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PASS
ALEPH P SERVICE MANUAL
REV 1.7
PASS LABORATORIES
24449 FORESTHILL RD.
FORESTHILL, CA 95631
TEL (916) 367-3690
FAX (916) 367-219
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
Pass Laboratories
Aleph P Owner's Manual
Introduction
The Aleph P is a single ended Class A audio preamplifier, the first produced by Pass
Laboratories. It combines completely new design thought applied to a traditional topology and
the experience of twenty five years of amplifier design. This manual applies to the second
version of the Aleph P, which incorporated circuit improvements and remote control operation.
This preamplifier flows from a commitment to create the best sounding product: a simple circuit
with the most natural characteristic. The Aleph P integrates power Mosfet devices and single
ended Class A operation in a simple topology in order to deliver the most natural sound
possible.
The circuitry of the Aleph P breaks new ground in simplicity and performance. Each gain stage
consists of a single Mosfet power transistor operated "common source" mode. One such circuit
amplifies each signal polarity of a balanced input signal, and their operation is inter-coupled to
allow optimal operation with both balanced and unbalanced input signals while still providing
balanced and unbalanced outputs. It retains the important simple single-ended characteristic
while operating either single-ended or balanced.
It is unique in providing superior flexibility and performance with balanced and unbalanced inputs
and outputs, converting one to the other as desired without switches or adapters.
The Aleph P absolutely minimizes the number of components in the signal path, and yet retains
exemplary objective performance specifications. More importantly, it pushes the edge of the art
in exploring how much subjective quality is obtainable with a new but very elementary gain stage.
A very few people are involved in the production of this product. I supervise all phases of the
construction, and I test and listen to each preamplifier myself. If you have questions, comments,
or problems, please feel free to contact me directly.
Thank you for purchasing this preamplifier. It is my sincere hope that you will enjoy its sound as
much as I do.
_________________________________
Nelson Pass
Serial # ____________________
Date: ____________________
Next page: Distortion curve of your particular preamplifier at 1 kHz from .1 to 10 volts
Setup
The preamplifier has four sets of input connections, two sets of output connections, an input
selector and three level controls. Optionally, it also has a remote control.
It also has an AC line power connection. The amplifier's voltage and current rating are indicated
on the rear. It will be either 240 volts, 120 volts, or 100 volts. A .5 amp 3AG slow blow fuse is
provided with 100-120 volt units, and a .25 amp slow blow fuse is provided with 220-240 volt
units. The frequency rating of the power supply is 50 to 60 Hz. The preamplifier typically draws
25 watts during operation.
We have provided a standard AC power cord which fits into the line receptacle at the rear. The
preamplifier is equipped for operation with an earth ground provided by the AC outlet. Do not
defeat this ground. The chassis and circuit ground of the preamplifier is connected to earth
through a power thermistor, which gives a ground connection but helps avoid ground loops.
The four input connections on the rear are pairs of XLR and RCA connectors with right and left
channels indicated. If your signal source is balanced, you may use the XLR input connectors.
On these connectors, pin 1 is grounded, pin 2 is the positive signal input, and pin 3 is the
negative signal input.
If your signal source is unbalanced, input will occur through the RCA input connector, which is in
parallel with the XLR connections 1 and 2. For operation with unbalanced inputs, a shorting plug
is provided between pins 1 and 3, shorting the negative input to ground, and providing optimal
performance.
The unbalanced input impedance of the preamplifier is a nominal 10 kOhms. In balanced mode,
the input impedance is higher, with a differential impedance of 20 kOhms.
Next to the inputs on the rear panel, the preamplifier offers tape outputs through both XLR and
RCA connectors. This output is a direct connection to inputs 1, 2, and 3 when they are selected
from the front panel. Input 4 is deliberately not available through the tape output connection.
Input 4 is designated for use with a tape recorder if you have one, and we have arranged that it
will not place its output on the tape out, which will prevent you from accidentally creating a
feedback connection with your tape machine.
At the left hand side (viewed from the rear) of the rear panel, two male XLR connectors and two
RCA connectors are used for the main output. On the XLR, pin 1 is ground, pin 2 is positive, and
pin 3 is negative. The RCA connector's ground is in parallel with pin 1 and the RCA hot is
attached to pin 2. You may use either of these connectors for balanced or unbalanced
operation.
The circuitry of the preamplifier is such that it will drive any impedance without distortion,
however as the load impedance goes down, you will experience a reduction in gain. The
preamplifier will act as a maximum plus and minus 20 ma current source into a dead short.
The operation of the front panel input selector is straight forward. The selected input is indicated
by a blue LED above the knob, and the selection can be moved by rotating the knob. The knob
is attached to an optical encoder indicating the direction of rotation to the microcontroller which
drives the input relay switches.
