Double down, good or bad?

Ralph, sorry but to my mind that's cheating. Were the tube amps driving auto-transformers or Apogees?

Unsound, thanks for the link to the Threshold Stasis article. From the description given, there are two main differences between the Stasis circuit and a conventional solid-state amplifier.

First the output stage -- a standard bipolar emitter-follower output section can be thought of as using 100% local voltage feedback, purely a function of the transistors' exponential Vbe/Ic characteristic, which delivers very good linearity and low output impedance (even before global feedback) as a result of the bipolar transistor's high transconductance. It seems that the Stasis circuit instead uses local current feedback to effectively bootstrap the output transistors to the voltage amp, and its linearity and output impedance will be a result from the particulars of the "current sensing" circuit employed (which I assume to be quite effective).

The other main difference isn't so much a Stasis thing, but it's the fact they use FET transconductance voltage amps, rather than a bipolar transresistance voltage amp. This means there's a fraction of the raw open-loop gain available. According to the text, there's no overall feedback, so I'm not quite sure about the hows and whys of the differential input stage, but suffice it to say that a low-gain FET voltage amp is a symbiotic choice when paired with a low- or zero-global-feedback design.

But in terms of clipping, I think the overall output impedance is pretty low, so it should still behave very similarly to a conventional amplifier as far as "doubling-down" is concerned.

Kirkus, thank you for sharing your insight.

This is a very enightening thread. I have followed along as I used to be real SS die hard, considering even to delve into ultra ML or Classe stuff until I heard a pair of excellant tube amps that were designed to operate on 4 ohms, and drive low impedence speakers:

Push-pull mono output stages with new driver electronics for the output tubes. This all-new design improves the transient response with complex loads and reduces distortion - particularly when low impedance speakers are connected. Ideal for critical loudspeakers such as Martin Logan, Avalon, Audio Physik, Caldera, Isophon Vertigo and Europa.

Clearly, I can agree with Atmasphere on how a design is capable of perforimng such tasks as driving low impedance speakers, it is most likely the topology in that implementation that needs to be considered. My amps, the Octave MRE 130 are rated at 130, tested and reviewed at 140, but 100 watts into 8 ohms.

All I can say is with technology like that, I can drive easily a pair of B&W 800's in my 25x18.5 listening room with thunderous bass and more than enough volume to make it concert level. The addition of the so called "Super Black Box" and their promised benefits in bass control and 2-ohm stability. The Super Black Box has an additional 100 plus amps of current and capacitance. I am not sure how other companies would implement such a feature, perhaps larger power supplies, but this concept works with these amps.

I believe that that tube amp builders who design amps capable of driving low impedance speakers from the start are actually doing the audio community justice.



Ciao,
Audioquest4life

Audioquest4life:
I see a few posts back that you state the Magnepans are a difficult load to drive.
Not So. The impedance is pretty flat and at least for the 1.6s, the phase angle is moderate at worst. The only fly in the ointment is they seem to like current.
That being said, even my old Rotel RB1070 of 130x2 into 8 with NO 4ohm spec did well until pushed.

Stereophile measures phase / impedance but few manufacturers publish such data.