RoXX-Series
301 Personal Headphone Amplifier / Line Checker Manual
The RoXX-Series 301 by ASL is a versatile professional stereo headphone amplifier for
personal and portable use. The unit is battery powered and accepts all commonly used
audio signals: balanced and unbalanced signals at microphone, line or speaker level.
This Personal Headphone Amplifier has been designed with great precision. However, designing equipment like this
is always a matter of finding compromises between: max. gain, noise, max. output power, distortion, power
consumption, flexibility, mechanical aspects, costs etc. The starting point of this design was to make a versatile unit,
which can be used for many purposes in different circumstances, with long battery life and as little as possible
controls. We hope we managed to meet your requirements. Read this user manual first.
Applications for the ASL RoXX - Series 301 Personal Headphone Amplifier
Practicing through headphones Checking microphone signals
An electronic instrument (guitar, keyboard) or a CD Very usefull to, for instance, boom operators.
player can be connected to the (stereo) Jack input(s),
and a dynamic microphone can be connected to the
XLR input. These signals can be mixed to the stereo
headphone output to practice. Even two headphones
can be connected in order to allow a singer to practice
together with an instrumentalist.
Creating a monitoring system for studio or on
stage use
An output of the mixing console can be connected to
the stereo input of the PHA, and a microphone or
instrument can be connected to the XLR input with a
loop through to a mixing console input. The performer
can make his own mix between his own voice /
instrument and the signal from the mixing console.
Headphones can be used in a studio, ear plugs on
stage.
Boosting a headphone output
Whenever the headphone output power of any equipment is not sufficient, the Personal Headphone Amplifier can be connected to that headphone output.
The output power of the PHA will be high enough for
most circumstances/purposes.
Trouble shooting
A sound engineer can check lines, microphones or
outputs, to locate signals or to solve problems.
Routing two headphone outputs to one headset
For instance the monitor outputs of two mixing consoles, or the monitor output of a mixing console and
an intercom signal.
Setting up a (battery powered) two station intercom system
A dynamic (headset)microphone can be connected to
the XLR input of each of the two PHA’s. The loop
through XLR’s are interconnected. The two users can
hear eachother (and themselves) through the connected head(set)phones. Two identical microphones
need to be used, and the microphone signals are
summed by there output resistance.
Sending an audio signal to many headphones
Two headphones can be connected to one PHA.
Personal headphone amps can be interconnected
using the loop through connectors.
More applications to be discovered by yourself!
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
FRONT PANEL
1 Power switch
With this switch the unit is turned on and off.
5 -20dB switch
With this switch the gain of the unbalanced inputs
can be reduced by 20 dB for monitoring speaker or
2 Power-on LED indicator
This LED illuminates green when the unit is
switched on.
high level line signals.
In the normal position the input impedance of the
unbalanced inputs is very high (1M Ohms),
enabling the monitoring of electric guitar signals (or
* When the Personal Headphone Amplifier is
other pick-ups).
prepared for charging Ni-MH batteries within the
unit, this LED also indicates fast charging of the
cells. During fast charging, while the unit is
switched off, this LED lights up burns red. During
6 Stereo/mono switch
In mono position a stereo signal on the unbalanced
input(s) is made mono.
fast charging, while the unit is switched on, this
LED lights up orange.
7 Unbalanced input Volume control
With this knob the volume of the unbalanced input
3 Stereo headphone 3,5mm mini jack
signal can be controlled
Two headphones can be connected to this
headphone amplifier at the same time. A headpone
with a 3,5mm mini jack can be plugged in this
connector.
8 Balance control
With this knob the left / right balance of the
unbalanced input signal can be adjusted.
Both high and low impedance headphones can be
used (when using 2 headphones at the same time,
9 Mic / Line switch
use headphones of the same type). The internal With this switch the sensitivity of the balanced input
power amplifiers are short-circuit and overload can be selected.
protected.
10 Balanced input Volume control
4 Stereo headphone jack 1/4”
With this knob the volume of the balanced input
A headphone with a 1/4” jack can be plugged in signal can be controlled.
this connector.
11 Pan control
With this knob the left / right panning of the
balanced input signal can be adjusted.
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
REAR PANEL
12 Balanced input + loop thru XLR’s
These XLR-3 female and male connector are
paralleled and are the input & loop thru for a
balanced signal (mic. or line level).
13 Mono left / stereo input jack
This 1/4" jack connector is the input of the left
signal of a stereo signal when a mono jack is used
(input 14 is the right input), or the left and right
signals when a stereo jack is used (input 14 is not
used). It accepts signals of line or speaker level.
