best triode for active XO?

[keto]

Hey all, I'm states-side for a couple weeks, looking to stock up on supplies and am wondering which double triode would be best for a sallen-key type active crossover circuit. With a +150V -150V PSU, I've come up with something based in the 12AU7, but seem to remember having read that a higher-mu triode can be better for cathode followers because there's less negative feedback, and there's a famous XO maker who uses the 12AX7... So, there's the 12AU7, the 12AX7, and somewhere in between, and reasonably priced is the 5670. Any suggestions as to which of these very different triodes might be optimum? I suppose one advantage of going with the 12AU7 for me is that I could also get away with just a B+ supply and attach C4S current draws to the cathode(-follower-)s. Happy Holidays!  --keto

[Paul Joppa]

The higher-mu triodes will have more negative feedback, not less. Higher transconductance will produce a lower output impedance, always a good thing. An interesting approach would be to design for the 12AU7 and then plug in a 12AT7 - they can operate well at the same current and plate voltage, but with substantially different mu you will be able to make a useful comparison.

A 431 chip regulatro between the negative power supply and ground will give you up to 30v negative, if you want to avoid the -150v supply...