Please explain amplifier output impedance

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.

Atmasphere,

Interesting. Perceived loudness clearly can be different from actual loudness. I have experienced this too. My speakers go well over 100 db continuous SPL levels at the listening position and yet the perceived sound is less loud than when my daughter plays iTunes at maximum distorted levels from the mini 10W speakers connected to our PC in the study.

In fact I have to ask her to turn it down even though the SPL levels are miniscule in the kitchen which is about 20 feet from the study; distortion is tiring, distracting and it seems we are very sensitive to low levels of it.

This may also explain why compression and limiting applied to modern pop CD's makes them sound very loud and unpleasant, especially at higher listening levels (when perceived loudness due to distortion and real loudness become most unpleasant). The CD "loudness wars" are a way to intentionally manufacture distortion in order to get a unpleasant & louder sounding music that gets everyones attention.

I read somewhere that IMD distortion occurs in the ear and this is how we perceive loudness, therefore, distortion that is added before the sound reaches our ears is interpreted as loud.