14 Mono right / mono input jack
When no signal is connected to the left jack input
(13), the mono signal on this input is sent to the left
and right cans of the headphone. When there is
also a mono signal connected to the left jack input
(13) the signal on this input is sent only to the right
input (input 13 is the left input). It accepts signals of
line or speaker level.
15 Stereo loop thru connector
This jack connector is always configured as a
stereo loop thru for the signals connected to the
unbalanced inputs 13 and/or 14.
When a stereo signal is connected to input 13 this
connector provides a stereo link. When mono left
and mono right signals are connected to inputs 13
and 14, this connector outputs those signals as a
stereo signal (left=tip, right=ring). When a mono
signal is connected to input 14, this connector
outputs the mono signal (on tip and ring). This
connector is especially useful for connecting
multiple headphone amplifiers to one source.
SIDE PANEL
16 DC adaptor input
An external DC adaptor can be used to power the
RoXX-Series 301. The internal batteries will
automatically be switched off. The unit accepts DC
voltages from 9 V to 24 V.
* When the Personal Headphone Amplifier is
prepared for charging Ni-MH batteries within the
unit, the external DC voltage can range from 12 V
to 18 V. The adaptor must be capable of delivering
a current of at least 700 mA.
17 Internal battery compartment
Accepts 4 AA-type 1.5 volt batteries or 1.2 volt
rechargeable cells.
The Personal Headphone Amplifier can be
modified so that Ni-MH cells can be charged in the
unit. Consult your distributor for more information
18 Beltclip
Powering the Personal Headphone Amplifier
Battery Operation Open the battery compartment on the side of the unit
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
by turning the knurled screw anti-clockwise. Take out
the battery holder. Insert four AA-size 1.5V batteries in The ASL RoXX -Series 301 Personal Headphone
the holder. Put the holder back into the unit, with the Amplifier can be modified for charging four Ni-MH
holders connection terminals pointing to the side cells in the unit. Consult your distributor for more
where the beltclip is mounted. Close the battery information. Whenever a RoXX 301 is prepared for
compartment by putting back the little coverplate and charging Ni-MH cells, normal batteries should not
turning the screw clockwise. The Personal Head- been used anymore and reverse voltage protection for
phone Amplifier is now ready to operate on batteries. the cells does not work anymore!
Always choose four identical batteries of the same
age. Rechargeable NiCd or Ni-MH 1.2V cells can be Mains Operation
used as well. The RoXX 301 is protected against The ASL RoXX -Series 301 Personal Headphone
reverse voltage. Amplifier can be powered by mains by using a mains
The ASL RoXX -Series 301 can be switched on by adaptor (not supplied with the RoXX 301). Choose an
pushing the power button (1). The unit will start adaptor with an input voltage that matches the local
whenever the total battery voltage is higher then 4.0V. mains voltage. The output of the adaptor needs to
The unit will work until the total battery voltage drops supply a DC-voltage between 9 and 24V at a current
below 2.5V. of 500mA (12 to 18V DC at a current of 1A when the
When the unit switches off because the battery RoXX 301 is prepared for charging Ni-MH cells). The
voltage is lower than 2.5V, the batteries will start to DC-voltage can be either unregulated or regulated.
recover a bit. The battery voltage rises, and when it Connect the adaptor via an appropriate plug to the
exceeds 4.0V, the unit will switch on again. By then, DC-in socket (16) at the side of the RoXX 301. The
the battery voltage will again go down, resulting in inner pin of the socket is hot (+), the sleeve is ground
switching off the unit at 2.5V. This frequently and (-). The unit is protected against reverse voltage.
automatically switching on and off means that the
batteries are empty and that they should be replaced.
Warning !
Always turn the unit on and off without the headphone
on your head.
Connecting and Monitoring Balanced Signals
The ASL RoXX -Series 301 Personal Headphone The Pan control knob (11) can be used to pan the
Amplifier has an independent section for balanced signal between the left and the right channel of the
signals and one for unbalanced signals. Balanced RoXX 301.
signals can be both microphone and line-level signals.
Speaker signals should not be connected to the The unbalanced input is DC and overvoltage protected
balanced input (they are mostly unbalanced and can (except for speaker signals with the Line/Mic. button in
be monitored with the unbalanced section of the ASL the Mic. position).
RoXX -Series 301).
