QSC USA 370 Power Amplifier User Manual

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

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