Yet another passive summing box! (Pics Included)

[phatmateo]

can anyone give any advice on this?  I changed the 220ohm shunts to 180ohms and it seemed to help a small amount with the noise.  If I go any lower the hum gets worse.  I also tried putting a .1uf cap from ground to chassis, but there was no change.  The copper wires are completely separate from the chassis.  I'm using 10k resistors in series on each channel as suggested.  Thanks for any ideas gentlemen!

 

[3nity]

Hey man are you able to show some pix?
you sure all xlr pin 1 are grounded?
take a look.

Jorge Aristondo.

 

[phatmateo]

the pictures are strikingly similar to marc's.  The only thing that is different is the resistors are soldered directly to the trs jacks instead of on the copper buss wire.  I am using shielded cable for connections.  Everything is soldered correctly and connected to ground.  There is no chassis ground.  I can probably take some pictures tomorrow. 

 

[phatmateo]

Pictures:
http://img134.imageshack.us/i/sum1.jpg/
http://img134.imageshack.us/i/sum2.jpg/
http://img188.imageshack.us/i/sum3.jpg/

 

[craptical]

There´s very little point in using shielded cable (and chassis) if not connected to (some) ground. Try connecting your chassis and see if the hum goes away. Good luck!


/Dave

 

[phatmateo]

I figured shielded cable wouldn't matter, especially with bare copper wire, but it was easy to run them and its tidy.  Grounding to chassis didn't seem to work either.  Do I have an impedance mismatch?  I've noticed that when I've tried my Seventh Circle Audio A12's as makeup gain, there isn't enough gain and everything sounds thin.  My N72's and Green Pre's work though.  I'm sending subgroups out via my Lynx Aurora.  Here are specs:
Input Impedance Balanced mode: 24k Ω
Unbalanced mode: 12k Ω
Output Impedance Balanced mode: 100 Ω
Unbalanced mode: 50 Ω
Output Drive 600 Ω impedance, 0.2 μF capacitance

For the Summing mixer I'm using 10k Resistors in series and 220ohm shunts.  I took the shunts out at the time when the pictures were taken fyi.  The shunts are used to create more separation between channels, correct?  I'm trying to understand the math, according to http://electronicdave.myhosting.net/miscimages/mixnetwork.gif
It seems simple, but I don't understand how...
R(which is 10k)= (N-1)/(N+1)*Z
N=8 channels and Z should be around 1200ohms(the input Impedance of my SCA N72 Preamp)
My calculation is 933.3
Divide that by 2 since its balanced
and it should be 500ohm resistors.  
How did we end up at 10kohms?
or 5kohms? as in http://www.forsselltech.com/downloads/schematics/8chsum_2.pdf

??  Sorry I should break out my yamaha book haha
and sorry for hijacking this thread Marc!

 

[craptical]

Hmmm... On a closer look, aren´t that type of TRS connectors breaking the signal to one of the sides when a plug is inserted? In that case either pos or neg side of your channel inputs gets disconnected.

/Dave

 

[3nity]

Craptical you;re right it looks like when a jack inserted it cuts the signal.
take a look at that man!

 

[0dbfs]

Please correct me if I am wrong but I believe the shunts create a source impedance of 220 for the mic pre that normally follows the summing-box outputs for makeup gain.

If that is the case, would switched shunt's be adviseable to match them to your pre-amp impedance settings?

As for hum, I would try lifting some of the GND connections to see if you can get it to go away.

Cheers,
j

 

[phatmateo]

I didn't realize that it would break the signal!  I'll have to create a jumper between the two.  Thanks for the catch! I'll let you know how it goes.  Thanks for the quick replies! 

 

[3nity]

that might be the cause of the db loss.

 

[phatmateo]

all fixed and super quiet!  Thanks again guys.  I definitely see an improvement in the mixes after using this

 

[Nacho]

Hey Marc,

What's the crosstalk value on that summing network?

I built one and I think we are using the same diagram, just different building methods.  My crosstalk value is -50dB.  Its too high.

I would love to know the value for yours.  If yours is better, I will re-do my box to look more like yours. 
Thanks for any help!!.

Nacho

 

[danjpiscina]

Mark, what mic pre to you go into after this summing stage?

 

[prescott]

Thank U folks fot this topic!

I've built a summing box very similar, and works fine!

Thanks

 

[cromwell]

Hi guys,

could I use this summing mixer as well for unbalanced signals?
I would like to summ my M-Audio Delta 1010LT outputs but it seems they are unbalanced...

Thanks for helping

Cromwell

 

[daggertX]

cromwell look at phatmateo's link to the forsell schematic above.  it shows both balanced and unbalanced. 

 

[cromwell]

Thanks daggertX; I should have seen this :

 

[Colorblind]

Hmmm... On a closer look, aren´t that type of TRS connectors breaking the signal to one of the sides when a plug is inserted? In that case either pos or neg side of your channel inputs gets disconnected.

/Dave

I'm about to build one of these.  Can somebody tell me how to identify (and avoid) the TRS connectors that break the signal to one side?    Also, what kind of wire should I use for the grounding?  this will be my 1st DIY project, so any help would be great!

Thanks,

C

 

[phatmateo]

You'll easily be able to tell what side breaks the signal on the trs connection after observing the action of the 1/4" plug being inserted into the jack.  It will lift the side it breaks.  I was dumb/blind that I didn't see this when I first built mine.  You can get the ground wire from radio shack.  Its bus wire.  Any wire will do just fine though.  I do recommend the carare install cable for the signal routing.  http://www.redco.com/shopexd.asp?id=541

-Matt