Monitoring phantom powered signals
Before connecting any balanced signal to the XLR The ASL RoXX -Series 301 Personal Headphone
connectors, turn the Balanced input Volume control Amplifier is not equipped with a phantom-power
(10) down. Press the Line/Mic. button (9) if you plan to facility. Nevertheless, sources which are phantomconnect a microphone level signal. Release the powered by another device can be monitored.
Line/Mic. button if you plan to connect a line-level
signal. Connect the balanced signal to one of the XLR Connecting the RoXX 301 to the circuit of source and
connectors (the other one is for loop thru, the male phantom-powering device will cause clicking of both
and female connectors are paralleled). Turn up the the headphone amplifier and the other device (the
Balanced input Volume control (10) carefully and you input capacitors of the RoXX 301 need to be charged).
will hear the connected balanced signal. To reduce or prevent this clicking, a so-called ground-
lift cable (e.g. pin 1 not connected in one of the XLR
The volume control button not only acts as a volume connectors of that cable) can be used.
control but it in fact controls the gain of the
preamplifier and the signal level send to the poweramps at the same time. This reduces noise and
creates the capability to handle a great range of input
signals with only one knob.
Connecting and Monitoring Unbalanced Signals
The ASL RoXX -Series 301 Personal Headphone Unbalanced signals may be signals produced by
Amplifier has an independent section for unbalanced instrument pick-ups (electric guitar), line outs of
signals and one for balanced signals. mixers, CD-players etc. or speaker signals. They can
range from -16dBu to (almost)+4. For your own safety
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
do not connect AC-signals higher than 40Vrms (e.g. When the connected signal is higher than +6dBu, the
outputs of power amps >200 Watt into 8 ohm; a 1/4” -20dB button (5) needs to be pressed. Turn up the
jack is not a safe connector for these kind of signals). Unbalanced input Volume control (7) carefully and you
will hear the connected unbalanced signal(s). The
Before connecting any unbalanced signal to the 1/4” balance of the signal can be controlled by the Balance
input jacks, turn the Unbalanced input Volume control control knob (8). A stereo signal can be made mono
(7) down. by pressing the Stereo/Mono knob (6). This is usefull
when a single muff headphone is used or when you
want to pan the unbalanced signal to one side of your
There are three possibilities of connecting unbalanced headphone.
signals: The unbalanced inputs are DC and overvoltage
protected.
* Suppose you want to connect an unbalanced mono
signal on a mono 1/4” jack. This signal should be
connected to the Jack input labeled "Right/Mono" Linking the unbalanced signal(s) to another ASL RoXX
(14). The input labeled "Left/Stereo" (13) should be -Series 301
left unused. The mono input signal will be sent to The connector labeled ‘Unbal. Stereo Loop Thru’ (15)
both left and right channnels of the Personal will allways provide a stereo signal of the signals
Headphone Amplifier. connected to the input connectors. The unbalanced
input signal(s) can be linked to another Personal
* Suppose you want to connect an unbalanced Headphone Amplifier (or other device) by connecting a
stereo signal on a stereo 1/4” jack. This signal stereo jack-jack cable between the Unbalanced
should be connected to the Jack input labeled Stereo Loop Thru (15) connector of the first and the
"Left/Stereo" (13). The input labeled "Right/Mono" Left/Stereo (13) of the second. Never use a 1/4” mono
(14) should be left unused. The stereo signal will jack on the loop thru connector (the right input signal
be sent as a stereo signal to the left and right will be short circuited).
channel of the Personal Headphone Amplifier.
* Suppose you want to connect an unbalanced Using the RoXX 301 with an electric guitar (or any
stereo signal on two mono 1/4” jacks (e.g. Left and other instrument with pick-ups)
Right). Connect the left signal to the input labeled A pick-up will sound best if the load of it is very low
"Left/Stereo" (13) and connect the right signal to (e.g. input impedance is very high, like an active DIthe input labeled "Right/Mono" (14). The signals will box). ASL RoXX -Series 301 has a very high input
be sent to the corresponding channels of the ASL impedance on the unbalanced inputs (as long as the RoXX -Series 301. 20dB button is not pressed). So an instrument pick-up
can be connected to the RoXX 301! Normally the
Right/Mono input connector would be used for this
mono signal. But then the pick-up is loaded by both
the left and right input amplifiers of the Personal
Headphone Amplifier, making the input impedance
half its value. It would be better to connect the pick-up
to the Left/Stereo input and pressing the Stereo/Mono
button. The input impedance is now 500k ohms and
the pick-up will sound best.
