bass preamp

[shawnee]

Hey guys, I am putting together a bass preamp and was wondering how to incorporate an XLR out. Would I just have to add a buffer to create low impedance?

[stecykmi]

xlr's used balanced output, meaning the you need to create a copy of the signal that's 180deg out of phase (kinda like an "upside-down" copy of the signal).

there are plenty of schematics out there that will show you how to create a balanced output, many of them are fairly simple op-amp based implementation that should work fine. googling will yield lots of results.

if you're unfamiliar with balanced signals (how and why they work), you may want to read up on them a little. the wikipedia article would be a good place to start.

[jimilee]

Amz has a board for that I believe

[shawnee]

Great thanks!

[peterc]

Here is a balancing amp PCB design.

Peter

http://twin-x.com/groupdiy/thumbnails.php?album=487

[shawnee]

awesome. thanks peter.

[dwmorrin]

Shawnee, you don't have to use a balanced output to use an XLR.
I'd figure I'd mention this, since your original post mentions nothing about balancing.
Here's some more good reading on the subject: http://www.douglas-self.com/ampins/balanced/balanced.htm

There are many ways, and many types, of XLR output schemes, most of which deal with pro audio or other signal problems that a bass preamp doesn't necessarily need to address.  The "common sense" seems to be that balanced is always better, but in practice a unbalanced output is usually OK.

For a basic unbalanced XLR output, yes you should buffer it and set the impedance to 1k or less, and attach the output to pin 2 of the XLR.  Tie pins 1&3 to ground.  Balanced inputs will accept this.  No extraneous circuitry required.