Still as confused as ever
So, if you select the lower of the two settings for speaker impedance, the voltage rails will be lowered, and the signal level at which clipping distortion occurs will shift to a lower signal level.
The question that immediately begs to be answered is this:
When the volume is set adequately low to avoid clipping when using the lower impedance setting, will distortion be the same regardless of which setting is selected?
Moreover, there are other questions that I really, really wish that someone as knowledgeable as Gene would answer:
From the standpoint of how transistors behave, how is it, exactly, that in the low-impedance setting, idle current in the output devices is lowered?
The Onkyo receiver that I recently purchased runs much, much cooler when I select the low-impedance setting than when I select the high-impedance setting. The difference is night and day even when the volume control is turned down fully and left down, which suggests to me that it has nothing per se to do with clipping.
It also has nothing to do with the actual speaker impedance, manifestly, because the output load does not affect idle current. Nevertheless, reducing the idle current is a highly effective means to lower the heat dissipated within the output devices, notwithstanding that idle current is not coupled to output load.
It thus seems apparent to me that either (a.) bias is intrinsically, unalterably coupled to the supply rail, or (b.) the supply rail affects idle current without altering bias, or (c.) the switch is altering the bias directly, i.e., adjusting the operating class.
Of course, for an amplifier that is already operating very close to class B when in the high-impedance setting, the question is moot. But for many amplifiers, possibly the majority, it is likely possible to effect a significant reduction in heat by adjusting bias. And it seems to me that if I wanted to reduce the amount of heat within the output devices, to protect them from a modest reduction in output load, that I would take this approach when possible, notwithstanding that idle current is independent of load.
In any case, the behavior of my Onkyo amplifier suggests to me that in the high-impedance setting, it is operating very close to class A, and that the principal effect of the low-impedance setting is to cause it to operate much closer to class B. As such, it seems reasonable to me that the increased distortion at higher volume setting, when the low-impedance setting is selected, does not necessarily imply clipping per se, since the amplifier becomes increasingly less linear as the input signal amplitude increases.
If my reasoning is flawed, than I hope that Gene will enlighten me. But if my reasoning is sound, then it seems to me that there is a simple test to discover, for a given amplifier, whether the speaker impedance switch directly changes the operating class of the amplifier. If this switch has a pronounced effect on the amount of heat given off by the amplifier even when the volume is turned fully down and left down long enough for the temperature to stabilize, then it seems to me that the primary effect of the switch is to alter the amplifier's operating point directly.
Regardless, the bottom line is this: if your amplifier runs cooler in the low-impedance setting and you never hear any difference at all, then why would you not use the low-impedance setting, even if you have 8-Ohm speakers? Do you not trust your own ability to discern what you can and cannot hear? What possible reason would there be to use the setting that causes your amplifier to run hotter, if you are unable to discern any difference?
Gene, I would very much appreciate it if you would try to answer some of these questions. Thanks in advance.