Volume Control
The unbalanced input section and the balanced input Series 301 can be ear damaging and destructive to
section both have their own volume control knob as
your headphone.
descriped in the above sections. The unbalanced
input(s) and the balanced input can be used at the
same time with the possibility to make a mix between
the signals with the two volume control knobs (and the
balance and pan knobs).
Whenever only one input section is used turn the
volume control of the not used section down (anti
clockwise).
Warning !
Always turn down the volume controls down before
connecting any headphone or input signal and turn up
volume carefully. The output levels of the RoXX -
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
Choice of the Headphone(s) The Built-In Limiter
The choice of the headphone(s) is highly individual. The ASL RoXX -Series 301 Personal Headphone
However, the ASL RoXX -Series 301 can handle Amplifier has a built-in stereo limiter to prevent the
almost any headset. output amplifiers from clipping.
The headphone impedance should preferably have a The limiter range is 30 dB for the balanced input and
value higher than 32 ohms. The headphone outputs of 25 dB for the unbalanced inputs. So when the output
the unit are short-circuit proof so loading the outputs level of the RoXX 301 reaches the threshold level of
with a headphone-impedance lower than 32 ohms the limiter, the input signals can become 25 or even
won’t damage anything (it may eventually cause 30 dB higher before the output amps will actually clip.
clipping of the output amplifiers before the built-in
limiter begins to operate). If clipping occurs in one of the input stages of the ASL
RoXX -Series 301, the limiter can’t do anything against
In general, low-impedance headphones create a it. Carefull fixing of the -20dB button, the Line/Mic.
higher Sound Pressure Level on your ears (also button and the volume control knobs remains
depending on their efficiency), but using them will necessary (the balanced volume control knob also
reduce battery life. These headphones can be a good controls the gain of the preamplifier)
choice in noisy environments. High-impedance
headhones (>200 ohms) often have a lower SPL, but The stereo limiter is combined for the balanced and
will increase battery life. the unbalanced signals. This means that whenever the
limiter limits for instance the balanced signal, the
In case you want to use two headphones with the ASL unbalanced signal will go down in level even when the
RoXX -Series 301, choose headphones with the same unbalanced signal itself still has not reached the limiter
impedance (> 50 ohms) and efficiency. threshold level.
Please check the specifications of your headphones if
they can handle the output power of the Personal
Headphone Amplifier.
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
SPECIFICATIONS ASL RoXX - Series 301
Frequency response
Balanced input 50 Hz - 36 kHz +0, -3dB
Unbalanced input 40 Hz - 40 kHz +0, -3dB
Total Harmonic Distortion
< 0,3% (20 Hz - 20 kHz, 3 dB below limiting)
< 0,5% (100 Hz - 20 kHz, 10 dB over limiter threshold)
Equivalent Input Noise
- 125 dBu (22 Hz - 22 kHz, Rsource = 50 Ohm, max.
gain)
Headphone output Impedance
< 2 Ohms, Short Circuit Proof
Limiter
Limiter Threshold 2,5V rms (10,4 dBu)
(measured at headphone output)
Limiter Range Balanced input 30 dB
Unbalanced input 25 dB
Power
Internal 4x 1.5V AA-Battery
or 4x 1.2V rechargable NiCd or Ni-MH AA-Cell
External 9 - 24 V DC
Supply Current (idle) 80 mA with 4x 1.5V AA-Battery
Battery Life 24 hours under normal conditions
with 4x 1.5V Alkaline Batteries
Dimensions 43 mm H x 94 mm W x 138 mm D
Weight 500 grams nett
Maximum Voltage Gain, Input Impedance and
Maximum Input Levels before clipping
Maximum Input Maximum
Voltage gain Impedance Input Level
Balanced Input
@ mic level
Balanced Input
@ line level
Unbalanced Inputs
pad @ 0
Unbalanced Inputs
pad @ 30 dB
71 dB 28 kOhms -10 dBu
36 dB 20 kOhms +34 dBu
26 dB 500 kOhms +12 dBu
7 dB 23 kOhms
f = 1 kHz, Rsource = 50 Ohm
Maximum Headphone Output Level + Power
Max. Output Level Output Power
R load = 200 Ohm
R load = 32 Ohm
4.1 V rms 84 mWatt
3.2 V rms 320 mWatt
+44
ASL COMMUNICATION SYSTEMS
Zonnebaan 42
3606 CC Maarssenbroek
HOLLAND
tel. : +31 - (0)30 - 241 19 01
fax.: +31 - (0)30 - 241 06 38
ASL reserves the right to alter
specifications without further notice.
A S L C O M M U N I C A T I O N S Y S T E M S RoXX-Series 301 Manual
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