Here's something else to think about folks. That is, the lower the impedance, the less time that the amp stays in Class A and the sooner it switches over to Class B. The more switching of operating Class that takes place, the "grungier" and less consistent the amp will sound ( in most cases ).
With that in mind, a "typical" amp that is of a "rich AB" design is typically 6 - 10 wpc at 8 ohms in Class A and then switches over to Class B at power levels above that point. If one has a 4 ohm load, the same amp will only run in Class A up to about 3 - 5 wpc. At 2 ohms, you get 1.5 - 2.5 wpc in Class A and at 1 ohm, you end up with .75 - 1.25 wpc in Class A. As such, one would have to have an amp that is VERY richly biased if you wanted to stay in Class A mode for the majority of listening on a very low impedance, low sensitivity speaker. In order to do this, you need an amp that has GOBS of heatsinking and current capacity to say the least. Otherwise the amp will both starve itself and cook itself to death at the same time.
With that in mind, a rather unique approach regarding bias levels were chosen when John Curl designed the Parasound JC-1's. The JC-1's are switchable between "rich biasing" and "very rich biasing" for these purposes. In the lower bias mode, it runs in Class A up to 10 wpc and then switches over to Class B mode. In the high bias mode, it runs in Class A up to about 25 - 30 wpc* and then switches over to Class B above that point. Using this approach, one can let the amps idle in "low bias" mode when not in use, which will keep them as warm or warmer than most other amps, and then switch over into high bias mode when listening. This approach allows one to have the benefits of "ultra high bias" in terms of sonics yet retain the benefits of "medium to high bias" by cutting down on power consumption and heat build-up. I'm not suggesting that this amp is the right amp for Apogee's or any other specific speaker, but that it is a very unique and "user friendly" approach to designing real world products.
Doing the math with the JC-1 as a reference and starting off with the conservative rating of 25 wpc of Class A operation available at 8 ohms, this would give us something like 12.5 wpc @ 4 ohms, 6.25 wpc @ 2 ohms and slightly over 3 wpc @ 1 ohm. All of these figures are based on the amp operating in Class A and then switching to Class B operation above that point. Given that most listening occurs with less than a watt or two being used ( except for rock music ), the JC's should retain their sonic characteristics relatively consistently for most uses with just about any load. This is due to the consistent electrical characteristics that the amp would display while remaining in Class A operation and the lack of switching distortion that all amplifiers will demonstrate when pushed harder.** Sean
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* Stereophile measured 27 watts in Class A on their review samples.
** This is all based on theory and not first hand familiarity with this amp. I chose it as i was familiar with the electrical characteristics of the amp as advertised and as measured by Stereophile. Other amps, like the Bear Labs Symphony, Pass Labs X series, larger Krell's, etc..., may perform quite similarly due to the very high levels of bias applied to the output stage.
