QSC CX-12-T Owners manual

CX Series

USER MANUAL

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CX4

CX4T

CX6

CX6T

CX12

CX12T

Rev. B

*TD-000057-00*

*TD-000057-00**TD-000057-00*

*TD-000057-00*

*TD-000057-00**TD-000057-00*

31

T able of Contents

PART 1—CX SERIES USER MANUAL

I. Warning Notices ....................................................................................................................................... 2

Speaker Damage ............................................................................................................................................ 3

Speaker Output Shock Hazard.......................................................................................................................3

Rack Mounting Precautions...........................................................................................................................3

II. Overall Description ................................................................................................................................... 4

III. Inputs............................................................................................................................................................ 6

Input Sensitivities..........................................................................................................................................6

IV. Outputs ......................................................................................................................................................... 6

V. Controls & Displays ................................................................................................................................... 8

VI. Operation ..................................................................................................................................................... 8

AC Power ....................................................................................................................................................... 8

Operation .......................................................................................................................................................8

Troubleshooting .............................................................................................................................................9

Open Input Architecture™ Level I................................................................................................................. 10

Parallel, Stereo, or Bridged Operation ....................................................................................................... 11

Maximum Long-Term Output Power ........................................................................................................... 13

AC Current Consumption ............................................................................................................................. 13

Heat Emissions............................................................................................................................................. 14

Protection Circuits ....................................................................................................................................... 15

VII. Specifications.......................................................................................................................................... 16

PART 2—CX APPLICATION GUIDE.................................................................................. 19

VIII. Distributed Lines ..................................................................................................................................... 19

IX. Designing the Distributed Sound System .......................................................................................... 20

Loudspeaker Coverage and Placement....................................................................................................... 20

Determining Power Levels...........................................................................................................................22

Selecting the Amplifier ...............................................................................................................................23

Using Components with Different Voltages...............................................................................................24

X. How Many Speakers? .............................................................................................................................25

XI. Other Design Considerations ................................................................................................................26

Speaker Transformer Saturation ................................................................................................................26

Connecting Both a Distributed Line and a Direct Loudspeaker on One Channel...................................... 26

Wire Loss .....................................................................................................................................................27

AC Current Consumption ............................................................................................................................. 29

Heat Emissions............................................................................................................................................. 29

XII. Address & Telephone Information ......................................................................................................30

1

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 of sufficient magni­tude to constitute a risk of electric
shock to humans.

The exclamation point within an equi­lateral triangle is intended to alert
the users to the presence of impor­tant operating and maintenance (ser­vicing) instructions in the literature
accompanying the product.

EXPLICATION DES
SYMBOLES GRAPHIQUES

Le symbole éclair avec point de flèche
à l’intrérieur d’un triangle équilatéral
est utilisé pour alerter l’utilisateur de
la presence à l’intérieur du coffret de
“voltage dangereux” non isolé
d’ampleur suffisante pour constituer
un risque d’elétrocution.

Le point d’exclamation à l’intérieur
d’un triangle équilatéral est employé
pour alerter les utilisateurs de la
présence d’instructions importantes
pour le fonctionnement et l’entretien
(service) dans le livret d’instruction
accompagnant l’appareil.

ERKLÄRUNG DER GRAPHISCHEN
SYMBOLE

Der Blitz nach unten zeigendem Pfeil in
einem gleichseitigen Dreieck weist den
Benutzer auf das Vorhandensein einer
unisolierten,
Spannung“ im Gehäuse hin, die stark
sein kann, einer Person einen
elektrischen Schlag zu versetzen.

Das Ausrufzeichen in einem
gleichseitigen Dreieck weist den
Benutzer auf wichtige Betriebs- und
Wartungs- vorschriften in den
beiliegenden Unterlagen des Gerätes
hin.

gefährlichen

”

CAUTION

RISK OF ELECTRIC SHOCK

DO NOT OPEN

CAUTION: To reduce the risk of electric shock, do not
remove the cover. No user-serviceable parts inside.
Refer servicing to qualified service personnel.

WARNING: To prevent fire or electric shock, do not
expose this equipment to rain or moisture.

AVIS

RISQUE DE CHOC ÉLECTRIQUE

NE PAS OUVRIR

ATTENTION: Pour eviter les risques de choc électrique,
ne pas enlever le courvercle. Aucun entretien de
pièces intérieures par l’usager. Confier l’entretien
au personnel qualifié.

AVIS: Pour eviter les risques d’incendie ou
d’électrocution, n’exposez pas cet article à la pluie
ou a l’humidité.

VORSICHT

GEFAHR EINES ELEKTRISCHEN

SCHLAGES. NICHT ÖFFNEN!

VORSICHT: Um das Risiko eines elektrischen Schlages
zu vermindern, Abdeckung nicht entfernen! Keine
Benutzer Wartungsteile im Innern. Wartung nur durch
qualifiertes Wartungspersonal.

WARNUNG: Zur vermeidung von Feuer oder
elektrischen Schlägen, das Gerät nicht mit Regen
oder Feuchtigkeit in Berührung bringen!

SAFEGUARDS

Electrical energy can perform many useful func­tions. This unit has been engineered and manufac­tured to assure your personal safety. Improper use
can result in potential electrical shock or fire hazards.
In order not to defeat the safeguards, observe the
following instructions for its installation, use and
servicing.

DECLARATION OF CONFORMITY

for all CX and CXT models

We declare as our sole responsibility
that this product is in compliance with
the EMC Directive 89/336/EEC and con­forms to the requirements of the Har­monized Product Standards EN 55013
(Product Emissions), and EN 55020
(Product Immunity).

L’énergie électrique peut remplir de nombreuses
fonctions utiles. Cet appariel a été conçu et réalisé
pour assurer une sécurité personnelle entiére. Une
utilisation impropre peut entraîner des risques
d’électrocution ou d’incendie. Dans le but de ne pas
rendre inutiles les mesures de sécurité, bien ob­server les instructions suivantes pour l’installation,
l’utilisation et l’entretien de l’appareil.

2

PRECAUTIONS

I. Warning Notices

SPEAKER DAMAGE

The CX Series amplifiers are among the most powerful professional amplifiers available and are capable of
producing much more power than many loudspeakers can handle. It is the user’s responsibility to use suitable
speakers with the amplifier and to use them in a sensible way that will not cause damage.

QSC will not be responsible for blown speakers. Consult the speaker manufacturer for power-handling
recommendations.

Even if you reduce the gain using the amplifier’s rear panel attenuator controls, it is still possible to reach full
output power if the input signal level is high enough.

A single high-power crescendo can blow high-frequency speaker drivers almost instantaneously, while low­frequency drivers can usually withstand very high, continuous power levels for a few seconds before they too
die. Reduce power immediately if you hear any speaker “bottoming out”-harsh pops or cracking distortion that
indicate that the speaker voice coil or diaphragm is striking the magnet assembly.

QSC recommends that you use amplifiers of this power range for more headroom (cleaner sound) rather than
for increased volume.

Optional rear rack
mounting ears.

SPEAKER OUTPUT SHOCK HAZARD

A CX amplifier is capable of producing hazardous output voltages. To avoid electrical shock, make sure the cover
is in place over the output terminals, and do not touch any exposed speaker wiring while the amplifier is operating.

RACK MOUNTING PRECAUTIONS

To minimize twisting stress on the front mounting ears, the amplifier’s internally mounted output transformers
are located close to the front panel. Nevertheless, when rack mounting any CX amplifier, make sure it is well

supported at all four corners to avoid damage to either the amplifier mounting ears or the mounting rails.

Contact the QSC Technical Services Department to order rear support brackets.

