QSC USA 370 Power Amplifier User Manual
- June 3, 2024
- QSC
Table of Contents
QSC USA 370 Power Amplifier
Explanation of Graphical Symbols
The lightning flash with arrowhead symbol, within an equilateral triangle, is
intended to alert the user to the presence of uninsulated “dangerous voltage”
within the product’s enclosure that may be sufficient magnitude to constitute
a risk ot electric shock to humans.
The exclamation point within an is intended to alert the users to the
presence ot important operating and maintenance (servicing) Instructions in
the literature accompanying the product.
CAUTION: To REDUCE THE RISK OF ELECTRIC SHOCK, DO NOT REMOVE THE COVER. No USER-SERVICEABLE PARTS INSIDE. REFER SERVICING TO OUALIRED SERVICE PERSONNEL.
WARNING: To PREVENT FIRE OR ELECTRIC SHOCK, 00 NOT EXPOSE THIS EQUIPMENT TO RAIN OR MOISTURE.
INTRODUCTION
This manual contains imponent information on the roper installation and use of
the amplifier. Please read it care fully in
order co obtain the best performance from your new equipment Do not hesitate
to call your QSC dealer for resistance with questions about installation, use,
or servicing of the amplifier.
THE AMPLIFIERS
The USA 370, USA 850 and USA 1300 amplifiers arc intended for professional
sound reinforcement and .studio monitoring applications. They arc also useful
in distributed sout1d and page in applications. These are all two-channel
amplifiers, differing primarily in terms
of out >ut power. The USA 370 produces 185 watts at 4 ohms; the USA 850
produces 425 watts per
channel at 4 ohms; and the USA 1300 produces 650 watts per channel at 4 ohms
(all values are RMS, both channels driven}.
The USA 370 is passively cooled; the higher powered USA 850 and USA 1300 have
dual-speed fans.
Each channel in an USA series amplifier is isolated by separate power
transformer secondary windings, each its own bipolar power supply. Balanced
or unbalanced inputs can be made with screw lugs to the barrier scrip, or with
1/4-inch plugs (ring-tip-sleeve for balanced inputs). Speaker arc made with
five-wav handing posts.
The ch,L5Sis includes integral rack mounting ears. The recess . m the front
panel house., the AC ,-witch, circuit breaker, and
LF.D indicators for clipping, power on, and protect mode. The gain controls
arc mounted on d1e rear pa11d.
Complete protection is provided for open circuit, short circuit, and
mismatched loads. The amplifier will down
temporarily if it overheats, and a front panel AC circuit breaker protects the
power supply against unusual overloads.
All protection systems except the main circuit breaker will reset
automatically as soon as safe operation is assured. internal protects user’s
loudspeakers from
unexpected damage by muting the amp during turn-on and turn-off and by
blocking DC faults (uncontrolled power
breakdown, whether caused by the amplifier or preceding components}. More
detailed explanations are found on subsequent pages.
PRECAUTIONS
BACKGROUND – Speakers have several limits which should not be for
reliable operation. It is the user’s responsibility 10 determine these limits
and operate the accordingly. we offer protection against many types of
n:ianomahcs, but you must still select speakers of the
appropriate type and power capacity.
DC Fault Protection – All QSC Audio Produces, Inc. amplifiers the speakers
against amplifier, so no special protection _is require! for full range.
loudspeakers. The USA series amplifiers the response below 20Hz [0 procec1
the amp and load from possible damage caused large subsonic transients such
as breath pops, dropped 1mcrophoncs, etc.
very powerfi,1, with extra peak power ( dynamic headroom) in reserve. Observe
the hookup and operating QSC Audio Products, Inc. is not liable for and
damage to loudspeakers caused by overpowering, frequency operation or
faults.
