Please explain amplifier output impedance

Impedence curves of amps is not usually part of the spec's. But, when you read reviews where they have been measured you will usually find them referenced. One of the reasons is that SS amps generally have very low output impedence, well less than 1 ohm. The effect of this is that the speaker will see a very flat electrical signal.

Tube amps are notorious for having high output impedence '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. That's one reason some tube amps sound warmer/looser etc than others, and most warmer than SS. I'm unaware of a loudspeaker spec other than its impedence curve which would be meaningful. What I think (and I really mean think) would be helpful is if your speakers impedence curve dipped in the same frequency that the amps impedence rose they might produce a flatter tone than either would on its own.

Clear as mud? With any luck Sean will read this and sign in with a crystal clear explaination. As I said, FWIW.

I'm not an expert on the subject but always heard that solid state amplifiers are low voltage, high current devices and work better with lower impedence speakers (8 ohms or lower), whereas tube amplifiers are high voltage, low current devices and work better with higher impedence speakers (8 ohms or higher; up to 16 ohms). Eventhough amplifiers put out more power into a 4 ohm load vs. an 8 ohm load, when the impedence of a speaker goes lower (4 ohms or less) it becomes an increasing difficult load for many amplifiers. Like Newbee pointed out, the impedence curve varies throughout the freqency range; some 4 ohm speakers, depending on their crossover design, may dip as far down as 2 ohms or even 1 ohm, which is a very difficult load for most amplifiers (except Krells which don't seem to mind). Hope this makes sense.

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.