LIFTING PRECAUTIONS
In order to safely move or install the amplifier, it is recom­mended that two persons share the weight when lifting and
positioning the unit.

SICHERHEITSVORSCHRIFTEN BEIM HOCHHEBEN
Um den Verstärker sicher zu verschieben oder einzubauen,
wird empfohlen, das Gewicht des Verstärkers beim Hochheben
und Verschieben gleichmäßig auf zwei Personen zu verteilen.

3

II. Overall Description

The CX Series from QSC comprises six high-efficiency professional power amplifier models, each with two
independent channels. The table below lists the range’s audio power ratings.

MODEL 8

1

ΩΩ

Ω

ΩΩ

1

ΩΩ

4

Ω

ΩΩ

2

ΩΩ

2

Ω

ΩΩ

25 volts

3

70 volts1100 volts

1

CX4 150 watts 225 watts 350 watts
CX4T 150 watts 225 watts 350 watts 150 watts4175 watts4175 watts

4

CX6 200 watts 300 watts 450 watts
CX6T 200 watts 300 watts 450 watts 200 watts3250 watts3250 watts

3

CX12 425 watts 600 watts 900 watts
CX12T 425 watts 600 watts 900 watts 400 watts3500 watts3500 watts

1: FTC watts per channel, 20 Hz–20 kHz, 0.1% THD
2: EIA watts per channel, 1 kHz, 1% THD
3: Watts per channel, 45 Hz-15 kHz, 0.25% THD
4: Watts per channel, 50 Hz-15 kHz, 0.25% THD

3

Each channel has its own power transformer secondary. The CX4, CX4T, CX6 and CX6T utilize complementary
Class AB linear output circuitry. For improved efficiency, the CX12 and CX12T utilize Class H step linear
complementary output circuitry and have bipolar multi-rail power supplies. The power transformers, as are the
output transformers, are mounted in the front of the amplifier chassis, as close to the front mounting rails as
possible; this keeps the unit’s center of gravity forward to minimize the twisting force on the front
mounting ears. Still, the amplifier should be supported at all four corners, especially if it is in a

MODEL NET WEIGHT

portable rack.

CX4 13.6 kg; 27 lb

The CX4 and CX4T are two rack spaces high; the CX6, CX6T, CX12, and CX12T are three rack spaces
high. The adjacent table lists the amplifiers’ weights by model.

CX4T 18.2 kg; 37 lb

The amplifiers require a rack depth of 18 inches (45.7 cm) to clear the rear support ears. Allow
some additional clearance for input and output connectors at the rear panel, which is 16.9 inches
(42.9 cm) behind the plane of the frount mounting rails. The built-in cooling fan draws air in at
the rear of the chassis and exhausts it through vents in the front panel. The flow-through cooling
scheme allows you to rack-mount the amplifiers one atop the other, with no clearance necessary
in between. This mounting technique also helps support the weight of the upper amplifiers.

The CX Series derives much of its design from our award-winning MXa Series amplifiers. They are engineered
for stability and exceptional reliability, with protection for open or short circuits and mismatched loads. To protect
your loudspeakers, the outputs mute during turn-on and turn-off, and also in the event of a DC fault. The CX4
and CX4T are AC coupled and cannot pass DC to the load. Therefore, the DC fault muting is not required. All
protection circuitry automatically resets to normal when conditions assure safe operation.

Every CX amplifier features thermal management and protection in the form of a two-speed fan and extruded
aluminum heat sinks with direct-mounted output devices. If a heat sink overheats to 85½ C or higher, a protection
circuit mutes the amplifier channel output.

4

CX6 20 kg; 42 lb
CX6T 25 kg; 55 lb
CX12 22.7 kg; 50 lb

Front
features of
CX4 or CX4T
amplifier

Rear
features of
CX4
amplifier

Rear features
of CX4T
amplifier

13

7 78

13

7 78

2 4112

109

109

11 10 11

1 — Power Switch
2 — Power Indicators (Ch1, Ch2)
3 — Signal Indicators (Ch1, Ch2)
4 — Clip Indicators (Ch1, Ch2)
5 — Protect Indicators (Ch1, Ch2)
6 — Accessory Slot Cover
7 — Gain Control
8 — Detachable Input Header
9 — Cooling Fan

10 — Shrouded Output Connector

(Direct Outputs)

11 — Shrouded Output Connector

(Isolated Outputs)

12 — Mounting Holes for

Optional Handles
13 — Rear Support Ear
14 — Parallel/Stereo/Bridge

Switch

Front
features of
CX6, CX6T,
CX12 or
CX12T
amplifier

Rear
features of
CX6 or CX12
amplifier

Rear features
of CX6T or
CX12T
amplifier

13

6

7 78

6 11

13

7 78 14

2 3 4 5

112

109

14

109

10 11

5

III. Inputs

CX amplifiers feature balanced inputs, connected via a Euro-style 7-terminal
detachable header. Attach input signal wires as shown. If you are connecting an

Loosen screw,

insert wire here,

then tighten screw.

unbalanced signal to an input, use the non-inverting (+) input and the ground
terminals of the header, and also connect a wire jumper between the inverting (-)
input and the ground terminal. The wire jumper will prevent a reduction in gain
caused by a floating input.

If you need XLR or 1/4” RTS inputs on your CX6, CX6T, CX12, CX12T, use the optional MX-1
input expander (available through QSC’s Technical Services Department).

Typical of QSC amplifiers, the input impedance of a CX Series amplifier is 20 kilohms balanced
and 10 kilohms unbalanced.

Each CX Series (except CX4, CX4T) amplifier features a Level I Open Input Architecture slot.
See the Operation Chapter for more information about Open Input Architecture.

INPUT SENSITIVITIES

Audio signals of these levels will produce full rated output power at 8 ohms.

CX4, CX4T 0.98 volt (+2.0 dBu)
CX6, CX6T 1.02 volt (+2.4 dBu)
CX12, CX12T 1.00 volt (+2.2 dBu)

Wire jumper

between

input and

Unbalanced input
connections

Ground

Channel 2+

Channel 2-

ground

Shield

Ground

Ground

Channel 1-

Channel 1+

Balanced input

connections

Signal conductor

IV . Outputs

Covered barrier strips located on the rear panel allow speaker cable connections to the amplifier
outputs. See the diagrams for details on connecting speakers and/or distributed (25-, 70-, or 100­volt) lines. For attaching wires, we recommend using insulated connectors of the type shown. Always
make sure the amplifier is turned off before you change any output connections or lift the safety
shroud.

If you are operating the amplifier in bridged mono mode to achieve higher output voltages, neither
speaker wire is grounded. Therefore, use the same precautions in handling and dressing the wire
that you would for the normal “hot” outputs. See the section on bridged mono operation for
additional important instructions.

If you are driving a distributed line, make sure the sum of all the power demands of the speakers
does not exceed the amplifier’s power rating. In fact, it is good practice to allow about a 25% safety
margin. For example, if the load on the distributed line is 400 watts, use an amplifier with a power
rating of 500 watts. See the Application Notes in the back of this manual for more details on distributed lines.

6

Lift safety shroud.
Loosen screw

terminals.
Insert wire connectors

under screws.
Tighten screw

terminals.
Lower safety shroud.

If your application calls for connecting an 8-ohm speaker and a distributed line to the same amplifier channel,
the CX4T, CX6T and CX12T can do that. However, since most of the audio power is drawn by the direct-connected
speaker, you must derate the distributed line; the distributed line should have a total power load of no more than
one-fourth the amplifier’s normal distributed line power rating. Make sure the direct-connected speaker can
handle the 8-ohm power rating of the amplifier. See Applications Notes section for more details.

Output connections for CX, parallel or stereo mode.Output connections for CX, bridged mono mode.