INSTALLATION
UNPACKING AND INSPECTION
All CSA series amplifiers are carefully inspected before shipment from the
factory. Despite the protective carton and rugged amplifier design, damage in
transit is always a possibility. Check for obvious carton damage while
unpacking the unit, and check the amp for loose internal parts, broken control
knobs, or other physical damage. If shipping damage is evident, notify the
transportation company with the carrier for shipping damage. Be sure to save
the carton for the shipper to inspect. Please retain the carton. It is the
best and safest means of packing the amplifier if needs to be shipped to the
factory for repair. QSC docs not warranty against damage resulting from the
shipment by you of improperly packaged amplifiers. immediately. Only the
consignee can c a claim
IMPORTANT PRECAUTIONS
The power must bc OFF when making any connections. If you connect plugs with
the power on, especially in dry environments, static sparks or bad cables can
cause pops or hums that damage speakers. When first powering up the amp, have
the amplifier gain controls all the way off, in case of decoctive cables or
hookups. Turn the gain controls up gradually until normal operation is
verified. Check the AC voltage printed on the serial number label. Make
certain it matches the AC voltage supplied in your area before plugging the
unit in. CONNECTION TOA VOLTAGE SOURCE OTHER THAN THE ONE SPECIFIED WILL
IMMEDIATELY DAMAGE THE AMP. Never connect the speaker terminals (red binding
posts) for wo channels together on any power amplifier. The two channels will
fight cache other and possibly fail. Do not
Never connect the spikier terminals (red binding posers) for two channels
together on any power amplifier The two
channels will tight each other and possibly fail. no not und terminals (black
binding posts) to chassis or signal grounds, as the resulting ground loop
could cause ultrasonic oscillations. Keep all wiring separate for each
channel, and separate. from input wiring.
Do not remove the amplifier cover, as dangerous voltages inside. Do not
expose to rain or moisture. Refer all servicing co qualified personnel. Please
the factory for service center information.
High voltages be present on the speaker terminals. Always connect speaker
terminals with the power oft: and use cable with 110 frayed strands or damaged
insulation.
Power am have high power circuity inside. The is an electrical hazard, and a
potential for fire in the of a short circuit or other electrical malfunction.
Never plug in a damaged amplifier condition of the insulation is checked. If
a circuit blows when turning amplifier on, the equipment is defective and
should not be restarted until repaired by a competent technician, or replaced.
Failure to observe these precautions to fire or electrical shock.
Power amplifiers arc heavy and may become hot after use. Provide adequate
support and be careful how you hold the when handling it.
OPERATION QUICK INSTRUCTIONS
Stereo Operation These instructions cover the normal use of the amplifier in
two-channel or stereo applications5. AC Power- Connect the AC cord to a
standard outlet only. The amplifier will operate satisfactorily over a tl0%
range of voltages, but full rated performance will be achieved only at the
rated voltage. Heating Chassis Ground There is no provision for lifting signal
ground relative to chassis ground on these amplifiers. Electronic balanced
inputs are provided for hum rejection. Use balanced input cables to avoid hum
and interference. For safer reasons do not remove the ground pin on the AC
cord. Input Connections-The input polarity is as follows inch plug-Tip is
minus or inverting, Ring is plus or non-inverting, Barrel is ground (as
always). Barrier Strip input, – is the minus input. When making unbalanced
connections, the unused terminal must be grounded for proper response.
balanced inch plugs will work automatically. When connecting an unbalanced
input to the harrier strip, connect the signal wire to+ the shield to “” and
the “” to GND. Speaker Connections You may connect banana plugs, Spade lugs,
or bare wire ends to the five-way binding posts. correct polarity (red/black
terminals) so all speakers move in direction. Power Up-When first powering up,
start with the gain controls off until proper operation is verified. Upon
turning on the AC switch, the Power LED should comes on. After two seconds the
muting will stop and sound will be chard. In case of difficulty refer to
Section 4. Operation and Indicators Gain should be kept in the upper half of
its range for full performance. Fact channel has a red Clip indicator that
will show any distortion in the amplifier. The mute circuit will cut the sound
off as soon as you turn the amp, and mute for two seconds before restoring
power to the speakers. This blocks turn-on and turn- oft thumps. GND is
circuit ground, + is the plus.
Cooling-Thc USA 370 is passively cooled. n the SA 850 and USA 1300, an
internal fan forces air across heatsinks. Air flow is from the rear and warm
air is exhausted to the front. This prevents the recirculation of heated air
and reduces rack temperatures. Be sure that plenty of inlet space is allowed
in the rack for free air flow. When installing a USA series amplifier that has
a fan in the same rack with passively cooled amps or fan cooled amps that
exhaust into the rack, locate the USA fan coolcd amp on the bottom. This will
assure the coolest air for all amps
INPUT connections
Input Circuit- An electronic balanced input is standard. For best performance
in the balanced input mode, the source should have equal impedances for both
signal conductors, so that the loading effect on cache leg will be the same
for common mode (noise) signals. Minor mismatches will result in slight loss
of common mode rejection, but will still provide much greater noise rejection
than unbalanced inputs. Balanced Inputs For proper balanced line operation,
the cable shield must be kept separate from both signal conductors. The cable
shield is connected to the barrel of a % inch plug, or to the GND terminal of
the barrier strip.
