Lehmannaudio Linear User manual

Linear USB
Headphone amplifier / Dual Source Preamp
User manual

Lehmannaudio Manual Linear USB - page 2 -
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Dear customer,
congratulations for buying the Lehmannaudio Linear USB. You will be rewarded with the best
possible sound from your headphones. You can use practically all dynamic headphones on the
market. Carefully selected components and a highly sophisticated circuit design guarantee a very
high level of audio reproduction.
Please read this manual carefully and keep it nearby the unit for future reference even if you have
experiences with other headphone amplifiers!
Precautions
Please read the following before operating your Linear USB.
While installing the Linear USB
- Be careful to use an appropriate cable for the connection between your power outlet and the Linear USB.
Make sure that you have the appropriate version for your mains voltage.
- Be careful to prevent Linear USB from getting wet; do not allow water to enter the unit, especially when
raining or snowing, or near a body of water. Otherwise fire or electrical shock may result.
- Do not place heavy objects on the power cord. If the cord is damaged, fire or electrical shock may result.
In particular, it is possible that one might accidentally place a heavy object on a carpet that covers the cord;
definitely avoid these situations!
- Do not install the Linear USB in a place where it might be exposed to oil, smoke or steam (for example,
near a cooking table or humidifier). Otherwise, fire or electrical shock may result.
- Do not place the Linear USB on an unstable surface, such as an unstable bench or slanted surface.
Otherwise, the device may fall or drop, resulting in an injury.
- Do not place the power cord near a heating device. Otherwise, the cord sheath may melt, resulting in fire
or electrical shock.
- Do not locate the Linear USB in a place subject to excessive heat, such as inside a car with all the windows
closed, or in direct sunlight. Otherwise, fire may result.
- Do not handle the power plug with wet hands. Otherwise you may receive an electrical shock.
- When you remove the power plug, be sure to hold the plug. Never pull on the cord. Otherwise, the power
cord may become damaged, resulting in fire or electrical shock.
- The Linear USB offers superb sonic quality. To ensure best possible results, you should use the best
quality connecting cables that you can afford. Regular maintenance involves keeping all connections clean
using a quality contact cleaner.

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Using the Linear USB
Do not touch the Linear USB under the following circumstances:
- If you hear thunder, remove the power plug from the AC outlet as soon as possible. If you fear a lightning
hit and the Linear USB has been connected to an AC outlet, do not touch the power plug. Otherwise you
may receive an electrical shock.
- Do not attempt to modify this equipment. Otherwise, fire or electrical shock may result. Warranty is void on
modified devices.
- If you think the Linear USB needs to be checked for maintenance or repair, consult your dealer.
- Do not place a container of water or any small metal object on top of the Linear USB. If water is spilled or
if the metal object gets inside, fire or electrical shock may result. This applies to vases, potted plants,
glasses, cosmetic bottles, medicine, etc.
- Do not damage, process, bend, twist, stretch or heat the power cord. If the cord is damaged, fire or electrical
shock may result.
- When you are connecting other audio devices to the Linear USB make sure that you first turn off the power
to all devices to be connected. Refer to the user´s guide for each device and use the specific cable for
connection.
- Set the volume levels of all devices (including that of the Linear USB) to minimum before connecting the
Linear USB to your actual setup and to the AC outlet. Otherwise, an extremely loud noise could damage
your loudspeakers or even your hearing.
- If you plan not to use the Linear USB for a long period of time (such as when you are on vacation), remove
the power plug from the AC outlet. Otherwise, a fire could possibly result.
If any abnormality occurs while using the Linear USB, remove the plug from the AC outlet. In this
case first shut off the power amplifiers or turn down the volume controls and so on to avoid possibly
damaging transients
- If you notice any abnormality - such as smoke, smell, noise, etc. - remove the plug from the AC outlet.
Confirm that the abnormality is no longer present, then consult your dealer for repair. If you continue using
the Linear USB under abnormal conditions, fire or electrical shock may result.
- If a foreign object or water enters inside the equipment, remove the Plug from the AC outlet and consult
your dealer for repair. If you continue using the Linear USB under this condition, fire or electrical shock may
result.
- If the power cord is damaged (for example if it is cut or if the core wire is exposed), ask your dealer for a
replacement. If you continue using the Linear USB under this conditions, fire or electrical shock may result.
- If the Linear USB is dropped, or if the case is damaged, remove the plug from the AC outlet and consult
your dealer. If you continue using the Black Cube under abnormal conditions, fire or electrical shock may
result.
Maintenance
- Before cleaning the Linear USB remove the power plug from the AC outlet for safety. Otherwise, an
electrical shock may result.
- Do not open the Linear USB. There are hazardous voltages inside that might result in an injury or an
electrical shock.