Output connections for CXT, bridged mono mode.

Output connections for CXT,

25 volt line.

Output connections for CXT,
bridged mono mode, 140 volt

line.

Output connections for

CXT, 70 volt line.

Output connections for CXT, bridged

Output connections for CXT, direct low

impedance

Output connections for

CXT, 100 volt line.

mono mode, 200 volt line.

7

V . Controls & Displays

CX4 AND CX4T

The channel attenuator controls, labeled in dB of attenuation , are located on the rear panel. They have 11
detents, adjustable from 0 dB (full gain) to × (full attenuation). The LED displays for each channel function
as shown at right.

CX6, CX6T, CX12, AND CX12T

The channel attenuator controls, labeled in dB of attenuation , are located on the rear panel. They have 11
detents, adjustable from 0 dB (full gain) to × (full attenuation). The LED displays for each channel function
as shown at right.

VI. Operation

AC POWER

The amplifier is available for 100, 120, or 220-240 VAC, 50 or 60 Hz operation. Unless it is specially ordered
otherwise, each amplifier is configured for the line voltage in the market it is sold in. For example, those sold
in Japan are 100 VAC; for North and Central America, 120 VAC; for Europe and most of Asia, 220-240 VAC.

LED Display for CX4 and
CX4T:

LED Display for CX6, CX6T,
CX12, and CX12T:

The power cord has a plug for connecting to a standard AC source. For 120-volt operation, the plug is a standard
15-ampere grounded NEMA fitting; it meets safety agency requirements for current consumption of less than
12 amperes during “normal” operation, but peak current draw under heavy usage can be higher.

Models built for 220 to 240-volt use have a standard Schuko (continental Europe) connector on the power cable.

OPERATION

CX4 and CX4T

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 come on, and after three seconds, the muting will stop and sound
can be heard). The amp should now be working.

Gain should be kept in the upper half of its range for full performance. “POWER” LED indicators are provided
to monitor the operation of each channel. Each channel has a red “CLIP” indicator that will show any
distortion in the amplifier. The mute circuit should cut the sound off as soon as you turn off the amp, and
mute for three seconds before restoring power to the speakers. This blocks turn-on and turn-off thumps.

8

CX6, CX6T, CX12 and CX12T

When the amplifier is first turned on, the red “PROTECT” LED on each channel will light for about two or
three seconds, during which the output relays will stay open to mute the speakers. After the turn-on muting
interval, the “PROTECT” LEDs turn off, the green “POWER” LEDs light, and the output relays close to enable
the speaker outputs. Even during the muting interval, the yellow “SIGNAL” and red “CLIP” LEDs operate
normally if there is a signal present. If the “CLIP” LED is on while the amplifier is muted, cut the gain back
immediately to avoid a full-power blast of sound when the output relays close. If a channel stays muted
with “PROTECT” lit, or if its “SIGNAL” or “CLIP” indicators light up when the gain is turned all the way down,
it may be defective; see the troubleshooting segment for more information.

The “SIGNAL” LED indicates signal levels that are -30 dB (referenced to full rated output power) or higher.
Whe the amplifier is shut off, the amplifier should mute both channels virtually instantaneously, with the

“PROTECT” indicators lit until the power supplies are discharged.

TROUBLESHOOTING

Problem: Channel will not come out of muting

• If reducing the gain control to × attenuation does not release muting, the channel is defective or
overheated (see “Overheating,” below).

• If reducing the gain releases the muting, raise the gain back up slowly while you watch the “SIGNAL”
and “CLIP” indicators; the problem may be an abnormal signal (with excessive ultrasonic energy, for
example) that could otherwise damage your speakers.

PROBLEM: No sound (CX6/CX12)

• Is the channel muted? (If the “PROTECT” indicator is lit, the channel is muted; see below.)

• Is the “SIGNAL” LED lit or flashing? (If so, the speaker is open or blown, there is an open circuit in the

speaker wiring, or there is an open circuit in the internal output wiring of the amplifier.)

• If the “SIGNAL” indicator is dark, there is probably not enough signal, or even none at all. Try turning
up the rear panel attenuators or boosting the signal level at the input.

• If the “SIGNAL” indicator shows little or no activity but the “CLIP” LED is lit or flashing, there is probably
a short circuit in the speaker wiring, especially if the “PROTECT” indicator starts flashing. It is also
possible, but less likely, that the channel’s output relay is defective and will not open, thereby short­circuiting the channel output and producing the same symptoms.

PROBLEM: Channel goes into muting, with “PROTECT” LED on (CX6/CX12)

• If the fan is running full speed, the channel probably suffers from severe overheating. Unless there
is a blockage in the flow of cooling air, the channel should return to normal within a minute or so (see
“Overheating,” below).

9

• If the fan is not running at full speed, or the channel does not reset to normal after a cool-down period,
the muting is probably because of a DC fault or other amplifier failure, especially if the “SIGNAL” or
“CLIP” indicators are lit even with the attenuator turned all the way down.

PROBLEM: Overheating

• The thermal management system on a CX Series amplifier features a two-speed fan that
modulates the cooling air flow over the heatsinks in response to the cooling needs.
If the air flow is blocked, however, or if the amplifier is overdriven into very low
impedance loads, the amplifier could overheat even though the fan is running at full
speed.

CX4 & CX4T

• At 85½ C both channels will mute. The “POWER” LED will remain lit, although the “CLIP” indicator will
be dark. The amplifier will remain this way until the temperature drops to a safe level.

CX6, CX6T, CX12 & CX12T

• At 85½ C, the channel’s output relay will mute the output. The channel will remain muted until the
temperature drops to a safe level. Even while the channel is muted, the “SIGNAL” and “CLIP” indicators
will function normally. If the “CLIP” indicator is flashing or continuously lit, reduce the gain to hasten
the cool-down and prevent repeated thermal shutdowns.

Tightening input panel
mounting screws

PROBLEM: Hum in the audio

• Because of its grounded-collector output transistor configuration, which maximizes thermal efficiency,
the signal ground on a CX amplifier cannot be lifted. The amplifier’s balanced inputs are meant to reject
hum, but if hum remains a problem, check the tightness of the two mounting screws on the input panel
(except CX4 and CX4T); if they are loose, the panel itself might not be well grounded to the chassis.
If you have any problems with these screws, contact the QSC Technical Services Department. In some
cases, such as when the audio signal cables are routed near lighting dimmers that use triacs, you may
need to use input isolation transformers because of the extremely high noise field produced by the
dimmer circuitry. See the paragraphs on Open Input Architecture for further information on input
transformers.

OPEN INPUT ARCHITECTURE™ LEVEL I (CX6, CX6T, CX12 and CX12T only)

On the rear of a CX amplifier are an upper and a lower input panel; both are easily removeable for future upgrades.
This is the concept of Open Input Architecture, and the opening in which the panels mount is called the “slot.”
As installed at the factory, the lower is the CX standard input panel and the upper one is a blank. A ribbon cable
connects the “slot” panels to the rest of the amplifier. It carries input signals, speaker output monitor signals,
muting status, and clip activity; these are for supporting future accessories and remote control systems. Although
the physical dimensions of the CX, MXa, and EX panels are the same, their internal cabling is different and the
CX platform supports most (but not all) of the MXa functions but doesn’t support all of the EX functions. The
CX and MXa Open Input Architecture slot thus is designated as “Level I” and the EX as “Level II.”

10

The standard input panel installed on the CX Series amplifier has balanced Euro-style detachable header inputs,
gain controls, and the Parallel-Stereo-Bridge switch. The panel’s circuit board has solder footprints for passive
roll-off circuit components, input isolation transformers, and other special customizations.

