di box and testing 2sb737

[kirshman]

Hi,
I tried building the phantom powered di box here:
http://guitar-fx-layouts.42897.x6.nabble.com/Phantom-Powered-Active-Direct-Box-td16400.html

I ordered some 2sb737's from china on ebay, and got it all wired up and all I get is a loud hiss.  I wired 2 1/4 ts plugs in parallel on this thing, so I don't know if that's the problem or not.  I'm wondering if there is any way to test these 737s like you can test the jfets.  

Any help would be great.  Thanks!

[Silver Blues]

2SB737 is just a PNP BJT. Your multimeter may have a built-in test function, although I don't know of how much use the hFE reading will be to you.

[motterpaul]

when you say you have two TS jacks in parallel - that should be okay, but you do also have the XLR output? A DI box is a way to take high impedance guitar jacks and make it low impedance XLR (which has three leads 1,2 & 3)  for recording.

[kirshman]

Yes I have the xlr out as well.  I just didn't know if plugging 2 - 1/4 inches in would mess it up.  Our keyboard has left and right outs so I wanted to be able to plug them both in and send it to the board through the xlr in one channel.  It doesn't matter what I plug into it though, its just a loud hiss.  I've cut tracks and looked it over and can't find where its messed up.  Pretty simple build so not much ground to check.  

[motterpaul]

In general, any time you have two inputs you need to combine them through a mixer, In other words - separate gain structure for each input. Just combining two analog signal passively does not work. But it should be pretty easy for you to find a way to create a dual input summing box.

Phasing issues could also crop up, tho. So, the easy thing to do, if you want mono at the board, is to give him a mono DI box and take the "left/mono" output from his keyboard.

However, if you have "hiss" chances are it is something else. Ground problems usually sound like a loud hum (not hiss).

[motterpaul]

I was just looking at this...

http://tagboardeffects.blogspot.com/2010/07/guitar-and-bass-paralooper.html

and it says that if you want a stereo (two different input sources) that one should go in "input" and the other one into the "return" jack...

So, basically you are "mixing" the two inputs through the blend circuit.

(Sorry, had to edit "inout" to say "input")

[motterpaul]

Also - here is a stereo input (for a keyboardist, ironically) mixer with one output. You probably could put this up front (before the direct box).

http://tagboardeffects.blogspot.com/2013/01/2-channel-mixer.html