The Left and Right input level controls and the Master level control are designed not only to
provide level adjustment and balance adjustment, but also to optimize the performance of the
circuit against various input levels.
The input of the gain stage is directly coupled to the input selected, without an intervening volume
or balance control as seen in other preamplifiers. This provides the highest performance from
the Mosfet gain devices.
Because there is a wide variety of possible source levels, the intrinsic gain of the circuit should
be adjusted to provide optimal performance. This is something you will not see on other
preamplifiers. The intrinsic gain of the circuit itself is adjusted by the Left and Right level
controls. We set the gain lower for sources which have high levels, and higher for sources with
low levels, minimizing the distortion and noise for each.
We have set the maximum input so that the circuit will remain linear with balanced input peaks up
to about 18 volts. If you are concerned that your source will approach or exceed this level, you
may use the internal attenuator switch to reduce the input by 12 dB.
The switch default position is 1,2,7,8 = ON and 3,4,5,6 = OFF. For a 12 dB input attenuation,
1,2,7,8 = OFF and 3,4,5,6 = ON.
This switch can also be used to effectively lower the gain of the preamp by 12 dB. It does not
otherwise affect the performance of the preamp.
At the output of the active gain circuits is the master level control. This is a low impedance
discrete level control which sits between the preamp active circuitry and the input to your power
amplifier. It attenuates the output of the preamp in the same manner as you would experience
with a "passive preamp" except that it has a precision 256 position 4 pole attenuator for both
balanced channels matched to .1%. It also has a low impedance, which assures that the typical
unbalanced output impedance is between 100 and 1000 ohms.
There are good reasons for having such an arrangement:
The gain stage operates at a constant level regardless of the setting of the master level control,
and so the sound of the this circuit will not alter at various level settings.
The preamplifier will drive a low impedance load without alteration in the signal, in fact it can be
used as a current source driving a 0 impedance mixer junction without any distortion.
Any noise characteristic of the preamp circuit is attenuated along with the signal, unlike circuits
where the volume control is before the input.
If you have a concern that the output impedance may be too high to properly drive the
capacitance of a cable, keep in mind that that 1000 ohms still will allow bandwidth to 160 kHz
into 1000 pF of capacitance. The maximum output impedance only occurs at the volume control
maximum, and quickly goes to a low figure below the maximum level position. In typical
operation, the output impedance will be between 100 and 300 ohms.
The sixteen relays which operate the level control allow greater than 48 dB range and are driven
by a microcontroller which reads the optical encoder which serves as a front panel volume
control. In this manner tracking of the volume of the two balanced channels is possible with an
accuracy unavailable on any ordinary volume control, assuring precise level steps and high
common mode rejection in balanced circuits.
The relays may lack the high speed switching characteristic of the solid-state state switches
used by other preamplifiers, but they also have none semiconductor distortions and noise which
accompany analog gates and voltage controlled amplifiers. If it bothers you to hear the click of
the relays as the level is adjusted, just consider the artifacts you won't hear after the level is
adjusted.
Adjustment to optimize the level controls against an input level is easy. Select the most
important input. Set the Right and Left level controls at minimum gain (counterclockwise) and
adjust the Master level control to as high as you will want to listen. If you don't have enough gain,
then increase the levels on the Left and Right level controls until you do.
For maximum performance, the idea is to keep the Master control as clockwise as possible,
and the Left and Right level controls as counterclockwise as possible.
In other words, go ahead and turn it up.
This is not a big deal to worry about, but the ability to get the most performance is available in
this preamp in a manner not seen elsewhere, and you will want to take advantage of it.
Using the remote control is very simple. Point it at the preamplifier and press one of the four
buttons on the remote.
The side to side buttons will change the input selection.
The top to bottom buttons will change the master level.
The end.
Product Philosophy and Design Theory
When I started designing amplifiers 25 years ago, solid state amplifiers had just achieved a firm
grasp on the market. Power and harmonic distortion numbers were king, and the largest audio
magazine said that amplifiers with the same specs sounded the same.
We have heard Triodes, Pentodes, Bipolar, VFET, Mosfet, TFET valves, IGBT, Hybrids, THD
distortion, IM distortion, TIM distortion, phase distortion, quantization, feedback, nested
feedback, no feedback, feed forward, Stasis, harmonic time alignment, high slew, Class AB,
Class A, Pure Class A, Class AA, Class A/AB, Class D, Class H, Constant bias, dynamic bias,
optical bias, Real Life Bias, Sustained Plateau Bias, big supplies, smart supplies, regulated
supplies, separate supplies, switching supplies, dynamic headroom, high current, balanced
inputs and balanced outputs.