The MX-1 Input Expander panel may be installed in the upper panel position. It features XLR inputs and 1/4" RTS
inputs. Installation instructions are included with the expansion kit.

Other available accessories include a universal filter and a stereo power limiter; more are in development. Contact
QSC for more details before using accessories with CX amplifiers. See the CX Application Notes also.

PARALLEL, STEREO, OR BRIDGED OPERATION (CX6, CX6T, CX12 and CX12T only)

The Parallel-Stereo-Bridge switch is located on the input panel. Always turn off the amplifier before
changing the position of this switch.

The most commonly used operating mode for a power amplifier is “Stereo,” in which the two channels
are independent and separate all the way from input to output. Set the switch in the center position
for stereo operation.

“Parallel” ties the two channel inputs together so that both will be driven by the same signal, without
the need for external jumpers or wiring. After the inputs both channels operate independently; though
they carry the same signal, their gain controls affect only their respective channels, and they must use
separate speakers. Never parallel the speaker outputs!

The “Bridge” position uses both channels to provide about three times the power to a single speaker
load that a single channel does. In this position, the switch feeds channel 2 with an inverted signal from
channel 1. Thus, when one channel “pushes,” the other “pulls,” providing twice the voltage swing of
a single channel.

11

BRIDGED-MONO MODE
CAUTION:

ALL MODELS—Output
voltages as high as 160
volts rms are available be­tween the CX amplifier’s
bridged terminals (200
volts at isolated outputs).

CX4T, CX6T and CX12T
(140V and 200V opera­tion)—Fully insulated
CLASS ONE wiring must
be used to connect the
amplifier to the loud­speakers

MODE BRIDGÉ MONO:
ATTENTION

TOUT LES MODÈLES—
Des tensions de sorties
aussi élevée que 160
volts rms sont
disponible aux bornes
du amplificateur CX en
mode bridgé mono (200
volts aux sorties
isolés).

CX4T, CX6T ET CX12T
(opération 140V et
200V)—On doit utiliser
du câblage entièrement
isolé de CLASSE 1 pour
relier l’amplificateur
aux haut-parleurs.

Instructions for bridged operation

VORSICHT BEI MONO­BRÜCKENBETRIEB:

ALLE TYPEN—Zwischen
den Ausgangsklemmen
der CX-Verstärker
können Ausgangs­spannungen bis 160 Volt
RMS anliegen (200 Volt
bei isolierten
Ausgänge).

CX4T, CX6T UND CX12T
(140V- u. 200V­Betrieb)—Der Anschluß
muß vom Verstärker bis
zum Lautsprecher mit
vollisoliertem Kabel
erfolgen.

PRECAUCIÓN PARA EL
MODO MONO PUENTE

TODO LOS MODELOS—
Voltajes de salida de
hasta 160 VRMS existen
entre los terminales de
puente del amplificador
CX (200 voltios a las
salidas aisladas).

CX4T, CX6T Y CX12T
(operación 140V y
200V)—Utilice
cableado de CLASE
UNO totalmente aislado
para conectar las
altoparlantes.

CX4 and CX4T

1. To engage the bridged mono mode, jumper the barrier strip inputs on the rear, in accordance with the
diagram printed under the barrier strip inputs. Connect the amplifier input to channel 1 only, with the
jumpers as shown to channel 2. Channel 1 and channel 2 gains must be matched. 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.”

CX6, CX6T, CX12 and CX12T

1. Set the switch to the “Bridge” position.

2. Connect the signal to channel 1’s input only. Do not connect an input signal to channel 2. However,
as a convenience for routing the same signal to multiple amplifiers without using Y connectors, the
mode switch connects the channel 1 and channel 2 inputs in parallel. Therefore, you can use channel
2’s input terminals to pass the audio signal on to other amplifiers.

3. Use only channel 1’s gain control to set the level. Both channels’ “SIGNAL” and “CLIP” indicators should
flash identically when the amplifier is operating.

4. Connect the single speaker load across the + amplifier outputs as shown in the Outputs chapter. The
positive speaker wire goes to channel 1, and the negative to channel 2.

12

MAXIMUM LONG-TERM OUTPUT POWER

In a properly designed sound system, you can obtain the desired sound level without using the full output power
of the amplifier; if the “CLIP” indicators flash once in a while, that is perfectly normal.

If the amplifier is operated at extremely high power levels, it may overheat or may even damage the speakers.
The amount of stress on the amplifier depends largely on the load and how hard the amplifier drives it. In general,
the lower the load impedance, the more stressful the load is.

8-Ohm Loads and Lightly Loaded 25V, 70V, or 100V Lines

The amplifier can operate at practically any power level without risk of overheating. However, if it is pushed hard
enough to continually light the “CLIP” indicator, the amplifier’s average output power can reach 110 to 475 watts
(depending on the model), which is more than many speakers can handle.

4-Ohm Loads and Heavily Loaded 25V, 70V, or 100V Lines

If the “CLIP” indicator flashes occasionally, the amplifier is approaching its maximum long-term power capacity.
If it is lit about half the time, the amplifier channel will probably go into thermal protection within a few minutes.

2-Ohm Loads

Except for an occasional flash, keep the “CLIP” indicator dark to avoid overheating the amplifier channel.

AC CURRENT CONSUMPTION

A major objective in the design of the CX Series amplifiers-even the higher-powered models-is to permit their
operation from readily available, standard AC power sources.

“Normal conditions” in power amplifier rating means operating with a random program source (pink noise), using
pink noise as a source, at an average power level equal to one-eighth of maximum power. This is recognized by
most of the world’s safety agencies as the loudest you can play music through an amplifier and still keep the
incidence of clipping to a reasonable minimum. An amplifier’s peak current draw at full output power into 2 ohms
is several times what the “normal” draw is, but its various protection circuits will prevent this condition from
lasting more than a minute or two.

When you plan the AC power hookups for your amplifiers, use the table in the Specifications section to predict
the current requirements per amplifier. You can use the one-eighth power figures to predict the normal
continuous current draw, then add a safety margin to allow for occasional crescendos.

13

HEAT EMISSIONS

Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts
it again into an analog of the input signal to send out to the loudspeakers. Any AC power that enters the amplifier
through the power cord and does not exit through the speaker outputs turns into heat, which the amplifier must
rid itself of by exhausting it to the outside. In indoor use this may present a sizeable challenge to a building’s
air conditioning system. Use the table below to predict the heat that will be emitted by your amplifier.

Full Power 1/3 Power 1/8 Power Idle

Model Channel Load BTU/hr kcal/hr BTU/hr kcal/hr BTU/hr kcal/hr BTU/hr kcal/hr

ΩΩ

CX4, CX4T 8

CX6, CX6T 8

ΩΩ

Ω + 8

Ω 645 163 615 155 510 128 85 22

ΩΩ

ΩΩ

ΩΩ

ΩΩ

4

Ω + 4

Ω 1265 318 1060 267 830 210 85 22

ΩΩ

ΩΩ

ΩΩ

ΩΩ

2

Ω + 2

Ω 2340 590 1700 429 1295 326 85 22

ΩΩ

ΩΩ

70V + 70V 1365 344 1050 264 800 202 85 22

ΩΩ

ΩΩ

Ω + 8

Ω 1406 354 1020 257 604 152 229 58

ΩΩ

ΩΩ

ΩΩ

ΩΩ

4

Ω + 4

Ω 2525 637 1617 408 921 232 229 58

ΩΩ

ΩΩ

2

70V + 70V 2010 507 1503 379 1141 288 229 58

CX12, CX12T 8

4
2

70V + 70V 3772 951 2338 589 1280 323 297 75

Heat emissions of CX
amplifiers under various
conditions

ΩΩ

ΩΩ

Ω + 2

Ω 4658 1174 2532 638 1290 325 229 58

ΩΩ

ΩΩ

ΩΩ

ΩΩ

Ω + 8

Ω 1791 452 1239 312 805 203 297 75

ΩΩ

ΩΩ

ΩΩ

ΩΩ

Ω + 4

Ω 3330 840 2081 525 1222 308 297 75

ΩΩ

ΩΩ

ΩΩ

ΩΩ

Ω + 2

Ω 5698 1437 3146 793 1611 406 297 75

ΩΩ

ΩΩ

14

PROTECTION CIRCUITS

The design goal in high-efficiency amplifiers such as the CX Series is to increase the amplifier’s ability to deliver
peak power without increasing its size, weight, and cost. However, the higher power flow through the output
components makes effective, responsive protection circuitry absolutely vital. To this end, the design of the CX
series takes a comprehensive approach to protection.

