Some people argue about transformer hysteresis distortion in PP mode (transformer crossover), due to signal polarity reversal and crossing zero magnetization. In attachment I show a -100dB signal generated by ARTA, using a "big" toroidal transformer (80x50x35mm core), measured at secondary side. No feedback or connection to primary, driven by 5687 in PP. So we don't need to worry about low level signal from trafos. Even the 60Hz mais are present as a little interference!
About this...
Is a PP transformer made for 6k:8R. Actually I'm using it for driving a STAX SR-507, like a PP anode choke, but sometimes I attach a normal headphone to secondary.
Since this toroidal core is huge for this application, it tolerate some DC. Each 5687 uses catode resistor, and a trim for reducing LF HD (perhaps a CCS wil be easier in long run
). A cap couples the cathodes for a PP operation, ensuring a almost differential output (not fully due to cathode resistors).The first stage is a common ECC88 RC gain stage with common cathode, driven directly (DC) from a ESS9018 chip.
Since both projects (Soekris and mine) use 2 stages with full PP operation, some similarities are inevitable
About DC balancing act... for parafeed without cap:
One can sample the anode DC voltage and apply to the CCS, for making the CCS self-adjusted for compensating tube variations, achieving same anode voltage, for not relying to mu-stages, if desired.
But the capacitor parafeed option is more easy
Oops.: about the core size, is approximate; I don't remember now what core size I used.
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