Amp Designs That Are Able To Drive 4/2 Ohms Load

My one tube amp is stable at 2 ohms, but the same wattage output as 8 ohms. My other mono block tubes are stable to 1.4 ohms but put out the same wattage as they do at 16 ohms or 70 volts.

Pardon my ignorance. How do you know your tube amp is stable at 2 ohms? Is the amp capable of driving speakers at 2 ohm load or did the spec specify this? I am just curious on the differences in the output wattages for various amp manufacturers when the impedance is halved. As others have pointed out, the power supply is the critical factor in determining the power output. Now I'm more confused to hear of tube amps that will output a similiar wattage at varying impedances.

It's a little more involved than just the power supply being unable to provide the current to double the power as load is halved. You can design two amplifiers with the same power rating at 8 ohms and have one being able to double power at 4 ohms and the other with the power remaining the same. Yet the amp that doubles the power can sound worse regardless of its heftier power supply and yet have nothing to do with the quality of the components.

When a speaker draws current, the power supply gets taxed. If the power supply serves both the input and output stages, the voltage sag affects both. This will cause the input stage to distort which sends a distorted signal to the output stage which then gets amplified to the speaker.

However, if you split that power supply with two stages of regulation (expensive), then the current demanded by the speaker only goes through the output stages, leaving the input alone. The input signal is left undisturbed by voltage fluctuations but the output power delivery is limited. So that's one answer as to where the "missing power has gone".

Another answer is that the power supply may have the capability, but poor design (such as high impedance) may cause the power supply to waste heat, thus robbing the output stages of the ability to maintain voltage during speaker impedance dips. There are other reasons, but basically the missing power is either diverted for better fidelity, is wasted as heat or is simply a victim of the manufacturer's budget.

Gs5556, thanks for posting. That had clarified most of the doubts I had previously.

Sorry, but designing for compromise is crappy design...period! I've found Krell for example sounds best driving a difficult load. Also, class A (at least in the pre-driver stage) is more musical. Case in point, the new Evo amps (no longer class A), which I owned for a time, sounded cold and flat compared to even the 400xi integrated. One must have an ear for exquisite tonal colors however...otherwise Bose will do just fine! Of course not everyone will agree, but i'll take a "Quantum of Solace" in the knowledge that JValin and Martin Colloms might just agree with me.

Ryder, take a look at
http://www.atma-sphere.com/papers/paradigm_paper2.html

In a nutshell, 'Doubling power' as the impedance is cut in half says nothing about how the amp sounds, in fact there is nothing in the rules of human hearing to suggest that this is helpful.

The result is, there are amplifiers that don't/can't double power as impedance is halved that nevertheless sound better than many amps that can double power, because the former are designed to the rules of human hearing.