As in all QSC amplifiers, the inputs are resistively buffered for overload and RF protection. Chassis bypass
capacitors at inputs and outputs further improve RF rejection.

CX amplifiers use the proven Ouput Averaging™ short circuit protection system. This circuit permits full output
current even into resistive or reactive 2-ohm loads, but reduces the current safely by about 75% if the output
is shorted.

Turn-on/turn-off muting keeps transients-both from the amplifier itself and from upstream equipment-from
reaching the speakers when the amplifier is turned on or off. The turn-on delay is approximately three seconds
to allow the power supplies and circuitry to stabilize. Turn-off muting occurs almost immediately after power
is shut off. Muting occurs whether power is turned on and off using the front panel power switch or externally
at the AC source.

Inside a CX6, CX6T, CX12, or CX12T amplifier, an NTC (negative temperature coefficient) thermistor in series
with the power switch and transformer primary limits inrush current. The thermistor initially has a high resistance,
which then diminishes rapidly as it warms, to avoid power loss. Typically, the inrush current of an CX Series
amplifier thus is equal to that of another amplifier of about 1/3 to 1/2 its power rating. The low inrush of the
smaller CX4 and CX4T does not require an NTC thermistor.

When the DC fault protection circuitry of a CX6, CX6T, CX12, or CX12T senses a DC voltage on a channel output,
it actuates a relay which shorts the output and load to ground. The amplifier’s Output Averaging protection
circuitry will limit current to further protect the output devices. If the DC problem persists, that channel will go
into thermal protection mode and should be serviced by a qualified technician.

The output stage of a CX4 or CX4T channel has AC coupling to prevent it from passing DC.
Temperature sensors on the channel heatsinks and the power transformers are part of the thermal management

circuitry. The temperature sensors govern whether the dual-speed fan should run at low or high speed. Below
55½C, the fan runs at low speed; above, it runs at high speed. At or above 85½ C, the channel mutes to go into
thermal protection.

Built-in second-order 50 Hz (CX4T) and third-order 45 Hz (CX6T and CX12T) Butterworth high-pass filtering helps
prevent saturation in speaker transformers on distributed lines by sharply reducing the amount of ultra-low
frequency energy the amplifier puts out. However, some lower-quality speaker transformers have rated low­frequency responses that do not even extend as low as these filters; if you have any of these in your distributed
line system you should insert corresponding high-pass filtering in the audio path before the amplifier.

15

VII. Specifications

POWER OUTPUT

Direct output, watts per channel,
both channels driven

8Ω, 20 Hz–20 kHz, 0.1% THD 150 200 400
8Ω, 45 Hz–20 kHz, 0.1% THD** 150 200 400
8Ω, 1 kHz, 1% THD 170 170 220 220 475 475
4Ω, 20 Hz–20 kHz, 0.1% THD 225 300 600
4Ω, 45 Hz–20 kHz, 0.1% THD** 225 300 600
4Ω, 1 kHz, 1% THD 270 270 350 350 700 700
2Ω, 1 kHz, 1% THD* 350 350 450 450 900 900

Isolated outputs, watts per channel,
both channels driven

70V or 100V, 45 Hz–16 kHz, 0.25% THD** 175 2 50 500
25V, 45 Hz–16 kHz, 0.25% THD** 150 200 400

Direct Outputs, bridged mono

8Ω 20 Hz–20 kHz, 0.1% TH D 450 600 1200
8Ω, 45 Hz–20 kHz, 0.1% THD** 450 600 1200
4Ω, 1 kHz, 1% THD* 700 700 900 900 1800 1800

CX4 CX4T CX6 CX6T CX12 CX12T

Isolated outputs, bridged mono

50V, 45 Hz–16 kHz, 0.25% THD** 300 400 800
140V, 45 Hz–16 kHz, 0.25% THD** 350 500 1000
200V, 45 Hz–16 kHz, 0.25% THD** 350 500 1000

*typical

DISTORTION

SMPTE-IM, less than 0.05%

FREQUENCY RESPONSE**

CX4, CX6, CX12: 20 Hz–20 kHz, ± 0.2 dB
CX4T direct outputs: -3 dB @ 50 Hz, -0.2 dB @ 20 kHz
CX4T isolated outputs: 50 Hz–16 kHz, +0, -3 dB
CX6T, CX12T direct outputs: -3 dB @ 45 Hz, -0.2 dB @ 20 kHz
CX6T, CX12T isolated outputs: 45 Hz–16 kHz, +0, -3 dB

**Low frequency response on “T” versions is limited by 18 dB/octave 45-Hz high-pass filter. Low frequency response
on CX4T versions is limited by 12 dB/octave 50-Hz high-pass filter.

DAMPING FACTOR

200 (@ direct outputs)

OUTPUT REGULATION

100V 1.0 dB 1.0 dB 1.1 dB
70V 1.1 dB 1.1 dB 1.2 dB
25V 2.0 dB 2.0 dB 2.0 dB

16

NOISE

100 dB below rated output (20 Hz to 20 kHz)

VII. Specifications (cont.)

CX4 CX4T C X6 CX6T CX12 CX12T

VOLTAGE GAIN

@ direct outputs: 35× (31 dB) 35× (31 dB) 40× (32 dB) 40× (32 dB) 56.5× (35 dB) 56.5× (35 dB)

INPUT SENSITIVITY, VRMS

for rated power, 8 ohms 0.98 0.98 1.03 1.03 1.043 1.043

INPUT IMPEDANCE

10K unbalanced, 20K balanced

CONTROLS

Front: AC Switch
Rear: Parallel/Stereo/Bridge Switch (except CX4, CX4T)
Ch.1 and Ch. 2 Attenuator Knobs (11 detents: 0, -2, -4, -6, -8, -10, -12, -14, -18, -24, off)

FRONT PANEL/INDICATORS (per channel)

PROTECT: Red LED (except CX4, CX4T)
CLIP: Red LED
SIGNAL: Yellow LED (except CX4, CX4T)
POWER: Green LED

REAR PANEL/CONNECTORS (each channel)

Input: Euro-style detachable header
Output: Covered barrier strips

COOLING

2-speed fan, with back-to-front air flow

AMPLIFIER PROTECTION

Output Averaging™ short circuit protection, open circuit, ultrasonic, RF, thermal muting.
Stable into reactive or mismatched loads.

LOAD PROTECTION

Turn-on/turn-off muting, DC-fault load grounding relay with internal fault fuses.

