Dgarretson, of course I did have to design for the real world, which includes not limiting myself to very low output impedance sources. In fact I do have one input from my input selector that is fed a high 6K output impedance coming from my phono stage. The thing you have to do to make that work is increase the number of winding turns and use that slightly oversized core of laminations.
That increases the LF inductance which pumps back up the LF extension. With that 6K input, the frequency response does reduce with LF -3dB cutoff at about 15 Hz. Also, there will be more FR deviations and some HF ringing as well as increased distortion. So I was forced to design accordingly.
That's why it's important to optimize the core size and the core material and the number of windings for these designs. For one thing, I believe I wound twice as many turns than Dave Slagle's AVC. More turns creates other problems, like possibly degrading performance for low impedance sources by increasing winding capacitance. The capacitance is what limits the HF cutoff.
I built a MC phono autotransformer step-up on the exact core form and material as the AVC and it turned out to be the best I've heard of any step-up as well (total serendipity for me). It's driven by a very low 4 ohm output MC cartridge (Koetsu Urushi) and is stepped up 1:10. Low impedance like that really drives down distortion. In fact the laws of physics says that nonlinear distortion approaches zero through a transformer as the winding resistance and driving impedance approaches zero.
But I think SuperPerm 80 is the good stuff responsible for making most of the low distortion and good sound. It's very good at low magnetization, and with copper windings it's slightly warm and sweet, but VERY slightly. Almost dead nuts neutral, but I don't think much of anything is truly dead nuts neutral. And the sound is extraordinarily detailed (transparent) and it actually gets more dynamic as you turn the volume down (increases turns ratio). I have an all Holco (the good old Holcos) 100K L-PAD passive step attenuator on the front end of a 27 line stage (now used as a haedphone amp). It's vastly better than the volume pots I had before, but inferior in every way to the AVC.
I've put the AVC in front of the 27 tube with the step attenuator at max position (0 series resistance and 100K shunt.) This is the best way to eliminate the sound of the step attenuator for trying out how the AVC would do as part of an active tube preamp. Well it wiped clean away the sound of the high quality Holco resistors at all levels. What I was left hearing was the sound of a very good tube, the 27, and its coupling caps. It was dissapointing really. How? Because it sounds so much better to just eliminate the 27 tube circuit and clean it up just that much more. I mean it sounded best standing alone, and it wasn't sounding too "sterile" either.
That's my story and I'm sticking to it.
Kurt
