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question abouts ohms, chaining speakers...

Ok, so basically, I'm trying to think about how i'm gonna set up the pa system my band is going to be buying, we're gonna have 2 15" Yamaha clubv mains which run at 8 ohms. now i think i read somewhere that if you chain 2 speakers together, if they were originally at 8 ohms, they would then run at 4 ohms each? is this true. also, how exactly do you bridge a power amp, and if the power amp is bridged, is it ok to chain the speakers together?

I know that was a little lengthy, but it'd really help me make some final decisons i need to make such as cabling and what power amp i wanna buy. i already have a Yamaha mg16/4 mixer.

Comments

RemyRAD Fri, 09/01/2006 - 20:15
It's really simple mathematics and since I am a simple person..... that's the extent of my mathematical capabilities. No really, if your speakers are 8 ohms each and you connect them in parallel, i.e. ganged together, the combined impedance will then be a total of 4 ohms. Most amplifiers produce more power into a 4 ohm load than they do into an 8 ohm load.

Conversely, if your amplifier has a provision for "bridged" output, that will generally allow for twice the total output power, from a single output. If loaded into a pair of 8 ohm speakers that are combined together to create a 4 ohm load (i.e. in parallel), you are talking about maximum transference of power! Generally, amplifiers that can be bridged usually require a single input plugged into the amplifier either left channel or right channel depending on what is specified by the manufacturer but both of your speakers that are connected together in parallel will only connect to the amplifiers output left and right channel RED binding posts. That's right. The black binding posts are not used and you should connect nothing to them. Of course doing that turns your stereo amplifier into a super monaural amplifier. Beautiful for your bass cabinets. Now this could be disastrous if you are connecting 4 speakers together in parallel, which would create a 2 ohm load, a near short! You won't damage the speakers but your amplifier probably won't like you much for doing that, it will completely defeat the purpose of bridging, produce less power than rated and may even sound bad. If you want to do that, don't bridge the amplifier.

Let me know if you have any further questions?
Ms. Remy Ann David

RemyRAD Sat, 09/02/2006 - 14:04
The speakers would utilize the full 450 W of the amplifiers output. Provided they are connected together in parallel and you're connecting to the bridged output of the amplifier then yes, the speakers will be receiving 450 watts of clean power.

Of course, if you insisted on stereo, then each speaker at 8 ohms, connected to each channel, would probably deliver 120 watts, to each speaker. Still quite usable. Still quite loud. 6 DB less headroom than the previous afermentioned hookup. Still not shoddy to say the least. Nobody ever really plays their system at a continuous high power rating. What it buys you is greater transient handling capabilities since you'll have more headroom.

No sweat
Ms. Remy Ann David

dpd Sat, 09/02/2006 - 19:24
Theoretically, bridging a power amp should increase it's power output by a factor of 4 into a given load impedance - IF the power supply can provide the increased current, the transistors can survive that current and the associated increased heat dissipation.

When you bridge the amp, the L/R channels are driven with inversed polarity; therefore, when one channel is at its maximum positive voltage, the other channel is at its maximum negative voltage. This means that there is twice the output voltage across the load impedance (speaker). Power is V^2/R so, for twice the output voltage the amp should supply 4x the power.

Audio power amplifiers are essentially voltage sources - they should double their power when the load impedance drops in half (e.g. from 8 ohms to 4 ohms). However, just like the bridging issue, the power supply, output transistors and/or the heat dissipation always puts a limit on the amount of current the amp can supply which keeps the amp from doubling its output each time the load impedance drops in half.

Kev Sun, 09/03/2006 - 14:33
dpd wrote: Audio power amplifiers are essentially voltage sources - they should double their power when the load impedance drops in half (e.g. from 8 ohms to 4 ohms).
yep

and this doesn't change for any of the audio sources from Mics and Guitars through to line level and speaker level amps.

in fact some small amplifiers are NOT louder than a Line Amp in that they both may not have much more level than say ... 26dBu
the line level unit may be capable of 26dBm ... load of 600 ohms
Headphone Amp ... load of 40 to 100 ohms
Speaker/Power Amp ... 8 ohms ... . perhaps 4 or 2 ohms.

It is the load that determines the current required.

Then it all comes down to CURRENT available.
The amp doesn't really drive the speaker. The speaker is a load and the load draws the current. If the current is available the Watts will follow.

Current capability usually requires big heavy transformers and lots of capacitance to smooth things at the high currents and FAT wires internally and output devices capable of carrying current and probably some heat to go with it.

The newer digital amps are a little different in that they have more in common with a lighting dimmer than they do, an old school analog power amp.

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