Autoformer vs Speaker impedance Curve

Atmasphere, regarding your link:

1. In general do reactive speakers match better with voltage paradigm amps?

2. What happens to linearity with a voltage paradigm amp?  Does the sp increase where the ohms dip (ie watts increase)—with an increase in distortion—and decrease where the ohms increase? 

3.  Same question as above, but with power paradigm amp. What happens to sp where ohms dip or increase...is it the opposite of voltage paradigm amps? (

(I’m sure the above varies with speaker and amp design, negative feed back and such, but just in the most general of terms)

Mac led the way in the late 1950s towards developing the idea that speakers be ’voltage driven’. 

Does this mean Macs are voltage paradigm amps? Doesn’t this conflict with the way autoformers function?  Seems they would be power paradigm, especially given their multiple output taps. A little over my head here, but learning, so please forgive my ignorance!

Even though I addressed this to atmasphere, I welcome and want all input!

Rob

Does this mean Macs are voltage paradigm amps?

Yes. Which paradigm an amp fits into is determined by its output impedance. (And btw, that categorization is along a continuum, rather than being a purely black and white distinction. Especially in the case of tube amps, which vary widely in their output impedances).

If the output impedance of an amp is a tiny fraction of an ohm, or is at least a very small fraction of the impedance of the speaker at any frequency, it will behave as a voltage source. Which means that for a given input signal to the amp, it will output a voltage which essentially has no variation as a function of the impedance of the speaker at whatever frequencies may be present, as long as the amp is operated within the limits of its maximum voltage, current, power, and thermal capabilities.

And in the case of McIntosh solid state amps having autoformers, the combination of their solid state output stages, the autoformers, and what I’m pretty certain is the liberal application of feedback is most of their designs, results in a very low output impedance. The MC302 I referred to earlier being an example.

Tube amps, on the other hand, will just about invariably have relatively high output impedances, usually somewhere between a large fraction of an ohm and several ohms, as I mentioned earlier. That will bring just about all tube amps much closer to the power paradigm end of the spectrum.

Regarding your questions 2 and 3, it follows from Ohm’s Law (I = E/R) and the definition of power (P = E x I, for a resistive load), where I is current, E is voltage, and R is resistance, that if a constant voltage is maintained into a varying load (as it would be by a voltage paradigm amp) more current and hence more power will be delivered into low impedances than into high impedances (assuming at least that all of the impedances are mostly resistive). It also follows that a power paradigm amp will come much closer than a voltage paradigm amp to maintaining constant power into those varying impedances, for a given input voltage to the amp, rather than maintaining constant voltage.

It may help to clarify some of this, btw, if you take a look at the Wikipedia writeup on voltage dividers that I referred to earlier. In the first figure on that page, consider Z1 to represent the output impedance of the amp, and Z2 to represent the impedance of the speaker. And consider Vin to be the voltage the amp is "trying" to put out, meaning the voltage it would supply without a speaker or other load being connected, and Vout to be the voltage seen by the speaker.

None of this necessarily means, however, that frequency response flatness (which you appeared to be referring to when you mentioned "linearity") will be compromised if additional power is or is not supplied into impedance dips. Depending on the design of the speaker its efficiency (SPL out vs. watts in) may or may not vary in a manner that is consistent with its impedance curve. As Ralph (Atmasphere) has said in a number of past threads, a tonal imbalance is especially likely to result when the paradigms to which the speaker and the amp conform are not the same.

Regards,
-- Al

Thanks Al. Very informative!  Will take a look at the Wikipedia reference 

an Autoformer would work, because it’s one of the only speakers that presents a very benign 3-4 ohm impedance load.

This statement ignores the fact that loop feedback compensates for this sort of thing.

Atmasphere, regarding your link:

1. In general do reactive speakers match better with voltage paradigm amps?

That has to do with the intentions of the speaker designer. In a general fashion, another way to answer this is that most speakers with highly reactive loads are Voltage Paradigm devices and so the answer would be 'yes'.

2. What happens to linearity with a voltage paradigm amp?  Does the sp increase where the ohms dip (ie watts increase)—with an increase in distortion—and decrease where the ohms increase?

Linearity usually refers to distortion... to the latter question, the sound pressure should stay constant, while the power input fluctuates.

3.  Same question as above, but with power paradigm amp. What happens to sp where ohms dip or increase...is it the opposite of voltage paradigm amps?

If the amp is on a speaker that is designed using Power rules, the output will vary only a little since the designer isn't expecting the amp to throttle its output power back on higher impedances. A good example of this phenomena is an electrostatic loudspeaker, whose impedance is based on a capacitor rather than the impedance of a driver in a box. The idea here is that distortion is kept down, with a preference for lower distortion rather than perfectly flat frequency response, since **the latter doesn't exist** despite what speaker or amp is used.

Does this mean Macs are voltage paradigm amps? Doesn’t this conflict with the way autoformers function?  Seems they would be power paradigm, especially given their multiple output taps. A little over my head here, but learning, so please forgive my ignorance!

Mac amplifiers have always been voltage source amplifiers since the 1950s. It does not conflict with the use of an autoformer; such use is not the defining aspect. Most transformer coupled amps are voltage sources too. The taps are used to optimize the interface between the output devices and the speaker to minimize distortion. To make a power paradigm amp you have two pathways- either no feedback at all, or current feedback and voltage feedback of equal amounts. Since all forms of loop feedback are known to add higher ordered harmonic distortions as well as intermodulations, zero feedback is preferred, if adequate means are employed to otherwise suppress distortion in the amplifier. In this way the result can be considerably less colored, despite likely not having perfectly linear frequency response, due to the way the ear perceives distortion. 

Put another way, the ear hears tiny amounts of higher ordered harmonics with striking ease. So if an amp has 0.001THD, but all the distortion it has is the 5th harmonic and above, it will sound bright and harsh. **That** is a coloration, and not a particularly pleasant one. This is why tubes vs transistors has been such an on-going debate and why tubes are still around.

an Autoformer would work, because it’s one of the only speakers that presents a very benign 3-4 ohm impedance load.

This statement ignores the fact that loop feedback compensates for this sort of thing.

Loop feedback compensates for the autoformer

How the designer set up the amplifier with the transformer plays a big role. The reason you do this BTW is to reduce distortion- all amps have higher distortion playing lower impedances, so if you can raise the overall impedance seen by the output devices distortion will be reduced. This is why Mac uses autoformers. 

Autoformers reduce distortion

Since all forms of loop feedback are known to add higher ordered harmonic distortions as well as intermodulations.

Loop feedback causes distortion 

Okay.  Got it:

Autoformers reduce distortion but need to be compensated for with loop feedback which adds higher order harmonic distortion...the worse kind!  🤯

Sorry, couldn’t resist having a bit of fun with this!  🤪

I’m sure the interplay between these forces is well beyond the scope of this discussion  and certainly well beyond my very limited knowledge.

Thanks all for your input!  Much to think about.

Rob