X2.5 - Pass Labs
X2.5 Owner’s Manual
10, 2006
X2.5 Owner’s Manual
The X2.5 is an ultra high quality linestage audio preamplifier
featuring remote control and internal power supply. It combines
a completely new circuit topology with traditional construction
representing thirty five years of preamplifier design.
This preamplifier flows from a commitment to create the best
sounding product: a simple circuit with the most natural sonic
characteristics. Derived from US Patent # 5,376,899, the X2.5
integrates power MOSFET and JFET devices in a simple Class A
topology in order to deliver the finest sound possible.
The Super-Symmetric gain circuitry is unique in providing superior
flexibility and performance with both balanced and unbalanced
inputs and outputs, converting one to the other as desired without
extra circuitry and without performance degradation.
The X2.5 minimizes the number of components in the signal path,
and yet retains exemplary objective performance specifications. It
pushes the edge of the art in exploring how much subjective quality
is obtainable with a new but very elementary gain stage.
The X2.5 employs a patented new type of volume control that
offers significantly better performance than seen before in an
electronic control. It provides over 60 dB of range, large voltage
swing and very good measured performance. The distortion and
noise is at 1/10 and the dynamic range 100 times the previous state
of the art in active attenuation. Most importantly it is subjectively
outstanding; better than any potentiometer we have evaluated.
The fluorescent display is a very special device. Not only will
you find its blue color highly attractive and readable, it is far more
reliable than the usual LED or LCD display.
Very few people are involved in the production of this product. I
supervise all phases of the construction, and I test 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.
Wayne Colburn
Vice President, Design
X2.5 Owner’s Manual
The preamplifier has four sets of input connections, both singleended and balanced output connections, a tape loop and a full
function remote control that mirrors the front panel functions.
It also has a 5-way binding post that is used for remote amplifier
turn-on. The preamplifier’s voltage and current rating are indicated
by affixed tag on the bottom of the preamplifier. It will indicate
either 240 volts, 220 volts, 120 volts, or 100 volts. A 0.5 amp 3AG
slow blow fuse is provided with 100-120 volt units, and a 0.25 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 30 watts during operation. Please verify that the
indicated voltage requirement of this preamplifier is consistant with
the voltage at your location.
We have provided a standard IEC AC power cord that fits into
the IEC line receptacle at the rear of the preamplifier chassis.
The preamplifier is equipped for operation with an earth ground
provided by the users 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 for
safety but helps avoid ground loops.
There are a total of four inputs and a separate tape loop. Two
of the four inputs can be used single-ended or balanced. The
other two inputs are single-ended only. The tape loop has both
single-ended and balanced inputs and outputs. Balanced inputs
and outputs are via XLR connectors. Single-ended inputs and
outputs are via RCA connectors. On the XLR connectors, pin 1
is grounded, pin 2 is the positive signal, and pin 3 is the negative
signal. Left channel RCA connectors are marked with “L” from
below. Right channel RCA connectors are marked with “R” from
below. Balanced XLR inputs and outputs are marked with “L” for
left and “R” for right channel.
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. When single-ended inputs are used on input 1 and input
2, a shorting plug is provided between pins 1 and 3 of the XLR
connection, shorting the negative input to ground, and providing
optimal performance.
The unbalanced input impedance of the preamplifier is 33 kOhms.
In balanced mode, the input impedance is higher, with a differential
impedance of at least 66 kOhms.
X2.5 Owner’s Manual
Next to the inputs on the rear panel, the preamplifier offers tape
inputs through both XLR and RCA connectors. This output is a
direct connection to inputs 1, 2, 3, and 4 when they are selected
from the front panel. Input 5 (TAPE IN) is deliberately not
available through the tape output connection. Input 5 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
The main outputs are located at the left-hand side (viewed from the
rear) of the rear panel, two male XLR connectors, and two RCA
connectors, “R” (right) and “L” (left). The RCA connector’s ground
is in parallel with pin 1 of the XLR outputs and the RCA hot is fed
from an independent summing junction that maximizes the X circuit
benefits. The RCA, single-ended, outputs are buffered from the
balanced outputs. You may use both the single ended and balanced
outputs at the same time. On the XLR, pin 1 is ground, pin 2 is
positive, and pin 3 is negative.
