Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blown240 on October 31, 2011, 07:20:41 pm

Title: 12AX7 Balanced XLR Out
Post by: blown240 on October 31, 2011, 07:20:41 pm

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.

(http://www.sound.westhost.com/p87-f4.gif)

Title: Re: 12AX7 Balanced XLR Out
Post by: HotBluePlates on October 31, 2011, 07:45:50 pm

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.

Title: Re: 12AX7 Balanced XLR Out
Post by: stingray_65 on November 01, 2011, 01:39:08 am

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

https://taweber.powweb.com/store/balun.jpg (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 (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 (http://www.parts-express.com/pe/showdetl.cfm?Partnumber=092-045)

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

Title: Re: 12AX7 Balanced XLR Out
Post by: blown240 on November 01, 2011, 10:09:08 am

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

Title: Re: 12AX7 Balanced XLR Out
Post by: stingray_65 on November 01, 2011, 10:36:33 am

Just to be clear!

These Webers are low impedance, 150 -150

Title: Re: 12AX7 Balanced XLR Out
Post by: terminalgs on November 01, 2011, 10:42:41 am

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 :

(http://i.imgur.com/LkC98.png)

Telefunken 72A,  driven by a CF stage:

(http://i.imgur.com/ud6wN.png)

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):

(http://i.imgur.com/6CNpY.png)

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

(http://i.imgur.com/i14KO.png)

Title: Re: 12AX7 Balanced XLR Out
Post by: blown240 on November 01, 2011, 01:31:03 pm

Those are cool. Thanks for posting them!