speaker impeadance question

@stringreen 

yeah I am past that part. I just find the topic interesting. I have already made that determination on which way sounds better.

Thanks

I have already made that determination on which way sounds better.

I note that the OP accomplished the selection in a day or two. In decades of tube amp ownership, I've never found it to be so cut and dried, often taking many weeks / months to come to a determination. 

I find myself switching back and forth all the time

Whist I have marked levels on program for decades, I settle on a transformer tap for a particular speaker for a selection of well recorded material and be done with it. One might argue that if one continually switches, neither is ideal.

Examine Fig 1 response curve here in https://www.stereophile.com/content/mcintosh-mc501-monoblock-power-amplifier-measurements-0 for an AutoFormer MC501 and Fig 1 here https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements for a Ref150 and this https://www.stereophile.com/content/parasound-halo-jc-1-monoblock-power-amplifier-measurements for a SS JC-1

Now examine the schematic and impedance/phase diagrams for Stereophile's simulated load here https://www.stereophile.com/reference/60/index.html. [Oh, how I wish Stereophile would run 'transformer' amps into the simulated load from all taps.]

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. 

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.

@ieales

"I note that the OP accomplished the selection in a day or two. In decades of tube amp ownership, I've never found it to be so cut and dried, often taking many weeks / months to come to a determination"  

Funny you should say that. Sometimes you have to digress a little. Im no different. 

Here is my observation so far. 

Moving the Olympica iii's to the 8 ohm taps definitely makes the system sound more lively, but at a small cost. I use subs in my system and initially I listened with the subs on. With the subs off its apparent that on the 8 ohm connections without the subs playing as someone earlier stated the bass response is not quit as tight. Not that big a deal to me because obviously the subs make up for that. 

I have also noticed a slight decay in the sound stage however. This could just be an adjustment with speaker placement, however that is no small task. It has taken me months of dialing these speakers in to where I think they sound perfect so for this fact alone that could be a deal breaker for me. Now with that said the system does seem to have a little more airiness which I like! 

Along those lines definitely a little brighter sound, but not to the point that its annoying so far. No sibilance that I can detect on recordings that I think would reveal that. My room is rather flat so the brighter sound really is not enough to make me switch back if I were just measuring that only. With that said however I have to admit the speaker is not quite as natural sounding as it was for sure. One of the the things SF is good at is they're natural sound quality in this line of speaker anyway, and I have to admit that on the 8 ohm taps they sound a little top heavy, but not really in a bad way if that make sense. 

For me at this point its about trade offs. While the systems sounds a bit more lively, and airy which I like, but loosing some of the lower octave quality, and sound stage quality.  Could I get used to it and keep things the way they are probably , but being honest sound staging, and a natural sounding speaker is of high priority in my case. I think I would have to go back through speaker placement tweaking to make a good determination, and for me I don't think I want get in a cycle of speaker placement doing compare and contrast between the two taps. I did not intend on this being a laborious exercise. 

I will report back and let you know what I decide in a few weeks. Excuse my fickleness for lack of a better word, but I guess that's how this works in this hobby. 

-Keith

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.