Tried to build a passive summing box.. failed

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kodaz

New member
Joined
Jun 19, 2010
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2
My first attempt at building something electronic.. I thought it would be easy but I can't seem to get this thing to work.

I basically followed the same design http://www.groupdiy.com/index.php?topic=16731.0

I used 1/2 watt 10K resistors from the direct + and - terminals, and 220 ohm resistors across the two main busses. I pretty sure I got the terminals right because I have a multimeter and tested the tip, sleeve, ring terminals before I started building it.

After finishing, I brought up a mix, 5 stereo busses, sent them out via my echo audiofire12, used 2 gap pres for the makeup gain and sent these back to the daw using the last 2 channels. Nada... The pre's don't register any signal coming from the summing box, even when I turn the gain right up there is just nothing coming out. Does anyone have any ideas where things have gone wrong or why it might not be doing what it should?

There are pictures over at my post on gearslutz: http://www.gearslutz.com/board/so-much-gear-so-little-time/503150-tried-build-passive-summing-box-failed.html

I wasn't entirely sure what to do with the grounding either.. I just connected a shielded copper wire to each sleeve terminal on the TRS jacks, and pin 1 on each XLR, then soldered this to a screw going into the case.

Cheers
 
Hard to tell what went wrong in your case, I basically built the same thing last night and it worked out great (just on a TT patch...). Probably some silly mistake you overlooked. You did use the right terminals at the TRS jacks?

Michael
 
Yeah, exactly what I was thinking...

from the photos, it looks like the TRS jacks are shorting or whatever its called. It means that when you insert a jack, it pushes a sleeve and breaks the connection. This is useful in some designs. But in this case if you soldered your resistor to the wrong solder lug it won't work when you insert a jack.

Can you post a link to where you got you jack sockets? We could then look at the data sheet and find out which lug to solder the resistor too and we can rule out the jack plug as a source of the problem.

J
 
It doesn't get much simpler than this...

The easiest way to troubleshoot is with a VOM to measure the resistance between circuit nodes.

With no sources or preamps plugged in (all input R's floating) you should measure only the bus termination resistance across the outputs. I don't see a schematic but guessing from your description you should measure 220 ohms from output pin 2 to output pin 3. With the same ohm meter, measuring from each input ring to input tip (using a short cable plugged in) you should get 20.2 kohms. You should also measure open circuit from any input tip/ring, or output 2/3 to ground..

JR

 
You should be able to test proper resistance of 10K between each contact of a plugged TRS connector and the XLR pins.  You should have 20k2 resistance between Tip and Ring of each TRS.  That will reveal if the TRS's are wired incorrectly.
Mike
 
don't worry. this project is impossible to screw up.
once i was building a guitar pedal and that thing didn't wanna work.
"bad caps, bad transistors, bad opamp, bad resistor, what?" turned out i forgot to solder the tip of the input jack..

so, look at it.
and all in all the total cost of the resistor is..  how?  1.5€? 


OT. are summing OTB?
just summing? if you need some mojo a GSSL on the buss is way better.
bye
 
Hey dood,
I responded over at gearslutz as I noticed in the pics you posted that it really looked like you
had the 1 and 2 pins switched on your XLR outputs (you have hot to pin 1 and shield to pin 2)...
 
Hey guys, I solved it.. it was just inexperience, I didn't take steps to remove the lacqure on the copper before I soldered it. So I went back, filed all the contacts and resoldered and now it's working beautifully with no noise.

Thanks for the responses.
 
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