Thrax Audio Libra User manual

Libra
Differential balanced 300B tube preamplifier
by Thrax Audio
Operating Manual
Manual issued 28/04/2020
CAUTION
THE UNIT CONTAINS NO USER SERVICEABLE PARTS. DO NOT REMOVE THE COVERS. LETHAL
VOLTAGES ARE PRESENT WITHIN THE CHASSIS.
NEVER OPERATE THE PREAMPLIFIER WITH THE CHASSIS COVERS REMOVED.
DO NOT OPERATE THE UNIT IF WET OR NEAR WATER.
THE PREAMPLIFIER IS FAIRLY HEAVY. BE SURE TO USE PROPER LIFTING TECHNIQUES TO AVOID
BACK STRAIN AND INJURY. BE CERTAIN TO INSTALL IT IN A SECURE LOCATION FROM WHICH
IT CAN NOT FALL OR TIP OVER.
TIME SPENT FAMILIARIZING YOURSELF WITH THIS MANUAL WILL BE WELL REWARDED.

1. Important Safety Instructions …………...................….................................... 3
2. Introduction ………........................................................................................… 4
3. Specifications …..........................................................................................… 5
4. Unpacking and Installation
- Unpacking ……....................................................................................… 6
- Installation ……....................................................................................… 6
5. Front Panel ……..............................................................................................… 7
6. Back Panel
- AC Fuse …….........................................................................................… 9
- Inputs …..…...........................................................................................… 9
- Outputs ………......................................................................................… 9
- Tape output ………............................................................................… 10
9. Remote Control ............................................................................................. 11
8. Operation ……….........................................................................................… 12
CONTENTS
2

READ ALL INSTRUCTIONS: All the safety and operating instructions of your Thrax Audio
equipment should be read before power is applied to the equipment.
RETAIN OWNER'S MANUAL: These safety and operating instructions should be retained
for future reference.
HEED WARNING: All warnings on the unit and in the operating instructions should be
adhered to.
FOLLOW INSTRUCTIONS: All operating and use instructions should be followed.
CLEANING: Unplug the unit from the wall outlet and leave for at least 30 min. - make sure
the unit is not hot before you start cleaning. Use dry or damp soft cloth to clean the
surfaces. Do not use sprays, abrasive or corrosive cleaning products.
VENTILATION: Do not block any ventilation openings. Do not install near any heat sources
such as radiators, heat registers, stoves, or other apparatus (including amplifiers). The unit
should not be installed in a built-in location such as a bookcase or rack unless proper
ventilation is provided.
GROUNDING: Do not defeat the safety purpose of the grounding-type plug. A
grounding-type plug has two blades and a third grounding prong. The third prong is
provided for your safety. If the provided plug does not fit into your outlet, consult an
electrician for replacement of the obsolete outlet.
POWER CORD PROTECTION: Protect the power cord from being walked on or pinched
particularly at plugs, convenience receptacles and the point where they exit from the
apparatus.
LIGHTNING: Unplug this apparatus during lightning storms or when unused for long
periods of time.
SERVICING: Refer all servicing to qualified service personnel. Servicing is required when
the apparatus has been damaged in any way, such as if the power-supply cord or plug is
damaged, liquid has been spilled inside the appliance or it has been exposed to
moisture, the appliance does not operate normally, or has been dropped.
DO NOT OPEN THE CABINET: There are no user serviceable components inside this
product. Opening the cabinet may present a shock hazard, and any modification to the
product will void your warranty.
WARNING: To reduce the risk of fire or electric shock, do not expose this apparatus to rain
or moisture.
IMPORTANT SAFETY INSTRUCTIONS
3

The Libra preamplifier is the world’s finest audio preamplifier and an absolutely unique
component. It’s unusual topology, premium parts, and flexibility make it a pleasure to
hear and operate.
Libra allows you to experience and be awed by a full 300B signal amplification path - it
uses two 300B tubes per channel in true dual differential mono pure class-A triode
operation with two independent outputs.
Features
6 inputs – 3 XLR line inputs, 1 XLR Tape Input and 2 RCA inputs
3 XLR Outputs - 2 line outputs and 1 Tape Output
100-stepped volume control
Tape Loop mode for use with tape recorders
Home Theatre mode for assigning different settings to each output
Inside you get
Balanced differential topology for noise immunity and subtle dynamics.
Auto bias for compensation of tube ageing and correct operation of any 300B.
The case is machined from plates of different thickness aircraft grade aluminum.
INTRODUCTION
4

