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Why are low impedance speakers harder to drive than high impedance speakers

I don't understand the electrical reason for this. I look at it from a mechanical point of view. If I have a spring that is of less resistance, and push it with my hand, it takes little effort, and I am not working hard to push it. When I have a stiffer spring (higher resistance)  I have to work harder to push it. This is inversely proportional when we are looking at amplifier/speaker values.

So, when I look at a speaker with an 8 ohm rating, it is easier to drive than a speaker with a 4 ohm load. This does not make sense to me, although I know it to be true. I have yet been able to have it explained to me that makes it clear.  Can someone explain this to me in a manner that does not require an EE degree?

Thanks

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geoffkait's avatar
geoffkait
23,365 posts
 
 
timber77

bdp24
In a related matter, Roger Modjeski of Music Reference recommends hooking up your speakers to a tube amp on the lowest impedance tap that provides the power you need. So if an amp puts out 45 watts at 8 ohms and 30 at 4, and 30 watts is enough for your needs (with the combination of speaker sensitivity, room size, listening level, etc.) with an 8 ohm speaker, use the 4 ohm tap for lowest power amp distortion and best sound.

THE last part of the sentence is usually true, however the logic is totally off.
No audiophile amplifier will go down in output in that fashion , if its 45 watts at 8 ohms then it should be 90 watts at 4 ohms....If the wattage is dropping then the amplifier should not be trying to drive the lower impedance load in the first place.

the impedance of the speaker generally dictates the best sounding amplifier tap. Sometimes speakers have widely varying impedance curves that means experimentation is probably necessary to see which tap sounds best. Depends on the impedance curve of the speaker, though. You can't get something for nothing. There's no free lunch. 😄
almarg's avatar
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almarg
9,670 posts
 
I agree with Geoff’s post just above. Also, regarding:
Timber77 1-15-2017
No audiophile amplifier will go down in output in that fashion , if its 45 watts at 8 ohms then it should be 90 watts at 4 ohms....If the wattage is dropping then the amplifier should not be trying to drive the lower impedance load in the first place.
While the maximum power capability of high quality solid state amps will of course often be twice as much into 4 ohms as into 8 ohms, tube amps do not behave in that manner. A tube amp which has an output transformer and provides 4 and 8 ohm taps will generally be designed to have a maximum power capability that is the same or similar when a 4 ohm load is connected to the 4 ohm tap as when an 8 ohm load is connected to the 8 ohm tap. And an output transformerless tube amp will typically have a greater maximum power capability into an 8 ohm load than into a 4 ohm load (and often an even higher capability into 16 ohms).

In the situation bdp24 referred to, where an 8 ohm load is connected to a 4 ohm tap, maximum power capability will usually be reduced in comparison to the amp’s capability when an 8 ohm load is connected to the 8 ohm tap or when a 4 ohm load is connected to the 4 ohm tap. The degree of that reduction will depend on the specific design, as will the desirability of the "light loading" (i.e., 8 ohm load connected to 4 ohm tap) that Mr. Modjeski recommends.

Regards,
-- Al


bdp24's avatar
bdp24
9,821 posts
 

Excellent clarification from Al, with more specifics. No surprise there! Timbre77, you missed the important qualifying adjective of the amplifier scenario I described---"tube". As Al stated, while the power solid state amps create increases with dropping impedance, the opposite is generally true of tube amps, with the notable exception of the unique Music Reference RM-200, which actually behaves more like a ss amp in that regard.

As Al detailed, the different impedance taps on a tube amp allow the amp to provide similar power to all the taps---that's one task of tube amps output transformers. However, if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response.

atmasphere's avatar
atmasphere
11,841 posts
 
if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response.
While generally true, a problem that can turn up when doing something like this is that the transformer can 'ring' if insufficiently loaded. In addition, with such a load, it will not be as flat across its bandwidth, as the transformer will tend to express less of its turns ratio and more of its inter-winding capacitance. The 'lighter' you load the transformer the more of a problem this becomes.

So the result, while possibly reducing distortion in the output tubes, will be to **increase** distortion from the transformer (ringing) and degrade the frequency response. Of course, if the amp employs negative feedback some of this will get sorted by that, but a problem with negative feedback is that while reducing distortion overall (in particular lower ordered harmonics), it actually **introduces** higher ordered harmonics that otherwise may not have been present at all! Its best not to give feedback too many places to screw up.

 

bdp24's avatar
bdp24
9,821 posts
 
Excellent additional information from the always illuminating Ralph Karsten! Thanks as always. Modjeski designs his transformers and has them built to his specs (even winding them personally, for those willing to pay him to do so). Perhaps he does so in a way that takes light loading into consideration. I don’t employ it myself, needing all the power I can get for the rather insensitive 8 ohm loudspeakers I use the RM-200 with.
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