In the realm of avant-garde thermionic reproduction, the phenomenon of "Sag" (the dynamic collapse of the power supply) is frequently distorted. Commercial lore describes the amplifier as an isolated entity and the loudspeaker as a fixed resistor. The physics of electroacoustic systems dictates a fundamentally different reality: an amplifier does not connect to a static load, but modulates energy as a function of a highly reactive system. 

Purposely designing a sagging power supply causes a systemic collapse of the nominal rails, shifting the operating points of the preamplifier stages and destroying the micro-dynamics of the musician's touch. To understand where the true compression and the natural "sagging" of an output stage originate, let us analyze the actual impedance curve of a classic 12-inch ceramic transducer Celestion G12H Greenback (Versione 16 Ω)  measured in free air (whose fundamental behaviors remain geometrically analogous in closed boxes, whereas in reflex systems the resonance splits into a double peak with an intermediate saddle). 

Physical Analysis of the Operating Points 

• Horizontal Line (SAG): Represents the resistive illusion of boutique blogs. Elementary software assumes the loudspeaker is a flat, static load (8 or 16 Ohms). In this short-sighted view, compression depends entirely on the B+ voltage drop. This interpretation completely ignores the reactive nature of magnetic coupling and the variation in actual transferred power, which decreases drastically as the impedance rises. 

• Point Fs (Resonance Frequency): Near the mechanical resonance of the moving assembly (55-60 Hz), the electrical impedance undergoes a massive spike, approaching 250 Ohms. The Output Transformer (OT) acts merely as an impedance matching device based on the turns-ratio squared (𝑅𝑎𝑎=𝑛2⋅𝑍𝑙). When the load impedance (𝑍𝑙) at the secondary multiplies, the dynamic load line on the plate curves flattens instantly. The power tubes run out of voltage headroom long before running out of current, and the output stage "sags" on its own due to geometric saturation, limiting the power delivery. There is no escaping physics—this happens whether you use a cheap transformer or the most sophisticated custom core. 

• Point F1 (DC Resistance Saddle): The concave window between 200 Hz and 300 Hz. Here, the reactive component temporarily zeroes out, and the curve hits its minimum at roughly 11-12 Ohms (the actual DC voice coil resistance 𝑅𝑒). This is the sole stationary point where the amplifier interfaces almost exclusively with the intrinsic ohmic resistance of the copper conductor, delivering power linearly according to its nominal ratings. 

• Points F2 and F3 (The Inductive Rise): Moving from point F2 (400 Hz and above) toward point F3 (full upper audio spectrum), the voice coil's self-inductance dominates according to the inductive reactance law (𝑋𝐿=2𝜋𝑓𝐿). The real impedance climbs linearly, exceeding 35 Ohms at 5 kHz. Consequently, the reflected 𝑅𝑎𝑎 at the plates multiplies again, progressively reducing the useful power output. The amplifier encounters a natural electromechanical brake when reproducing high-frequency transients. The organic compression and sweetness a musician feels under their fingers in the upper register stem from this reflected magnetic friction, not from a collapsing power supply rail. 

The Atelier Philosophy: The Power Supply as a Rock 

An amplifier is, for all intents and purposes, a direct extension of its power source. For this reason, the esoteric school of pure research rejects the concept of an intentionally sagging power supply. Our power networks are calculated to be rock-solid and bulletproof against macro-fluctuations of the B+, guaranteeing absolute stability of the preamplifier operating points and preserving the transient attack.

Acoustic sweetness is not achieved by starving the circuit of current, but by leveraging the intrinsic physics of the vacuum—such as the progressive conduction of the GZ34 rectifier tube, free from the harsh switching transients and hash of silicon—allowing the magnificent complexity of the speaker's reflected load to shape the biological dynamics of the instrument.