PassLabs Aleph P Service manual

PASS

ALEPH P SERVICE MANUAL

REV 1.7

PASS LABORATORIES
24449 FORESTHILL RD.
FORESTHILL, CA 95631

TEL [phone]
FAX [phone]

PRODUCT DESCRIPTION Version 1.7
The ALEPH P is a high performance Mosfet Class A audio

preamplifier, intended for maximum performance in reproduction of
music. It is a simple design, having only a single balanced gain stage
consisting of a balanced pair of power Mosfets. The gain stage of the
preamplifier is a single Mosfet operated common source and biased
by a Mosfet constant current source.

To best understand the operation of the preamplifier, refer to the
schematic of the basic gain stage. Q16 and Q19 form the active
portion of the gain stage, with Q16 receiving the (+) balanced input

signal, and Q19 receiving the (-) balanced input signal. The inputs are
capacitively coupled through C9 and C13. R50 and R65 are employed
to prevent parasitic oscillation, and the D5-6 and D7-8 protect the
Mosfets from electrostatic high voltages at the input.

The Mosfets amplify the input signals and present them at the outputs,
with the polarity inverted, so that the output of Q16 becomes the (-)
signal, and the output of Q19 becomes the (+) output.

When using the preamplifier with a single-ended input, the negative
input should be shorted to ground for best performance. This is
accomplished by using shorting plug between pins 1 and 3 on the XLR
input connector.

The power supply transformer delivers approximately 80 volts (for 120
or 240 VAC operation) unregulated DC into C28 and C30, which is
then actively regulated down to 60 volts at the output of Q23. Q23 is
driven by a stack of Zener diodes and the regulated output is
additionally filtered by C27.

The output muting relay is controlled by the circuits of Q21 and Q22.
For the relay to activate, both Q21 and Q22 must be conducting. Q22
turns on slowly via the charge on C31, while Q21 will only conduct
when there is at least 8 volts difference between regulated and
unregulated supplies. Thus Q22 delays relay turn on, and Q21 shuts it
off quickly.

The input selection system schematic shows the simple use of relays
to switch four inputs with four poles of connection.

The input pad settings are as follows (only 3 settings)

1.3 dB 3,4 5,6 on

9.7 dB all off
15 dB 1,2 7,8 on

The volume control system schematic shows the 8 bit binary relay
ladder circuits which perform precision attenuation of the output of the
gain stages. In order to achieve good common mode rejection
performance, the output attenuation must be matched far more

precisely than can be accomplished with potentiometers, and so
relays and precision resistors are used.

The control circuitry is located on the front panel PC board of the
preamp. The main elements are the micro-controller, latches, infrared
receiver and optical encoders.

U201 is a Phillips 87C752 micro-controller running at 12MHz. It
receives input from the infrared receiver module via Q204 inverter
buffer. The input select and volume controls (OPT200, OPT201) are
optical interrupters that generate a quadrature pulse train on pins A
and B. These feed interrupt pins on the micro-controller which
recognizes rotational direction and speed.

Test points on the front panel:
Collector of Q204 for infrared received commands.
Pins 10 and 11 of U201 for 12 MHz.
Pin 11 of both latches when volume is being changed.
Pins A and B of the optical encoders when they are being turned.
The indicated 5 volt test point or any 5 volt location on the schematic.

Schematics
Fig. 1 Mother board main gain stage.

Fig. 2 Power supply.
Fig. 3 Volume control.
Fig. 4 Input select.
Fig. 5 Display board.
Fig. 6 Remote control.

Fig. 7 Reference designator drawing for mother brd.
Fig. 8 Reference designator drawing for display brd.

SPECIFICATIONS
Max Gain 22 dB bal / 16 dB unbal
Freq. Response -3 dB @ 1Hz, 100 KHz
Distortion < 1% THD
Max Out 20 volts rms. bal out

10 volts rms. unbal out
Output Impedance 0-800 ohms
Input Impedance 20 KOhm bal

10 KOhm unbal
CMRR -60 dB
Output noise 5 microvolts, 20-20KHz
Power consumption 25 watts
Dimensions 19” W x 11.5” D x 4” H