Please explain amplifier output impedance

Output impedance is defined by the amplifiers Damping Factor. It is the ratio of the output impedance to the nominal speaker input impedance or load impedance.

In practice the higher the damping factor the better because cabling and the interaction of the amplifier and load will modify the response as amplifier input impedance rises and damping factor falls. A damping factor of around 100 is worth having if you seek accurate audio reproduction (above this there are still benefits but they may not be audible).

In short, an amplifier with high ouput impedance will be much less linear when coupled with a speaker than one with low output impedance. Warm one resonant bass response is also common with high output impedance amps as the natural low frequency resonance of the speaker drivers are not shunted/damped. (Some seek this warm bass response - in this case a high ouput impedance can be regarded as a tweak to get the desired sound. Some like to change cables to twak the soud an in this case too, a high output impedance amp and may be desirable as it magnifies the affects of cables.)

I agree with most of what Newbee said. Output impedance and speaker impedance together give the damping factor of the system, which is an indication of hte amp's ability to control the speaker. The higher the speaker impedance is over the amplifier output impedance, the higher the damping factor. It would be better, therefore, if your speaker and amplifier impedances dipped in the same spots. This is why many tube amplifiers (especially OTL, like AtmaSphere) like high impedance speakers, in the 8 - 16 ohm range. These are rare today, because solid state amplifiers have such low output impedances, the difference between 4 ohm and 8 ohm impedance has a vanishing differnece on the damping factor, but a solid state amp can put out more power into a 4 ohm speaker than an 8 ohm.

>>a solid state amp can put out more power into a 4 ohm speaker than an 8 ohm.<<

In some but not all solid state amplifiers.

Bob's explanation is a more accurate one than mine. The point is that impedance varies with frequency....therefore the response function of the combined system Amp-Cable-Speaker can vary greatly for amplifiers with high ouput impedance.....not necessaily a simple roll off....in fact the speaker impedance variation with frequency will color the response when coupled with a high output imepdance amp. Newbee describes very well what this means in practice: tube amps often have higher ouput impedance and therefore tend to sound warmer (a non linear but desired coloration to the sound that can help tune a system towards audio nirvanna)

A low amplifier output impedance (very high damping factor) makes the effects of cable and speaker load impedance close to negligible....although an almost imperceptible roll off naturally exists due to skin effect in cabling (higher frequencies see every so slightly more impedance)

Thanks for all the replies.

In short, an amplifier with high output impedance will be much less linear when coupled with a speaker than one with low output impedance.
Shadorne (System | Reviews | Threads | Answers)

Tube amps are notorious for having high output impedance 'curves' (one of mine has a rise to 3.5 ohms in the mid bass frequencies. The net effect of this is in the frequency where this rise occurs I will hear a corresponding rise in frequency response in the speaker.
Newbee (System | Threads | Answers)

From the limited research I've done, these two statements sum up the effect the best.

According to this article, a high output impedance amplifier will contribute to a loudspeaker reproducing music that is out of balance across the frequency range. Frequencies in which the loudspeaker has high impedance will be proportionately louder than those frequencies in which the speaker has low impedance. Therefore, treble could be significantly louder than the midrange, etc, thus contributing to a situation where the speaker seems "bright".

Shadorne touched on the second effect of amplifier output impedance, the damping factor, which has the effect of controlling the motion of the drivers...and therefore "tighter" or "looser" bass.

The article to which I refer above states that:

The lower the amplifier output impedance, the lower the speaker produced voltage is. The lower the speaker produced voltage, the less effect it has on cone motion. A good rule to follow is that amplifier output impedance must be at least ten times lower than speaker impedance for good control of cone motion. This is called damping factor. Amplifier output impedance must be less than one ohm for best performance.

Therefore, if one agrees with the author, an amplifier's output impedance must be less than 1 ohm.

I suppose that answers my question, although I'd like to hear from an expert or two who might specifically state what output impedance measurement is considered too low (X ohm - low), too high (X ohm - high), and just right (X ohm - ideal). Let's call it the Goldilocks Output Impedance Rule.