Aikido Mic Preamp July 27 2012
The brilliant John Broskie over at tubecad.com has devised a very clever circuit that has lots of interesting properties and is supposed to sound very good. He calls it the Aikido amplifier.
Basically the aikido offers 3 benefits: It takes the power supply noise and injects it into the cathode of the cathode follower's current sink to null that noise. It loads the gain stage and the cathode follower with a copy of itself, so that the load reflects the nonlinearities of the tube being loaded. Lastly, since the tube and the load are the same in both stages, you can swap in just about any tube with the same pinout.
I have it set up with Altec/Peerless 15095 input and output transformers, and I expect to get a bit less than 36dB of gain. Into my Alesis IO2 usb box, it is plenty since the IO2 has gain adjustment on the line out. The output impedance before the output transformer is just less than 350 ohms, so a 1:1 output transformer like the Altec 15335 would be perfect. Or I could use an Altec 4722 on the input. I like the idea of the 15095 on the input because I think it probably can take a lot more signal before distorting... and they're cheaper :)
The switches are (L to R) -20dB pad, phase, and +48v phantom power.
I really like the ground bus connected at the input wiring technique. I even used untwisted heater wiring connected to convenient grounds, with snubber caps, and it is DEAD quiet. Into my recording setup, there is more noise from the usb box and laptop than from the mic pre.