Please explain amplifier output impedance


I have recently read a few loudspeaker reviews which mention that the speakers would likely work best with low output impedance (or high output impedance) amplifiers.

So, what measurement defines low output impedance (or high output impedance) on an amplifier? What's the numerical value of low and high output impedance, and what is "average"?

Also, what specification of a loudspeaker provides info that would indicate using an amplifier with particular output impedance?

Thanks in advance for explaining this in laymen’s terms. :)
tvad
I have been following this thread with interest and doing my own novice research on this topic and it seems that there is no right answer here in terms of whether or not damping factor is a relevant spec. Here are some of the tidbits I have picked up:

1. The mechanical damping of the speaker plays the largest role in terms of how the cone movement is damped

2. The effect of any damping factor above 10 is impossible for humans to hear

3. A static damping factor number does tell the entire story since both amp and speaker impedance changes with frequency.

4. A high damping factor is misleading as it could mean the use of a lot of negative feedback which could have adversely affect the sound

These are just some bits of info I have picked up from reading about dampiing factor so please refrain from blasting me with flames. I am by no means an expert in this area.

I think there is no real good rule of thumb. Everyone has to just listen to the amp/speaker combo and decide for themselves.
I think there is no real good rule of thumb. Everyone has to just listen to the amp/speaker combo and decide for themselves.
Tboooe (System | Reviews | Threads | Answers)
Not to necessarily critique Tboooe's comment, but it prompted a thought.

It seems to me that if considerable discussion is had by several contributors to this thread, all of whom appear to know what they're talking about (at least to this non-technical layman), specifically identifying output impedance and/or damping factor as it relates to speaker impedance and how they affect sound. And, if mathematical relationships are developed that correlate and explain amp/speaker performance, which is what I gather has been discussed in these posts...then, to toss it all away by saying that there is no good rule of thumb and everyone simply has to listen to amp/speaker combinations and decide for themselves, is to disregard science and engineering and instead throw up our hands at any attempt to use specifications to narrow down choices for the best potential amp/loudspeaker matches.

I come away from these discussions believing the audiophile hobby is overcomplicated. This is undoubtedly due to the large numbers of boutique specialists who build their products in a semi-vacuum choosing to place ease-of-use and ease-of-matching at the bottom of the priority list. I understand this can be a benefit insofar as it allows almost infinite possibilities to tailor a system's sound, but it can also work to a cross purpose by keeping the goal elusive.

Please accept this as a comment from a music loving end-user who is not an electrical engineer, technophile, or DIY'er.

Someone could make a fortune by simplifying. I suppose Linn has been the most successful at this concept...and perhaps NAIM to a lesser extent, but these two manufacturers still haven't cornered enough of the market for me to believe there isn't room for others on a smaller scale and budget.

Thank you. Please remember to tip your waiter or waitress... and try the veal.
Tvad: Unfortunately, there is no "Big Dummy's Guide To Assembling A High Fidelity Music Reproduction System". It's NOT that easy. But then again, if it were that easy, you would have to find some other frustrating and equally expensive hobby to spend your time on : )

Personally, i have always thought that a "good" product was a "universal" product. Anything that requires very specific mating of componentry is strictly a "specialty" item. If one is getting involved with that type of product, they should be prepared to deal with the specifics that come with it. This is why i tend to "knock" certain products, as they are very specific in what they do. This makes them less than ideal for the average user that wants to maintain some semblance of balance.

Having said that, there's nothing wrong with a manufacturer targeting a certain market. So long as they are forthright in their product descriptions and are willing to make specific recommendations as to how to achieve optimum performance with their product(s) and others that will mate well with them, more power to them.

Atmasphere / Ralph: Your comments pertaining to damping factor are evidenced in how you design and impliment your products, which is good. This shows a form a consistency in both thought and action. This makes it easier for the consumer to understand what they are getting and what to expect from a given manufacturer / product line.

Having said that, i don't think that your amps are a "universal" product as described above. This moves them over into the "specialty" category ( in my opinion ). Given that you are willing to work with your customers and offer specific recommendations with reasoned explanations, your company is a great example of the "specialty" market.