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Why headphones?
When you use headphones you can reach a very high level of sound reproduction at a fraction of
the price you would have to spend for a regular hifi setup with high quality loudspeakers. Detail
resolution and dynamics of even the best loudspeakers can easily be reached with the best
headphones. In most cases headphones will be used to enjoy music either to acoustically disclose
a noisy environment to just to be able to listen to louder levels without disturbing other people.
What makes headphones different?
There is a number of parallels between (dynamic) headphones and loudspeakers. Technically
they are complex loads for the amplifiers driving them and should therefore be used with stable
outputs. There are headphones with higher impedance that need a higher signal voltage and low
impedance types which need more current for optimal sonic results. Just connecting a headphone
to any given headphone output without considering the headphone’s electrical characteristics will
most likely yield dissatisfactory results.
Impedances of dynamic headphones can vary from 30 Ohms to 600 Ohms. Transferred to
loudspeakers this would result in impedances from 4 Ohms to 80 Ohms. The same signal
amplitude results in a power consumption difference of factor 20!
Loudspeakers and headphones are manufactured in a dazzling variety. Irrespective of their
impedance headphones and loudspeakers can have very different efficiencies which means that
the electrical signal is converted more or less effective to acoustic energy. When comparing
loudspeakers the dimension used is the sound pressure level (SPL) reached at a given power of
1W (at 1m distance). There is a similar parameter for the efficiency of headphones except for the
fact that the standard power is 1mw (=1/1000W) The figure is given as dB/mw which means the
resulting sound pressure level at 1mW (not per 1mW).

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If the power is doubled then the resulting sound pressure level is 3dB higher. If this is calculated
the other way round it means that only half of the power is needed when using a headphone with
3dB more efficiency to reach the same sound pressure level. The 3dB formula is valid for
headphones and for loudspeakers.
From the given efficiency the power needed for a certain sound pressure level can be derived.
The power for the same sound pressure level can differ up to a factor of 500 for the different
headphones on the market. In the end what counts when selecting a headphone or a headphone
amplifier is if the desired volume can be reached without distortion. At least equally important is
to avoid damage for the headphone and – most important – to avoid damage for your hearing.
The maximum power rating is a figure that can be found in the datat sheet of the headphone.
From this power rating the maximum voltage can be derived. If the voltage will be higher then the
voice coils of the headphones will be destroyed. Today most headphones allow hearing levels
well above a secure level for the listener. This means that a standard pair of cans might survive
extremely high levels without any degradation of long term performance but the hearing of the
listener might be seriously damaged. Especially interesting are the standards of different regions.
Taking a look at laws in the US and in Europe reveals substantial differences in what is thought
to be suitable sound pressure levels for the listener:
Permissible noise exposures in Germany (dBA)1
SPL /dBA
Duration
85 88 91 94 97 100 103 106 109 112 115 118 121
hours per day 8 4 2 1
minutes per
day
30 15 7,5
seconds per
day
225 112 56 28 14 7
Permissible noise exposures in the USA (dBA)2
SPL /dBA
Duration
90 92 95 97 100 102 105 110 115
hours per
day
8 6 4 3 2 1,5 1
minutes per
day
30 15
1 Roughly harmonized throughout Europe
2 Figures taken from US departemnt of labour noise regulations

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Overview of power consumption of different headphones at different signal
levels:
level
(veff)
Headphone impedances
32 Ohms
60 Ohms 120 Ohms 300 Ohms
600 Ohms
0,1V 0,31mW 0,16mW 0,08mW 0,033mW 0,016mV
0,5V 7,8mW 4,2mW 2,1mW 0,83mW 0,42mW
1V 31,3mW 16,7mW 8,3mW 3,3mW 1,7mW
2V 125mW 66,7mW 33,3mW 13,3mW 6,7mW
3V 281mW 150mW 75mW 30mW 15mW
4V 500mW 267mW 133mW 53mW 27mW
5V 781mW 420mW 210mW 83mW 42mW
6V 1,13W 600mW 300mW 120mW 60mW
7V 1,53W 817mW 408mW 163mW 82mW
8V 2W 1,06W 533mW 213mW 107mW
9V 2,53W 1,35W 680mW 270mW 135mW
Voltage needed for 1mW power
units Headphone impedances
32 Ohms
60 Ohms 120 Ohms 300 Ohms 600 Ohms
Volts 179mV 245mV 347mV 550mV 775mV
dBu
(0dB = 775mV)
-12,7 -10 -7 -3 0
dBV
(0dB = 1V)
-14,9 -12,2 -9,2 -5,2 -2,2