OUTPUT CIRCUIT TYPE

CX4, CX4T, CX6, CX6T: Class AB complementary linear stage
CX12, CX12T: Class H complementary linear stage, with 2-step high-efficiency circuit

POWER REQUIREMENTS:

100, 120, 220-240 VAC, 50/60 Hz

17

VII. Specifications (cont.)

POWER CONSUMPTION

Normal operation: 1/8 power @ 4Ω per channel
Worst case continuous program: 1/3 power @ 2Ω per channel
Maximum: full power @ 2Ω per channel
Multiply current times 0.5 for 220-240 VAC operation

Model Channel Load AC Current, Full Power AC Current, 1/3 Power AC Current, 1/8 Power AC Current, Idle
CX4, CX4T 8Ω + 8Ω 5.7 A 3.4 A 2.3 A 0.4 A

4Ω + 4Ω 9.2 A 5.2 A 3.5 A 0.4 A
2Ω + 2Ω 13.3 A 7.4 A 4.9 A 0.4 A

25V, 70V, 100V 8.7 A 5.0 A 3.3 A 0.4 A

CX6, CX6T 8Ω + 8Ω 7.7 A 4.9 A 3.3 A 0.6 A

4Ω + 4Ω 12.5 A 7.4 A 4.8 A 0.6 A
2Ω + 2Ω 18.8 A 11.0 A 7.2 A 0.6 A

25V, 70V, 100V 11.2 A 6.7 A 4.5 A 0.6 A

CX12, CX12T 8Ω + 8Ω 14.7 A 8.4 A 4.5 A 0.8 A

4Ω + 4Ω 23 A 12.4 A 6.1 A 0.8 A
2Ω + 2Ω 33 A 16.5 A 8.0 A 0.8 A

25V, 70V, 100V 21 A 10.9 A 5.7 A 0.8 A

(see chart on page 13)

DIMENSIONS

Faceplate Width Standard 19" (48.3 cm) Rack Mounting
Chassis Depth 17.9" (45.5 cm) deep (to rear support ears)
Faceplate Height CX4 & CX4T: 3.5" (13.3 cm)

CX6 & CX6T; CX12 & CX12T: 5.25" (13.3 cm)

WEIGHT CX4 CX4T CX6 CX6T CX12 CX12T

Shipping 33 lb; 16.4 kg 42 lb; 21 kg 49 lb; 24 kg 62 lb; 28 kg 58 lb; 26.3 kg 75 lb; 34 kg
Net 27 lb; 13.6 kg 37 lb; 18.2 kg 42 lb; 20 kg 55 lb; 25 kg 50 lb; 22.7 kg 67 lb; 30.4 kg

18

Part 2—CX Series Application Guide

8

COM

4

8

16

COM

10W

5W

2.5W

1.2W

70V

4.5V

ith the helpful advice and input from contractors and consultants around the world, engineers at

QSC designed the CX Series amplifiers to be a versatile and reliable foundation for

W

This applications guide will help you design your sound system properly and utilize your CX amplifier(s) effectively.

VIII. Distributed lines

The best way to distribute audio power from a common amplifier among numerous relatively low-powered
loudspeakers is to use a distributed line, whether all the loudspeakers are meant to receive the same or different
power levels.

A distributed line is usually referred to by its maximum voltage­25-, 70-, 100-, or 140-volt lines are most common. An amplifier’s
power rating is determined by how much current it can put out
at a standard voltage level; a low-power 70-volt amp has less
current capability than a high-powered 70-volt amp, even though
they can both put out the same voltage. The high-powered amp
can drive more speakers and at higher power levels than the
lower-powered one can. This is somewhat analogous to AC
electrical distribution, where you might have circuits rated for
15A, 20A, 30A, etc., at 120 or 240 VAC, and you can connect any
number of appliances to them as long as you don’t exceed the circuit’s current capacity.

high-quality installed sound systems.

Example of a loadspeaker connected to a
distributed line through a transformer

Example of a 70 volt
distributed line

Amplifier

Loudspeakers on a distributed line use tapped
transformers to match their power requirements
to the line. A loudspeaker with a 70-volt trans­former tapped at 2 watts, for example, will
receive that amount of power when the amplifier
puts out 70 volts on the distributed line. Trans­former taps are usually set at 3 dB intervals, i.e.,

B

each successive tap is twice the power level of its
predecessor. An example would be a transformer
tapped at 0.5, 1, 2, and 4 watts. This allows you
to select a power level that is most appropriate
for the situation while still retaining the versatil­ity of configuring other loudspeakers on the same

70V

0

1.2W

2.5W
5W

10W

COM

0.5W
1W
2W

SPKR

COM

16
8
4
COM

1.2W

2.5W
5W

10W

COM

A

8

16
8
4
COM

C

88

line for different power levels.

19

IX. Designing the Distributed Sound System

There are several main steps in designing a distributed sound system:

• Determining loudspeaker coverage and placement

• Determining power levels for each loudspeaker

• Choosing the right amplifier

LOUDSPEAKER COVERAGE AND PLACEMENT

The goal in placing the loudspeakers in a distributed system is to acoustically cover the required area effectively
but economically. An effective coverage would be one where the sound from the system’s loudspeakers is not
only audible, but intelligible, over all of the area. An economical coverage would be one that achieves the goal
with the fewest loudspeakers necessary.

A loudspeaker in a partially or fully enclosed area produces two sound fields: the direct field, which is sound coming
directly “line-of-sight” from the loudspeaker; and the diffuse field, which is sound from the loudspeaker which
has reflected off of surfaces in the area, such as floors, walls, tables, ceilings, etc. The direct sound field decreases
with distance from the loudspeaker; every time you double the distance, the sound pressure level drops 6 dB.
This is called the Inverse Square Law because the result of a 6 dB sound pressure level decrease is the same
as 1/4 of the sound intensity (i.e., the sound power per square centimeter), which varies inversely with the square
of the distance.

Even though it too is subject to the Inverse Square Law, the intensity of the diffuse field is more difficult to predict,
especially with multiple acoustically reflective surfaces. The problem
is that moving away from one surface may also be moving closer
to another. Controlling the diffuse field often involves

1watt in

sensitivity = 92 dB, 1 watt@1m

architectural planning as well as acoustical treatment
with absorptive surface materials.

1 meter

To maintain intelligibility, maximize the
ratio of direct field to diffuse field;

92 dB SPL

2 meters

this generally involves placing
the loudspeakers as
close to the listeners

86 dB SPL

as is both physically
and economically prac­tical.

Sometimes, a distributed line

82.5 dB SPL

system can help a coverage
area with difficult acoustical char­acteristics. For example, a very re-

80 dB SPL

verberant room with all large hard,

3 meters

4 meters

20

reflective surfaces—wood floors and ceiling, concrete and plaster walls, etc., the type of room that tends to
make speech unintelligible—might benefit from using many low-powered loudspeakers placed throughout the
room and close to the listeners, instead of using a couple high-powered, centrally-located loudspeakers.

With ceiling-mounted loudspeakers, one general rule of thumb is to make the center-to-center distance between
them no greater than twice the floor-to-ceiling distance. Used with loudspeakers that have a 90° angle of
coverage, this is often suitable for background music systems, but for paging and public address applications
it leaves some erratic coverage at listening positions higher than floor level.

A better technique is to space the ceiling speakers at twice the distance from the listeners’ ears to the ceiling.
This requires more loudspeakers spaced closer together than the previous scheme does, but provides good,
uniform coverage at realistic listening positions. In a room where people are standing, you’ll need closer spacing
than if people tend to be sitting. For example, if a lunchroom has a ceiling height of 2.9 meters (9.5 feet), and
the height of an average listener’s ear, when seated, is about 1.1 meter (3.5 feet) above the floor (and therefore
about 1.8 meter or 6 feet from the ceiling), then you should space the loudspeakers no more than 3.6 meters,
or about 12 feet, apart.

Even better, more uniform coverage will result from spacing the loudspeakers at 1.5 times the ceiling-to-ear
distance. In the above example, this would require spacing the loudspeakers about 2.7 meters (9 feet) apart.

