C C
1 1 1 C101 R105
G

221K

1N4004

6.8K 3W
R8

9.1V
D6

Z5
221

R3
INPUT GAIN 10UF IRF610 S
200-240V
D
Q2
6.81K
R104

R107
221K

+25V
9.1V
Z102

221
+4V G

R9

.25A SLO
IRF610 S

9.1V
1N4004
3 3

Z6
220UF

F1
D7
C5
OUTPUT 1

POSITIVE RELAY

9.1V
Z7
LEVEL CONTROL

OUTPUT 2
RELAY NC
C
18.2

22.1

27.4

33.1

39.2

47.5

56.2

68.1

82.5

100

124

150

182

221

274

331

392

475

562

750
10

15

2 L
2
RV22

RV21

RV20

RV19

RV18

RV17

RV16

RV15

RV14

RV13

RV12

RV11

RV10

RV9

RV8

RV7

RV6

RV5

RV4

RV3

RV2

RV1
4 Title 4
C
L
1 ALEPH L PREAMP
1
Size Number Rev

B PL20
Date 2/1/96 Drawn by PASS
Filename PL20.S01 Sheet of

A B C D
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 (530) 367 3690

fax (530) 367 2193
 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.

 4
2

3
1

INPUT 1
INPUT 2
INPUT 3
INPUT 4

OUTPUT 2
OUTPUT 1

1
2

C
C
Z102 Z101

1
C

2
L

A
A

2
C

RELAY NC
9.1V 9.1V

1
L
1
S
3
C

R107
4
C

221K S1

RV22 10 SELECT

RV21 15

RV20 18.2

RV19 22.1

RV18 27.4
R120

C101

10UF
RV17 33.1 6.81K
R109

LEVEL CONTROL
RV16 39.2 221K R110
+4V

220uF 22.1K
RV15 47.5
221
R104

C104

B
B

RV14 56.2 R113

1K
Q102

G
G

RV13
+30V

68.1
IRF610
Q101

R106 R111
S
S

D
D

RV12 10pF
82.5 221 10

C104
R115 R114
RV11 100
IRF9610

470 221
C102

10UF

RV10 124 221

R112
V+ REG

RV9 150
+60V

R8
+4V
L

RV8 182 221K

4.75K 4.75K
R7

220UF D6
2.21K

RV7 221
R11 R10
LED1
C5 1N4004
RV6 274
REGULATOR
R9
221
S
D

RV5 331 1000 UF R21

Q1

10
C

C4
C
G

RV4 392
IRF610

R
+85V

1000 UF R22
IRF610
Q2
NEGATIVE RELAY

RV3
S

475
D

10
R4

221

C3
+65V

RV2 562 R5

4.75K
RV1 750
D7 Z2 Z1
R6

+25V
2.21K

1N4004 9.1V 9.1V

V+ DEC

IRF9610

Q3

R3
S

POSITIVE RELAY 6.8K 3W 1000 UF

C2
Z7 Z6 Z5 Z4 Z3
Title

Size
R1

Date
B
3.3 3W

Filename

9.1V 9.1V 9.1V 9.1V 9.1V TH1

1000 UF
THERMISTOR
C1
Number

9/24/96
5
1
1

A
A
V+UNREG

D
D

0
-

A
B
+

200-240V
D1-4

100-120V
5
1
1

B
C
AVEL 4007

Sheet

F1
ALEPH L PREAMP

PL20MB12
0
B
D

Drawn by

.25A SLO
4 X 1N4004

of
Rev

PASS
4
3
2
1
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 (530) 367 3690

fax (530) 367 2193