Please explain amplifier output impedance

Damping factor has less effect on a speaker than most might imagine. Keep in mind that any waveform will cause an amplifier to produce power (and incidently, it is *power* that drives all speakers- voltage cannot be produced in the absence of current and current cannot be produced in the absence of voltage...). That power will cause a voice coil (or other motive mechanism) to obtain a particular location with respect to rest. As the power level changes, the VC will follow it- in effect power driven to excursion and back again. Damping factor only plays a minor role.

What is really happening is that we are able to hear what negative feedback does to sound. Too much (more "damping") and the sound dries up, too little and with *some* speakers you loose flat frequency response.

In general tube amps have less feedback and many have none. This is not because it is not somehow available (this is the 21st century after all and we *do* have the technology). The problem is that feedack is a failed concept and many designers recognize that.

IOW having a 'constant voltage' output characteristic is a thought model and does not have a basis in the real world where our ears exist. As humans we are often looking for ways to place things in neat cubbyholes but Life itself does not care what we think- it exists in spite of our thought. Feedback and constant voltage are examples of cubbyholes that are thus not actually real.

Yes, it is impossible to make voltage without current. Power=Voltage X Current. Even in a preamp the voltage is there because current is there also. This fact is inescapable and is the result of Ohm's Law, the basic law of all electricity.

The reason Negative Feedback does not work is two-fold. Propagation delays inside all audio amplifiers insure that negative feedback arrives with a delay with respect to the input signal its supposed to provide correction for. At bass frequencies this problem is not profound, but at treble frequencies it is responsible for added odd-ordered harmonic content which (although in small levels) is something that the human ear uses as loudness cues- in effect a source of unnatural harshness to the human ear.

Negative feedback runs counter to the rules our ears use and we're stuck with the ears we have. If we could eliminate the propagation delays inside audio amplifiers and gain stages, NF would work, but until then Negative Feedback is a failed concept.

Hi Bob, in the case you mention, no work would be done. IOW this has no bearing on driving a speaker.

Shadorne, in fact we operate in a world of paradigms. If this was glossed over in school, paradigms are a set of rules that are accepted as fact until the flaws in the rules are perceived. Then a new paradigm emerges; the old paradigm comes to an end. We are living in an era of transistion (which has been going on for the last 10?-15? years) now: some of the stuff that you (and me, and thousands of others) were taught is now being found to be not so truthful. Take a look at Duke's post above- it points directly to the problem that negative feedback causes- in fact negative feedback is a failed concept (old paradigm) in audio. Astrology too was taught in the world's major universities as fact less than 400 years ago :)

What the theory of negative feedback overlooks is that propagation delay exists. Since propagation delay is a fact of the real world we are now witnessing the emergence of a new paradigm.

The alternative paradigm has a different set of rules. It too looks for low distortion, but achieved in a way that does not offend the human ear (i.e. no feedback).

No worries Sean! I agree with much of your comments regarding the ideal amplifier also. The issue for me personally when I set forth on the designs I did was how to get around the ill effets of negative feedback. Some of our early designs used it, and we pressed the limits of the tubes to produce super wide open loop bandwidth, in an attempt to put the propogation delay effects out beyond 20KHz. The result was full power bandwidth to nearly 30 MHz!

It didn't solve the feedback issues though (which I have previously discussed). We had set the feedback up so that we could switch it. In the end we chose zero feedback, and in the process discovered that a different set of design rules emerges.

I've had some discussion of these rules and the competing (more accepted, Voltage Paradigm) rules in the past. Both have challenges to overcome- the Power Paradigm has to find ways to make amplifiers more universal as you say and the Voltage Paradigm has to find a way to get loop feedback to arrive at the input of the amplifier in time to make the correct adjustment, not an approximate one (in order to create a musical presentation). Both are serious challenges!

Hi Shadorne, if you look at the quote you will see that it is in fact *harmonic distortion*, not IMD. Several studies now have shown the same thing: the human ear/brain system uses odd-order harmonics beyond the 7th harmonic or so as loudness cues. In nature these harmonics are quite low- and even very slight enhancement of them is easily detected by the human ear.

I learned this years ago while servicing an amplifier on the bench. The output of the amplifier was connected to a loudspeaker and a VU meter. The amp had a sine wave at the input. While malfunctioning and making less than 20db of its normal output, it still sounded louder than the normal undistorted output. Once you experience this you will not forget it!

Shadorne- Yes. This is a major reason why tubes continue to be popular- they create less loudness cues.

Success? I would consider damping over 20:1 to be excessive. I use master tapes for reference and one thing has become clear over the years: high damping factor equates to not getting the bass right; retentive (not in a good way) IOW punch without real definition and the first thing to go is always low frequency ambience.

Some speakers are intentionally designed for amps of higher output impedance (new paradigm, BTW) in order to take advantage of the benefits such amps offer. IOW there is no *ideal* value for output impedance- it all depends on the speaker...

The lesson here is that you have to pay attention to the speaker/amplifier interface regardless of the amplifier or speaker that you have chosen. To ignore this means you could flush thousands of dollars away to no good effect.

Hi Sean, I can't think of an amplifier that that qualifies as 'universal use'. For any example you can think of I can think of a speaker it does not work with. On that account your argument seems to fall apart. It is also not true that I 'prefer vented speakers', in fact what I prefer is speakers that sound right regardless of their technology. I use headphones (Stax, Grado) and master tapes for reference to avoid the pitfalls that you (?might?) have assumed that I have fallen prey to.

We've used a variety of speakers over the years including acoustic suspension ('sealed'), horns, bass reflex, magnetic and ES planars. On all of these speakers it was easy to demonstrate that our amps were making correct bass against the typical power house transistor amps which do not.

I should also point out that I do use a set of design rules based on reality and easily proven. Nor do we lack in the measurement department for them (see our website for details). IOW we respect the human ear and the rules that *it* uses, rather than made up rules that it does not use.