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Table of efficiency of common headphones3
Type Impedance
/Ohms
Effektivity
dB/1mW
V
oltage for
100dB SPL
Power for
100dB SPL
AKG
K141Studio 55 101 209mV 0,8mW
K240DF 600 88 3,1V 16mW
K501 120 94 694mV 4mW
K1000 120 74 6,93V 400mW
Beyerdynamic
DT660 32 97 253mV 2mW
DT770, DT880,
DT990
250 96 800mV 2,6mW
Grado
All 32 98 225mV 1,6mW
Koss
Portapro 60 101 218mV 0,8mW
Sennheiser
HD500 32 97 253mV 2mW
HD555 120 94 694mV 4mW
HD650 300 97 775mV 2mW
Ultrasone
HFI-15G 32 94 716mV 4mW
HFI-550 64 100 252mV 1mW
HFI-700 75 94 548mV 4mW
It can easily be seen that even among the products of one manufacturer there can be huge
differences as far as impedance and/or efficiency are concerned. This makes it very important
that you either choose headphone and headphone amplifier as a matching combination or go for
a headphone amplifier with switchable gain that can be used with a wide variety of headphones.
Purchasing the Linear USB was a perfect step in the right direction.
3 All figures were taken from freely available data sheets of different manufacturers and have been partly recalculated
from other dimensions. Voltage and power are referring to effective values and not to peak values. All figures are given
without any guarantee.

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Elements at the front section
1. Power LED
When this LED lightens the Linear USB is connected to mains voltage switched on and
ready for being used.
2. Headphone output 1
6,3mm headphone jack for common headphones. When a headphone is connected to
this headphone output the line output at the rear side of the unit will be muted. Warning:
when you are also using the line output please make sure that the volume knob is turned
to minimum position (counter clockwise) to avoid sudden signal peaks. Please have in
mind that the same signal amplitudes can result in very different volume levels connected
to power amps or to headphones.
3. Headphone output 2
6,3mm headphone jack for common headphones. When a headphone is connected to
this headphone output the line output will not be muted. This enables you to listen to
headphones and your loudspeakers at the same time
You can connect to headphones of the same type at the same time to the Linear USB.
Please avoid using two different headphones. This may result in very different levels
4. Volume knob
Here you can adjust the desired output level. Turn clockwise to get a higher volume and
counter clockwise to lower the volume
W A R N I N G :
While using headphones you can easily reach volume levels that can seriously damage your hearing.
Every time before you put on a pair of headphones you must make sure that the volume is set to an
appropriate level!

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Elements at the rear side
1. USB input (Stereo)
USB connector. Please connect your computer here. Usually no driver installation is
necessary because the Linear USB will be automatically recognised as external audio
device. When connected to a computer the USB signal is automatically priorised. As as
soon as the Black Cube Linear detects a valid audio data stream this signal is routed to
the amplifiers input. Without any USB audio data stream present the analog input will be
used. This means that no input switching is necessary.
2. Status LEDs
These LEDs show the status of the USB connection. The red LED will light when no USB
connection with a computer at all can be recognized. The green LED will show a valid
audio data stream. When no LED lights then a USB connection is recognized but no valid
audio data stream is present.
3. 3,5mm mini jack, Stereo (loop out or input)
Mini Jack (stereo) for the connection of any analog source to the Linear USB like for
example a tuner, a MP3 player's analog output or the output of an external phono stage.
The 3,5mm mini jack is hard wired in parallel to the Linear USB’s RCA input connectors.
If you are using the 3,5mm input jack for the input signal then the RCA connectors can
be used as feed through loop outputs. DON'T use both input jacks with sources
connected at the same time.

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4/5. Line input, Stereo (input or loop out)
RCA connectors for the connection of any analog source to the Linear USB like for
example a tuner, a MP3 player's analog output or the output of an external phono stage.
The RCA jacks are hard wired in parallel to the Linear USB’s 3,5mm mini jack. If you are
using the 3,5mm input jack for the input signal then the RCA connectors can be used as
feed through loop outputs. DON'T use both input jacks with sources connected at the
same time.
6/7. Line output (Stereo)
RCA connectors for the connection of your power amplifier or active loudspeakers.
Depending on which headphone output you are using on the front side of the unit the
signal is either fed through the Linear USB to the line output or muted. The RCA jacks are
connected in parallel to the Linear USB’s output stage and not just passively connected
to the input connectors so that you benefit greatly when you use the unit as single source
preamp.
8. Power switch
This switch separates the primary coil of the power supply transformer from the mains
voltage. There is no stand by mode.
9. Fuse holder
Fuse holder for the primary fuse. In case of replacement you must use the same type
(100V – 120V AC: 630mA slow or T; 200V – 240V AC 315mA slow or T). If there is a
mechanical noise (rattle) when you move the unit the Linear USB is equipped with a spare
fuse which is also placed in the fuse holder.
10. Mains connector
IEC receptable for the connection to mains voltage. Please connect the power cord here.
You must make sure that you connect the Linear USB to the appropriate mains voltage.
The mains voltage and fuse type needed are both printed on a sticker underneath the
unit.