Please explain amplifier output impedance

Tvad: There isn't a specific point where an amp is either considered to be high or low output impedance as it is a relative thing. Having said that, a lower output impedance is typicall considered to be a more desirable trait in most cases. As previously mentioned, this sole criteria should not be used to judge whether one amp is "better" than another, but looked at as part of the total package when considering system synergy / component compatability.

If one wants to avoid potential problems in this area, look for speakers that maintain a relatively higher nominal impedances. Some speakers are specifically designed to stay above 10+ ohms. As you probably know, these are designed to be more compatible with tubed amps that typically demonstrate higher output impedances. Sean
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10-12-06: Sean
...lower output impedance is typically considered to be a more desirable trait in most cases.

In your view, what measurement is considered to be a lower output impedance?

BTW, I recently did an extensive search for loudspeakers. I'd say the list included 20 loudspeakers from 20 manufacturers. They were all dynamic designs. Only one model had a nominal impedance above 10 ohms (Coincident Super Eclipse III). I don't believe a 10+ nominal impedance is a realistic goal when shopping for loudspeakers...even on esoteric horn loaded loudspeakers.

Perhaps you could offer three loudspeakers that have 10+ ohm nominal impedance?

Tvad: The speakers that came to mind right away were the Coincident's that you mentioned. I don't know of others that i can rattle off though, as i really don't pay attention to / go out of my way looking for such designs. Most all of my gear / installations revolve around lower impedance designs, hence my lack of familiarity. Having said that, my preferences in loudspeakers also tends to dictate my preferences for higher powered SS amps that run in a very rich Class AB or Class A mode.

As to output impedance on an amp, my personal opinion ( and that is all that it is ) would be to "draw the line" at about .2 of an ohm or so*. Obviously, there's a bit of a "fudge factor" involved here and what i would consider to be "acceptable" would somewhat depend on the load that it would be driving. Obviously, i would like to see it lower than this, but with some designs, that would require the use of massive amounts of feedback. At that point, it becomes a balancing act as far as which evil you want to subdue the worst.

With that in mind, most tubed amps are noticeably above this level, hence the looser low frequency response and greater variations in performance from speaker to speaker. I have seen some tubed amps that exhibited output impedances that were quite high i.e. above 8 ohms on certain taps. These amps would tend to be extreme sonic chameleons i.e. drastically changing their bandwidth, transient response and tonal balance as the load that they saw was varied. It would be a situation like this where some speaker cables can DRASTICALLY make very audible differences. Sean
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* For those that can't do the math, an output impedance of .2 ohm would produce a damping factor of 40 as referenced to an 8 ohm speaker. This would be an acceptable starting point for someone trying to drive a larger woofer with a decent sized motor structure. Smaller diameter woofers with smaller motors and / or limited excursion might get away with a slightly lower DF ( damping factor ) without any really noticeable problems.

In some cases, a lower damping factor might actually be desirable, as it might lend a fuller tonal balance to an otherwise thin sounding system. Only problem is, that loss of DF would typically come at the expense of transient response, so once again, system synergy comes into play.

Thanks for that, Sean.

Sean, I recently spent several weeks designing two loudspeakers specifically to work well with high output impedance tube amps. I wanted them to also work well with low output impedance solid state amps, though that was a secondary priority.

Getting the speakers to work well with a high output impedance amp was not difficult as long as I kept the speaker's input impedance about 15% higher than the amplifier's output impedance. Now granted a higher impedance speaker would have theoretically worked better, but the 16-ohm drivers I tried didn't sound as good. So I opted for what sounded better to me.

A much greater challenge was meeting my secondary priority - that the speakers still sound good with a solid state amp. It took me a very long time to get the impedance curves smooth enough that there wasn't a significant tonal balance difference depending on which amp I used. Easy to smooth the impedance curves, but hard to do so without screwing something else up. And in the end I'd still say that optimium bass tuning with the solid state amp is a few Hz higher than with the high output impedance tube amp because I left the bass impedance peaks intact (didn't try to smooth them by overstuffing the cabinet). To accomodate both amp types (as well as variations in room boundary reinforcement) I went with a port system that is somewhat user-adjustable.

Tvad, PHY and Lowther both make full-range drivers that have a nominal impedance of 16 ohms, and I don't think they dip below 12 ohms.

Let me mention two other speaker lines with models that work well with SET and OTL amps: Silverline and Reference 3a.

The Silverline Sonatina III, Bolero and Panatina II don't have particularly smooth impedance curves, but with a solid state amp their frequency response curves dip where their impedance curves peak. So with a high output impedance tube amp, their frequency response will be smoother than with a solid state amp.

The Reference 3a DeCapo has a very smooth impedance curve, varying between about 6 and 11 ohms above the bass region. And, the 11 ohm maximum is in the region where there's a frequency response dip with a solid state amp, once again helping to smooth the frequency response with an OTL or SET tube amp.

Duke