Question, why do some amps double power from...

8 to 4 ohms and others don't?

Is it because their power supply rails are not quite high enough in voltage? Is it because they don't have enough reserve storage in their caps? Are there some other factors I have missed.

What you refer to is called "power supply droop" and is mostly due to the limited size of the power transformer. It takes a large transformer to be able to maintain a high enough regulation factor to handle a full doubling into 4 ohms - and a large transformer costs lots of money.

As to whether this doubling always correlates with better sound, I have doubts. There are numerous design issues that also need to be considered, such as the power supply rail voltages and the current load on the transformer, not to mention the circuit topology and gain devices. In my experience, there is little correlation between power output and sound quality so long as the amp is powerful enough for the chosen speaker.

There is little correlation between 'doubling power' and the rules that human ear uses.

There is however a design rule that takes advantage of the concept, and there is another design rule that says that what is really important is making an amplifier that obeys the rules of human hearing to the best of technology's ability. The two rules have been in conflict in the audio world for close to 50 years, since slightly before the advent of the transistor.
Thank you for your responses.

The reason I ask is that I intend to drive a pair of Thiel CS3.6's and they spend a lot of time around 2.3 ohms. It has been said that the they are driven best by amps that double their power into 4 ohms. I guess they are real current hogs.

I just purchased an amp that does 200w into 8 and 350w into 4. Not quite up to snuff considering the doubling rule and I'm just wondering if it would be possible to tweak the amp into the doubling range. I'm always tinkering with something. Perhaps I could add stiffening caps and a larger transformer. Of course I'll wait to see how it works without any mods first but besides always tinkering I'm always thinking!

Thank you,
Hi Ron, The main thing you want to look at is the idea of constant voltage. For speaker like the Theil, which is a Voltage paradigm device, the amp must also conform to the same paradigm. To do that it *does not* have to double power! The Voltage paradigm relies on negative feedback as part of the platform of the paradigm itself; in this case the negative feedback acts to limit the total output power in such a way that the amplifier, even though it does not double power *at full power* it is nonetheless capable of flat frequency response on the speaker, which is the design goal.

Bottom line: your amp is able to do the job on your speaker.