Crown MA-1202 Operation Manual page 29

MA Series Power Amplifiers
9 AC Power Draw andThermal Dissipation
This section provides detailed information about the amount of power
and current drawn from the AC mains by Macro-Tech 602, 1202 and
2402 amplifiers and the amount of heat produced under various condi-
tions. The calculations presented here are intended to provide a realistic
and reliable depiction of the amplifiers. The following assumptions or
approximations were made:
• The amplifier's available channels are loaded, and full power is being
delivered.
• Amplifier efficiency at standard 1 kHz power is estimated to be 65%.
• Quiescent power draw is 90 watts (an almost negligible amount for
full-power calculations).
• Quiescent thermal dissipation equals 105btu/hr at 90 watts.
• The estimated duty cycles take into account the typical crest factor for
each type of source material.
• Duty cycle of pink noise is 50%.
• Duty cycle of highly compressed rock 'n' roll midrange is 40%.
• Duty cycle of rock 'n' roll is 30%.
• Duty cycle of background music is 20%.
• Duty cycle of continuous speech is 10%.
• Duty cycle of infrequent, short duration paging is 1%.
Operation Manual
Here are the equations used to calculate the data presented in Figures
9.1, 9.2 and 9.3:
Total output power with all
channels driven (watts)
AC Mains Power
=
Draw (watts)
Amplifier Efficiency (.65)
The quiescent power draw of 90 watts is a maximum value and includes
power drawn by the fan. The following equation converts power draw in
watts to current draw in amperes:
AC Mains Power
Draw (watts)
Current Draw
=
(amperes)
AC Mains
Voltage
The power factor of 0.83 is needed to compensate for the difference in
phase between the AC mains voltage and current. The following equa-
tion is used to calculate thermal dissipation:
(
Total output power with all x Duty
Thermal
channels driven (watts) Cycle
Dissipation =
(btu/hr) Amplifier Efficiency (.65)
The constant 0.35 is inefficiency (1.00–0.65) and the
factor 3.415 converts watts to btu/hr. Thermal dissipation in btu is
divided by the constant 3.968 to get kcal. If you plan to measure output
power under real-world conditions, the following equation may also be
helpful:
(
Total measured output power
Thermal
x
from all channels (watts)
Dissipation =
(btu/hr) Amplifier Efficiency (.65)
x Duty
Cycle
+ Quiescent Power
Draw (watts)
Power
x
Factor (.83)
)
.35
x
Quiescent Power
x 3.415
+
Draw (watts)
)
.35
Quiescent Power
x 3.415
+ Draw (watts)
page 29