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Front Panel
Setting Up and Using the Servo 120a Amplifier
The Servo 120a Protection Circuitry
Bridge Mode
8
9
10
11
12
13
Appendix A: Linearity vs. Frequency Sweep
Appendix B: Power Output vs.THD
Specifications
Copyright 2004, Samson Technologies Corp.
Printed December, 2004
Samson Technologies Corp.
575 Underhill Blvd.
P.O. Box 9031
Syosset, NY 11791-9031
Phone: 1-800-3-SAMSON (1-800-372-6766)
Fax: 516-364-3888
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Congratulations on purchasing the Samson Servo 120a Power Amplifier! Although designed for easy operation,
we suggest you first take some time to go through these pages so you can fully understand how we’ve imple-
mented a number of unique features.
The Servo 120a is a single rack-space stereo power amplifier which is optimized for use in both professional and
project recording studio environments as well as for live performance. It delivers 60 watts of power per channel
into 4 ohms (or, in Bridge mode, 120 watts into 8-ohms) over the full frequency spectrum, from 3 Hz to 65 kHz.
Both balanced 1/4" TRS and unbalanced RCA-type input connectors are provided, along with 5-way binding
post output connectors. Front-panel controls and displays include a power switch, independent left- and right-
channel input level controls, ten-segment LED meters, a Protection LED, and a convenient headphone jack and
speaker on/off switch for private cue monitoring. Suggested applications for the Servo120 include:
Amplification for nearfield monitors or cue headphones in both professional and project studios Its
superb audio specs and whisper-quiet performance (its convection cooling design means there is no fan),
along with its unique front-panel speaker on-off switch and dedicated headphone jack, make the Servo
120a the perfect complement to any recording studio.
Home Theatre applications The power and flexibility offered by the Servo 120a (it can be used either as a
60 watt stereo amplifier, or, in bridge mode, as a 120 watt monophonic amp) makes it the perfect addition
to any home theatre, in conjunction with any DVD player or other surround sound system. For example, use
it in stereo mode to drive your main front left/right speakers, using your home stereo to drive the rear satel-
lite speakers—or use the Servo 120a in Bridge mode to drive your center channel or subwoofer with power
galore.
As an extension to a home hi-fi system - The Servo 120a boasts professional specs which far exceed that
of most consumer products. For a real “studio”experience in your own home, try connecting your home
hi-fi amplifier’s auxiliary outputs to the Servo 120a’s inputs (thus using your hi-fi amp as a preamplifier) and
then connect your existing speakers to the Servo 120a’s outputs. If you fall in love with the sound (as we’re
sure you will), you might want to substitute a professional pre-amplifier for your existing hi-fi amp—and,
from there, you may well end up graduating to higher-level speakers. High quality sound is addicting—
don’t say we didn’t warn you!
Powering public address systems in permanent installations - The Servo 120a can be used to effectively
drive PA speakers in installations such as classrooms and corporate conference rooms.
Onstage monitoring - In small and medium-sized onstage areas (such as in clubs, lounges, etc.), the Servo
120a can be used to drive stage monitors, allowing the performers to hear themselves without having to
rely on onstage equipment amplification.
MIDI rack monitoring - Taking only a single rack space, MIDI musicians can easily incorporate the Servo
120a into their existing rack of gear. Combined with a pair of high quality stage or studio speakers, this
makes for an excellent monitoring system that can accurately reproduce the broad range of frequencies
typically output by devices such as synthesizers, samplers, and digital audio workstations.
In these pages, you’ll find a detailed description of the many features of the Servo 120a Power Amplifier, as
well as a guided tour through its front and rear panels, step-by-step instructions for its setup and use, reference
appendices, and full specifications. You’ll also find a warranty card enclosed—please don’t forget to fill it out
and mail it in so that you can receive online technical support and so we can send you updated information
about these and other Samson products in the future.
SPECIAL NOTE: Should your unit ever require servicing, a
sary. Without this number, the unit will not be accepted. Please call Samson at 1-800-372-6766 for a Return
Authorization number prior to shipping your unit. Please retain the original packing materials and, if possible,
return the unit in its original carton and packing materials.
