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

100

R112

TH1

THERMISTOR

A B C D

D1-4

4 X 1N4004

1000 UF

3.3 3W

1000 UF

IRF9610

6.8K 3W

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

RV21

RV22

RELAY NC

SELECT

10UF

C102

R111

IRF9610

Q102

R110

221

R109

221

MPSA92

Q103

R108

10K

C103

680 UF

R106

47.5K

C104

680 PF

IRF610

Q101

R105

221

C101

10UF

R107

6.81K

0 DB

+6 DB

+12 DB

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

OUTPUT 1

4.7K 3W

.25A SLO

221K

2.21K

9.1V

1N4004

1000 UF

9.1V

IRF610

221

4.75K

220UF

221

IRF610

1N4004 R10

4.75K

R11

4.75K

LED1

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

DCBA

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 (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.

R106

221

10UF

C102

R11

4.75K

1000 UF

C3 10

R22

R21

2.21K

221

R112

TH1

THERMISTOR

A B C D

D1-4

4 X 1N4004

1000 UF

3.3 3W

1000 UF

IRF9610

6.8K 3W

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

RV21

RV22

RELAY NC

SELECT

R111

IRF9610

Q102

R109

221K

C104

220uF

IRF610

Q101

R104

221

C101

10UF

R107

221K

9.1V

Z101

9.1V

Z102

OUTPUT 2

INPUT 1

INPUT 2

INPUT 3

INPUT 4

9.1V

OUTPUT 1

.25A SLO

221K

2.21K

9.1V

IRF610

221

4.75K

220UF

221

IRF610

1N4004 R10

4.75K

LED1

1N4004

R120

6.81K

R113

C104

10pF

R115

470

R114

221

R110

22.1K

9/24/96

LEVEL CONTROL

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

DCBA

BRevNumber

Title

Size

Date

Filename

Drawn by

ofSheet

Table of contents

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