Matching amplifier power::impedance rating important?
Mar 16, 2006 at 12:35 AM Thread Starter Post #1 of 6

jpelg

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My receiver amp section's power output is rated at 6 ohms. I understand that it is important to not place a load of less than that (ie. 4 ohms) on it.

But will I see any increase in performance when using 6-ohm rated speakers vs. those with specs listed at 8-ohms? Am I on a level playing field as long as I stay with any speakers at or above 6-ohms? Or will the 8-ohm speakers present a less demanding load to the amp, and thereby increase its headroom, making them an even better choice?
 
Mar 16, 2006 at 3:51 AM Post #2 of 6

Wodgy

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It won't make any difference for a solid state amplifier. The impedance rating just indicates the lowest impedance for which the amplifier is unconditionally stable. It has nothing to do with sound quality.

For a tube amp, it does make a difference. Using speakers with a nominal impedance higher than the output tap's rated impedance, distortion goes down, but power also goes down. Using speakers rated lower than the output tap, distortion goes up, but power output also goes up. The amp may also become unstable. However, what makes a bigger difference with tube amps is whether or not the speaker has a relatively flat impedance curve, because that always affects affects the frequency response of the speakers. For an illustration of that, see the frequency response curves of these tube amps into the "NHT dummy-speaker load" (cyan lines):
http://www.soundstagemagazine.com/me...s/aes_six_pac/
http://www.soundstagemagazine.com/me...n_premier_140/
 
Mar 16, 2006 at 5:05 AM Post #4 of 6

Wodgy

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Quote:

Originally Posted by TheSloth
Well it actually depends on the output stage of the Solid State amplifier. Some SS amplifiers, such as the Musical Fidelity A1 have a higher rated output into 8 ohms than into 4, and even lower into 2, due to their rather stubborn output stages that don't provide much current.


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In general, a solid state amplifier should deliver roughly twice the power into a 4 ohm load as into an 8 ohm load. If it cannot do this, that's a sign the power supply design is inadequate.
 
Mar 17, 2006 at 6:40 AM Post #5 of 6

SemperFidelity

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It's a bit more complicated; I'm assuming a solid state amplifier.

First, most speaker impedances are nominal values only; 6 ohms is a typical figure these days, but often there are dips to 4 ohms or less at certain frequencies. Decent amplifiers can easily handle a four ohm load, and often less. An 8 ohm speaker for the same power requires a greater voltage swing, but the voltage swing is fixed, so maximum power available is less. But, although power for the 8 ohm load is cut in half compared to 4 ohms, which sounds drastic, the difference to the ear is not very great at all. One dB is the generally accepted minimum step the ear can hear, and half power is only 3 dB. Most people feel that the sound has been "halved" at 1/10 the power, or 10 dB.

Any decent amplifier will be stable at any reasonable impedance, from an ohm to open.

So the message I'd suggest that you take away, is to get speakers that you really like; try them with your amplifier, and I think you'll find that impedance and power considerations are not that important.
 
Mar 17, 2006 at 5:19 PM Post #6 of 6

jpelg

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Thanks guys. Great responses and interesting discussion.
 

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