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The Samson Servo 120a Power Amplifier utilizes the latest technology in professional power amplifier design.
Here are some of its main features:
• Power to spare - Each channel delivers 60 watts of power into 4 ohms (or, in Bridge mode, 120 watts into 8
ohms).
• Clean, crisp sound - Impressive audio specifications such as .01% THD, S/N of 105 dB, crosstalk of 75 dB,
and frequency response of 3 Hz to 65 kHz guarantee ultra-clean sound quality in any live or recording envi-
ronment.
• Independent input level controls for each channel with 41-position detents.
• Ten-segment LED meters for each channel continuously display power output levels and allow you to cor-
rect for overloading (clipping) conditions.
• Front-panel Protection LED shows you at a glance overheating or faulty wiring conditions.
• Innovative front-panel Speaker on/off switch and dedicated headphone jack enables private cue monitor-
ing of the input signal.
• Both electronically balanced 1/4" TRS and unbalanced RCA-type input connector jacks are provided along
with 5-way binding post output connectors
• Convection cooling (no fan) makes for whisper-quiet operation even in critical listening environments such
as recording studios.
• Unique bipolar circuit design that continuously keeps DC output during idling at or near 0 volts (thus keep-
ing idle speakers at their 0 point). This serves to minimize heat overload problems by effectively preventing
the Servo 120a from applying power when unnecessary.
• Protection relay circuitry (linked to the DC offset circuitry) that prevents “thumps”when powering on or
off. This means that you can use the Servo 120a with a single power strip into which a mixer or other audio
devices are connected, without danger of damage to connected speakers.
• Toroidal transformer power supply.
• Rugged construction (an all-steel chassis with a titanium finish and a lightweight anodized aluminum heat
sink) makes the Servo 120a eminently roadworthy.
• Flexible design allows the Servo 120a to be used free-standing or, with the use of included rack ears,
mounted in any standard 19" rack (taking just a single rack space).
• Three-year warranty.
• Last but certainly not least, value. The Samson Servo 120a has been designed from the ground up to deliver
excellent yet affordable sound quality.
5
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1: Channel input level controls - These 41-position detented knobs allow you to adjust the input level of the
signal arriving at the rear-panel input jacks (see #4 and #5 on the following page). At their fully counterclock-
wise position, the signal is attenuated by 70 dB (essentially completely off). At their fully clockwise position, the
signal is at unity gain (that is, no attenuation). When +4 dBu of signal arrives at the input jacks and the Channel
input level controls are set to their fully clockwise “0 dB”position, the Servo 120a delivers full power output.
2: LED meters These ten-segment LED meters continuously monitor the power output level for the corre-
sponding channel. For example, when the -8dB segment is lit, the amplifier is operating at 20% of it's maximum
power. When the -4dB segment is lit, 40% of it's maximim power, 0dB segment, 60% power, +2 segment lit, 80%
power, +4 segment lit, 100% power. When the PEAK LED lights, the amplifier is producing a distorted (clipped)
output signal For the best signal-to-noise ratio, the PEAK segment should light occasionally during maximum
levels: If it lights frequently, the amplifier may be overloadedTry turning down the input level controls. If the
amplifier now delivers too low an output level for your application, consider using Bridge mode (see the “Bridge
Mode”section on page 10 in this manual for more information).
3: Headphone jack Connect any standard stereo headphones to this jack (via a standard 1/4" TRS plug) for
private monitoring of the final output signal. NOTE: The Servo 120a speaker outputs are not automatically
muted when headphones are inserted into the Headphone jack—to monitor incoming signal in privacy, press
the Speaker on/off switch (see #7 below) so that it is out (in its “up”position)—the Speaker on/off LED will go off.
The built-in Servo 120a headphone amplifier delivers 400 mW into 100 ohms.
4: Power LED - Lit whenever the Servo 120a is powered on.
5: Power switch - Use this to power the Servo120 on or off.
6: Speaker on/off LED - Lights when the Speaker on/off switch (see #6 above) is pressed in and the Servo 120a
is delivering signal to its rear panel output terminals (see #7 on the following page).
