Practical Devices XM4 User manual

XM4
Headphone Amplifier
User’s Manual

©2007 Practical Devices Corporation. All rights reserved.
Thank ou, and congratulations on our purchase of the XM4 Portable
Headphone Amplifier!
We’ve worked ver hard to provide ou with what ma be one of
the most advanced, feature packed, and best sounding headphone
amplifiers on the market toda . The XM4 is designed to be durable, flexible,
eas to use, practical, and above all, sound great. Each unit is carefull
manufactured, and then rigorousl tested for performance, durabilit , and
sound qualit .
If ou have an questions that are not answered b this manual,
please feel free to send us an email. Enjo the amp!
Yours trul ,
James Forest
President & CEO
A Word About Protecting Your Hearing : The most
valuable piece of audio equipment ou own are our ears. Smart people protect
their hearing. When using the XM4, or an headphones, get in the habit of
removing the headphones from your ears whenever ou:
• Turn the XM4 on;
• Connect or disconnect an cables,
• Do ANYTHING that ma cause a sudden burst of loud sound.
Your onl have one set of ears, and ou can’t replace them. Never listen at
excessive volume. Always listen to music at reasonable volume, and most
importantl , use good common sense.
PLEASE
READ!

XM4 Block Diagram ....................4
Connecting Your XM4 .................5
Power Button Functions ..............6
Crossfeed Controls......................7
Disassembling the Unit................8
Replacing the Batter ..................9
Gain Boost Jumpers ..................10
Lithium Batter Charger.............11
External Power Suppl ..............12
Testing the Batter ....................13
Frequentl -Asked Questions .....14
Troubleshooting.........................15
Warrant and Guarantee ...........17
Technical Specifications ............18
3
Table of Contents

Output
Connect to
Headphones
Po er Button
Press button:
1 time = Turn on power for 1 hour
3 times = Turn on power for 3 hours
6 times = Turn on power for 6 hours
7 times = Turn on power forever
2 times = Toggle flashlight mode
5 times = Toggle dimming of blue LED
Press and HOLD to turn unit off.
Press once to displa batter voltage. For
example, if batter is at 8.5 volts, the amber
LED will flash 8, then 5 times. Input
Connect to MP3 pla er
or other audio source.
Main Volume
Amber LED
Flashes batter voltage when Power Button
is pressed.
Flashes a one-flash warning when the batter
is low (below 6.5 volts). Flashes a two-flash
warning when the batter is ver low (below
5.2 volts).
Blue LED
Indicates
power is on
Crossfeed Enable S itch
IN position: Crossfeed enabled.
OUT position: Crossfeed disabled.
Crossfeed Adjuster
Turn this potentiometer to
adjust the amount of
Crossfeed, from near-stereo
to 100% mono.
This onl has an effect when
the Crossfeed Enable Switch
is in the IN position.
Po er Connector
“Barrel” connector with 1.3mm
center pin. Center pin must be
positive.
Used to power and recharge
the XM4.
Battery Charger
Jumpers
Change these onl if ou
want to use a NiMH
batter . See Users
Manual Pages 12 and 13
XM4 Block Diagram
Gold-Plated Amp Chip Socket
Allows for future amp chip
upgrades and modifications for
those who like to experiment.
Gain Boost Jumpers
Jumpers installed: = +10db extra boost
Jumpers not installed: = +0db extra boost
Bass +6dB
4

Connecting Your XM4
1. Connect a suitable source to the input jack, as
pictured. Sources ma be MP3 or CD pla ers, iPods,
or an line-level devices.
2. Connect our headphones to the output jack.
Your headphones can have an impedance of 24 to 600 ohms.
5

6
The power button functions are activated b pressing repeatedl
within a three second window:
1 press: Turn unit on. The XM4 turns off
automaticall after one hour.
3 presses: Turn unit on for three hours.
6 presses: Turn unit on for six hours.
7 presses: Turn unit on. The XM4 will not turn itself off.
Press and hold: Turn unit off.
The unit will respond to button presses by flashing the
blue LE once for each hour it will stay on.
Additional po er button functions:
2 presses: Toggle flashlight mode.
5 presses: Toggle LED dimming.
Power button
Po er Button Functions

