speaker impeadance question

An amplifier having "constant power" aka "constant wattage" characteristics will, for a given input voltage, tend to deliver less output voltage at frequencies for which speaker impedance is low, and more output voltage at frequencies for which speaker impedance is high. That will result in loosely approximating delivery of constant power into those varying impedances. Most tube amps fall into that category, to a loose approximation. How loose that approximation is will depend on both the output impedance of the particular amplifier (which varies widely among different tube amps), and on how the impedance of the particular speaker varies over the frequency range.

@almarg  Just as a bit of clarification on this bit: any tube amp can be made to act as a voltage source if you can add enough voltage feedback. If the amp is zero feedback it will behave more like a power source. Further muddying the waters is the simple fact that just because an amp doesn't double power as impedance is halved does not mean that it can't behave with a constant voltage characteristic. Many tube amps do exactly that.


In the 1950s before the idea of voltage driven speakers caught on, it was seen as a good thing to make sure that the amp had low distortion as well as the correct damping factor for a given speaker. Now some open baffle speakers need a damping factor of only 1:10 and you did read that correctly: where the amp has 10x the output impedance as opposed to the loudspeaker. OTOH some speakers do need 20:1 to sound right.


To allow this to occur, in the 1950s a number of amps were made with a 'Damping Control' which balanced voltage feedback against current feedback. So at one extreme the amp behaved with constant voltage; at the other with constant current, and in the middle where the voltage and current feedback were in equilibrium, constant power. BTW its worthy of note that this idea has nothing to do with solid state vs tubes.


IME/IMO, while the Voltage Paradigm has taken over decades ago, the simple fact is that there are speakers out there that don't and **can't** work under the voltage rules- specifically ESLs, a number of magnetic planars, a good number of horns, 'full range' drivers and certain box speakers that designers noticed that they sound better with tubes than solid state...  I think the damping control isn't a bad idea at all; if you want plug and play, the damping control is more likely to do the job for you.

I don't really like to use the phrase "constant voltage" here. I think that the term "voltage source." is more clear.

In the industry the two are pretty well interchangeable.

Compare how the three WELL REGARDED amps react to the impedance curve of the simulated load: JC-1 barely, MC501 some and Ref150 plenty. Amps do not have a constant output impedance across the power band. Coupled to a non-constant loudspeaker impedance, the combination of the two are a tone control.

Looking at the phase diagram for the SF Olyiii, we see it leading the 500 to 2k region and abruptly lags @ ≈3k. Given a selection of transformer amps, some may sound best on 4Ω, others on 8Ω [2Ω/16Ω anyone?] and some not at all depending how their impedance reacts with that of the loudspeaker IN THE SYSTEM.

Combined with program phase and level anomalies one has the makings for a Sisyphean task of finding an ideal solution.

Ironically it was MacIntosh and ElectroVoice that led the charge back in the late 1950s to the idea of driving speakers with an amplifier that acts with a constant voltage behavior, i.e. Voltage source. This was accomplished by simply having enough feedback that the output impedance was low enough that constant voltage was possible **with most speakers available at the time**.

As time went by, lower output impedances became possible and as a bit of a self-fulfilling prophecy, necessary.


The problem with this sort of approach has to do with how humans perceive sound pressure, which is done by sensing higher ordered harmonics. While feedback is well-known to suppress distortion, its also well-known to add higher ordered harmonics of its own through a process of bifurcation. This is why we are seeing SETs and other amplifiers that have no feedback and relatively high output impedances, since:
1) you can make a speaker designed for a high impedance output in the amp and get very nice linear response
2) in a nutshell as pointed out above, making an amplifier to act as a voltage source isn't foolproof- its a "Sisyphean task". So after 60 years we still don't have the plug and play flat frequency response that was the goal so long ago. A pragmatic observer might easily come to the conclusion that we never will.

You won't damage the speaker. The feedback in the Mac will handle that impedance spike just fine.