What's the deal with 'fake' PI's?

[ax84ch]

I built a single ended amp (AX84 SEL) some years ago with an LTP to try to get some of what I was told was missing in larger push-pull amps. One side just fed nothing.

I never bothered to do it again.

Part of that is likely because the kinds of amps I gravitate towards get their tone mostly from the preamp, the speaker impedance curve effects, and the negative feedback impact on tonality. The LTP had nothing to do with any of those in a meaningful way. Of course a wide variety of amps get their magic tone from the PI and Power amp and I think this is what the OP is referring to.

Synergy (and "Boutique Amps Distribution" more broadly) have been incorporating the "zero watt power amp" idea into some products and I think it may be just a phase splitter but I've not studied them to see.

[didit]

Hello --

Yup, Zero Watt --> https://ampgarage.com/forum/viewtopic.php?t=37748.

Best ..

[drewspriggs]

Hello --

Yup, Zero Watt --> https://ampgarage.com/forum/viewtopic.php?t=37748.

Best ..

So with 0.5w from a 12AX7, it could possibly be a PI driving a transformer directly - would be interesting to see what's happening inside specifically

[jbrrrrr]

So with 0.5w from a 12AX7, it could possibly be a PI driving a transformer directly - would be interesting to see what's happening inside specifically

I haven't gotten to see one torn down, but that's my best guess - there's one tube responsible for phase inversion per Tone King, but it's not clear whether that's a single triode or both. There's not enough triodes, nor does it make much sense, to run a full push-pull output section, so I bet it's something like a cathodyne and taking whichever output (plate or cathode) was preferable wrt phase and impedance, and then doing whatever they're calling "power amp simulation" - which could be anything from actually making a single-ended power tube from one triode and reactively loading it for the line out, and/or possibly involving a small little transformer, or just calling the phase inverter the "power amp simulation" and doing some clever buffering and shaping after the PI to allow for shaping the signal before it hits the IRs and effects.

The more I think about it, the less I believe they went all that far to preserve most factors of a scaled-down driven power amp. Instead, I think they worked out a way to create discrete sections of gain where the preamp does one thing, the phase inverter reacts to it differently based on gain structure, the attenuation is maybe just a Post-PI MV to help allow for using a ton of preamp gain or choosing to get it from the back/power amp end.