Passive summing, warning: noob

[Grooveteer]

Sorry for the noob-question. I did search, but could not find what I'm looking for.

What I’m looking for is a no-nonsense 12 or 24 channel summing box. (Passive & balanced). I don’t need pan, mute or level, I just want to be able to sum 6 or maybe 12 stereo stems from my DAW.

D/A > normalized patch bay for inserting comp/eq > summing box > Pre-amp

I have been searching here, and the summing boxes I came across are either unbalanced or have options that I don’t need. I’m too much of a noob to convert those designs.

I’m guessing that all I need are a bunch or resistors, right? Could anyone help me get a better understanding on how to do this? I’m sure it’s a no-brainer for most of you, but I’m not quite there yet.

Thanks a lot.

Cheers! :sam:

Grooveteer

 

[pstamler]

Well, yes, but they have to be the right resistors.

First off, you don't want them to load down the output of your DAW/Comp/EQ too heavily, so let's make the balanced input impedance 20k (10k on each leg, and they really should be tightly matched, an easy-enough job with a digital multimeter).

At the same time, if you're feeding a preamp with a transformer-coupled input, you want it to see a source impedance, coming out of the summing box, of about 150 ohms, so the transformer's damping will be right.

So okay, you have a stack of 10k resistors in each leg. Assume the impedances feeding the box are negligible (because they almost certainly are); looking back from the box's output, for a six-input box each leg looks like 10k / 6 = 1667 ohms, or 3333 ohms total. To get that to look like 150 ohms you need to add a parallel resistor:

1/Rpar = 1/150 - 1/3333 = .00637

So Rpar = 1/.00637 = 157 ohms, or the nearest E96 value, which is 158 ohms. For a 12-input box, the parallel resistor would be 165 ohms.

(I'm assuming that the box will have all of its inputs connected at all times, or if not connected, then shorted to ground.)

How much make-up gain will you need? For each input, you have a 20k series resistance and a shunt resistance made up of the other inputs in parallel, plus Rpar in parallel, plus the input Z of the preamp, which we'll assume to be about 1.5k.

For a six-input box, there are five resistances in shunt (= 20k/5 = 4k), in parallel with 158 ohms and 1.5k; the total shunt resistance is about 132 ohms; for a series resistance of 20k, the total attenuation for each channel is

132 / (132 + 20000) = .00655x

which means you'll need about 43.7dB of make-up gain. Most preamps will provide that happily. Of course, you'll be combining multiple channels, so figure about 7.8dB boost, on average, for 6 inputs; that means you'll really need only about 35.9dB of make-up gain with everything playing.

For a 12-input box, the shunt resistance works out to about 137 ohms, and the attenuation is about 43.3dB per channel. More channels mean more signal to add up, of course, so expect to need about 32.5dB of make-up gain. Piece of cake.

Of course you'll also need input and output jacks. I recommend TRS for the inputs, with swingers on them to short the connections when nothing's plugged in. XLRs or TRS for the outputs, no swingers.

Peace,
Paul

 

[Grooveteer]

Thanks! Great info :thumb: :thumb: :thumb:

Cheers :sam:

G

 

[Grooveteer]

And another question... :?:

Would it make sense to use premium resistors (like Dale) or not? Assuming that I match the 2 resistors for each input.

 

[MartyMart]

[quote author="Grooveteer"]And another question... :?:

Would it make sense to use premium resistors (like Dale) or not? Assuming that I match the 2 resistors for each input.[/quote]

Matching would be fine, for my own ( yet to do ) I bought Dale RN60 resistors from
Ebay VERY cheaply, was $1.99 for a set of 25 10k's plus a bit of postage.
They are 0.1% tolerance IIRC I got 2x sets of 25.

MM.