Apart from digitally recorded source material, things have not changed very much in twenty five
years. Solid state amplifiers still dominate the market, the largest audio magazine still doesn't
hear the difference, and many audiophiles are still hanging on to their tubes. Leaving aside the
examples of marketing hype, we have a large number of attempts to improve the sound of
amplifiers, each attempting to address a hypothesized flaw in the performance. Audiophiles
have voted on the various designs with their pocketbooks, and products go down in history as
classics or are forgotten. The used market speaks eloquently: Marantz 9's command a high
price, while Dyna 120's are largely unwanted.
There has been a failure in the attempt to use specifications to characterize the subtleties of
sonic performance. Amplifiers with similar measurements are not equal, and products with
higher power, wider bandwidth, and lower distortion do not necessarily sound better.
Historically, that amplifier offering the most power, or the lowest IM distortion, or the lowest THD,
or the highest slew rate, or the lowest noise, has not become a classic or even been more than a
modest success.
For a long time there has been faith in the technical community that eventually some objective
analysis would reconcile critical listener's subjective experience with laboratory measurement.
Perhaps this will occur, but in the meantime, audiophiles largely reject bench specifications as
an indicator of audio quality. This is appropriate. Appreciation of audio is a completely
subjective human experience. We should no more let numbers define audio quality than we
would let chemical analysis be the arbiter of fine wines. Measurements can provide a measure
of insight, but are no substitute for human judgment.
As in art, classic audio components are the results of individual efforts and reflect a coherent
underlying philosophy. They make a subjective and an objective statement of quality which is
meant to be appreciated. It is essential that the circuitry of an audio component reflects a
philosophy which address the subjective nature of its performance first and foremost.
Lacking an ability to completely characterize performance in an objective manner, we should
take a step back from the resulting waveform and take into account the process by which it has
been achieved. The history of what has been done to the music is important and must be
considered a part of the result. Everything that has been done to the signal is embedded in it,
however subtly.
Experience correlating what sounds good to knowledge of component design yields some
general guidelines as to what will sound good and what will not:
1) Simplicity and a minimum number of components is a key element, and is well reflected in the
quality of tube designs. The fewer pieces in series with the signal path, the better. This often
true even if adding just one more gain stage will improve the measured specs.
2) The characteristic of gain devices and their specific use is important. Individual variations in
performance between like devices is important, as are differences in topological usage. All
signal bearing devices contribute to the degradation, but there are some different characteristics
are worth attention. Low order nonlinearities are largely additive in quality, bringing false warmth
and coloration, while abrupt high order nonlinearities are additive and subtractive, adding
harshness while losing information.
3) Maximum intrinsic linearity is desired. This is the performance of the gain stages before
feedback is applied. Experience suggests that feedback is a subtractive process; it removes
information from the signal. In many older designs, poor intrinsic linearity has been corrected out
by large application of feedback, resulting in loss of warmth, space, and detail.
High idle current, or bias, is very desirable as a means of maximizing linearity, and gives an
effect which is not only easily measured, but easily demonstrated: Take a Class A or other high
bias amplifier and compare the sound with full bias and with bias reduced. (Bias adjustment is
easily accomplished, as virtually every amplifier has a bias adjustment pot, but it should be done
very carefully). As an experiment it has the virtue of only changing the bias and the expectations
of the experimenter.
As the bias is reduced the perception of stage depth and ambiance will generally decrease.
This perception of depth is influenced by the raw quantity of bias current.
If you continue to increase the bias current far beyond the operating point, it appears that
improvements are made with bias currents which are much greater than the signal level.
Typically the levels involved in most critical listening are only a few watts, but an amplifier biased
for ten times that amount will generally sound better than one biased for the few watts.
For this reason, designs which operate in what has been referred to as "pure" Class A are
preferred because their bias currents are much larger than the signal most of the time.
As mentioned, preamp gain stages and the front ends of power amplifiers are routinely single
ended "pure" Class A, and because the signal levels are at small fractions of a watt, the
efficiency of the circuit is not important.
Problems with push-pull amplifier designs associated with crossover distortion have been
discussed elsewhere at length, and one of the primary results is non-monotonicity. Class B and
many AB designs have distortion products that dramatically increase with decreasing signal.
This is reduced greatly by Class A mode, but crossover distortion remains as a lower order
discontinuity in the transfer curve.
A very important consideration in attempting to create an amplifier with a natural characteristic is
the selection of the gain devices. A single ended Class A topology is appropriate, and we want
a characteristic where the positive amplitude is very, very slightly greater than the negative. For
a current gain device, that would mean gain that smoothly increases with current, and for a tube
or field effect device a transconductance that smoothly increases with current.
