Double down, good or bad?

Ralph & Kirk,

I think that the confusion (or lack thereof) between power at the clipping point and output impedance comes down to what I said in my post dated 11/24. It is important to clearly maintain a distinction, in discussions such as we have in this thread, between doubling down of an amplifier's RATED power output into 4 ohms vs. 8 ohms (which is what this thread was originally about, and which amplifiers having very low output impedance may or may not be able to do), and the fact that ANY amplifier having very low output impedance WILL deliver double the power into a 4 ohm speaker impedance vs. an 8 ohm speaker impedance, provided that it is not driven to the point where its output is limited by clipping or other factors.

Best regards,
-- Al

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