21

DETERMINING POWER LEVELS

After you’ve determined where to place the loudspeakers, you need to calculate the power each one requires.
If the installation is in an existing facility already in use, use an SPL meter, set for slow response, to measure
the A weighted ambient noise at the listener’s ear position. Try

0.5

20.0

18.0

16.0

14.0

12.0

10.0

8.0

6.0

4.0

2.0

0.0

-2.0

-4.0

-6.0

-8.0

-10.0

dB(ref1watt)

to take this measurement at the noisiest time—in a factory,
when the machines are running; in a restaurant, when it’s full
of patrons, etc. Background music will require an SPL at least
10 dB above the ambient noise. For good paging intelligibility,
you’ll need an SPL approximately 15 dB higher than the
ambient noise; 25 dB above ambient will yield excellent
intelligibility.

Next, use the distance from the loudspeaker to the listener’s
ears, along with the loudspeaker’s sensitivity rating (typically
expressed as “n dB @ 1 watt, 1 meter,” which means n dB of
SPL with 1 watt input, measured at a distance of 1 meter) to
determine how much power the loudspeaker needs to get from
the distributed line. Use the inverse square law guide below
to convert distance to dB; you’ll need to add this figure to the
desired SPL and then subtract the sensitivity rating to deter­mine how much more or less than 1 watt the loudspeaker
requires.

0.1 10.2 10 1005 20 502

Power(Watts)

Decibels
referenced to
1 watt

22

Distance from speaker

Power increase in dB; referenced to loudspeaker

sensitivity rating (1 watt @ 1 meter)

EXAMPLE

A loudspeaker (sensitivity: 94 dB @ 1W, 1 meter) in a busy office covers an area with an ambient noise
of 72 dBA, measured at a seated person’s ear position at the desks. The client wants superb
intelligibility, so your goal is to provide an SPL of 97 dB (72 plus 25) to the intended listeners. The ceiling­mounted speaker is about 6 feet, or 1.8 meters, above the listeners’ ears. The dB figure from the inverse
square law conversion guide for that distance is 5.2 dB; this means that the loudspeaker would have
to produce 102.2 dB (97 plus 5.2) at a distance of 1 meter to achieve 97 dB at a distance of 1.8 meters.

The loudspeaker will require a power level 8.2 dB (102.2 minus 94) greater than 1 watt; this works out
to 6.6 watts. The speaker transformer taps are 0.5, 1, 2, 4, and 8 watts; the closest choice would be
the 8-watt tap, which turns out to be only 0.8 dB higher than the design ideal.

SELECTING THE AMPLIFIER

After you’ve determined the power taps for each loudspeaker transformer, add them up. The sum will be what
you determine your amplifier requirements from. If you have 16 loudspeakers tapped at 2 watts, seven at 1 watt,
and eight at 10 watts, the total audio power the loudspeakers want is 119 watts.

Thanks to a transformer phenomenon called insertion loss, though, your amplifier needs to provide more power
than the loudspeakers will draw. High-quality speaker transformers typically have an insertion loss of about 1 dB,
meaning that it takes about 1.25 watt going into the transformer to put 1 watt into the loudspeaker. A lower­quality transformer may have a loss of 2 dB, which requires approximately 1.6 watts for every watt that the
loudspeaker receives. Poor-quality transformers may have even higher losses, but they will probably degrade
the system’s audio performance severely even if there is adequate amplifier power to overcome the losses.

To compensate for the insertion loss, add a corresponding percentage to the sum of the transformer power taps.
For transformers with a 1 dB loss, add about 25%; in the example above, that would increase 119 watts up to
149 watts.

Next, it is a good engineering practice to add another 25% to the figure. This allows a margin both for dynamic
audio headroom and for some future adjustments to the system—an added speaker or two, a few transformer
tap changes, etc. That would bring the total power requirement for the example system up to 186 watts. This
would be the

minimum

power rating for the amplifier you choose.

INSTALLATION TIP:
Once the sound system is installed and operational, turn the amplifier off and take an impedance

measurement across the distributed line at the amplifier output, using an audio impedance meter
(

not

an ohmmeter). Record the measurement for later use. If you ever have to make a service call
on the system, measure the impedance again and see how it compares to the recorded figure; it’s
a quick and easy way to see if anything in the distributed line system has been changed. Likewise,
measure the impedance anytime you’ve changed a transformer tap, added or removed a loud­speaker, or made any other adjustment to anything on the distributed line.

23

USING COMPONENTS WITH DIFFERENT VOLTAGES

Sometimes it may be practical to use a transformer or loudspeaker/transformer combination with a different
voltage system from what it was originally intended. For example, a 70-volt transformer could be used in a 25­volt system, although you would have to derate the power taps similarly. Never use a transformer with a higher
voltage than what it is designed for, though; i.e., you couldn’t use that same transformer on a 100- or 140-volt
line.

To properly derate the transformer tap power levels, locate the column with the voltage the transformer is rated
for in the table below, then locate the row with the system voltage you want to use it in. The table will indicate
the dB reduction to use with the derating chart below it.

24

EXAMPLE

You have a 70-volt transformer with taps labeled 0.6, 1.2, 2.5, and 5 watts. What power points will
those taps provide on a 50-volt line?

The table shows that 50 volts is 3 dB less than 70 volts. The chart confirms that the taps will provide

0.3, 0.6, 1.25, and 2.5 watts, respectively.

X. How Many Speakers?

How many loudspeakers can you connect to the distributed line? The amplifier itself determines the amount of
audio power available to the line, and from that figure you must compensate for insertion loss in the speaker

transformers and for headroom when you determine how much power is available

1.56 watts

for the speakers.

…+25% for
headroom.

…approx. 1/4 watt

more to compensate

for transformer

insertion loss*

For every watt put
into the speakers,

you need…

*assuming a typical insertion loss of 1dB

†a good engineering practice

†

1.25 watts

1 watt

For example, let’s suppose you have an amplifier rated at 500 watts into a 70-volt
line, and your speaker transformers each have an insertion loss of 1 dB, which is
typical. The losses in the transformers mean that they will draw 25% more audio
power than they deliver to the speakers. For headroom, as mentioned before, you
add at least another 25% to the total amount of power drawn from the amplifier.
The additional 25% headroom is optional but highly recommended. It will allow
for future additions to the system and reduce the stress on the power amplifier.

1.25 x 1.25 = 1.5625
So you’ll need at least about 56% more power than the total sum of the transformer

taps. The reciprocal of 1.5625 is .64, so conversely you can figure on allowing a
maximum of 64% of the amplifier’s rated power for the loudspeakers. Therefore,
the highest combination of transformer taps the 500-watt amplifier can drive
satisfactorily is a sum of about 320 watts, e.g., 32 speakers each tapped at 10 watts,
or 64 speakers tapped at 5 watts, etc.

This table lists the recommended maximum load (sum of all speaker transformer
power taps on one line) for the CX6T and CX12T amplifiers. These figures assume
a typical transformer insertion loss of about 1 dB.

You can, of course, use different output configurations on
the two amp channels if it’s not operating in bridged mono
mode. For example, one channel of a CX12T could drive a
70-volt line loaded at 320 watts while the other channel
drives a 25-volt line, or even a direct loudspeaker.

25

XI. Other Design Considerations

SPEAKER TRANSFORMER SATURATION

Speaker transformers tend to be fairly small, and some vary widely in quality. They tend, therefore, to be prone
to core saturation at low frequencies. Saturation occurs when the magnetic field induced in the transformer’s
iron core by the audio signal waveform has built up as high as the core can handle. Even if the signal voltage
increases, the magnetic flux cannot increase any further, so it “clips,” in a way. High frequencies are less likely
to cause saturation because the signal waveforms, rapidly alternating in polarity, reverse the direction of the
magnetic flux before saturation can occur.

