Which high power SET

Al, they have an output impedance of about 1.2R, which is in large part due to their 13K : 4 output transformers (Monolith Magnetics S-833); they spec them at 12k:4 but my measurements say 13k. The OPTs weigh 62lbs each and have an amorphous double-C core with varilay winding and teflon interwinding insulation. I'm running the output tubes at 2.3kV and 160mA, which is well within the specs of the 833C with forced air cooling (400W max plate dissipation). I had plans to add in some local NFB around the 833 to drop the output impedance further, but listening tests told me it wasn't needed - bass is detailed and well controlled as is.

The Infinity speakers are actually a relatively benign load below 100Hz, ranging between 6R down to 14R at 40Hz and about 10R at 20Hz. They dip to 1.6R at 9kHz, but there's not much energy up there and the top end sounds just fine even with the low damping factor in that region.

The power supply on the amps is massive (about 3/4 of the amp chassis is PS) with 400 joules of energy storage just for the output stage! That helps a lot with transient power demands, as does the additional 35 joules of energy storage for the driver stage; very helpful when grid current is demanded in A2, at full power it consumes 200mA of grid current!

So, the robust power supply combined with a relatively benign low frequency region in the speakers, allows me to get away with a zero NFB SET amp where you might not think it possible. Again, it's all about the match among system components, a speaker that dipped to 1R in the LF region would most likely be a boomy disaster.

PS: do you live anywhere near central NJ, Al? If so, you could stop by for a listen any time. Just let me know...

Ait, that's awesome. When I read you first post, I thought 200WPC SET was a typo and you meant 20WPC! That must be quite an amp. Congrats!

Ait, thanks for the detailed response. Wrm's choice of the word "awesome" is an understatement, if anything :-)

I'll add to your comments, relevant to the thread, that the 1.2 ohm output impedance is of course significantly lower than that of more typical SETs. And it is low enough so that the effects on frequency response resulting from the interaction of that output impedance with widely varying speaker impedance are not likely to produce unintended results when used with speakers designed with the expectation they would be used with either push-pull tube amps using feedback or even solid state amps.

And thanks for the invite. I'm not in NJ, but every once in a while I have occasion to visit northern NJ, and I'll keep your invitation in mind.

Best regards,
-- Al

The first is the size of the listening room and the second is the listening level preference. The impedance curve is important as well, but to me speaker efficiency is a much lower priority.

For me, #1 priority is amp's ability to drive speaker sufficiently. If not, SQ will suffer whether low or high listening levels from my experience.

Listening level is not a factor in my book since I listen to all different levels depending on the situation.

Ait, have measured the bandwidth of these behemoths?