Matching speaker impedance

Fungtank

I have a set of 8ohm speakers and am looking at purchasing a separates system.

What effect would powering the speakers off a 4ohm amplifier have on the speakers physically and their resultant output?

robbie1876

I take it then that your amp has specifications rating the power at something like "120w @ 4ohms" or something like that? Post up the model number of your amp and your speakers and we'll check it out for you.

In reality, hi-fi amps can generally drive speakers between 4 and 16 ohms with no problems. Some amps can go right down to 2 ohms, but the lower the impedance, the more load on the amp, shortening the lifespan of the amp. You won't have any problems at all with an 8ohm load, that is pretty normal.

Manufacturers sometimes rate the power of their amplifier at lower impedances to make them look more powerful than they actually are. In the hi-fi world, it's normal for amps to be rated at 8 ohms, as most hi-fi speakers are 8 ohms. In the professional PA world, most loads are at 4 ohms, so amps tend to be rated at this.

As a very general rule of thumb, 4 ohm loads are usually just about twice the wattage of 8 ohm loads, so if the amp is rated at 120w at 4 ohms, it could put out 60w or thereabouts at 8ohms. It should be pointed out that you won't hear much audible difference between a 4ohm load and an 8ohm load, one isn't twice as loud as the other. It's just that the amp has to work twice as hard with the 4ohm load, so it's putting out twice the volume, but has twice the resistance to drive.

In summary, post up the model numbers to be sure, but I wouldn't worry about the load at all!

Robbie

Fungtank

Thanks man. That kinda what i reckoned but wasn't sure.

The amp is sony entry level, can't remember the model number. It gives 80W @ 4Ω. The speakers are 90W @ 8Ω.

Thanks again for your input.

robbie1876

Yeah, you'll have no problems there at all.

As a slight aside, using an amp that is less powerful than your speakers is more likely to blow the speakers than a more powerful amp. In your case for example, the amp will be able to put out about 40w at 8ohms, but the speakers can take 90w. When the amp is driven close to it's maximum volume (say beyond the halfway point on the volume knob), the speakers can take yet more volume, which the amp tries to output, resulting in overdriven clipped signals. These clipped signals can sometimes be exponentially louder than the maximum volume on the amp, and can cause the speakers to blow pretty easily.

On the other hand, having an amp that is way more powerful that the speakers is obviously bad news too. Best practice is that your amp is rated about 20% higher than your speakers, but it's not something to worry about too much. Just be aware that if you hear distortion, turn your amp down immediately, because it will be the amp distorgting, not the speakers.

Robbie