The rear 5-way binding post can be used to turn on any X series
power amplifier.
Description of Operation
In application the X2.5s front panel controls and alphanumeric
display are quite straight forward, and intuitive. We encourage you
to become familiar with their operation prior to establishing any
input connections with this unit. Experience indicates that engaging
in a few minutes of exploration and playtime will add much to your
ultimate enjoyment of this exceptional Pass Labs product.
The two front panel mode switches move the preamplifier and it’s
display through 10 different functions. From left to right these
functions are as follows: Volume, Input, Mute, Tape, Balance,
Display, Mono, Unity, Amp (power-amplifier control), and Gain.
Actuating the mode < switch moves the function to the feature
immediately to the left, and conversely actuating the mode > switch
moves the function to the feature that is immediately to the right.
The two Select switches (designated by up and down pointing
arrows) then alter the displayed active function. The select arrows
toggle the following functions either off or on (the up pointing
arrow representing “on” and the down pointing arrow representing
“off ”) Mute, Tape, Mono, and Amp.
The leftmost function as selected by the two mode switches is
X2.5 Owner’s Manual
volume, and the up/down arrows select 32 steps incrementally,
indicated on the display as 0 - 31. A more traditional knob, located
at the far right of the front panel, which is always active, exactly
duplicates this control function.
The next mode to the right is the input selector, for inputs 1
through 4. Input 4 is unique in that it is linked with the Unity
function. (The UNITY FUNCTION is discussed at length later in
this manual.) The two select arrows logically control selections of
the various inputs to the preamplifier.
The channel balance function is affected by the up and down
arrows. The up arrow is balance right and the down arrow
conversely balance left.
The display function has three iterations of brightness selectable by
the up/down arrows; they are bright, dim and off.
The unity function bears special attention. This control is associated
only with input number 4 and has two positions selected with
the up/down arrows. The up arrow toggles to a straight through
function with neither attenuation nor gain. This function is useful
in that it allows the preamplifier to function as a unity buffer for use
with components that best function with their own volume controls,
such as surround processors. When the right-front and left-front
outputs of the processor are routed through input 4 of the X2.5
(with the gain of the X2.5 set for Unity gain), the volume will be
under full control of the processor and the X2.5’s action will be
totally transparent. When the user chooses Unity this sets the X2.5’s
gain to the low (+4dB) setting, regardless of how it was previously
configured. When at Unity both left and right level controls
on the X2.5 will have a displayed value of 24, and any previous
balance settings will be lost. Both of these default conditions are
intentional, and intended to make the most advantageous use of
your source electronics. You may also set any input to unity by
selecting low gain and setting the volume at 24.
Before selecting this option, it is imperative that the input device on
input number 4 has its gain set to minimum. Failure to follow this
precaution could result in a volume level intolerable to both ears
and speakers. To further protect your hearing and valuable audio
equipment, Wayne has designed this product, so that the volume
setting of the preamplifier will always revert to zero as you change
inputs. We trust that the remote control, with its four buttons that
duplicate those of the preamp front panel will enable you to quickly
select inputs and establish levels of output.
X2.5 Owner’s Manual
The rear mounted pair of five way binding posts are intended to
control the on/off function of our own X series power amplifiers
from the X2.5 front panel, or via the four button remote. When
the preamp mode “Amp” is selected it allows the user to switch a
nominal 12 volts DC at maximum draw of 50 milliamps to these
binding posts. In some instances this voltage may have the ability
to control products from other manufactures, but we by no means
have comprehensive listing of their compatible features. If you have
questions about compatibility your service technician and dealer are
an excellent resource of information.
The X2.5 to our knowledge is unique in that its gain may be
attenuated to match the requirements of a wide variety of
electronics and speakers. The gain control toggles between two
settings, offset from one another by 10 dB. The up pointing arrow
selects +14 dB and the down pointing arrow selects +4 dB. When
used with our own X series power amplifiers, which are quite high
gain (30 dB), we find the +4 dB setting to be most satisfying. With
other product we suggest that you let your ears determine which
setting conveys the most sonic benefit.
