Quick and silly - passive unbalanced line level summing ...

[snaper]

Hi there guys,

Since I was able to find schematics only for balanced things, I've started this quick thread, if other came across this problem too.
I've seen a summing cable on Reverb...
...after that I realised, it would be awesome if I could just "sum" some of my synth outputs in my patchbay into one input (stereo pair).
I have cables, I have connectors, I could "DIY" this cable.
It doesn't need to be balanced (these are really old, vintage digital synth, used for techno, I'm OK with lo-fi).

How to calculate the resistor values for an 8 to 2 summing cable?

 

[ruffrecords]

Simplest answer is to choose a value you know the synths can drive. I would hope that even vintage synths could drive a 10K load so use that as a starting point.

Cheers

Ian

 

[snaper]

Yeah, I assume they'll be around 10k, like old FM and PD things (CZ5000, TX Yamahaz) and some PCM stuff (Wavestation SR).
If 10k, how should I proceed with the calculation?

 

[ruffrecords]

You just feed every unbalanced input through a 10K resistor and tie all the other ends together to mix them. Assuming the actual source impedances of the synths are relatively low then the gain you need to restore the loss in the mix bus is equal to the number of inputs. So with 8 inputs would will need a gain of 8 times which is 18dB.

You should use sockets with shorting contacts so that unused inputs are shorted. This avoids the mix loss varying with the number of inputs in use.

Cheers

Ian

 

[snaper]

Thanks Ian, I'll try to "construct" an 8 to 2 cable and we'll see, I'll use all the inputs actually.

 

[efinque]

From the limited knowledge I have 10kOhm shouldn't be an awful lot for a synthesizer output but like stated thermal noise could become an issue with a makeup stage (I built a passive summing bus using 10k resistors which was fed from line preamplifiers capable of something like 3Vrms, another mixer could drive the summing bus at moderate level)

The real problem you're facing is how to build it into a cable without using a "wart" at the fanout (I'm not sure if this is the correct word but there isn't really much space for a single component, say 1/4W summing resistor inside a standard 6,3mm plug which I assume you'll be using, not to mention if it's some kind of large 1-5W dummy load)

For longer cable runs the cable itself too (depending on the material and thickness) has a certain resistance and inductive coupling usually in the range of picofarads (ie. it acts as a low-pass filter)

EDIT : another thing is if one or more of your sources is mono.. then you'll have to deal with possibly poor stereo separation between L/R when using low resistor values (for example in my 10kOhm summing bus I had a mono mic channel to a stereo L/R bus, the separation provided would be the bus resistance x 2 so 20kOhm but I switched them to 220R later which makes 440R although I further improved this with additional resistors).. you could also be interested in resistor bridge networks which usually come in SIL or DIP-16 but these don't let you alter the resistor value (from what I read they're basically a bunch of fixed value resistors in series inside a standard IC package, something I didn't know when I started building mine)

 

[ruffrecords]

Thermal noise will not be an issue. It is simply the noise in a 10K resistor which is below -110dBu.

Cheers

Ian