CrossFeed Controls
The XM4’s Crossfeed circuit simulates the sound stage of
loudspeakers, through our headphones.
• Press the Switch IN to activate Crossfeed.
• Leave the button OUT for full stereo operation,
b passing the Crossfeed circuit entirel .
Use a small screwdriver (Philips or Straightblade) to adjust
the Crossfeed Potentiometer:
• All the wa clockwise to produce a mono signal;
• All the wa counter-clockwise to produce a near-
stereo image.
Crossfeed Adjuster Crossfeed Enable Switch
Wh would ou want Crossfeed? Most music is mixed to be
listened to on loudspeakers. The studio technicians mixing the recording
assume there will be alot of crosstalk, since with speakers the listener's left ear
clearl hears the right speaker's output, and vice versa. Thus, most music is mixed
with a ver wide stereo soundstage.
With headphones, however, the left ear onl hears the left channel, and the right ear
onl hears the right channel, with nearl zero crosstalk. This can, for man recordings,
cause the soundstage to appear to be "in the center of our head". Further, some
recordings (earl Beatles recordings are infamous for this) have each instrument and
vocal track panned 100% to either left or right, which can almost immediatel cause
listener fatigue. Crossfeed corrects this b bleeding some of the left channel into the
right, and vice-versa.
7

To re-assemble the unit:
1. Slide the unit back together
(make sure the circuit board
slides smoothly on the case's
guide rails)
2. Re-install the two screws. Do
not overtighten.
3. Re-install the batter and rear
panel.
Disassembling the Unit
Remove
t o scre s,
and push
circuitboard
from rear.
8
Help! I think I stripped the scre s
hen I tightened them! No, ou
didn’t. You can gentl pinch the case’s
screw receptacles with a pair of pliers,
as shown, to tighten things up.
If you need to change the Gain Boost Jumpers or Battery
Charger Jumper, you ill need to disassemble the unit.
Here's ho :
Note: there is a
notch in the case.
Make sure the case
is oriented so the
notch lines up with
the pushbutton, as
shown.
1. Turn the unit off, and remove the external
power adapter if connected.
2. Remove the rear panel using a Phillips
screwdriver. Remove the batter .
3. Remove the two screws on the front panel
(use the included 5/64” allen ke ). Note: ou
do NOT need to remove the volume knob!
4. From the rear, push the circuitboard out the
front of the case.

Replacing the Battery
Replace the batter b removing the rear panel, using a Philips
screwdriver.
• Connect the batter clip to the new batter
• Insert the batter clip-first into the XM4
• The batter should not protrude out of the rear of the unit (if it
does protrude, ou won’t be able to completel screw down the
rear panel. Check that the batter cable hasn’t prevented the
batter from being pushed in completel ).
If the batter cable has become entangled, ou ma need to
disassemble the unit as shown on page 8.
9

With the volume knob at full, our XM4 will
deliver +13dB of gain. Additionall , ou can
install the Gain Boost jumpers, which will give
ou an extra +10dB of gain, for a total of
+23dB.
In order to get the best possible noise floor,
ou should not install the Gain Boost jumpers
unless: a) the volume of our headphones is
too quiet, and, b) ou have alread tried turning up the volume of our
Gain Boost Jumpers
10
Usuall , the Gain Boost jumpers are onl needed when using
headphones which have an impedance greater than around 150
To install or remove the Gain Boost jumpers:
1. Disassemble the unit (see page 8)
2. To install the jumpers, place a jumper of each pair of riser pins.
(Hint: if ou are removing the jumpers, place each jumper over
one pin only, so ou don't lose them).
3. Re-assemble the unit.
Jumpers are installed.
Gain boost is +10dB
Jumpers are removed.
Gain boost is disabled.
Jumpers are removed, but
are placed over a single
pin so the don’t get lost.
Gain boost is disabled.
Gain Boost
Jumpers

