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Studio Reference
IIIII
&
II
II
II
II
II
Professional Studio Amplifiers
7 AC Power Draw and
Thermal Dissipation
This section provides detailed information about the
amount of power and current drawn from the AC mains
by
Studio Reference amplifiers and the amount of heat
produced under various conditions. The calculations
presented here are intended to provide a very realistic
and reliable depiction of the amplifiers. The following
assumptions were made:
• The amplifier’s available channels are loaded, and
full, standard 1 kHz 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 307 btu/hr at
90 watts.
• Duty cycle takes into account the typical crest factor
for a particular 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 uncompressed rock ‘n’ roll is 30%.
• Duty cycle of background music is 20%.
• Duty cycle of continuous speech is 10%.
• Duty cycle of infrequent paging is 1%.
Here are the equations used to calculate the data pre-
sented in Figures 7.1 and 7.2:
The estimated quiescent power draw of 90 watts is a
maximum figure, and assumes the fan is running at
high speed. The following equation converts power
draw in watts to current draw in amperes:
Current Draw
(amperes)
=
AC Mains Power
Draw (watts)
x
AC Mains
Voltage
Power
Factor (.83)
The power factor constant of 0.83 is needed to
compensate for the difference in phase between in the
AC mains voltage and current. The following equation
was used to calculate thermal dissipation:
Total output power with all
channels driven (watts)
Thermal
Dissipation
(btu/hr)
=
+
Quiescent Power
Draw (90 watts)
x Duty
Cycle
Amplifier Efficiency (.65)
(
)
x 3.415
.35
x
The constant 0.35 is inefficiency (1.00 – 0.65) and the
factor 3.415 converts watts to btu/hr. Thermal dissipa-
tion 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 be helpful:
Total output power with all
channels driven (watts)
Thermal
Dissipation
(btu/hr)
=
+
Quiescent Power
Draw (90 watts)
x Duty
Cycle
Amplifier Efficiency (.65)
(
)
x 3.415
.35
x
8 Ohm Stereo / 16 Ohm Bridge-Mono / 4 Ohm Parallel-Mono
L O A D
50%
40%
30%
20%
10%
1325
1075
830
585
340
1,780
1,485
1,190
900
605
1,925
1,555
1,190
825
460
630
520
410
300
190
15.9
12.9
10.0
7.0
4.1
23.1
18.7
14.3
9.9
5.5
Duty
Cycle
AC Mains
Power
Draw
(Watts)
btu/hr
Current Draw (Amps)
7.2
5.9
4.5
3.2
1.8
10.5
8.5
6.5
4.5
2.5
4 Ohm Stereo / 8 Ohm Bridge-Mono / 2 Ohm Parallel-Mono
Studio Reference
I
450
375
300
230
155
kcal/hr
2,500
2,060
1,620
1,185
745
100-120 V 220-240 V
Thermal Dissipation
btu/hr
Current Draw (Amps)
kcal/hr
100-120 V 220-240 V
Thermal Dissipation
AC Mains
Power
Draw
(Watts)
Fig. 7.1 Studio Reference
I
Power Draw, Current Draw and
Thermal Dissipation at Various Duty Cycles
