DIY Pass Labs First Watt F5 amp impressions

I think the XA25 is similar to the F7 but with balanced supersymmetry circuits. It's no more powerful than the F5 or F7 though. I've head that it's noticably better too. Of course you're going to pay a lot more for an XA25 than an F7. 
I think I read that the F6, F7, and F8 we're going to be revisits to the basic topologies of the F3, F4, and F5. I'm very curious to see what the F7 layout is like. I might be very interested in building one. 

Kosst,
Great information!  You clearly know your way around electronics. I am going to attend Burning Amp this fall down the road in San Francisco to meet "Papa" again.  It is funny that you mention the KEF LS50's.  I bought them shortly after they came out and drove them with a 175 wpc Plinius 8200 MK II integrated amp and the amp rather struggled to drive them properly. There is no chance a FW amp would be compatible with them.

However, with OB speakers like my Spatial Audio Holograms, DeVores, Omegas, etc, the FW amps are a great match it seems to me.  Forget about Martin Logans and the like, but a pair of Klipsch speakers would sign with these amps.  You and Kenny wisely pointed out in your posts the compatibility issues one needs to be aware of to get the the best performance from the FW variants.  

 

I’m using my F5 to drive a pair of Focal Aria 936’s. Those are an even more reactive load than the LS50’s and the F5 handles them with plenty of power, speed, and grip. I think it’s the current source designs that really struggle with the crazy low impedance and phase angles some speakers present. The output stage of the F5 provides nothing but current, but it’s totally unregulated. All the voltage gain in the F5 comes from the SJ74 and SK170 input pair, but they provide absolutely no current gain.
If I recall correctly, the outputs in the F6 provide both voltage and current gain. It just depends on the circuit the devices are in. The F5 is specifically designed to provide current to maintain the voltage gain. The F4 is kind of the opposite in that it’ll let the voltage flop around a bit to strictly follow the current gain. Benign loads do well with amps like that, especially high impedance loads. As the amp tries to ram more current down the load, the voltage demands will actually drop. It’s a different way of attacking the drive challenges of big horns and speakers like DeVores.
In all honesty I wish I knew a hell of a lot more. 2 years ago I barely knew how to solder in a capacitor. I LOVE Nelson's designs because though they're simple, there's a lot going on that compliments what other parts of the circuit are doing. A lot of other designers build far more complex circuits to do some of the things features in his circuits do, but he prefers to reduce the complexity and play weaknesses off weaknesses in a way that makes them strengths. That's the real Zen and allure to what he does. It's got a certain organic nature and beauty to it. 

My very simpleminded/layman observation is some amps put out more power as the load is reduced which implies a near constant voltage delivery. Others behave the opposite which seems to imply a constant current source. The former is better for low impedance loads whereas the latter better for higher impedance speakers.This is also evident when comparing solid state and tube designs. I know I’m stating what most folks already know but a good layman view of the amplifier differences and how to find matching speakers.

I think those are good statements of fact. Tubes are a whole different world. They have very high output impedance and high voltages. The transformers convert that too low impedance and current. Still, no transformer can achieve the vanishing impedance of many direct coupled designs. Seems to me, as a matter of principle, if you want the amp to have optimal command over the speaker, connect it directly to the active output devices. There are decent arguments to be made that the "tube sound" is at least in significant part "transformer sound".