Triodes and Mosfets share a useful characteristic: their transconductance tends to increase with
current. Bipolar power devices have a slight gain increase until they hit about an amp or so, and
then they decline at higher currents. In general the use of bipolar in a single ended Class A
circuit is a poor fit.
Another performance advantage shared by Tubes and Fets is the high performance they deliver
in simple Class A circuits. Bipolar designs on the market have between four and seven gain
stages associated with the signal path, but with tubes and Mosfets good objective specifications
are achievable with as few as one gain device in the signal path.
Regardless of the type of gain device, in systems where the utmost in natural reproduction is the
goal, simple single ended Class A circuits are the topologies of choice.
We recommend the use of the balanced output mode whenever possible. It will retain the
character of the input mode, but offers less distortion, less noise, more gain, and more voltage
swing, all without compromising the sound in any way.
The Aleph P uses power Mosfets exclusively for its gain stage. These Mosfets were chosen
because they have an excellent transfer curve for an asymmetric Class A design.
The gain Mosfets are rated at 40 watts each and peak currents in excess of 5 amps. Needless
to say, they do not work very hard when sourcing 30 milliamps into a load. The use of such
devices does provide very high transconductance and charge surface area over small gain
devices, and this shows in the excellent linearity obtained with only one device operated without
feedback.
Mosfets provide the widest bandwidth of solid state power devices, however they were not
chosen for this reason. The design of the Aleph P does not seek to maximize the preamplifier
bandwidth as such. The capacitances of the Mosfets provide a natural rolloff in conjunction with
the resistive impedances found in the circuit, and the simplicity of the circuit allows for what is
largely a single pole rolloff characteristic. Nevertheless, the bandwidth of the circuit will typically
extend to about 200 kHz (-3dB).
There is no such thing as a slew rate for this circuit, as it will retain the linear RC characteristic
for any input signal.
The common mode rejection of the preamp reflects the intrinsic common mode rejection of the
topology, the matching of the gain devices, and the matching of the output attenuator channels.
In this case we have been able to keep the total mismatch to about .1%, for a common mode
rejection of approximately -60 dB.
The input system of the preamplifier will exhibit full common mode noise rejection with passive
balanced sources, where the negative input is connected to ground at the source through the
appropriate source impedance. This allows adaptation of unbalanced sources to balanced
operation with passive cable connections in a manner that achieves the noise rejection of active
balanced sources.
Load impedances do not make much difference to the character of the output. The intrinsic
output of the impedance of the preamp is passive in nature, and no load will create nonlinearity.
The internal power supply for the Aleph P consists of a toroidal power transformer delivering an
unregulated 85 volts which actively regulated before feeding passive filtering before powering
the constant current sources which bias the gain stages. The power supply noise reaching the
circuit is on the order of a microvolt, and is differentially rejected at the output in a balanced
system. The relays and control systems are regulated independently.
The output of the preamplifier is guarded by muting relays which delay connection during turn-on
and shut off the output when insufficient power supply is available to maintain regulation. The
preamplifier is designed to run constantly, and will exhibit optimum performance within an hour of
turn-on.
The chassis of the Aleph p is made entirely of machined aluminum. We mill and engrave the
chassis components from solid aluminum material on computer controlled vertical milling
machines. No sheet metal is employed.
The Aleph P is warranted by Pass Laboratories to meet performance specifications for 3 years
from date of manufacture. During that time, Pass Laboratories will provide free labor and parts
at the manufacturing site. The warranty does not include damage due to misuse or abuse or
modified products and also does not include consequential damage.
SPECIFICATIONS
Gain 15 dB bal in / bal out 15 dB unbal in / bal out
9 dB bal in / unbal out 9 dB unbal in / unbal out
Freq. Response -3 dB @ 5 Hz -3 dB at > 100 kHz
Distortion < .1 % THD typically .01% @ 2 volts @ 1KHZ
Maximum Output 15 volts rms. bal out 8 volts rms. unbal out
Output Impedance 0 - 1000 ohms unbalanced
0 - 2000 ohms balanced
Input Impedance 10 kOhm unbalanced 20 kOhm balanced
CMRR typical -80 Db, 20-20 KHz
Crosstalk typical -90 dB, 20-20 KHz
Output Noise Noise floor < -120 dBV, 20-20 KHz
Power Consumption 25 watts
Dimensions 19 " W x 11.5 " D x 4" H
Shipping Weight 35 lb.
PASS
Pass Laboratories
PO Box 219
24449 Foresthill Rd.
Foresthill CA 95631
tel (916) 367 3690
fax (916) 367 2193