Question abouts ohms,

when I looked for a home theatre system generally the rule was you want your amp and speakers both at the same ohms. 4 or 8 usually.

I've been looking at PAs and a lot of them are rated at 4 ohms but the speakers they include are rated at 8 ohms.

heres an example of what I keep seeing:

Phonic SEM 715 2-Way PA Speaker

Power handling: 600W peak, 400W program), 200W RMS
Frequency response: 50Hz - 20kHz
SPL (@ 1W / 1M): 98dB
Load: 8-ohm nominal
Phonic Powerpod 740 Plus / SEM715 PA System Features:

Phonic POWERPOD 740 PLUS Mixer

220W + 220W / 4 ohm stereo power amplifier for Main L/R or Main/Monitor (Bridge mono, 440W / 8 ohm)

so if you use both outputs like you'd expect, considering 2 speakers are in the box, you won't be "matched"??

and then it mentions it's 8 ohms when bridged.

So if I send all power to 1 output when bridged is it then matched? isnt that better than unmatched? in home theatre they make a big deal about that.

I know I'm missing something, if anyone understands this a bit better and feels like explaining that would be great.

I'm not sure why they sweat matching ohmages. As long as an amp is stable for a given load (2,4,8,16 ohm) then it's fine. You're going to find that most amps handle 2 & 4 ohm loads while speakers are usually 8 ohms.

A speaker is going to draw what it can. Just because potential is there, that doesn't hurt anything. An 8 ohm speaker will draw much less current than a 4 ohm speaker if the amp is stable to 4 ohm, then that's fine. Hooking up a 2 ohm load to that 4 ohm amp will fry it, or cause it's circuit protection to kick.

Ask your home theater guys what they mean about matching. I guess what I'm saying is, is that if I have an amp that can theoretically switch from 4 ohm and 8 ohm output and I hook an 8 ohm load to it, then it's going to draw the same in either modes. Why - because the speaker's load determines what it draws and the amp provides the potential.

I'm sorry if I explaied any of that wrong... Someone will correct me if I did.

It's a good question.
I don't know why they mention it. Maybe I remember it wrong and they were referring more to the set of speakers all being the same ohms and not matching amp to speakers.
maybe it helps when getting the surround set up or something. :shrug:

for now based on what you said I'll assume it's fine.

plus, why would they sell it that way if it wasn't?

When the signal is dc you refer to the resistance in ohms.
However, for ac, it is impedence we're talking about...Z.

Load balancing and bridging are just shunting current around to even the amount passed onto the load - the speaker.

If they sell the amp & cabinet as one, why worry about it at all?

A lot of manufacturers will list the wattage rating into a 4 ohm load. It allows them to state a higher wattage rating.

It really important to understand when using PA amplifiers. You are more likely to run some speakers in parallel in PS systems thus causing the amp to be seeing a lower resistance.

RMS, or root mean square is the figure to use, not peak or any other.


Heat brought up a good point.