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.