7: Speaker on/off switch - When pressed in (the normal position), the Servo 120a delivers signal to its rear
panel output terminals (see #3 on the following page). When this switch is out (in its “up”position), outgo-
ing signal is muted, allowing personal monitoring of incoming signal through connected headphones (see #3
above).
8: Idle LED - Lit when there is signal present at the Servo 120a's input.
9: Protect LED - When illuminated, the Servo 120a is in protection mode and the outputs temporarily shut off.
10: Peak LED - Lit whenever the Servo 120a is producing a clipped output signal.
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1: AC Input - Connect the supplied standard 3-pin “IEC”plug here.
- Insert a 2.5 amp, SLO-BLO 125 volt fuse here for 115 volt operation, or a 1.5 amp, SLO-BLO 250
volt fuse for 230 volt operation.
WARNING: Fuses should only be replaced with the power cord disconnected.
3: 5-way Binding Post Output Connectors - Use these to connect each channel of the Servo 120a to loud-
speakers. Be sure to connect the loudspeaker correctly, with the red (+) terminal normally connected to the
positive input of the speaker and the black (ground) terminal normally connected to the negative input of the
speaker. See the “Bridge Mode”section on page 10 in this manual for speaker connection instructions when
using the Servo 120a in Bridge mode.
4: Balanced Inputs* - Connect incoming signal to these electronically balanced 1/4" TRS (Tip/Ring/Sleeve)
jacks, wired as follows: Tip hot, Ring cold, and Sleeve ground. Use balanced three-conductor cabling and TRS
plugs wherever possible (unbalanced two-conductor plugs can also be inserted into these inputs, but you’ll get
better signal quality and less outside noise and hum if you use balanced lines). Stereo signals should be con-
nected to both the left and right input jacks; when operating the Servo 120a in Bridge mode, use the left input
jack only (see page 10 in this manual for more information). The Servo 120a accepts input levels of any strength
but needs at least +4 dBu to achieve maximum power.
5: Unbalanced Inputs* - Connect incoming signal to these unbalanced RCA-type jacks. Stereo signals should
be connected to both the left and right input jacks; when using the Servo 120a in Bridge mode, use the left
input jack only (see page 10 in this manual for more information). The Servo 120a accepts input levels of any
strength but needs at least -10 dBv to achieve maximum power.
6: Bridge switch - For normal operation, place this switch at its right (“STEREO”) position. When placed at its
left (“BRIDGE”) position, the two amplifier sections (the left and right channels) are bridged, providing full 120
watt power into a single output. For more information, see the “Bridge Mode”section on page 10 in this manu-
al. WARNING: Due to the extremely high power output of the Servo 120a when used in Bridge mode, be sure
to use only 8 ohm loudspeakers sufficiently rated to handle the resultant wattage.
* If required, both the balanced and unbalanced inputs can be used simultaneously.
7
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Setting Up and Using the Servo 120a Power Amplifier
Setting up your Servo 120a is a simple procedure which takes only a few minutes:
1. Remove all packing materials (save them in case of need for future service) and decide
where the amplifier is to be physically placed—it can be used free-standing or mounted in a
standard 19" rack, requiring only one rack space. When installed, make sure that there is good
ventilation around the entire unit (we recommend using spacer panels, especially if multiple
amplifiers are used in a rack. Before rack-mounting, use a Philips screwdriver to remove the
Servo 120a’s feet.
2. Set the rear panel Bridge switch as desired (see page 10 in this manual).
Bridge switch
3. Make the speaker connections, using the 5-way binding post output connectors on the rear
panel. It is never a good idea to power up any amplifier that is not connected to loudspeak-
ers. In normal operation, any loudspeakers with a minimum impedance load of 4 ohms (that
is, 4 ohms or greater) can be used; however, in Bridge mode, 8 ohm speakers
Be sure to connect the loudspeaker correctly. In normal mode, connect them with the red (+)
terminal connected to the positive input of the speaker and the black (ground) terminal con-
Output connectors
nected to the negative input of the speaker. See page 10 in this manual for connection instruc-
tions when using the Servo 120a in Bridge mode.
