12AX7 Balanced XLR Out

[blown240]

I am going to have 1/2 of a 12AX7 to play with in a preamp build I am planning.  I found this schematic for a balanced XLR out and was wondering if there was a way to use the 1/2 12AX7 instead of the TL072.

[HotBluePlates]

If you really want an XLR output driven by a tube, you will very likely need a transformer between the tube and jack. Most tubes have simply a too-high output impedance for driving long lines.

[stingray_65]

Weber sells a nifty little transformer that's great for XLR.

https://taweber.powweb.com/store/balun.jpg

I try to put one on every amp I build that's less than 30 W

$2 a pop! I usually order 5 at a time. https://taweber.powweb.com/store/magnetic.htm all the way to the bottom.

I try to use these neutrik 1/4" XLR panel sockets.

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=092-045

get the switched kind and you can run a bal/unbal out.

[blown240]

That little webber tranny is cool.  Its ALOT smaller than I thought it would be.  And for $2, you cant go wrong!

[stingray_65]

Just to be clear!

These Webers are low impedance, 150 -150

[terminalgs]

you want to look to vintage studio gear of preamps and limiters/compressors for examples of tube driven balanced output circuits.  a lot of them are push-pull in the final stage of output,   but a few are "single ended",   here are some schematic excerpts of the tube & OT:

EMI's redd 47,  a little different than what you'd do with a single 12ax7 section,  but similar.  driven by a gain stage :



Telefunken 72A,  driven by a CF stage:



Universal Audio LA2A,  a complex CF driver   where the cathode load resistor is replaced with another triode section (not for you if you only have a single 12ax7 section,  but interesting nonetheless):



the above LA2A idea that is a little easier to read (taken from Bill Whitlock's Audio Transformers Chapter... from "Handbook for Audio Engineers" ):

[blown240]

Those are cool. Thanks for posting them!