Please explain amplifier output impedance

* 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.

Much earlier in this thread I suggested 80 as an "ideal" damping factor or 0.1 Ohm output impedance as a good number to seek for a nominal 8 Ohm speaker load (not too much negative feedback and not too lacking in linearity/control when coupled with a speaker).

I can also live with Sean's very close suggestion above. I think, at least for once, we are reaching a consensus on your question Tvad; you have your "Goldilock's" answer as to what may be considered too low, too high and "just right" for amplifier output impedance in relation to load.

Of course, I hope everyone understands that this is a huge generalization that applies to SS amps and I would never recommend choosing one component over another based on this criteria alone.

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.)

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)

Tvad,

If you accept solid state amplifier technology and you do not desire to tweak the sound and wish for a tight controlled bass and wish to simplify it to a damping factor then I would say 80 is adequate and perhaps an ideal number.... i.e. for an 8 ohm load the amp should have an output impedance of of around 0.1 ohms. There is very little to be gained with even lower impedance and probably not audible anyway unless the speaker impedance drops very low.

BTW: This discussion applies to equipment input impedance too.....ideally you want a high input impedance (nominally around 10K Ohm) on all equipment prior to your speakers or headphones. This reduces the effect of interconnects and coupling of equipment in your system to almost negligible levels.

What is really happening is that we are able to hear what negative feedback does to sound

negative feedback just ensures linearity - it makes sure that the output matches the input. An electrical circuit operates at close to the speed of light...I doubt anyone can hear negative feedback in modern circuitry. Slew rates of good amps are typically 50 volts per micro second or enough to accurately reproduce a signal of well over 100 KHZ without distorting the signal. Since it is accepted that people rarely are able to hear anything above 20 KHZ then it is extremely doubtful that any slight anomalies of this kind of order are actually audible.

Speaker cones will try to keep moving because of inertia, the suspension pulls them back and as they move they induce current (EMF) in the coil which the amp will sense and will dampen by driving the output to match with the input. Provided the drivers have low mass, a high Xmax and a strong magnet then a powerful low output impedance amp should be able to control the driver well. If the driver is low quality, with a weak magnet and a low Xmax then it doesn't matter as much if the high powered amp has a high damping factor as the cones become harder to control as they travel well outside the linear range of the magnetic field...in this case, ultimately, the suspension pulls them back in but unfortunately at this point you have large amounts of audible distortion).

Negative feedback in high levels is quite audible and it doesn't sound good.

Agreed. A badly designed circuit will sound terrible. Most manufacturers try to avoid building unstable circuitry. However, in the pursuit of ridiculously high damping factor specifications (for marketing purposes), it is certainly possible to build a dangerously unstable circuit. Extremely high amplifier gain will lead to instability and oscillations. This occurs when feedback times open loop gain approaches negative one. In this case, the closed loop gain will approach infinity...which is of course not possible and everything becomes oscillatory, distorted and clipped.

In general, typical SS amplifier circuitry (with negative feedback loops), although very linear when operated within tolerance, are not at all forgiving when they are over-driven; typically when over-driven they sound harsh and then damage speakers fairly quickly. Certainly, high levels of negative feedback are likely to lead more quickly to catastrophic behavior.

Since music is very dynamic, it is relatively easy to over-drive equipment. Some SS gear has built in protection circuitry that is designed to detect and protect equipment from damage.

Negative Feedback is a failed concept

I humbly beg to disagree. Done properly, negative feedback produces excellent accuracy, stability and linearity in electronic operational amplifiers. If this is not the case, then much of my first year electrical engineering and the techniques employed by hundreds of thousands of engineers is deluded. I doubt that the entire electrical engineering industry suffers such monumental delusions as to use failed concepts for most analog circuitry.

His study found that very high levels of low order distortion (30% second harmonic) were inaudible, but very low levels of the type of high order distortion produced by large amounts of negative feedback were quite audible and highly objectionable

I assume you are refering to Intermodulation Distortion or IMD distortion in an amp that is oscillating from large amounts of negative feedback. I agree that this is far more detrimental to the sound and something our ears seem quite sensitive to. Harmonic distortion is often indistingushable from the real sound of the instrument because the pitch of the note does not change (the note becomes fuller or leaner sounding). Even harmonic distortion is particularly hard to discern as your physicist friend points out. My understanding is that odd harmonics are more easily discerned, although still not nearly as easy to discern as IMD.