The remote control uses the same four-button system. The far left
button is mode < left the far right button is mode > right. The
top button is select ^ up and the bottom is select v down. The
display shows the active function. The display has two brightness
levels and off. If any control is used the display reverts to the last
level. The off mode shows the display for about 7 seconds and
then turns off.
The electronic volume control allows greater than 60 dB range
and is driven by a micro-controller that reads the optical encoder
serving as a front panel volume control. In this manner tracking of
the volume of the two balanced channels is possible with accuracy
unavailable on any ordinary volume control, assuring precise level
steps and high common mode rejection in balanced circuits. Wayne
Colburn’s exceptional volume control in conjunction with the
fluorescent display give the user ability to replicate volume levels
with absolute accuracy in steps of roughly 3dB.
Product Philosophy
X2.5 Owner’s Manual
1) Circuit simplicity and a minimum number of components is
a key element. The fewer parts in series with the signal path, the
better. Adding more parts and gain stages may improve measured
specs, but will seldom improve the perceived sonic quality or
reliability of a product.
2) The characteristic of gain devices and their specific use is
important. Individual variations in performance between like
devices are important, as are differences in topological usage. All
signal bearing devices contribute to the degradation, but there are
some different characteristics that are worthy of attention. For
example, smooth low order nonlinearities are additive in quality,
bringing false warmth and coloration, while abrupt high order
nonlinearities add harshness and false brightness.
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. The trick is to get good performance
without this excessive reliance on feedback.
The art of high end audio design lies in the approach to combining
these elements to get high reliability and simplicity, high quality
measured performance, and high quality subjective performance. It
takes imagination and hard work to get as much of these as possible
at once without trading them off against each other.
Other Design Comments
We recommend the use of the balanced output mode where
possible. It will retain the character of the input mode, but offers
less distortion, less noise, more gain, and more voltage swing,
without compromising the sound.
With balanced operation, 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 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.
The use of a micro-controller allows all of the preamplifier
functions to be repeatable and accurately controlled. The micro
processor only controls the functions of the preamplifier. At no
X2.5 Owner’s Manual
time does any of the input or output signal come into contact with
the digital control signals. The digital circuits are powered by a
power supply that is completely separate from the analog supply.
Should it ever be necessary to update the software that controls the
functioning of the preamp only the socketed micro processor need
be changed.
The power supply for the X2.5 consists of a toroidal power
transformer delivering an unregulated +/- 45 volts which is actively
regulated before feeding passive filtering and powering the constant
current sources which bias the gain stages. Each channel has it’s
own regulation. The power supply noise reaching the circuit is on
the order of one microvolt, and the little of that which get through
the circuit is rejected at the output in a balanced system. The relays
and control systems are regulated independently, and all digital
circuitry is isolated from the power supply.
Muting relays, which delay connection during turn-on and shut off
the output when insufficient power supply is available to maintain
regulation, guard the output of the preamplifier. The preamplifier
is designed to run constantly, and will exhibit optimum measured
performance within about 10 hours of turn-on.
The X2.5 consists of a single chassis.
For more information please contact:
Pass Laboratories Inc.
P.O. Box 219,
24449 Foresthill Rd.,
Foresthill, CA 95631
Tel: 530.367.3690
Fax: 530.369.2193
© 2006 Pass Laboratories Inc.
X2.5 Owner’s Manual
X2.5 Specifications
Gain adjustment
Number of volume control
Freq. Response
Distortion @ 1KHz
Output Voltage
Output Impedance
Input Impedance
Number of gain paths
Separate analog channels
Remote control
Input voltage before overload
Output noise floor
External amp turn on
Single-ended only inputs
Balanced / single ended
Mono operation setting
Unity gain setting
Power Supply
Power consumption
Number of chassis
4dB or 14dB
menu setting
-3dB @ 2Hz, -3dB @ 100kHz
<0.1% THD typ, 0.003% @ 2V
10 Balanced, 7 Single-ended
360 Balanced, 150 Single-ended
66K Balanced, 33K Singleended
10 volts
-55dB 20 - 20kHz
-125dBV ref to 10V output
typ. -85dB 1kHz
25 watts
17"W x 3.5"H x 11.5"D
“Pass”, “pass”, “Pass Labs”, “Pass Laboratories”, Supersymmetery”,
“Aleph”, and “Zen” and are all registered trademarks of Pass
Laboratories, Inc., and all rights thereto are protected by law.
