QSC Audio MX2000a, MX 1500a, MX3000a Heat Emissions, Protection Circuits, BTU/hr Kcal/hr MX 1000a

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 Table 3 to predict the heat that will be emitted by your amplifier.

Table 3: Heat emissions of MXa amplifiers under various conditions:

	Load	1/3 Power, both channels		1/8 Power, both channels		Idle, both channels
MODEL		BTU/hr	kcal/hr	BTU/hr	kcal/hr	BTU/hr	kcal/hr
MX 1000a	8Ω	1035	265	580	150	75	20
	4Ω	1430	365	270	915	75	20
	2Ω	2140	545	305	1035	75	20
MX 1500a	8Ω	1165	295	770	195	110	28
	4Ω	1875	475	1010	255	110	28
	2Ω	2770	700	1415	360	110	28
MX 2000a	8Ω	1850	470	1080	275	145	40
	4Ω	3100	785	1755	445	145	40
	2Ω	4730	1195	2760	700	145	40
MX 3000a	8Ω	2980	755	2015	510	200	50
	4Ω	7100	1795	3600	910	200	50
	2Ω	10280	2600	5040	1275	200	50

PROTECTION CIRCUITS

The design goal in high-efficiency, lighter-weight ampli- fiers such as the MXa Series is to control more power with fewer or smaller load-bearing components, e.g., resis- tors, transistors, transformers, etc. However, the higher power flow through these components makes effective, responsive protection circuitry absolutely vital. To this end, the design of the MXa series takes a comprehensive approach to protection.

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

MXa amplifiers use the proven Output 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 reach-

ing 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.

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 MXa Series amplifier thus is equal to that of another amplifier of about 1/3 to 1/2 its power rating.

When the DC fault protection circuitry senses a DC voltage on the amplifier output, it activates a relay which shorts the output and load to ground. If the DC is due to a fault in the amplifier channel’s output circuitry, it will probably produce enough current flow to blow the fault fuse in series with the output. This will disable the channel

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