4. Next, make the signal input connections, using the electronically balanced 1/4" and/or the
unbalanced RCA-type input connectors on the rear panel (if operating the Servo 120a in Bridge
mode, use the left input only—see page 7 in this manual). If your mixer or crossover network
has balanced outputs, use the Servo 120a’s electronically balanced inputs (unbalanced two-
conductor plugs can also be inserted into these inputs, but you’ll get better signal quality and
Input connectors
less outside noise and hum if you use balanced lines).
5. On the front panel of the Servo 120a, turn both Channel input controls fully counterclock-
wise. Then connect the supplied 3-pin “IEC”cable to the rear panel IEC connector and to any
grounded AC socket. Because of the relay protection circuitry built into the Servo 120a, you
can even plug it into the same power strip that other audio devices (such as a mixing console)
are connected to. You can then turn on all devices at once with the single power strip on-off
switch, with no danger of damaging connected speakers by generating “thumps.”
6. Press the front panel Speaker on/off switch (so that it is in, in its “down”position) and then
press the front panel Power switch in order to turn on the Servo 120a. The Power LED will light
and the Protection LED will go on. After approximately five seconds, the Protection LED will
go off and the Speaker LED (in the Speaker on/off switch) will light instead (you’ll hear a click
when this occurs).
Channel Input control
Protection LED
7. Apply an input signal to the Servo 120a at or about +4 dBu (if sending signal from a mixer,
drive the output meters at approximately 0 vu). While the input signal is present, slowly raise
the Channel Input controls until the desired sound level is achieved. The ten-segment LED
meters next to each Channel input control will show you the continuous power output of the
Servo 120a as signal is being passed. For the best signal-to-noise ratio, the Servo 120a should
normally be run with the Channel Input controls at or near maximum and the right-most (+4
dB / 100%) segment should light occasionally (but not frequently) during peak levels. If you
are using a mixer that has a master output level control (sometimes called “Control Room
level”), use it to attenuate the signal as necessary to achieve the desired speaker level.
Ten-segment LED meters
8. You can monitor the output signal in privacy by connecting a pair of headphones to the
front panel headphone jack and pressing the Speaker on/off switch so that it is out (in its “up”
position), at which time the Speaker LED will go off.
WARNING: Because the Servo 120a is capable of generating extremely high headphone
signal levels, always start with the Channel Input knobs turned down and then slowly
Headphone jack and
Speaker on/off switch
If you encounter difficulty with any aspect of setting up or using your Servo 120a, you can call
Samson Technical Support (1-800-372-6766) between 9 AM and 5 PM EST.
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The Servo 120a Protection Circuitry
As noted in the “Guided Tour”section of this manual, the Servo 120a's front panel
Protection LED indicates the activity of the relay speaker connection circuitry. When
the Protection LED is lit, this circuitry is inactive, and all connected speakers are muted
(provided with 0 volts DC), thus protecting them and preventing any audible “thump”
from occurring.
The following conditions will cause the Protection LED to go on:
Initial power-up: For approximately five seconds after initial power-up, the relay
speaker connection circuitry is deactivated and the speaker output is muted. If every-
thing is operating normally, you will hear an audible click at the conclusion of this
brief period, as the circuitry is activated and the Servo 120a begins delivering signal
to connected speakers (at which point the Speaker LED will light if the Speaker on/off
switch is in its “in”position). It is normal for the Protection LED to fade gradually after
the amplifier is powered off.
Overheating: A temperature sensing device in the Servo 120a will cause the relay
speaker connection circuitry to be activated (and the Protection LED to go on) when-
ever the operating temperature of the unit rises above a safe level. To guard against
this problem, make sure the Servo 120a receives adequate ventilation on all sides
(see page 8 for more information); in extreme environmental conditions, you may also
want to consider the use of cooling fans.
Severe overcurrent conditions: This occurs whenever the signal being output
from the Servo 120a rises to a level above 20% THD (Total Harmonic Distortion).
Shorted speaker cables: This will occur if, due to faulty wiring, the hot and ground
signals being output by the Servo 120a short to one another.
Output impedance drops below 2 ohms: This can occur if the Servo 120a is con-
nected to inappropriate speaker systems (see the “Setting Up and Using the Servo
120a”section in this manual for more information).