X2.5 Owner’s Manual
Warranty Information
Please check with the factory-authorized distributor in the country you are
purchasing this product for specific warranty information.
All Pass Laboratories products purchased from an authorized Pass Laboratories dealer in North America are covered by a transferable, limited 3-year
warranty. This warrantee includes all parts and labor charges incurred at the
repair facility in addition to return shipping to the domestic customer, exclusive of subsequent damages. Damage due to physical abuse is specifically
not covered under this warranty.
For this warranty to apply the customer is responsible for returning the
product unmodified to the factory within the warranty period. The customer
assumes all responsibility for shipping and insurance to the factory or a factory specified repair facility. The conditions and stipulations of this Pass
Laboratories warranty only applies to units sold new in North America.
Non-North America customers should consult with their original Pass Labs
dealer or distributor for warrantee repair instruction prior to contacting the
factory or shipping product for repair.
Non-North American product must be returned to the country of origin for
warrantee service. Foreign distributors are only required to offer warranty
service on Pass Laboratories product that they have imported.
Please note: Conditions of warranty service and customer rights for product
purchased outside the United States may vary depending upon the distributor and local laws. Please check with your local distributor for specific rights
and details.
Any modifications to Pass Laboratories products that have not received
written factory approval nullify all claims and void the warrantee. Should a
modified product be returned to the factory for repair the owner will be required to pay all necessary charges for the repair in addition to those charges
required to return the product to it’s original configuration.
In the case of safety issues, no product shall be returned to the customer
without those safety issues being corrected to the most recent accepted
Removal or alteration of original Pass Labs serial numbers voids the factory
warranty. Product with altered or missing serial numbers will be suspected
as counterfeit product.
Pass Laboratories will not repair or in any way indemnify any counterfeit or
cloned product.
Pass Laboratories does not offer products in voltages intended for international markets either to authorized Pass Labs dealers or to third parties
located in the United States or Canada.
X2.5 Owner’s Manual
For your protection please
read the following:
Water and moisture: Electrical devices should not be used near
water ( as per example, near a bathtub, washbasin, kitchen sink,
laundry tub, wet basement or swimming pool ). Care should be
taken such that objects do not have the opportunity to fall, and that
liquid is never spilled onto or into the device enclosure through
Power Sources: An electrical device must be connected to a mains
power source in strict accordance with the supplied product owner’s
manual. Please verify that the AC mains voltage specified in the
product manual matches those requirements indicated on the unit
and the AC voltage provided to your location by the power company.
Grounding: Adequate precautions should be taken so that the
grounding provisions built into an electrical product are never
Power Cords: Pass Laboratories provides a power supply cord that
meets all legislated requirements for the market in which the product
was originally sold. If you choose to substitute an after-market
product we urge you to choose one that is fully safety rated by the
necessary local authority.
Power Cord Protection: Power supply cords should be routed so
that they are not likely to be walked on, abraded, or pinched by items
placed on or against them, paying particular attention to cords where
they enter plugs or exit from a device. Never under any circumstance
insert a cut or damaged power cord into a mains power socket.
Power and Signal: Cables should never be connected /
disconnected with equipment powered up. Failure to heed this
warning may damage or destroy equipment.
Ventilation: Power-amplifiers run hot, but you should be able to
place your hands on them without discomfort. You must allow for
this heat in installation, by providing for free air circulation around
the product. Electronics should not be subjected to sources of
excessive radiant heat. Excessive heat can shorten the life of the
product and may cause the electronics to self-protect and shut down.
Servicing: To reduce the risk of fire, electrical shock or other
injuries, the user should not attempt to service the device beyond
that which is described in the operating instructions. All other
servicing must be referred to qualified service personnel.
X2.5 Owner’s Manual
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