Mackie M1200/M1400 owner manual 70V Distribution Systems, Load Gauge, Impedance Wire

The thickness of wire is rated in gauges. Use the chart below to determine the correct gauge of wire to use according to the distance between the speakers and the amplifier, and the impedance of the load the amplifier is driv- ing. This ensures that the power lost across the speaker wire is less than 0.5 dB.

	Load	Gauge of
Wire Length	Impedance	Wire
Up to 25 ft.	2Ω	14 gauge
	4Ω	16 gauge
	8Ω	18 gauge
Up to 40 ft.	2Ω	12 gauge
	4Ω	14 gauge
	8Ω	18 gauge
Up to 60 ft.	2Ω	10 gauge
	4Ω	12 gauge
	8Ω	16 gauge
Up to 100 ft.	2Ω	8 gauge
	4Ω	10 gauge
	8Ω	14 gauge
Up to 150 ft.	2Ω	6 gauge
	4Ω	8 gauge
	8Ω	12 gauge
Up to 250 ft.	2Ω	4 gauge
	4Ω	6 gauge
	8Ω	10 gauge

70V DISTRIBUTION SYSTEMS

A distributed sound system uses a constant- voltage, high-impedance network that feeds a number of tapped transformers which, in turn, deliver power to individual speakers. Each tap is rated in watts, so you can select the amount of power delivered to the speaker. Developed for distributed paging and public address sys- tems, one benefit of such a system is that it eliminates complicated impedance calcula-

tions when setting up a multi-speaker system. You just add up the total wattages of all the speakers in the system and make sure that it doesn’t exceed the total power rating of the amplifier (allowing at least 10% for insertion losses). Another benefit is that by using high voltage and low current, losses in the speaker distribution wiring are kept to a minimum. Standard voltage levels include 25V, 70V, and 100V, but 70V systems are most commonly used in commercial sound distribution systems in the U.S.

Because of the high power capability of the M•1200/M•1400 amplifiers, they can be used to directly drive 70V constant-voltage distribu- tion systems without the use of a step-up transformer. When the M•1200 amplifier is operated in BRIDGE mode, it can produce 800 watts into 8 ohms, or 80V. This is slightly higher than the standard 70.7V for which the system was designed. You can recalculate the actual power delivered to each tap by multiply- ing the tap’s rated wattage by a correction factor (K). The correction factor is P1/P2, where P1 is the power delivered by the amplifier into 8 ohms (BRIDGE mode), and P2 is the power delivered by 70.7V into 8 ohms (625W).

M•1200: K = 800W/625W = 1.28. Thus, a 2.5W tap becomes 3.2W, a 5W tap becomes 6.4W and a 10W tap becomes 12.8W.

M•1400: K = 850W/625W = 1.32. Thus, a 2.5W tap becomes 3.3W, a 5W tap becomes 6.6W and a 10 W tap becomes 13.2W.

CAUTION:A characteristic of tappedtransform- ers is that they saturate at very low frequencies, which causes their impedance to decrease, ap- proaching the DC resistance of the copper wire. This can result in overloading the amplifier if the signal contains lots of low frequencies. When using an FR Series amplifier in a 70V dis- tribution system, set the LOW CUT FILTER

FR SERIES

POWER AMPLIFIER IN BRIDGE MODE

+

CH 1

–

–

CH 2

+

RC NETWORK

+C1 C2+

R1

to 100Hz or higher. In addition, install an RC network at the SPEAKER OUTPUT of the amplifier, as shown in the figure below.

70V LINE

RC NETWORK VALUES

C1 = C2 = 660∝F @ 250VDC

R1 = 4Ω @ 100W

ALTERNATE RC NETWORK

Note: You can substitute	C3
a single capacitor for C1/C2.
C3 = 330∝F @ 250VDC,
NON-POLARIZED.
	R1

2.5W 5W 10W 2.5W 5W 10W

++

––

CONSTANT

VOLTAGE

TRANSFORMER

2.5W 5W 10W

POWER TAP

SWITCH

+

–

70V Constant Voltage	25
Distribution System