DC voltage detected at speaker output: The most likely cause of this is an internal
failure.
In general, any time the Protection LED lights up (other than during the approximately
five seconds following initial power-up), there is reason to be concerned. If this
occurs, turn the Servo 120a off immediately and carefully check all wiring and external
devices in order to locate and correct the condition that caused the LED to light up in
the first place.
9
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Bridge Mode
The Servo 120a provides a rear-panel switch that allows it to be used in a Bridge
mode. When this switch is placed in the “RIGHT”(Stereo) position, the Servo 120a
functions as a true stereo amplifier, where both of the two independent amplifier
channels (left and right) can receive different input signals and produce independent
output signals. However, when the switch is placed in the “LEFT”(Bridge) position,
both amplifier channels process only the signal present at the left input, thus produc-
ing a single output signal with a true 120 watt output (into 8 ohms).
The illustration on the left shows how this works. In bridge mode, the polarity
(phase) of the right output signal is reversed relative to that of the left output signal.
Both channels then process the same signal, with the speaker load connected so that
power is derived from both channels. The effective voltage swing seen by the load is
thus doubled so that the power output is doubled.
INPUT
LEFT(+)
OUTPUT
When using the Servo 120a in Bridge mode, be sure to connect each loudspeaker as
shown in the illustration below (and as silkscreened on the rear panel), with the red (+)
terminal of the left channel connected to the positive input of the speaker and the red
(+) terminal of the right channel connected to the negative input of the speaker. Do
not use the black (ground) output terminal of either channel (the speaker load
must “float” away from the amplifier chassis).
RIGHT(+)
OUTPUT
+
-
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Appendix A: Linearity vs. Frequency Sweep
AUDIO PRECISION Servo 120a LEVEL (dBr) vs FREQ (Hz)
0dB ref
5.0000
Ap
4.0000
3.0000
2.0000
1.0000
0.0
-1.000
-2.000
-3.000
-4.000
-5.000
10
100
1k
10k
50k
Linearity (0 dB Ref) vs. frequency sweep
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Appendix B: Power Output vs.THD
AUDIO PRECISION Servo 120a THD +N (%) & LEVEL (W) vs FREQ (Hz)
Watts
THD
80.000
1.000
Ap
73.000
61.000
56.000
48.000
40.000
32.000
21.000
15.000
8.000
.9000
.8000
.7000
.6000
.5000
.4000
.3000
.2000
.1000
0.000
200.0u
20
100
1k
10k
50k
Power output (60 W) vs. Total Harmonic Distortion
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Specifications
Rated Output Power, per channel (@ 1 kHz)
4 ohm, .<0.05% THD+N
4 ohm, 1% THD+N
8 ohm, <0.05% THD+N
8 ohm, 1% THD+N
60 W
80 W
50 W
55 W
Bridge mode
8 ohm, .<0.05% THD+N
8 ohm, 1% THD+N
120 W
150 W
Typical Distortion, per channel
THD+N (80 kHz LPF @ 1 kHz, rated output power)
IMD (SMPTE 4:1, 60 Hz & 7 kHz @ rated output power)
.01%
.03%
Signal To Noise Ratio
(22 Hz - 22 kHz bandwidth @ dB below rated output power)
105 dB
Frequency Response
10 Hz - 20 kHz +0, -0.5 dB
3 Hz - 65 kHz, +0, -3.0 dB
Input Sensitivity
-10dBv on the RCA inputs,
+4 dBu on the 1/4" inputs to achieve rated power.
(Level control set to maximum)
Crosstalk (adjacent channels)
Output DC Offset Voltage
-75 dB, 1 kHz
± 50 mV, DC Servo Controlled
Headphone Output Level
400 mW (into 100 Ω @ 1% THD+N)
17.5 in (w) x 11.5 (d) x 1.75 (h)
444 mm (w) x 292 (d) x 44 (h)
Weight
15.6 lbs • 7 kg
13
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Samson Technologies Corp.
575 Underhill Blvd.
P.O. Box 9031
Syosset, NY 11791-9031
Phone: 1-800-3-SAMSON (1-800-372-6766)
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