Hearing Harmonics

IMD distortion is one of the best arguments for promoting active speakers. Separate amplification for each driver over a limited bandwidth can only help to reduce IMD distortion significantly. Compare this to passive full range speakers where the amplifier must control the woofer at 40HZ and the tweeter at up to 20 Khz....hardly suprising that such a broadband system introduces audible IMD due to the combined interaction of all the drivers, the crossover and the amplifier fighting to control them.

Atmasphere,

Interesting. Perceived loudness clearly can be different from actual loudness. I have experienced this too. My speakers go well over 100 db continuous SPL levels at the listening position and yet the perceived sound is less loud than when my daughter plays iTunes at maximum distorted levels from the mini 10W speakers connected to our PC in the study.

In fact I have to ask her to turn it down even though the SPL levels are miniscule in the kitchen which is about 20 feet from the study; distortion is tiring, distracting and it seems we are very sensitive to low levels of it.

This may also explain why compression and limiting applied to modern pop CD's makes them sound very loud and unpleasant, especially at higher listening levels (when perceived loudness due to distortion and real loudness become most unpleasant). The CD "loudness wars" are a way to intentionally manufacture distortion in order to get a unpleasant & louder sounding music that gets everyones attention.

I read somewhere that IMD distortion occurs in the ear and this is how we perceive loudness, therefore, distortion that is added before the sound reaches our ears is interpreted as loud.

Success. I think we all agree if you accept that my persepctive is "old school" (from electronics text books, linearity, accuracy etc.)

Furthermore, I fully agree with Atmasphere that a low frequency ambience or bass definition is what a tube amp or an amp with a lower damping factor does to the sound.

Sean & Dave,

Thanks for talking this discussion even further.

There are quite different design philosophies behind the conflicting views on output impedance;

One philosophy is that the amplifier and cables should have a minimum affect on the speaker response - i.e. the speaker is designed for SS amps of many types and the speaker impedance variation with frequency is NOT intended to affect system frequency response. (provided the amp has enough power to drive the load this philosophy leads to consistent results => the flat speaker response will be maintained provided it is coupled with a low output impedance SS amp.)

The other philosophy is that amplifier and cables should be chosen in order to augment/adjust speaker response by behaving as a kind of "equalizer" or a tuned system; in this case, system response will vary as a function of speaker impedance. (amp ouput impedance - cable - speaker load act together as a filter/equalizer on the sound => this leads to a system which can be tuned to taste, according to the preferences of the user and the selected high output impedance amp)

It is the differing philosophy that leads to different views on amplifier output impedance. There are advantages to both. One is consistent and accurate; but you get what you get. The other approach offers greater flexibility for tweaking to taste.

This is why I qualified my earlier comments in this thread by saying they were generalizations for SS amps an not for tubes. Tubes seem to be desirable to those that like a certain flavor to the sound...a certain warmth or punch to the bass. (and there is nothing wrong with that. Doug Sax, one of the industry leading professional Mastering Engineers, uses tubes for mastering for this very reason. Indeed, most people seem to prefer an equalization curve with a boosted bass and a rolled off treble.

I repeat, a low output amplifier impedance does not guarantee proper speaker damping. It often actually overdamps the speaker. The subject of overdamping, critical damping and underdamping is a well-studied topic in engineering

I agree that damping is well studied in engineering, however, in engineering "damping" is NOT the same thing as the audio "damping factor" term used to describe an amplifier. Actually "damping factor" is simply a reflection on the amp output impedance relative to to the speaker load. This is an "indication" of how well an amplifier can be expected to handle a reactive load. It should not taken as gospel ...there is a lot more to an amplifier than damping factor.

Once the output impedance is low relative to the load then the "damping" of the driver (suspension/magnet/coil)and box will dominate the response...The amplifier simply puts the correct voltage over the speaker terminals which always pushes the driver towards the proper position (as determined by the audio signal) by supplying the appropriate current.

In theory more current can be supplied for a given amplifier voltage if the output impedance is lower...i.e more "push" in the right direction.

Apologies if I have oversimplified things but I don't see how amplifier damping factor can be regarded in quite the same way as the critical damped response that is often saught in most speaker designs.