ohms rating on speakers and amps

[simploerob]

could some one explain the difference between 4ohms and 8 ohms.

 

 

got a carlsbro grx7 (300watts 4ohs) running 2 jbl jrx 115 (250watts 8ohms)

 

want to replace it with a samson sd8 stereo mixer amp (250watts 4ohms) (it's sitting around doing nothing and the carlsbro has got a few channels down)

 

 

thanks simon

[Damon999]

The Ohm rating is the impedance (similar to resistance but not the same) that the speakers present to the amp. The lower the impedance, the easier it is for the amp to deliver the power. You really want an amp that will put at least 250watts into 8 ohms.

 

The Sampson one you mention will probably put around 125watts into 8 Ohms and could well lead to you pushing it into clipping and destroying your speakers.

 

Your current amp is still not really powerful enough and could also lead to you pushing into clipping. You certainly won't get the most out of the speakers.

 

This is all assuming that 250w is constant power handling and not the peak.

[revbobuk]

JBL's website recommends an amp rated at 250-500W into 8ohms to drive the JRX115, and JBL aren't known for being stingy about power handling, so if you want to get the most out of the speakers, you will need more power. But since half the power is only 3dB less loud (big approximation, but mostly true!), then if you don't mind lower volume levels - all depends on the application - then you might be fine. If all you are doing is vocals for an acoustic skiffle band, then you're laughing. You should still be hitting 119dB SPL at 1m, I think (from JBL's figures) (for 1 speaker - 3dB more for a pair)

[Bobbsy]

The Ohm rating is the impedance (similar to resistance but not the same) that the speakers present to the amp. The lower the impedance, the easier it is for the amp to deliver the power.

 

 

Whoa there! Saying that the lower the impedance the easier it is for the amp is giving a VERY wrong impression--and one that, taken to extremes, could damage the amp.

 

This is a difficult one to try and put in layman's terms but, if instead of thinking of the amp as "delivering the power" you visualise the speakers as "sucking power" out of the amp you won't be far wrong. Down to the specified lowest impedance the amp can tolerate, lower impedance speakers will, indeed, draw more power from the amp. However, if you go below that, the speaker will eventually draw more power than the amp can provide and, if you're lucky, the protection circuits will cut in and turn off the amp. If you're unlucky, the amp will overheat and be badly damaged. Think of it this way: the lower the impedance of the speaker, the closer you're getting to a short circuit across the amp outputs.

 

Four ohms is the practical minimum for most budget amps. Some good quality amps can work down to two ohms but that takes good engineering. Some amps start to heat even going below eight ohms.

 

I won't go farther here because there are MANY topics in the Blue Room about matching impedances of amps and speakers worth reading. For example, two 8 ohm speakers wired in parallel give an effective impedance of 4 ohms. And so on.

 

Bob

[simploerob]

thanks guys that makes a lot of sense.

 

 

it's used by a youth group for 2 hours a week (it's a put together what we have got, will check that the jbls are jrx 115 when I am down there next.

 

it's been running like that for 18 months and they only use it for a radio mic and playing music, never turned up that load.

 

simon

[Damon999]

Down to the specified lowest impedance the amp can tolerate

 

I suppose it might have been an idea for me to point that out :unsure: