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Stupid ohms question
Old 3rd September 2011
Here for the gear

Stupid ohms question

I always get this stuff backwards. All I have to do is build a small PA for a bridge tournament. I need to chain three (perhaps four but probably not) speakers onto one amp. I want to make sure I'm not making some horrible mistake.

The powered mixer's amp puts out 220W @ 4 ohms.
The speakers I want to chain into are 300 W (Peak), 150W (Program), 8 ohm nominal load.
I'll be using the second amp (also 220W 4 ohm) to drive a "monitor" mix over in a training room. I'll hook one or two speakers up to it.

What are the implications of doing this? Am I going to fry my amp by screwing up the ohmage?

The PA does not need to be super loud, just loud enough to be heard in a room of people playing bridge.

If you can respond to this username at gmail, that'd be handy, but here is cool too. Sorry for being so clueless.
Old 4th September 2011
Lives for gear
JohnRoberts's Avatar

If the amp makes 220W into 4 ohms, it will make roughly 1/2 that at 8 ohms (say 110W for this discussion) this is well within the speaker and amps capability so no smoke will be released.

If you connect two of those speakers to one amp channel each speaker will draw 110W and consume the 220W capability of the amp.

Putting more than 2 x 8 ohm speakers on one amp output will load it below 4 ohms, perhaps over heating it and causing it to shut down early.

Old 4th September 2011
Here for the gear

I'm perfectly ok with drawing only 110W on two speakers, it'll be plenty loud enough. Less than that would be enough, honestly. But yes, I see where 3 speakers brings me down to 2.7 ohms. Is there anyway to chain one more speaker onto this safely? Different ohmage speaker cabs? Different amp ohms? Chuck it all and use active boxes?
Old 4th September 2011
Lives for gear

You gotta be a little careful: an 8-ohm speaker may have an impedance curve that falls to 5 ohms (or lower) at some frequencies. One way to stay a little safer is to run 2 speakers in series, and that pair in parallel with the third, which would give you 5.3 ohms if they're each at 8 ohms, and 3.3 ohms if they're at a frequency where they are all running at 5 ohms. Yes there would be a 3dB difference between the pair and the single, but if it's for PA, who cares?

Another way is to get an old beat-up McIntosh autoformer-coupled amp - they are happy with just about any load, and if they're not pretty, they sometimes sell cheap, especially the solid-state ones.

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