Which high power SET

I run 85dB (if that!) Infinity RSIIb speakers with a homebrew 200WPC, zero NFB, SET amp and it's the best sound I've ever heard. Crystal clear highs, full lush mids, rock solid thunderous bass. Sounds like a fleawatt SET, with twin superchargers!
Unlike some of the reports here, I do not have a problem with bandwidth at any volumes I care to enjoy, including LOUD.

So, anything is possible.

Action Shot

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I guess Atma's point also has relevance to my situation, as 20% of 200W is still 40W, which should be able to push my speakers to a little more than 100dB, which is pretty loud.

The bonus is that at 40W, the amps are still in Class A1. Above that they begin to move into Class A2, which would mostly be for big dynamic peaks.

In any case, they work superbly with the RSIIb speakers. However, if you were to buy them commercially (if they were available) you'd be paying a LOT of money, as the parts cost alone was over $11K, with about a year of my time spent in planning and building. The 2.3kV voltage on the output tubes might also cause a little consternation in the family unit...

So I guess the moral of the story is...you can power a low efficiency speaker with an SET very effectively, but it's better if it's a monster-powered SET so you don't operate anywhere near full power for most music, and you'd better be prepared to spend a lot of money doing it.

Would you agree with that, Atmasphere?

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...

Atmasphere,

Small signal bandwidth is 7Hz - 66kHz, -3dB; the OPT is of course the limiting factor and those are the actual test results provided by Monolith Magnetics. When I checked the amp with a .05V signal using TrueRTA on my laptop, which only measures from 10Hz -20kHz, it was less than 3dB down at 10Hz, and still pretty much flat at 20kHz. At full power, saturation occurs at 32Hz; I can honestly say that I don't notice any frequency limitations soundwise even at very loud volumes. I had an old friend visiting for the past weekend and we listened into the wee hours to everything from Sonny Rollins to Black Sabbath, and it all sounded great...when we cranked up Yello the walls and floor were shaking from the subsonics.

06-30-14: Atmasphere
That is quite impressive to get that sort of bandwidth out of an SET of that sort of power! Do you know the configuration- by any chance is it a parafeed circuit?

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I designed and built it, so I'm very familiar with the configuration. It's a series feed two-stage direct-coupled single-ended design; no parafeed. The driver tube (the excellent Russian 6E5P tetrode, triode strapped) has an active gyrator/mu follower load to supply the grid current demanded when the amp goes into A2.

You can see the LTSpice schematic in the first post of this thread:

The Midlife Crisis

I simulated both parafeed and series feed as I was brainstorming this build; both have their advantages and disadvantages as you well know, but I decided to go for broke and make it series feed, which required me to get very expensive custom gapped transformers from Monolith (these are now part of their Summit line). I could have used a less expensive, smaller transformer with parafeed but then I'd need a custom plate choke to handle the voltage (which could swing to nearly 5000V!) and a high-voltage capacitor as well so it ends up not being much of a cost or weight advantage for parafeed. I opted for the simplicity of series feed.

As I said, the OPT is massive to be able to handle such high power; here's a picture of the wound core before strapping and potting, with an ECC83 tube for reference:

Core

Each monoblock weighs about 180lbs, and is built on a Landfall Systems aluminum chassis.

By the way, the actual schematic of the build differs slightly from the one in the first post of that thread (evolution!). The actual amp uses the AOT2N60 instead of the original AOT1N60 for better power dissipation, there are gate to source protection diodes around the 2N60, the resistor to the gate voltage supply is slightly larger, and there is a 6000V gas discharge tube across the OPT primary to protect it in case of overvoltage.