Please explain amplifier output impedance

His study found that very high levels of low order distortion (30% second harmonic) were inaudible, but very low levels of the type of high order distortion produced by large amounts of negative feedback were quite audible and highly objectionable

I assume you are refering to Intermodulation Distortion or IMD distortion in an amp that is oscillating from large amounts of negative feedback. I agree that this is far more detrimental to the sound and something our ears seem quite sensitive to. Harmonic distortion is often indistingushable from the real sound of the instrument because the pitch of the note does not change (the note becomes fuller or leaner sounding). Even harmonic distortion is particularly hard to discern as your physicist friend points out. My understanding is that odd harmonics are more easily discerned, although still not nearly as easy to discern as IMD.

Hearing Harmonics

IMD distortion is one of the best arguments for promoting active speakers. Separate amplification for each driver over a limited bandwidth can only help to reduce IMD distortion significantly. Compare this to passive full range speakers where the amplifier must control the woofer at 40HZ and the tweeter at up to 20 Khz....hardly suprising that such a broadband system introduces audible IMD due to the combined interaction of all the drivers, the crossover and the amplifier fighting to control them.

Hi Shadorne, if you look at the quote you will see that it is in fact *harmonic distortion*, not IMD. Several studies now have shown the same thing: the human ear/brain system uses odd-order harmonics beyond the 7th harmonic or so as loudness cues. In nature these harmonics are quite low- and even very slight enhancement of them is easily detected by the human ear.

I learned this years ago while servicing an amplifier on the bench. The output of the amplifier was connected to a loudspeaker and a VU meter. The amp had a sine wave at the input. While malfunctioning and making less than 20db of its normal output, it still sounded louder than the normal undistorted output. Once you experience this you will not forget it!

This is one of the reasons why i strive to build GOBS of dynamic headroom into my systems and believe in multiple amplifiers that are actively limited to covering only a small portion of the audible range. By reducing the stress on any given part of the system, and limiting the electrical interaction between various parts of the system, THD and IMD are drastically reduced. This is not to mention that dynamics are increased and signal purity remains quite high, regardless of drive levels.

On another note and as i've stated before, "clean" signals can be played at mugh higher spl's and they don't sound as loud as they really are. You also don't suffer nearly the amount of listening fatigue that one experiences on a "dirtier" system at lower spl's. Sean
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Tvad: "To bring this discussion back around to my original question, when a reviewer states that a particular loudspeaker would likely work best with a low output impedance amplifier"
When looking for an Amp, look for one that gives you a 4 ohm rating in the specs. Alot of amps give you the spes. for 8 ohms only. What you need is an Amp that will give you a 4 ohm rating and then you will be in the Ballpark to drive most low impedence speakers!!

PS. Did you have a certain speaker in mind or was it just a general question on low inpedence Speakers?

Benie, my question was not about low impedance speakers.