First Watt J2

I think that you've interchanged the mentions of the two amps. The J2, with its 0.4 ohm output impedance, behaves pretty much as a voltage source. The F2, with its 15 ohm output impedance, is the amp that behaves as a current source.

But to answer your question, while I have no experience with these amps, I would approach this endeavor with caution. See the following paper by Nelson Pass, about current source amps:

http://www.firstwatt.com/pdf/art_cs_amps.pdf

Note the many examples in which various passive components (resistors, capacitors, and in some cases inductors) are paralleled with the speaker to optimize sonics, with the values of those components depending on the particular driver. And note this statement in the conclusion section of the paper, especially the reference to "electrical networks":

Current sources and amplifiers with low damping offer interesting possibilities for improvement with these [full-range high-efficiency] drivers but they require considerable work to get the enclosures, electrical networks and acoustic environment just right.

Best regards,
-- Al 

Why not buy an actual tube OTL like an atmosphere???

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.

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

I’ve had several back loaded horns: CarderSound, Horning, Lamhorns, and Lowthers. Through trial and error, I’ve found they’re a much different animal than my front loaded horns, transmission lines, and conventional box loudspeakers. Like me, you may find a lot of what you thought you knew or how things look on paper quite different from how things work with these.

Although I’m not the biggest fan of the SET built around the bigger 211 and 845, they more than proved the best match with this type of loudspeaker, with the smaller SETs coming in next. The 211 and 845 simply put power into them in much better way, and came across as having much more seat of the pants power and better drive them than my push-pull tube, OTL, and solid state amplifiers

I had a J2 in my system for two weeks (borrowed from a friend).  It ran 99 db/w efficient system easily (I also don't play music that loudly).  I thought it was a very good solid state amp.  Unlike a lot of solid state gear, it did not sound lifeless and lacking in that indescribable "something" that keeps my attention; my mind tends to wander more when listening to solid state and I am just not as engaged.  It was also pretty decent with dynamics when listening at lower volume level (another strength of tube gear).  It was not as warm (bumped up in the upper bass) as some tube gear, but, then again it wasn't thin and harmonically bleached as some solid state and high-powered tube gear can be.  The only thing that really gave it away as being solid state is that it had a slight bit of an artificial edge or hardness to the initial attack of the note that I hear with most solid state amps.  I could easily live with the J2 and I am a committed fan of low-powered tube gear (I don't like most higher powered tube amps, with the exception of certain OTL amps).

I also heard a home built version of one of the First Watt SIT amps in the system of a friend (same person that loaned me the J2 built this amp).  It sounded really good driving a 105 db/w horn system.

kalali,I have 1.3 W SE EL11 Telefunken /in triod/ & EBF11 Telefunken /in triod/ & AZ1. This is my project. Im very glad. It drives my Fullrange 8 inch with added 12 inch bass speakers. If You are interesting, I can give You my amp for a few days to try with Your speakers and tell me Your opinion. I had projects 1 W amps, SEs 6P15P, 4P1L, 6CH6, 6LR8, 6BL7, 6N30P, 5687....I love 1 W SE!

Thanks for all the input. My current (integrated) amp for these speakers is a Sophia Electric Baby 10wpc push/pull version and they sound decent, especially vocals at lower volumes. Looking for something to upgrade and add more "body" in the mid/upper mid bass range. Definitely open to tubes as well. This is a second system so trying to keep the budget around $2K used.

You can use fullrange, bass and tweeter speakers. Fullrange 8 inch /no less/ for 50-14000 Hz /connected direct to amp, without crossover, that press the sound, that is most iportant/ for voices, simph. music, jazz...Bass 12 inch speaker for 30-35 Hz. And 2-3 inch speaker for 14-20 kHz. If You interest, I can explain about connection. I listen the same configuration and Im happy.

Has anyone driven their efficient speakers pretty loud with the j2? I've tried small watt tube amps (6 watts per channel) and things petered out even with my ZU Omen Defs when turning up a ways (not surprising). I'm not talking "head banging loud" but I do like to raise the volume every now and then fairly high. I've got my eye on a used J2

BTW, If I go the J2 route, I'll use my pre amp's balanced outputs which has 13 db of gain compared to 3db of gain thru RCA.

Second the Atma-sphere suggestion. You can choose between 30 or 60 w/channel. With the right load nothing I have ever heard comes close.

@kalali - I will build you a Class A power amp for under $1K that will beat up on the First Watt amp.  All point-to-point wired.

Has anyone really cranked up a j2? How did it do at high volumes?