5
SPECIFICATIONS
• Inputs
- 2 pair unbalanced RCA connectors
- 4 pair balanced XLR connectors
• Output
- 3 pair balanced XLR connectors
• Attenuation steps ………………..............................................................…….100
• Volume Control Range
- Minimum gain …………….........................................…….. -100 to +12dB
• Power supply ………………......................................................…….. 115 or 230 V
• Power consumption ………..……........................................................…….. 70W
• Dimensions ………………......................................….. 440W x 400D x 150H mm
• Weight ………………............................................................................…….. 65Kg
• Finish ………………...............................….. Black or Silver anodized aluminum
• Tube compliment :
- 4 x 300B Tube

This section describes the unpacking and installation procedures for your new
component.
Unpacking
All Thrax Audio shipping boxes have been specially designed to protect their contents
and special care has been taken to prevent damage under normal shipping conditions.
Mishandling should be evident upon inspection of the shipping container. Carefully
remove your new component from its packing and examine it closely for signs of shipping
damage. We strongly recommend saving all original packing to protect your
component from damage should you wish to store it or ship it at a later date.
Installation
• BE CERTAIN TO INSTALL THE UNIT IN A SECURE LOCATION FROM WHICH IT CANNOT FALL
OR TIP OVER.
• Provide adequate ventilation - the chassis will become warm in normal use. Do not
operate on carpet or any other surface that might block air flow.
• Do not allow the chassis to touch any metal parts, such as the frame of an equipment
rack. This might create a parallel ground path that could degrade the sound of your
system.
• Connect the input and output cables. Input connection is via single ended RCA jacks
or XLR jacks (which can be from balanced or unbalanced sources). The outputs are XLR
jacks.
IMPORTANT: READ THE SECTIONS “INPUTS” & “OUTPUTS”.
• The unit should be operated only with the type of power source indicated on the
marking label. If you are not sure of the type of power supplied to your home, consult your
unit dealer or local power company.
Connect the power supply to the power source indicated on the rear panel. Avoid
power conditioners that float the ground pin. ALWAYS connect power cord to
component before plugging it into an AC outlet, and make sure that unit’s power switch
is set to the „off” position before making the final connection. For best performance, try to
route the power cord away from signal cables.
Pay close attention to power quality, and be aware that different power cords can alter
the sound.
UNPACKING AND INSTALLATION
6

7
FRONT PANEL & MENU
Libra is operated using the two control knobs on the front panel which can be turned
or pressed, or by using the remote control.
“ON/OFF”
Press & hold the Left Control Knob for 10 seconds to turn the preamplifier On and Off.
Input Selection
The selected input is displayed on the main screen - toggle between inputs by turning
the Left Control Knob
Volume Control/Mute
Turn the Right Control Knob to toggle volume level (0-100). Pressing it Mutes the device
Menu
Press & Hold the Left Control Knob to access the Menu.
Menu Controls
Left Control Knob
- Turn to navigate through Menu items
- Press to select highlighted Menu item
Right Control Knob
- Turn to toggle a highlighted setting
- Press to return to previous page
Figure 1
Right Control KnobLeft Control Knob
Display

Home Theatre (HT) Mode
Enable HT Mode to set a separate gain setting for a selected input. The HT mode of
each input is saved in non volatile memory so even after a power outage this
programming is stored inside the preamplifier. Each time that source is selected the
level will jump to the programmed level and you use the HT processors level control to
set the volume - which also controls the other channels such as rear channels, etc...
If your HT processor already had its levels set for the same amplifiers directly
connected to the HT Processor then using level 24 (unity gain) on the preamplifier will
maintain proper surround channel vs main channel levels. If you are installing a new
processor or making other changes in the system then you should set the preamplifier
to an HT mode for that processor input BEFORE you set your surround levels. That way
the system levels will be correct each time that input is selected. The volume could
simply be set manually to the desired level each time you use that input. The HT
feature is just a handy way to make the level jump automatically to the same volume
each time that input is selected.
NOTE: In some systems switching to an unused input will cause a small amount of hum
at the speakers. This is because with no input connected the signal path is ’hanging in
the breeze’ picking up whatever noise is floating by. For unused inputs HT mode can
be engaged and set to volume level 1 or mute. Then when selected the level is so low
that there will be no hum.
Tape Loop
After connecting a tape deck or a digital processor to the Tape inputs and outputs,
enable the Tape Loop feature and select the type of device you’re using in the
settings (TAPE for analog and PROCESSOR for digital).
Output Settings
Use this function to offset the sensitivity of your outputs - generally used for bi-amping
purposes.
Input Settings
Use this function to offset the sensitivity of your Inputs and maintain an
equal gain level from all your sources.
System Settings
Date & Clock - set the correct time & date
Display - set display brightness and customize the interval the device would be
inactive before the display dims (0-60s).
8
FRONT PANEL & MENU

