Please explain amplifier output impedance

>>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.

I have spent most of the past few months designing a couple of loudspeakers specifically intended to work well with high output impedance tube amps. So hopefully what I learned (mostly the hard way) will add to the discussion.

I don't know enough about amplifier design to give a technical description of output impedance. From what I do understand, negative feedback is a means of significantly reducing the output impedance (raising the damping factor); unfortunately negative feedback often introduces audible problems. So, it's often an amplifier design trade-off.

Note that "damping factor" is calculated by dividing the input impedance of the loudspeaker (typically 8 ohms) by the output impedance of the amplifier. So discussions of damping factor are discussions of amplifier output impedance.

For ideal power transfer, the input impedance of the loudspeaker should be many times higher than the output impedance of the amplifier. Let's take Newbee's amplifier with the 3.5 ohm output impedance as an example of what happens with a high output impedance amplifier. We'll assume that the output impedance is 3.5 ohms across the spectrum (Newbee says it isn't, but I don't want to overly complicate this illustration).

Now let's say we pair up Newbee's amp with an "8 ohm" speaker whose impedance curve has a 40-ohm peak in the bass region, dips to 4 ohms in the midbass, rises to 20 ohms at the 2.5 kHz crossover, and gradually falls back to 8 ohms in the high treble. Assuming this speaker has a perfectly flat frequency response curve when driven by a low output impedance solid state amp, here's what will happen when it's driven by Newbee's amp:

The speaker/amp combination will have increased energy in the deep bass because the amp will deliver more than its 8-ohm rated power into the bass impedance peak, perhaps as much as 3 dB more. It will deliver only about half its rated power into the 4 ohm midbass dip, so we'll see a good 3 dB dip in that region. Into the high impedance in the crossover region we'll once again see increased output, maybe about 2 dB more. Since the impedance remains above 8 ohms across the treble region, the SPL will be remain slightly elevated in the top half of the spectrum. Overall, not a pretty sight. The increased deep bass energy might be beneficial, but above the bass region the net effect is clearly detrimental.

One solution would be to choose a loudspeaker that has frequency response dips in regions where the impedance curve has peaks, so that with a high output impedance amplifier the net effect will be a smoothing of the frequency response. Based on eyeballing frequency response and impedance curves, I think that Coincident and Silverline use this approach. Actually, I suspect that the designers used high output impedance tube amps in the design stage, but when SoundStage or Stereophile measures the frequency response they use a low output impedance solid state amp so we don't really see the frequency response that the designer intended.

Another solution is to keep the impedance curve as smooth as possible, so that the speaker's frequency response doesn't vary much with amplifier's output impedance. Obviously in the example above, if we'd used a "6 ohm" speaker whose impedance stayed between 4 and 8 ohms above the bass peaks, the result would be a much smoother frequency response than we got with our hypothetical "8 ohm speaker". The Reference 3a deCapo uses this approach, and according to SoundStage's measurements its impedance varies between about 5.5 and 11 ohms. Also, the 11 ohm maximum in the lower treble is in a region where the speaker normally has a frequency response dip. No wonder people with SET and OTL amps like it.

High amplifier output impedance certainly presents challenges in loudspeaker matching, but the reduced high order distortion (introduced by the high levels of negative feedback usually needed for ultralow output impedances) is in my opinion quite desirable. An in-depth study of distortion perception recently published in the Journal of the Audio Engineering Society points towards the same conclusion - namely, that high levels of low-order distortion are audibly insignificant while low levels of high order distortion tend to be quite audible and objectionable.

Duke