Roger Modjeski: The only loss (from light loading) is about 20% of the power rating or 1dB.Hi Paul,
As we had discussed some time ago, this statement by Roger strikes me as incorrect for most amplifier designs. I have no way of knowing how correct it may be for his own designs.
In general, connecting an 8 ohm load to the 4 ohm taps of an amplifier may reduce maximum power capability by up to 50%, compared to what can be delivered into the same 8 ohm load connected to the 8 ohm taps, and compared to what can be delivered into a 4 ohm load connected to the 4 ohm taps.
A loss of about 50% will occur in cases where maximum power delivery into a 4 ohm load connected to the 4 ohm tap is limited by voltage swing capability, and where the design is such that the voltage swing capability on that tap does not increase significantly as a result of the 8 ohm light loading.
Concerning your basic question, all of Ralph's comments make complete sense to me, and I would expect them to be true. However I would imagine that in some cases, presumably including Roger's designs, the transformer may be over-designed (or designed with the possibility of light-loading specifically in mind) such that the concerns Ralph raised would not have too much quantitative significance, and/or may be outweighed by the benefits Roger enumerated.
I third Viridian's excellent comments about speaker impedance swings, and about the need to try both connections and see what sounds best.
Your experiences with light loading?FWIW, my speakers are very close to 6 ohms throughout almost all of the spectrum, rising to around 10 ohms at deep bass frequencies. I tried both taps on my amp, and settled on the 4 ohm tap, although the difference was surprisingly small.
Best regards,
-- Al