First Watt J2

Al, thanks for the input, as always. My remark regarding the J2 as being a current source amplifier was solely based on the power output specs of 25w@8 ohm and 13w@4ohm vs. the F2 specs of 5w@8ohms and 10w@4ohms which seemed to imply a voltage source amplifier, e.g., doubling the power as the impedance is halved. I completely missed the (significant) difference in the amplifiers’ output impedance which seems to be more in line with what Nelson seems to recommend for the type of speakers in question, higher the better all other things being equal. Interestingly, I experimented with what he had suggested in that paper by adding different value resistors in line with the speakers and running them using my Adcom 545 amplifier. The results were not satisfying. The added resistors seemed to veil the sound and made the speakers quite dull sounding. 

In addition to the helpful input from Almarg, you might also benefit from emailing Nelson Pass.  He does reply.

How much did the f2 cost retail when it was available?

Get an SIT3...4 grand and it's yours! Note those supposedly work best with 4 to 8 ohm speakers.

@Kalali, yes, it is certainly understandable that the maximum power ratings of the two amps into 4 and 8 ohms would be misleading as to which is the current source amp and which is the voltage source amp, since **usually** doubled maximum power capability into halved load impedances signifies a voltage source amp. But as long as an amp is operated within the limits of its maximum voltage, current, power, and thermal capabilities, how its power delivery varies as a function of load impedance will depend on its output impedance, not on its maximum capabilities.

Also, it’s interesting to note that as indicated in its manual the power capability specs of the F2 are based on total harmonic distortions of a very high 4% for 5 watts into 8 ohms, and an even higher 9% (!!) for the 10 watts into 4 ohms. While the J2’s specs are based on 1% in both cases.

Best regards,
-- Al