4ohm or 8ohm

[DT426]

My receiver has a switch for 4 or 8 ohm speakers. My K-horns have 8 ohm writen by the speaker wire connectors, but when I checked them with a meter they are 4 ohm. Where should I set the switch on the receiver?

[Malcolm]

Set the switch to 8 ohms, the nominal impedance of your speakers. It is normal for speakers to have a DC resistance less than their nominal impedance. The resistance you measured is only part of the impedance. The rest of it depends on the capacitance and inductance in the circuit. The actual impedance of the speakers varies considerably, above and below the nominal impednace, depending on frequency.

[lynnm]

In line with what Malcolm has said:

An Ohm meter measures the resistance of the voice coil to the flow of a D.C. current ie. 0hz. The impedance figure quoted by the speaker maker refers to the voice coil's resistance to A.C. currents - typically at 1000hz which is an altogether different matter.

Given the capacitance and inductance seen by the amplifier a typical speaker rated at 8 ohms will present an impedance that may range anywhere from 1 ohm to 70 ohms at depending upon the frequency of the test signal. That is why speakers are rated as having a nominal frequency which as I have noted above is typically measured at 1000hz.

This is a reasonable benchmark and one can safely assume that a speaker with a nominal impedance of 4 ohms will present approximately 1/2 the A.C. resistance to A.C. currents that will be presented by a speaker with a nominal impedance of 8 ohms.

Eight ohm speakers are not superior or inferior to 4 ohm speakers - all else being equal. As a practical matter however it is true that speakers should not be rated significantly lower at a nominal impedance than the amplifier is designed to drive simply because a very low impedance speaker can drop to impedance values below which the amplifier can safely drive. If the impedance drops too low the amplifier may "see" what amounts to a dead short and the output stage may go into thermal runaway and essentially self destruct if its protection circuits are slow to react.

All of the above said however most modern amplifiers are capable of safely driving impedances that would have been considered impossible loads in the early days of audio.

The best policy is to use the 8 ohm outputs for 8 ohm speakers and the 4 ohm outputs for 4 ohm outputs - BUT NOT AT THE SAME TIME - Use either 8 ohm OR 4 Ohm speakers but not both.

Remember also that when one connects 2 or more pairs of speakers to a single pair of outputs the impedance is NOT the sum of the rated impedances but is in fact the product of the following formula:

(R1XR2)/R1+R2 therefore two 8 ohm speaker systems present: (8x8)/8+8 = 64/16=4 ohms. A second example

Assume 2 pair of 4 ohm speakers connected to a 4 ohm output :

(4X4)/4+4 = 16/8 = 2.0 ohms This results in a load that would very likely be a problem to the amplifier because as noted above the actual impedance range varies widely from the nominal value and the setup described above very well might effectively represent a "short" as far as the amp is concerned. The potential for trouble would of course be greater if that same load were connected to a circuit designed to expect an 8 ohm nominal impedance.

Hopefully I have not added to the confusion regarding this issue 8~)>

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It is meet to recall that the Great Green Heron rarely flies upside down in the moonlight - (Foo Ling ca.1900)

[DT426]

Thanks for your help.