Kneal7

Power amp & Speaker Ohms

7 posts in this topic

Hello, I'm just getting into setting up a live PA and I was wondering about the ohms - watt ratio. If i have 2 400w speakers @ 8ohms does it mean I need an 800w power amp at 4 ohms...

Also what happens to the ohms if you daisy chain the speakers....?

Is it possible to have 12 ohms? From memory i can only remember seeing 2,4,8,16..

Regards

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No, it means you need a 2 channel amp rated for 400Watts per channel into 8 ohms if you're running stereo.

If you hook the speakers up in parallel (i.e. feed the next one off the last one) then you will end up with a 4 ohm load - resistance decreases as you parallel up the speakers. There's an equation for it: http://en.wikipedia.org/wiki/Resistor . The more speakers in parallel, the lower the resistance will become.

IF you hook the 2 speakers up in mono off one channel of the amp (i.e. parallel) you need the amp rated 800Watts at 4 ohms. Most amps are stereo amps these days (or 2 channel) so IF you want to run mono, i suggest you hook the speakers up one per amp channel and then take a mono feed into the amp (i.e. a 400W x 2 @ 8Ohm amp will do you). This is better for the amp as loading one channel up and not the other can cause damage to the amplifier unless it's what the call a "Dual Mono" amplifier. Basically, to cut costs, most manufacturers share components between amp channels... so it's generally only the better amps that have totally independant channels and can operate with one under load(driving a speaker) and the other not under load (no speakers).

You can get 12 ohms... HOWEVER... most speakers are nominal 4 or 8 ohms. To get 12 ohms, you'd have to put a 4 ohm speaker in SERIES with an 8 ohm speaker. This isn't a normal or easy way to hook speakers up - they typically hook up in parallel and all PA speakers with a "speaker Out" connection are connecting the next speaker in parallel, so the resistance will go down.

Another tip is always use an amplifier with more power than the speakers you're connecting. This can help reduce noise floor and distortion - basically the system sounds better. Never use an amplifier of lower power than the speakers - this can cause the amplifier to saturate (overdrive) and put DC into the speakers, which will make smoke come out of them. Trust me - no matter how hard you try, the smoke never goes back in and the speakers will be dead.

If you're just getting started, i'd suggest a hire system would be a good way to go. The PA hire shop will match the amp and speakers for you and you can just start out by getting your head around the hookup and tuning the system. Or, go for a powered speaker system. Most sound really really good these days and the manufacturer matches the amp to best drive the speakers so you only need to worry about patching it together. I recently hired a couple of the new JBL EON's for a birthday party - i hate EON's, always have, but there's nothing an EQ can't fix. Much to my amazement this time, though, the new EON's sound really good out of the box. Powered speakers have come a long way and save a lot of mucking about until you need the big big systems. Check some out, if you get a chance. Could make life easy for you starting out.

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Jester, that answer is why I love sound punk. Thanks for such an in-depth reply. I'll get my head around it all eventually. Thanks again.

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Yeah, i read that and it is a bit confusing. Here's some pics that might help clear up what i've explained above. Should have done this yesterday... forgot how easy it is to do in Omnigraph :P

post-13736-0-23832700-1356911672_thumb.jpg

post-13736-0-69305800-1356911684_thumb.jpg

post-13736-0-38185900-1356911694_thumb.jpg

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One more for good measure and because it's quick/easy to edit the file... small band PA system using a 2 channel amp.

post-13736-0-61685700-1356912169_thumb.jpg

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