Saturation causes audible distortion. In extreme cases it can damage the power amplifier driving the line, because
as the transformers go out of saturation, their magnetic fields collapse and induce a voltage spike across the
line. That voltage spike travels along the line back to the amplifier, which has to absorb it. Interestingly, this
phenomenon is much more likely to happen on a lightly loaded line than on a heavily loaded one.

An effective way to eliminate the saturation problem is to filter out the frequencies most likely to cause it, without
adversely affecting the audio quality. The CX6T and CX12T both have built-in 45 Hz high-pass filters with an 18
dB per octave bass roll-off, while the CX4T has a 50 Hz high-pass filter with a 12 dB per octave roll-off. This is
adequate prevention for virtually all good-quality speaker transformers. However, if any speaker transformers
on the line do not have low-frequency responses that extend down to 45 Hz, you must insert corresponding high­pass filtering in the audio signal path before the amplifier.

CONNECTING BOTH A DISTRIBUTED LINE AND A DIRECT LOUDSPEAKER ON ONE
CHANNEL

The T versions of the CX amplifiers have simultaneous direct outputs (for loudspeak­ers) and 25-, 70-, and 100-volt isolated outputs (for distributed lines). This adds a
measure of versatility that few other amplifiers can match: being able to drive both
a loudspeaker and a distributed line on a single channel at the same time.

Setting up such an arrangement is simple, but it requires some precautions:

• Use an 8-ohm speaker for the direct connection. Make sure it has an
adequate power rating (150 watts for use with the CX4T, 200 watts for
use with the CX6T; 400 watts for the CX12T). Connect it across the low
impedance outputs.

• The distributed line must not be loaded any higher (sum of transformer taps
plus allowance for insertion loss plus allowance for headroom) than 40
watts for the CX4T, 60 watts for the CX6T, or 125 watts for the CX12T.
Connect it across the appropriate transformer terminals.

The directly connected speaker will constitute most of the load on the amplifier
channel, so it will receive considerably more power than the distributed line. If you
adjust the amplifier gain controls to vary the sound level from the directly connected

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speaker, it will affect the sound level from the distributed line’s speakers also, possibly altering them from your
intended levels.

An alternate method is to attach an 8-ohm speaker across the 25-volt outputs. This provides up to approximately
80 watts to the speaker and makes more power available to the 70V or 100V connections: up to 75 watts for
the CX4T, up to 150 watts for the CX6T; up to 400 watts for the CX12T.

WIRE LOSS

Even the highest-quality copper wire has a small amount of resistance to electrical current flow, and the wire’s
resistance is inversely proportional to the cross-sectional area of its conductor. Therefore, to minimize the power
lost to speaker cable resistance, you should use the largest stranded (always stranded) copper wire that is
practical for the job. This is especially important with direct low-impedance speaker connections; e.g., a half­ohm wire resistance would not affect a lightly-loaded 100-volt line noticeably, but it would reduce the amount
of power going to a 2-ohm load by 36%, a 1.9 dB drop. It would also reduce the damping factor to no better than 4.

If an amplifier could drive a speaker load through theoretical zero-resistance wire, no power would be lost in
the speaker cables. In the charts below we’ll compare the power delivered through real-world speaker cables
with the theoretical zero-resistance ideal and express it as a ratio called the power transfer coefficient. It is
determined by the formula

POWER TRANSFER COEFFICIENT = [R

LOAD

/(R

WIRE

+ R

LOAD

2

)]

Let’s say you have an 8-ohm speaker load. With that imaginary zero-resistance wire, all the power would be
delivered to the load, so the power transfer coefficient would be 1. If you then substituted wire with 0.2 ohm
of resistance, the load would only get 95.2% of the power it got with the zero-ohm wire, so the power transfer
coefficient would be 0.952 (a loss of 0.2 dB, by the way).

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28

Model

CX4, CX4T
CX6, CX6T

CX12, CX12T

AC CURRENT CONSUMPTION

A major objective in the design of the CX Series amplifiers—even the higher-powered models—is to permit
their operation from readily available, standard AC power sources.

“Normal conditions” in power amplifier rating means operating with a random program source (pink noise), using
pink noise as a source, at an average power level equal to one-eighth of maximum power. This is recognized by
most of the world’s safety agencies as the loudest you can play music through an amplifier and still keep the
incidence of clipping to a reasonable minimum. An amplifier’s peak current draw at full output power into 2 ohms
is several times what the “normal” draw is, but its various protection circuits will prevent this condition from
lasting more than a minute or two.

When you plan the AC power hookups for your amplifiers, use the following table to predict the current

requirements per ampli-

AC Current Consumption

IDLE

0.4A 6.0A 2.8A 2.5A 10A 4.5A 3.8A 15A 6.8A 5.6A

0.9A 7.9A 4.3A 3.2A 13A 7.5A 5.0A 19A 10A 7.0A

1.0A 15A 8.6A 5.1A 24A 12A 6.5A 33A 17A 8.5A

FULL 1/3 1/8

POWER POWER POWER

8Ω

4Ω or Distributed Line

FULL 1/3 1/8

POWER POWER POWER

FULL 1/3 1/8

POWER POWER POWER

2Ω

fier. You can use the one­eighth power figures to
predict the normal continu­ous current draw, then add
a safety margin to allow
for occasional crescendos.

Model

CX4, CX4T

(preliminary)

CX6, CX6T

CX12, CX12T

Btu/hr

kcal/hr

Btu/hr

kcal/hr

Btu/hr

kcal/hr

HEAT EMISSIONS

Essentially, a power amplifier draws electrical energy from the AC mains, converts it to DC, and then converts
it again into an analog of the input signal to send out to the loudspeakers. Any AC power that enters the amplifier
through the power cord and does not exit through the speaker outputs turns into heat, which the amplifier must
rid itself of by exhausting it to the outside. In indoor use, multiple amplifiers may present a sizeable challenge
to a building’s air conditioning system. Use the table below to predict the heat that will be emitted by your
amplifier. Use one-eighth power for normal, continuous usage.

Heat Emissions

FULL 1/3 1/8

IDLE

POWER POWER POWER

110 490 765 1545

28 123 193 389

250 890 735 735 1630 1655 905 3145 2205 1655

63 224 185 185 411 417 228 793 556 417

265 1725 1530 945 3250 2395 1360 5545 3785 2230

67 435 386 238 819 604 343 1397 954 562

8Ω

4Ω

or Distributed Line

FULL 1/3 1/8

POWER POWER POWER

FULL 1/3 1/8

POWER POWER POWER

2Ω

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XII. Address & T elephone Information

Address:

QSC Audio Products, Inc.
1675 MacArthur Boulevard
Costa Mesa, CA 92626-1468 USA

Telephone Numbers:

Main Number [phone]
Sales Direct Line [phone]
Sales & Marketing

[phone]

(toll-free in U.S.A. only)

Technical Services [phone]

[phone]

(toll-free in U.S.A. only)

Facsimile Numbers:

Sales & Marketing FAX

[phone]

Technical Services FAX

[phone]

World Wide Web:
http://www.qscaudio.com

BBS/World Group:
QSC OnLine Technical Support
1200-14400 bps; 8N1

[phone]
[phone]

CompuServe:

GO QSCAUDIO
ID: 76702,2635

30

1675 MacArthur Boulevard Costa Mesa, California 92626 USA PH: [phone] FAX: [phone]

32

“QSC” and the QSC logo are registered with the U.S. Patent and Trademark Office