From Kara Chaffee, posted here with her permission:
Hi Brett,
The heads in the 10-20mH realm are too low for the 222. Most other heads are 150mH and up, which work fine. I have customers with Flux heads, but you should verify the impedance of the ER head. The 222 uses a really special bipolar op amp as a head amp --then that couples to two 12AX7s , and they are coupled to a 5687 tube cathode follower. The tube sections are fed from a 6X4 tube rectified power supply. I tried to keep as much as possible to the spirit of the Ampex 351 , but you just need more gain than the basic 12AX7 can deliver.
Sincerely,
Kara
I'm guessing the OP amp she is referring too is the Muses02, can anybody confirm this?
So there you have it, I traced out the circuit to find out what I know about it - I'm a nerd. She also uses Russian K40 caps in it coupling over the op amp to the 12ax7s. This may be a source of consternation as well. The Russian caps are well liked in the DIY community - primarily for their price- although the price advantage has changed lately.
If I get the interest I will change out the caps with some of my favourites one day to test the difference. In the meantime I make progress with my own diy all tube design.. it will be ready one day . Maybe better maybe not.
So there you have it, I traced out the circuit to find out what I know about it - I'm a nerd. She also uses Russian K40 caps in it coupling over the op amp to the 12ax7s. This may be a source of consternation as well. The Russian caps are well liked in the DIY community - primarily for their price- although the price advantage has changed lately.
If I get the interest I will change out the caps with some of my favourites one day to test the difference. In the meantime I make progress with my own diy all tube design.. it will be ready one day . Maybe better maybe not.
Awesome info, thanks. I asked Kara if she had ever tried a transformer in place of the OP amp, she said she had not, as it would drive up the price considerably.
DIY is the path I may go down as well. A transformer to three tube stages and a cathode follower is what I have in mind. For interests sake I should try a Muses01/2 in place of the transformer.
I have had one of Kara's model 222's for a bunch of years now, it sounds great and has never given me any issues!!! I have used it on my Studer and my Otari, next I will try it with my Tascam...