The fact that you are willing to contribute to these threads and in other forums, share your opinion in a forthright manner without soft shoe dancing or pulling punches, etc... lets the customer know where you stand, what they can expect from you and your products and what will work best. Kudo's to you, as i wish more manufacturers were like this.

With all of that in mind, we obviously have differences of opinion on certain aspects of sound reproduction. You typically prefer vented speakers, whereas i typically prefer sealed speakers. You typically prefer a lower damping factor, i typically prefer a moderate to reasonably high damping factor. The sonic differences between each approach are typically easily identifiable when listening.

This does not make one approach "better" than the other sonically, as that all boils down to personal preference. Obviously, the differences could easily be debated electrically and acoustically. In that respect, i "know" that i'm right, and you "know" that you're right, so where does that leave us???

Suffice it to say that, it all still boils down to what one likes as an individual and system synergy. The only way that one would know for certain whether a product would be right for them would be to use & listen to that product for themselves. Knowing how things work and being able to decipher standardized test results can point us in a given direction, but they can't give you all of the answers. There's just too many variables involved and that's why the "Big Dummy's Guide" hasn't been written as of yet. Laura Dearborn, Robert Harley, etc... have tried, but the results are still pretty variable : )

As such, all one can do is to learn, listen and compare for themselves. When you find something that you like, sit back and enjoy both the music AND your new toy. After all, that's what it's all about, isn't it? Sean
>
There is no universal correct amplifier output impedance. This is why we and others offer a means to adjust output impedance/damping on some models. Many system designers have known this for years, and damping adjustment controls were found on some amplifiers going back to the 1950s or before. The reason that there is no one correct output impedance is because the speaker is a mechanical system with inertia and its transient response is very much determined by the source impedance driving it. Each speaker design behaves differently, and it is usually desirable to achieve what's called "critical damping". If the mechanical system (speaker) is under damped, the cone will overshoot and ring in response to a step transient. If it is overdamped, it will undershoot and be slow to reach its proper position. Underdamping happens when the output impedance of the amplifier is too high, where as overdamping occurs when the output impedance is too low. In terms of the bass reproduction, underdamping results in an overly warm resonant reproduction, and overdamping results in a overly dry and heavy presentation. Neither of these are correct. When the amplifier output impedance is optimum, the bass will have its most natural reproduction. A damping/feedback control on the amplifier lets the user best tune the speaker, and yes, the room acoustics enter into it as well.

I made a gadget for observing the optimum damping of a speaker cone in response to a step transient driven from various impedances. The device uses a lightweight conductive foil that can be taped to the cone of the driver, and a matching RF based capacitance probe is used to detect cone movement and display it on an oscilloscope. I can confirm various degrees of over and under damping, and have tested speakers that range from wanting to see as high as a 7.5 ohm amplifier output impedance (this was a large driver and quite unexpected) to speakers that are still underdamped with zero output impedance. I find that most high-end speakers fall in the range of working best with 2-4 ohms amplifier output impedance.

Of course, this is the bass driver movement and is easiest to measure. The higher frequency drivers in a particular speaker may want to see something different, and it seems still impossible to be completely scientific in determining what is really best when the whole frequency spectrum is taken into account. This is why it is really best to simply listen with an open mind, and not assume that there is a spec that will really tell you how good or accurate a speaker-amplifier combination is going to be.

Over an over again people get hung up on worrying about the issue of a high output impedance amplifier impedance affecting the frequency response. It is very easy to try to explain the sonic differences by making the argument that the frequency response of the system wobbles all over the place because the amplifier output voltage goes up at frequencies where the speaker impedance rises. Yes, this does happen, but how flat do these people think the frequency response in the listening room is if this were not to happen. Yes, if the output impedance of the amplifier was 50 ohms, and a very underdamped speaker reached a 50 ohm impedance, there would be a sharp peak in the system response. Whereas some speaker drivers can reach 50 ohms, an amplifier with an output impedance of as high as 5-10 ohms will largely tame it (but perhaps not properly damp it). In practice, the system frequency response flatness will likely have the similar degrees of deviation for all amplifiers from zero to 10 ohms output impedance. The particular frequency points of maximum and minimum response can be expected to shift around slightly, but this alone would not account for a significant enhancement or degradation of the sound. They will sound very different though, but because of the damping.