Hey, I built the Zero Field circuit today and I'm impressed, all the low end distortion is completely gone, it can handle +10dBu signals with no problem, I'm getting nice clean sinewaves at 20Hz, the distortion is 0.05% at 20Hz at +10dBu, nothing compared to what I used to get without the Zero Field circuitry, something odd is that the distortion at +22dBu @20Hz is actually lower (0.02%) than the distortion at +10dBu (0.05%), I just wish I could understand exactly whats going on in the circuit.
I see that there is positive feedback which causes a negative resistance to neutralize the DC secondary resistance, but what troubles me is that in the Lundahl schematic, R3 is going to ground and the capacitor connected to the top terminal of the secondary is also going to ground, I cant see exactly how this loop works. Also, whats the gain of this circuit? the 10K resistor in the NFB loop divided by the input resistors at the primary ?
Do you know if this type of arrangement can be applied to any other transformer, not just small core transformers, what about expensive line input transformers? I mean if a $6 part can perform like this, I cant imagine what a Lundahl or Jensen transformer can do.