Please explain amplifier output impedance

Bob and Duke, thanks for your great replies. Very helpful.

Still, no one has answered my basic question:

"What amplifier output impedance measurement is considered too low (X ohm - low), too high (X ohm - high), and just right (X ohm - ideal)?"

As an analogy, I know that a loudspeaker with an 8 ohm impedance is "just about right", given that a flatter impedance curve is more desirable than one that fluctuates wildly. Still, an 8 ohm nominal loudspeaker is a safe choice to be driven by a wide variety of amplifiers. On the other hand, a 4 ohm nominal loudspeaker starts to eliminate some amplifiers from contention, and a 12 ohm nominal loudspeaker opens up the possibility of mating with a wide variety of amplifiers.

So...what measurement of amplifier output impedance is analagous to these loudspeaker examples?

Duke, are you suggesting that amplifier impedence has a contstant value accross the spectrum, or were you only trying to simplify your explaination? You've confused the half of my brain I have still have left, I think! I thought amp output impedence was a varible over the frequency range.

Hi Tvad,

Sorry I completely overlooked your basic question.

I'm going to answer your question a little bit different from the way you asked it, and the numbers I'm pulling out of the air are somewhat arbitrary. I think whether or not an amp has "too low" and "too high" of an output impedance depends on the speakers as much as on the amplifier.

If the amplifier has an output impedance of .1 ohms or less (damping factor of 80 or more), I think it will have a flat frequency response into any speaker in production today. This may or may not be desirable! Many very high efficiency speakers have weak bass, but a relatively high impedance in the bass region, so they rely on an amplifier with a high output impedance to "warm up" the bass and restore proper tonal balance.

Between .1 and 1 ohms output impedance, the amplifier will work well with most speakers that do not rely upon high output impedance to warm up the bass.

Above 1 ohms output impedance, I think you better start looking at the speaker's impedance curve as well as its frequency response curve.

Above 4 ohms output impedance, the speaker's impedance curve is very important. Now as mentioned above, some speakers will definitely sound better when paired with an amplifier having a very high output impedance.

I think it's a good rule of thumb to avoid a speaker/amp combination where the speaker's impedance dips down to or below the amp's output impedance.

In practice, with most speakers the answer to your question is that there is no such thing as too high of an output impedance; only too low. With a few speakers, there's a "just right" range, but it varies from speaker to speaker.

I mentioned before that an amplifier may be trading off other sonic qualities to achieve a very low output impedance. Let me illustrate with a story involving the Wolcott monoblocks, which are push-pull tube amps with a variable output impedance control. On SoundLab electrostats at least, too low of an output impedance causes the sound to become dry and the soundstage depth to collapse. Unfortunately the variable output impedance control isn't marked so I don't know what value I'm choosing by ear. But at least in this case there are sonic tradeoffs to very low output impedance that I do not like.

Duke

Newbee,

I don't really know what amplifier output impedance curves really look like, and was just using an imaginary "flat" curve for my illustration. Capacitive or inductive behavior would of course change that.

Do you know what the output impedance curve on your amplifier looks like?

Duke

Thanks again Duke, that helps.

I have noticed the variable output impedance control on the Wolcott amplifiers. How cool is that?!