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


I have noticed that some amps like the Krell Evo 302 will output the exact wattage when halving the impedances ie. 300W/600W/1200W into 8/4/2 ohms respectively. On the other hand, some amps like the Bryston 7B-SST does not output the exact wattage when halving the impedances ie. 600W/8 ohms to 900W/4 ohms or the Plinius SA-100 (100W/8 ohms to 180W/4 ohms)

Can someone enlighten as to where the 'missing' watts have gone for amps that do not output the exact 2X the wattage when going from an 8 ohm to 4 ohm load? What are the criterias in amp design that determine which route to adopt, and what are the pros and cons of each design?

Thanks in advance.
ryder
As Bob noted, inability to drive a 2 ohm load does not reflect "Crappy" design. It reflects inteligent design (sorry about that) for an amp that will be used with 8 or 4 ohm speakers.

If you want to drive low impedance loads look to pro sound amps that are designed to do this.
One thing that came out at the recent CES is that there really is no longer any argument for low impedance speakers anyway. It turns out that transistor amplifiers, while easily able to *drive* a low impedance speaker like 4 or 2 ohms, actually sound their *best* driving 16 ohms.

Steve McCormick, as one example, makes transistor amps that have no problem at all driving 4 ohms and less, but he discovered that the amp *sounds better* when using a set of ZEROs to drive 4 ohms, rather than directly, much the same way MacIntosh did years ago (the ZERO was loading the amp at 16 ohms). By better: smoother, more detailed, more authority, and something easily heard.

So Eldartford and Bob are right- there are many amp manufacturers that don't care if their amp 'plays' a 2 ohm load- more likely is what they are really interested in is how the amp plays 8 ohms.
The original Eagle could almost be used for welding in that no speaker load bothered it.
Doesn't distortion go up as impedance decreases? Why design an amp for a lesser performing speaker? How many speakers actually are a stable 2 ohms, outside of car audio which has much higher distortion tolerances.

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. Does this mean I'm going looking for 2 ohm speakers, NO. I'm looking for the best sounding speakers in my budget but doubt it will be a 2 ohm speaker. 8 ohms will probably sound better, why push my amps to the extreme?
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.