Passive Speakers... confusion with impedences.

[fredbloss]

I'm looking into buying a new power amp for my speakers, but I've gotten myself all confused with impedences etc. trying to decide how much power I need/ want/ can take...

So I have 4 passive speakers, with 600W peak on each one but probably don't want to put through much more than 300W (right?). What happens with impedences when I plug 2 speakers into each channel on my amp?

For example, Im looking at this Electro-voice CP1800 that has power rating:

2 x 600w 4 ohms
2 x 350w 8 ohms

Now each of my speakers is ~ 8 ohms. So if I have 2 in parallel plugged into one channel, this will reduce the impedence per channel to 4ohms. But will this then send 600W to each speaker? Or will that be shared between the two giving 300W per speaker?

[Wonks]

You've got the impedance issue correct. And whatever wattage is being produced will be shared between the connected speakers so 600w into two speakers means they are handling 300w each.

What is the application? For PA, you'd normally select an amp between 1.5x and 2x the RMS speaker rating. You mention peak, which can be a very misleading description. You tend to get the RMS value, then a music power value (at 2x RMS) and then a Peak value (at 4x RMS), So for a 300W RMS speaker you may find it rated as 600W music power and 1200W peak.

For PA, the music power value is the best indicator of the power amp to match with it to get the most volume out of the speakers without destroying them. What kills the HF elements of PA speakers are high power clipped signals, so you don't want to underpower the speakers, set the amp on maximum to get the volume and then have power amp clipping.

This is not a good idea.

RMS values are continuous signal values and you don't get that with most live music (though pre-recorded heavily compressed and limited music might) . It's the average power in the signal that matters. So you can power the speakers with a bigger amplifier.

If it's for instrument usage, (say Bass) then you'll normally find that the speakers are rated at or above the amp rating. There aren't any HF elements (Piezo horns in bass cabs are pretty forgiving compared to PA compression drivers) that have the most trouble with clipped signals, but because most guitar and bass amps will produced distorted signals, you don't want to overpower them.