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David, my buddy's band has an Alesis RA300 which they use for practice PA.

It has 4 8-ohm monitors attached to it, 2 per side. This gives 4 ohms load into each channel. If they use only 2 monitors (one per side), then it still works fine but at less power.

You could put two monitors on one side (4 ohm load) and one on the other (8 ohm load) but I wouldn't guarantee that it's not gonna hurt the amp. (Comments from more experienced sound monkeys welcome.)

I definitely wouldn't bridge the amp into 3 (or more) speakers. The specs on this amp seem to indicate that it wants a MINIMUM load of 8 ohms when bridged.

Can you find one more monitor and run 2 monitors per side?
 

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I'm not the leading expert on this (Milkman?), but from the amounts of transistor-smoke I've generated over the years -

If an amp is rated at, say, 200 watts into 4 ohms, you'll get roughly half power (100W) at 8 ohms.

Most monitors have 2 jacks on the back, wired in parallel, so as you add speakers by plugging one speaker into the next, you are halving the ohmage.
(2 8-ohm monitors plugged one into the next = 4 ohm load to the amp)

When you bridge a stereo amp to mono, it requires twice the load as the 2 individual stereo channels did. ( minimum load for a stereo channel ( for this model amp, others are different) is 4 ohms, so the minimum load in bridged mode is 8 ohms.)
I have, however, read specs on some amps that say they'll work in bridged mode down to 2 ohms, so I guess it's not a hard-and-fast rule. However, all the specs I can find on Alesis amps say bridged mode has an 8-ohm minimum.
(If you run a solid-state amp into too small a load, you'll fry the main power transistors.)

After poking around the 'net some more, it looks like you'll be safe running 2 monitors off one side of the amp and one off the other. If the amp has separate volume controls for each channel, you should be able to crank up the channel with ONE monitor on it (it's 8 ohms, the channel with 2 monitors is 4 ohms) to compensate. Watch the clip lights.

I guess the bottom line is, wire it up to 3 monitors with the amp in stereo mode and see if it's loud enough. You won't damage the amp. I wouldn't run it in bridged mode into 3 monitors until you find out for sure if the amp can take it.
 

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No need to be confused....

- Monitor Out from your mixing board needs to go into both channels of the amp (hopefully there are 2 Monitor Out jacks on your mixer - if not, a Y cable is required)

( like this) http://www.directproaudio.com/product.cfm?directid=23399

- leave amp set for Stereo

- connect 1 monitor to one channel of the amp

- connect the other 2 monitors together and connect them to the other channel of the amp

- position the monitors where you need them

- adjust the volume controls of the amp as required

- Rock out!
 

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OK, David, thread-hijacking aside, I thought I'd sum up as the relevant information is spread over several posts.

You don't want to run everything in mono because your 3 speakers will blow the amp up in mono.

You need to split your monitor out signal with a Y cable ( a MONO Y Cable) to get the single signal from your mixer into both channels of the amp.

Connect 1 monitor to one channel of the amp and 2 monitors to the other channel.

300 watts is plenty to drive 3 monitors in a club setting, especially if you're mostly monitoring vocals. I'll be surprised if you need to go much above half-way on the volume controls of the Alesis.

Remember, you also need to fool around with your mixer's monitor mix controls to feed the correct amount of the various mixer channels into the mixer's Monitor Out jack. There should be a knob on each channel strip called MONITOR or something similar - that knob controls how much of that channel ends up in the monitor mix. Most mixers also have a monitor MASTER control that controls the final level of signal coming out of the MONITOR OUT jack.

For example, when we play small clubs, I run a 6-channel Yamaha powered mixer which drives 2 speakers-on-sticks at 200 watts each. This mixer also has a single MONITOR OUT jack which I feed thru a Y cable into a Yamaha stereo power amp which outputs 75 watts per channel. 2 monitors come off one side of the power amp and are placed at the front of the stage for the vocalists. The other side of the power amp runs a single monitor for the drummer.

On the mic channels of the mixer ( usually we run 3 mics for vocals, a mic on the kick drum and sometimes we run an acoustic guitar thru another channel), I set the MONITOR controls on the mic channels at around 4 or 5 and the monitor master control at 7. This feeds enough vocals into the monitors that everyone can hear themselves. The monitor controls on the kick drum channel and the acoustic guitar channel are set to 0 so none of those signals get into the monitors.
Both volume controls on the Yamaha power amp are generally set between 5 and 7 and left alone. If EVERYONE says they can't hear themselves, I adjust the volume of the monitors with the MONITOR OUT control on the Yamaha mixer. If ONE person says they can't hear themselves, I turn up just the MONITOR control on THEIR mic channel.

Should work pretty similar with your setup.

If I wasn't already booked, I'd drop by your show at the Congress this weekend and confuse you further, but alas I have prior commitments.

Rock On!
 
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