Tube Amp for Martin Logan Speakers

Ralph and I and others discussed his views on the relation between feedback and output impedance in this thread. It essentially comes down to a matter of terminology.

Regards,
-- Al

Al and Ralph, I read the article that George kindly provided. I also re-read the posts at the link Al provided. I think Al correctly states that there is a bit of friction caused by the use of terminology.

I was looking for, but didn't find the post where I coined the terms "synthetic" or "apparent" constant voltage source where negative feedback (NF) is used. I think that using such terminology is useful insofar as it helps one to better appreciate whether an amp becomes a true constant voltage source if NF is used or just acts like a constant voltage source to some extent.

Let me explain. My take-away from George's article is that if an amp has an output impedance that is greater than Zero, some part of its power output is being consumed internally, resulting in a voltage drop at the output taps when driving an external load.

The author illustrated this effect with some simple examples using an external speaker load of 8 ohms and two hypothetical amps -- one having an output impedance of: (a) .1 ohms; and (2) the other having an output impedance of 2 ohms. The math then follows illustrating how the amp's output voltage and corresponding power output changes under each scenario.

I surmise that the terminology confusion comes into play because of what NF does ... and does NOT do. That is, if an amp has an output impedance of say 2 ohms, then as Ralph says, that is its output impedance ... period. But if NF is used, the amp compensates for the voltage drop by increasing output voltage to comepensate, thereby causing the amp to perform like, but not actually be, a constant voltage source amp.

As such, with output voltage adjusted to offset the voltage drop at the output taps resulting from the interaction of the amp's own internal impedance and the speaker's impedance, the amp functions as though it was a constant voltage source amp, or like one to a greater extent.

But there is always a BUTT. This picks up on what Al, Ralph and the author of the EPDR article have been saying over and over again. Using NF cannot create energy. That is ... an amp using NF will be able to generate power only to the extent it is capable of doing so based upon its internal architecture. Or, as the EPDR article said, within its SOA (safe operatwing area). If the amp operates outside its SOA, it will cut back its power output, clip, become unstable, produce distortion, etc.

I said this at the beginning of this thread and I'll repeat it again. I think we are all agreeing with each other much more than not. Butt, I think we are talking past each other.

IMO, given that **most** (not all) speaker manufacturers seem to be making speakers with roller coaster impedance and phase angle plots, erring on the side of am amp having lots of SOA headroom and low output impedance is a safe play. That generally equates to a SS beast that can deliver lots of current/power.

OTOH, as I said here and in other threads, to illustrate my understand, my ARC tube amp functions within a pretty wide operating corridor like a SS amp, but to do so, requires NF, and a massive power supply. Or using my own terminology, my amp performs like a "synthetic" SS amp, to some extent within a large "SOA."

Hope this helps ... butt it probably doesn't.

Excellent post, Bruce, with which I agree. I also agree with the first two-thirds or so of the paper George referenced, although I would point out that the author has taken the reasonable step of simplifying the circuit analysis he presents by only taking into account the resistive component of the load impedance.

I would take exception, however, to much of the last part of his paper, beginning at the point where he states that:

Loudspeakers are not constant power devices. Loudspeakers must have a constant drive voltage to provide a constant acoustic output with changing frequency.

IMO (and I feel safe in saying in Ralph's opinion as well), that statement is incorrect and misleading. I would agree with it if it referred to the majority of loudspeakers, rather than being expressed as applicable to all loudspeakers. Many electrostatics being notable exceptions to his statement, IMO.

The fundamental error he makes, IMO, is that he assumes that with all speakers flat frequency response in (in terms of voltage) corresponds to flat frequency response out. That will be true, at least approximately, for the majority of speakers these days. However it will not be true for a substantial number of speakers, including many electrostatics. As Ralph has pointed out many times, it comes down to the intention of the designer.

Consider the classic Quad ESL-57 George referred to earlier, which is revered to this day (impedance curve shown here). It was designed before solid state amplifiers existed! And it is very successfully used with tube amplification by many audiophiles to this day. Although in fairness I'll say that I recognize that **at least among solid state amplifiers** the vintage Mark Levinson ML-2 which George referred to is recognized as being a particularly synergistic match, and that combination (with two stacked Quads per channel) was the heart of the highly regarded HQD system produced by Mark Levinson's original company (together with a large Hartley woofer and a Decca supertweeter, with a total of six ML-2 monoblocks driving the two channels in a triamped configuration).

Regards,
-- Al

Thanks Al. My apologies for the numerous typos in my last post. I was typing with my laptop literally on my lap, drinking coffee with one hand and typing with the other hand. Truly a multi-handed effort.

That aside, I wanted to say that this has been one of the best threads to get posted in a long time. So kudos to all of the contributors.

I believe that if and when I ever change out my speakers or my amp, the terrific comments posted in this thread will make selecting a good amp/speaker combo a less haphhazard (sp) exercise. At the very least, I'll be able to ask better questions ... and know what to expect in terms of getting better answers.

Thanks again to all, including of course Ralph, Al, George, Mapman, Unsound and Judy.

Cheers,

Bruce

Output impedance does go down with nfb - that's a basic tenet. Easy to check: take a non-nfb amp and change the load d
from 8 to 4 ohms. You will see that the output voltage drops a bit. Now apply nfb, do the same thing and you will see that the drop is less, showing that the output impedance has dropped. In fact, it drops exactly with the feedback ratio. Apply 20dB nfb and it drops by a factoe of 10.
Of course we all know that Zout is not physically a resistor or sumptin'....

Jan