Unbalanced Inputs and Polarity Since the input circuit responds to the difference between the plus and minus signals, if only an unbalanced (single- ended) signal is available the unused input terminal needs only be grounded for normal operation, without loss of gain. The agility to reject cable induced hum and noise is lost, but this may not be significant in well shielded environments with short distances between audio components. We have used the safest or most stable assignment (inverting) for the input polarity, of the 4 inch plug. Unbalanced 4 inch plugs can simply be fully inserted ín the % inch jack without special concern. You can wire to the – input of the barrier strip for stability, or the + input for non-inverting polarity, and ground the unused input and the cable shield to the central GND terminal NOTE- You can always reverse the red-black polarity to all the speakers to restore correct polarity even when using the more stable inverting mode. In any case. be sure to use the same polarity for all of the speakers so they work together. Cross Connecting Both Channels You can always connect the inputs of two or more channels to the same signal, but NEVER CONNECT TWO CHANNELS TO TŤIE ŠAME SPEAKER. Connect separate speakers to catch channel to avoid amplifier damage. Rather than using a Y cable, jumper wires at the barrier strip can be used. This connects the input jacks for Channel 1 and Channel 2 in parallel making it easy to cross patch to as many channels as desired. Bring the input signal into Channel 1. Connect jumpers between the “+” terminals of Channel l and Channel 2, and also between the-” terminals. This will send the signal from Channel 1 to Channel 2. Then you may connect another cable from Channel 2 of the first amp to Channel I of the next amp. By using the same procedure in cache amp, you can loop through to as many amps as desired. Each channels gain control will affect only that channel, not the signal reaching the other channels.
NOTE 1 – For balanced line operation, you must maintain balanced cables al the
way through. Any unbalanced cables will unbalance the whole network.
NOTE 2- The power of a given amplifier channel will not be increased by
connecting additional channels. You must connect additional, separate speakers
to each additional amp channel to multiply your total power rating. See below
for a discussion of mono bridging, which is a partial exception to this rule.
Mono Bridging To engage the bridged mono mode, there is a separate slide
switch located below the barrier strip inputs on the rear. Follow the
directions on the label to engage the switch in the *Bridge” position. Connect
the amplifier input to Ch. 1 only, and use only the Ch. 1 Gain control, which
now controls the entire amplifier. Do not faced another input into Channel 2.
The Ch. 2 Cain should be kept off for safety. A 4, 8, or 16 ohm speaker load
can be connected across the two red speaker terminals, using the red binding
post of Channel 1 for “” or hot. Mono Bridge Precautions Minimum load
impedance is 4 ohms, which will be the equivalent, to the amp, of 2 ohms per
channel. Be sure the spacers can handle the high power ratings (se
specilicauons page 8). The power supply is unable to support a 2 ohm load in
this bridging configuration, because bridging in this manner attempts to draw
four times the power from the
amplifier. This exceeds the rated output capacity, and would cause sever overheating. Both sides of the speaker cable arc hot or active. Do not connect any other circuit to either speaker conductor. With a 4 ohm load, the amp will be working at its rated limit (2 ohms per channel). Assure adequate cooling and AC power. Do not expect continuous full power operation without tripping the thermal protection circuits.
SPEAKER CONNECTIONS
Speaker Terminals Red and black five way binding posts, on standard 4 inch
centers, are located on the racer of the chassis. All speaker labels are shown
in red. Terminal Polarity-The red binding post carries the positive or hot
speaker output. The black binding post is the ground return for the speaker.
Do not ground the speaker common to other parts of the chassis as this might
cause audio ground loops and oscillations. Speaker Voltage- Because of the
amplifier’s high power capability there is a possibility of shock hazard at
the spacer terminals. Always make connections with the power off, and observe
good wiring practice and avoid stray wire strands. Speaker bales In order to
obtain the full benefit of the high power and high damping factor, use the
heaviest gauge, finest stranded wiring possibly. 12 gauge speaker cable is
available, and heavier gauge specialty cable is sold by audio dealers Cable
Polarity- Be sure to observe correct polarity at both the speaker and
amplifier end. The USA scries amplifier has adequate current capability to
fully drive loads down to two ohms. However, many high performance 8-ohm
loudspeakers, specially multi-way.