Lithium Battery Charger
Lithium Battery FastCharge™ System
If our XM4 came equipped with the optional Lithium rechargeable
s stem, just plug the charger into the powerjack on the rear of the
XM4.
• The batter will full charge within two hours. When charging is finished,
the LED on the charger will turn from red to green.
• The batter will charge even if the XM4 is turned on.
• You can leave the charger connected indefinitel .
• You can expect up to 40 hours of use per charge.
• Your Charger is a “World” charger—it will work with AC Mains voltages
from 105 to 240 VAC, at 50 or 60Hz. If ou are using the charger
outside of North America, ou ma need a plug adapter (available from
our local electronics store).
11

External Po er Supply
(Note: If our XM4 came equipped with the Lithium rechargeable s stem, then this
page does not appl to ou.)
External Po er Supply
If our XM4 did NOT come with the Lithium s stem, then it shipped from the factor
with an Industrial Alkaline 9v batter . You can use an external power suppl to save
batter life while ou are at home. The external power suppl must be:
• 6 to 15V DC (if ou want to use the suppl to recharge a NiMH or NiCd batter , then
the suppl must be Regulated 12V DC ± 5%)
• Have a barrel-t pe connector, with a 1.3mm center pin.
The center pin must be positive polarit :
• The milliamp (mA) rating on the suppl does not matter; an mA rating can be used.
• A suitable power suppl is RadioShack PN#273-1773 (make sure ou ask for
Adaptaplug "H" PN#273-1711, which is included with the adapter).
NiMH or NiCd Rechargeable Battery
A rechargeable NiMH or NiCd batter can be used instead of the alkaline, and ou can
use the XM4's integrated batter trickle charger to recharge it. RadioShack #23-529
batter will work, or an of the following t pes of rechargeable batteries can be used:
- Nickel Metal-H dride (NiMH), 8.4v or 9v
- Nickel-Cadmium (NiCad), 8.4v or 9v
Using a 7.2v batter is not recommended, as overcharging
ma result if the external power is applied for too long.
To use the trickle charger and a NiMH or NiCd batter :
1) Obtain an external power suppl described above
2) Install the NiMH or NiCd batter in the XM4
3) Set the Charge Jumpers in the XM4 as shown on bottom-center of Page 13.
• Charging a NiMH or NiCd batter will take approximatel 15 hours
• Batter life depends on the “mAHr” rating of our batter . For example, a
“250mAHr” batter can be expected to last approximatel 25 hours.
Use of an external po er supply hich is higher than the voltages sho n above
may cause battery overcharging, damage to the battery and/or the XM4, battery
electrolyte leakage, or possibly even battery explosion. If in doubt, measure your
po er adapter ith a voltmeter, or enquire at your local electronics store.
12
Barrel-t pe plug:
XM4 uses:
“1.3mm
center
pin”

13
Alkaline Battery Configuration:
Both jumpers are removed. Use
this configuration if ou are using
an Alkaline batter . This is the
Factor default for XM4’s
shipped with an alkaline batter .
Trickle Charger
Configuration. For NiMH
and NiCd rechargeable
batteries. Install the outer
one jumper, as shown:
Lithium Battery Fastcharge.
Both jumpers are installed.
This is the Factor default for
XM4’s shipped with the Lithium
option.
Charge Jumper Configuration
Onl change the charge jumpers from their factor defaults if ou want to install a NiMH
or NiCd rechargeable batter . (For advanced users onl !)
Testing the Battery
The XM4 also acts as a battery tester.
When the XM4 is turned on, the amber LED will displa the batter voltage
b flashing. For example, if the voltage is 8.5 volts, the LED will flash 8
times, then 5 times.
To see the batter voltage if the unit is alread on, press the power button
The XM4 will warn ou if the power falls below certain thresholds:
• At around 6.5 volts, the amber LED will begin to flash.
• Below around 5.2 volts, the amber LED will double-
flash, indicating the batter is ver low.

