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paulj 18th May 2004 07:40 AM

Speaker impedance too low for amps!
I was about to buy some particular speakers until I was advised they had a nominal impedance of 4 ohms: my amplifier set-up (2 x Cambridge Audio) can only handle 6 - 16 ohms, according to the instruction book. I am scared of damaging the amps. Is there any simple way of safely fixing this problem without degrading the sound or endangering the amps (or the 4 ohm Klipsch KG4 speakers, circa 1984, that I wanted to buy)?

lucpes 18th May 2004 10:24 AM

The KG4s are rated at 100 watts RMS power capacity and they are also high sensitivity (94dB) so you don't need a lot of power to get a lot of sound out of them. Your amps should be confy with those, as long as you don't try to rock the whole house.

paulj 18th May 2004 11:32 AM

Yes, but isn't the 4 ohm impedance of the speakers i8n danger of damaging the amps, which can only handle 6 - 16 ohms?

Frank Berry 18th May 2004 11:41 AM

Why not contact Cambridge Audio and ask them?

boholm 18th May 2004 12:23 PM


Originally posted by paulj
Yes, but isn't the 4 ohm impedance of the speakers i8n danger of damaging the amps, which can only handle 6 - 16 ohms?
No. It is not a question of matching 8 ohms to 8 ohms, 4 ohms to 4 ohms . . . .

It's more like having a big truck with a small engine. As long as you drive safe and carefully you will not notice the small engine. But if you want to go very fast or accelerate very quickly, you may destroy the engine.

If the amplifier has built-in protection you can go ahead and test the speakers on your amplifier. If not, then you can still test but you must turn up the volume nice, easy and slowly. If it is loud enough for you in mid position, then you are safe IMHO. Higher than that, then I would start to have second thoughts about the speakers.

kelticwizard 18th May 2004 12:39 PM

It's hard to say. Back in the seventies, they had amp circuits that simply were not comfortable with 4 ohm loads, regardless.

Assuming that Cambridge does not have a similar problem, (the seventies were a long time ago), then your maximum output into 4 ohms will be slightly reduced.

If your Cambridge setup gives 100 watts into 6 ohms, then when working at it's maximum, it is giving 24 Volts into 4 Amps, for a total power output of 96 Watts.

Since 4 Amps is as high as it goes, into a 4 ohm load it can only give 16 Volts into 4 amps, for a total power output of 64 Watts.

Working it out, that means the Cambridge can give 1.7 dB more maximum power into a 6 ohm speaker than into a 4 ohm speaker.

However, if these speakers are actually 94 dB @ 1M/1W, then they are far more efficient than normal speakers, and you won't need that extra 1.7 dB.

I think you should call up Cambridge and see what they have to say, as previously suggested. I am still a little skittish because there are some amp circuits that just don't like 4 ohm loads, although that seemed to be a problem from years ago more than now.

5th element 18th May 2004 12:58 PM

I find it very hard to believe that a comercial amplifier would not contain any kind of SOAR protection circuitry or a thermal cutout. I would say that driving a 4 ohm load is not going to cause the amplifier any problems unless you really want to crank the volume control. As statd before 94 is very high sensativity, I dont think you will have any problems reaching high output levels.

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