systems With passive crossovers, have impedances at some frequencies which are far lower than the average rating. An impedance minimum of 2 ohms or less is not uncommon. For this reason, speaker impedance curves should bc consulted before connecting spacers in parallel. We would expect the amplifier to do an outstanding job with any 8 ohm, full range speaker system, and we expect equally outstanding performance when driving 4 ohm loads without passive crossovers (as part of a bi- or tri amped system, for instance).Two ohm loads should be approached with caution, as three is no further margin for impedance dips. Two ohm operation will nor damage the amp, but high power operation into reactive 2 ohm leads may result in overheating or excessive AC current consumption, causing shutdowns. In addition, some power may be lost at those frequencies where the impedance dips below 2 ohms. For these reasons, operation with 2 ohm loads should be tested thoroughly before being put into use.
PROTECTION FEATURE
Short Circuit Protection Patented QSC Output Averaging short circuit
protection continuously monitors the actual load impedance. The result is full
performance into rated loads, ability to handle normal program peaks into
marginal loads, and good protection into short circuits. At no time will the
circuit cause abnormal distortion spikes or loss of Sound. Thermal Protection-
If the heat sink temperature rises to about 90°C, muting will be triggered,
permitting the circuit to rest until temperatures fall to safe levels. The
power indicator will sill be lit but no sound will come through. Reset should
occur within a minute or two. If thermal problems occur, check for blocked
ventilation, proximity to a heat source, short circuit, or improper load (too
many speakers). If the Power indicator goes our, this means the amplifier’s
circuit breaker has tripped and must be reset. DC Fault Protection The USA
series amplifiers have an AC coupled output with inherent DC protection.
Output transistor failures will not pass DC to the load. Turn-On/Turn-Off
Muting-The USA series amplifiers use solid state circuitry for muting. There
will be a two second muting interval after turn on. Alter turn off, or loss of
power for any reason, the amp will mute within a quarter of a second.
Input/output Protection isolated by 10KQ resistors, which are part of the
balanced input circuit. This protects the inputs from burnout to extremely
high input signals or RF interference. The amplifier output is isolated from
and inductive loads by an ultrasonic network that decouples the speaker
terminals slightly at frequencies above about 50kHz.
TROUBLESHOOTING
This section contains troubleshooting hints. There are no user serviceable parts inside the amplifier. This information is for use by qualified service personnel only. A problem can usually be isolated using a step-by-step evaluation, comparing the function of both channels, and using one channel to check the inputs and outputs of the other.
PROBLEMS WITH SOUND NO SOUND
Power LED does not come on: No AC power. Check AC plug, AC switch, AC circuit
breaker, and AC outlet with another device. Rest the circuit breaker, if
tripod. Power LED comes on: Input or output is not connected, or the channel
is faulty. If one channel is working, use it to test the inputs and speaker
wires from the bad channel. If there is still no sound, trace those
connections back to the preceding unit and speaker to isolate the fault. If
the good channel works using the bad channel’s cables, then the fault is
somewhere in the bad channel. Check the input jacks, trying an alternate type
jack if possible, and check the speaker binding posts for looseness. Is the
volume control turned up?
WEAK BUT CLEAR SOUND
Usually indicates lack of input signal or incorrect gain adjustment at some
point. If the other channel is working, swapping the connections. If the
sound is very thin or muffled, suspect that one driver in a multi-way speaker
has failed.
WEAK AND DISTORTED SOUND
The amplifier clip light comes on during the distortion: There is a shorted
speaker cable, the speaker is blown out, or the amplifier channel is
defective. The clip light does not show during the distortion: The distortion
is happening outside the amplifier. Check for misadjusted or defective units
before the amplifier, or bad speakers on the afflicted channel. Verify that
amplifier Gain is in the normal range (halfway up or higher) to prevent inpur
overload
SOUND cut’s IN AND OUT
Generally caused by a bad connection somewhere. To rest, shack the amp or the
input/output connectors. An intermittent connection to one side of the
balanced input can causa a 6dB fluctuation of input level. If the sound stops
for a minute or two, and then resumes by itself, cheek the amp for
overheating.
If the volume level diminishes without the Gain control having been touched, the problem may be with the speakers, rather than the amplifier. As speaker voice coils heat up, their resistance increases, and the amount of power Chey draw decreases. Volume will return when the speakers cool down. Some loudspeakers are more susceptible to this type of power compression than others. In a multi speaker system, be sure all of the speakers are still working. Your cars get used to high sound levels, and as the room fills with people more of the sound will be absorbed. Only a sound level meter, used with a standard signal level and at a standard distance from the speaker, can establish whether proper output levels are being produced. PROBLEMS
WITH NOISE HUM
Defined as a fairly rounded 60 cycle tone. Severe hum usually is caused by
broken cables, jacks with disconnected ground (shield), or corroded connectors
(specially 4-inch types). À mild hum, often with a little more “tone” or
harmonic content, is usually the result of ground loops. This problem is
caused by 60 cycle magnetic ficlds, which radiate from power transformers
including those in the amplifier. Try repositioning the cables away from the
various components. Note Tape recorder heads, phono cartridges, and electric
guitar pickups are especially sensitive to this ype of interference, and must
be kept away tromp high power electronics.
BUZZ
Defined as a very “raze” kind of hum. This is usually caused by interference
from solid-state light dimmer circuits. Follow the same precautions shown
above, and make sure the electronics’ are not connected to an AC outlet that
has a dimmer control.
HISS
Defined as a smooth “shah” noise. This is always a problem with sensitive
electronic inputs, and usually starts at the point of weakest signal. In a
properly designed system, this will be the initial microphone, phono, or tape
source There is a noise “floor” caused by random atomic vibrations. This
limits the signal-to-noise ratio of the original signal. The goal of a proper
system is to have a quiet pre-amp which immediately amplifies the input signal
to a standard “line level”, well above the noise floor, so that further
degradation docs not occur. The idea is to maintain a fairly constant signal
level after the initial pre-amp. To isolate the source of unwanted hiss, start
at the amp and work backwards, reducing, and then restoring gains. You should
hear a reduction of hiss and audio together at each point. When you find a
control which lowers the audio volume, but not the hiss level, you know the
hiss is coming in after that stage. Assuming that the hiss has not always
bacon there, this indicates defectives electronics. Certain special effects
units are rather noisy, so compare with other users.
CRACKLES
Defined as a “popcorn” noise. If the crackle persists during pauses, this
indicates defective electronics, and must be uraced down using the above
procedure. Crackles that occur during audio peaks or when the electronics arc
vibrated usually indicate bad connections.
SERVICING USER MAINTENANCE
There are no periodic adjustments required. The amplifier should provide stable performance until parts fail from age. Internal servicing must be referred to quantified personnel. The amplifier may be inspected for loose screws on the outside. If any loose parts rattle around on the inside, plicas have the amplifier serviced immediately, as a loose part could lodge in a dangerous place and causa further damage or a shock hazard. The faceplate and chassis can be cleaned with Cleaning a soft cloth and mild non abrasive eLearning solution. Avoid cleaning powders or scrubbing pads, as these will scratch and dull the paint. Be sure to unplug the unit prior to cleaning. Dampen the cloth with the cleaning solution and wipe gently. You may wish to buff the surface lightly wiry a dry soft cloth. Dust Removal- After prolonged use, especially in dusty environments, the heat sinks may become clogged with dust. This will interfere with cooling, lcading to higher temperature operation and reduced life. Some dust can be removed by directing an air jet in the fan intake on the rear. Severe buildups will require qualified service personnel to remove the top cover for thorough dust removal.
OBTAINING SERVICE
If the amplifier isn’t working properly, places consult the troubleshooting
chart in Section 4. If proper operation cannot bc restored, the amplifier
requires service. This must be performed by qualified technical personnel, to
avoid shock hazard or improper repairs. To obtain the location of the nearest
authorized Service Center, please contact your QSC dealer.
WARRANTY
QsC Audio Products, Inc. warrants this product to be free from defective material and/or workmanship for a period of three years from date of sac. We will replace defective parts and repair malfuncüoning products under this warranty when the defect occurs under normal installation and use, provided the unit is returned to our factory or authorized servicer via prepaid transportation. The returned product must, in our judgment, have a manufacturing or material defect. This warranty docs nor extend to any product that has been subject to misuse, neglect, accident, improper installation, alteration, unauthorized service, or where the serial number has been removed or defaced. QsC Audio Products, Inc. shall not, under any circumstances, be responsible for any direct, indirect, incidental or consequential damages, including but not limited to damage to the audio equipment or to any accessories used with the audio equipment. Some jurisdictions do not allow theca exclusion or the limitation of incidental or consequential damages, so this limitation may not apply to you.
Specification