Frequently-Asked Questions
Can I use a line-level input to the XM4, such as from a CD/
DVD player or VCR?
Yes, the XM4 will accept a line-level input.
Can I use the XM4 to drive speakers?
Usuall , no. The XM4 was designed as a headphone
amp, and will usuall not do a good job of driving
speakers, even small ones (most speakers have
impedances of around eight ohms). What ou are
looking for is a power amplifier, not a headphone
amplifier
For more FAQs, see our website:
http://www.practicaldevices.com/faq.htm
14

Troubleshooting
Here are some common problems, and their solutions:
Amp does not turn on or respond to pushing the power button:
- Disconnect the batter , hold do n the po er button
for 10 seconds, then reconnect the batter .
Headphone volume is too quiet, even with the volume knob at full:
- Increase volume of source (MP3 pla er). If it is still
too quiet, then install the Gain Boost jumpers (see
page 10)
Output is in one ear onl ; the other channel is dead:
- Check that all cables are completely plugged in all
the wa
- Check that our cable is a stereo cable, not a mono
cable.
15

16
Notes

Warranty and Guarantee
Practical Devices stands behind its products ith a full
no-risk arranty and money-back satisfaction guarantee.
Practical Devices offers a 30-da mone -back guarantee on all of
our products. If, for an reason whatsoever, ou are unsatisfied with
our purchase, ou ma return it for our mone back, including
our original shipping cost. (The fine print: Unit must be returned
within 30 days of sale; Unit must be returned in the same
undamaged, good working order as it was received; Practical
Devices will refund ou the original cost, plus the cost of the original
shipping. Customer must pa for return shipping cost).
Further, we warrant , for a period of one ear, that our products are
free from an defects in workmanship. Should ou have an
problems, we promise to make it right. (The fine print: warrant is in
effect for a period of one ear. Problems must be reported to
[e-mail] This warrant covers an defects in
workmanship. It does not cover abuse, ph sical damage, or the like).
Should ou need an service, please email us at
[e-mail]
17

18
Specifications
Description: Portable Audio Headphone Amplifier
Weight: 140 grams (4.9 oz)
Power Source: - 9-volt alkaline, NiMH, NiCd batter , or,
- 8.4-volt NiMH or NiCD batter , and/or,
- External Power Suppl .
- Option Lithium Rechargeable S stem
External Power Suppl : 1.3mm Barrel connector power jack,
center pin positive.
If using built-in NiMH batter charger:
12VDC ± 5%
If not using batter charger:
6-15VDC
Optional Lithium charger can also be
used.
Batter Charge Time Lithium Charge S stem: 2 hours
Trickle charge NiMH: 15 hours
Batter Life (t pical): Alkaline: 50 hours
Lithium: 40 hours
Maximum Gain: +23.1 dB (Gain Boost jumpers in)
+12.7 dB (Gain Boost jumpers out)
Frequenc Response (20-20,000Hz,
t pical):
+0.1dB, -0.4dB
-3dB Point (t pical): 21 kHz at top end;
9 Hz at low end
Total Harmonic Distortion
(t pical)
0.002%
Intermodulation Distortion
(t pical)
0.007%
Signal to Noise ratio
(20-20,000Hz, t pical): 100 dB

D namic Range
(20-20,000Hz, t pical): 100dB
Crossfeed
User-adjustable from mono to
stereo using potentiometer;
User can b pass using
pushbutton.
Auto Power-OFF User selects between 1, 3, or 6
hour poweroff interval, or unit
can be set to sta on indefinitel
Status Indicators Two LEDS (one blue, one
amber)
Integrated Voltmeter accurac
(t pical)
±100mV
Mechanicals Anodized aluminum case;
Gloss laser-anodized faceplate;
Polished aluminum volume knob;
FR-4 dual-la er Printed Circuit
Board with 1 oz. copper
Other Features
Flashlight Mode: Turns on both
LEDs to full brightness;
Dimming mode: Allows user to
optionall dim the blue LED.
Bass Boost +6dB @ 100Hz
19
Specifications (Continued)

XM4_v35 ©2007
www.practicaldevices.com