BACK PANEL
џTimer - set a time after which the display turns off completely - Disabled/30m/60m/
90m/120m/180m
џRemote Control - to customize buttons on your remote control, highlight a command
from the list and hold the button on the remote you want to assign it to.
џTube timer - displays how long your tubes have been in use.
џSerial Number - display the serial number for 15 seconds.
џAbout - display firmware info for 15 seconds.
9
FRONT PANEL & MENU
Libra
xxxxxx
OUTPUTS RIGHT POWER RIGHT POWER LEFT OUTPUTS LEFT
1 2 TAPE
OUT
TAPE
IN
21 3 5
6
5
6
3 2 1
TAPE
IN
INPUTS LEFTINPUTS RIGHT
TAPE
OUT
2 1
XLR Inputs (Right) XLR Inputs (Left)RCA Inputs (Right) RCA Inputs (Left)
XLR Outputs (Right) XLR Outputs (Left)
Power Plugs
Inputs
3x (pair) XLR - for standard balanced connections to sources, indicated 1-3 on the back
panel
1x (pair) XLR - for devices making use of the Tape Loop function, indicated as TAPE IN on
the back panel
2x (pair) RCA - for standard unbalanced connections to sources
Outputs
2x (pair) XLR - for standard balanced connection to power amplifiers, indicated 1 and 2
on the back panel
1x (pair) XLR - for devices making use of the Tape Loop function, indicated as TAPE OUT on
the back panel
Power Module
Libra’s power module is in a separate box to circumvent interference. Connect the
power module to the control module using the “Power Left” and “Power Right” plugs on
their back panels.

10
REMOTE CONTROL
The remote control allows for wireless adjustment of Source Select, MUTE, PHASE,
BALLANCE and VOLUME. The remote wand is powered by two “AAA” batteries. These
should be of the alkaline type. You will need to install them in the wand before using the
remote. Since batteries can leak, they should be removed from the wand if it is not going
to be used for an extended period of time.
Remote Handset Functions
On/off button
This button switches the unit ON / OFF, when in
stand-by mode.
Menu / Ok
This button pulls up the menu on screen, and
selects items from it.
Mute (center of volume/balance grid):
This button toggles between mute and normal
volume modes.
The mute will be cancelled when the volume is
adjusted by the remote handset or by the volume
knob on the front of the pre-amp.
Volume Up / Down
The volume up and down buttons step the volume
up or down 1 step (about 2db) per button press or
if held down they continuously adjust volume.
Channel +/-
Use these buttons to select an input or to navigate through the menu.
Source Select Buttons
The buttons labeled ’1’ through ’6’ on the bottom portion of the remote handset directly
access each of the 6 source inputs on the preamplifier. These direct buttons (rather than
next/prev) are used to make sure that macro functions programmed into a (user
supplied) advanced programmable handset function repeatedly.
On/off
Mute Volume
UP / Down
Channel
+/-
Source Select
Figure 3
Menu
Ok
Back

11
For initial power up press the main switch located on the back panel.
To switch the preamplifier on press & hold the Left Knob on the front panel or the ON/OFF
button on the remote control - allow the tubes to warm up for approx. 30 seconds.
The front panel display will be on while the unit is executing a command and then turns
off after a short timeout period, which can be customized in the System Settings menu.
To turn the preamplifier off press & hold the Left Knob again, or use the remote. All the
amplifier circuitry will be turned off for minimum power consumption leaving only a small
STANDBY circuit running. However if you are not going to use the amplifier for a long
period we recommend turning it off, using the power switch on the back panel, to save
power.
Refer to pages 7-9 of this manual for using the Menu.
As with all high fidelity products, the sound characteristic changes somewhat as it warms
up. Best sound will be achieved after about 15 minutes of operation.
Any time that the Libra Preamplifier has not been used for a few weeks the sound may be
different. This is also normal for high resolution audio equipment. Optimum sound should
return after a few hours of operation, preferably with an audio signal.
Note that although your Thrax Audio System has been run for 72 hours at the factory, it will
continue to ”break in” for approximately 150 hours. Also be aware that many
components display the need for a new break in period after being transported in
unheated cargo aircraft.
OPERATION

NOTES