The Tetra Sans PS phono stage uses two tube-based gain stages with a passive equalization network in between. The gain stage's topology is the Constant-Current-Draw Amplifier [CCDA], whose low distortion, fairly high gain, and low output impedance are desirable attributes in a phono preamp. The CCDA is a compound circuit that holds a grounded-cathode amplifier directly cascaded into a cathode follower. So what; what's so special about this obvious pairing? Its special status lies in the details.
The ACF-2 is an All-in-One design that holds both the B+ and heater power supplies and two Aikido-ized cathode followers. In other words, the ACF-2 is a unity-gain buffer for those who do not need any extra voltage gain, but do need more current delivery than a passive line stage would allow.