Passive mult best practices

Hey there,

My system is built up enough that I can start doing epic stuff, even without a decoder. LZX standards are close enough to broadcast voltages that I can just patch video directly into the system.

This topic might have been covered elsewhere, but I didn’t find anything in a search. What are the best practices for using passive mults in an LZX system? I’ve tried to optimize rack space for modules that process signals creatively rather than simply split or combine signals. I do have Bridge, Proc, SMX3, but I want to use those for mixing rather than simply summing.

So far I’ve had good results using Tiptop Audio Stackables… these are well made and give much of the flexibility that I was used to with the EAB Videolab. Before I go nuts with this, I did a little bit of research and came up against some warnings.

Specifically, combining signals with a passive mult or “dirty mixer” type patch not only connects multiple outputs to a single input, but it also connects those outputs to one another. So now we’re sending a signal into output jacks whether we know it or not. How dangerous is this with LZX modules? I would assume that the output jacks are protected against incoming voltages, just in case the user accidentally patches an output to an output. And I would furthermore assume that the tolerance for that would be the same as for system power. I.e. inputs and outputs can handle signal voltages (nearly) up to the system DC power rail. That should be +/- 12v, right?

So if those assumptions are accurate, there should be no danger in connecting outputs together as long as the total voltage at any point in time doesn’t exceed +/- 12v? E.g. multing three 1v video outputs should not be an issue?

And what about splitting a signal? Is that going to cause “potential” issues? Is the voltage drop significant? Will the patch draw more current from the connected outputs?

Thanks!

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Passively multing as many times as you like is fine in the LZX ecosystem. You’re not gonna break anything. All I/O is buffered so voltage drop is minimal.

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Best practice when signal integrity and performance is critical would be:

  1. Only use buffered summing amplifiers & multiples, such as the Sum/Dist module.
  2. Only use short patch cables (24-36 inches at most).
  3. Never connect an output to an output.

Standard expectation
Multing a signal from one output to multiple inputs using stack cables, headphone adapters, etc is OK. But it will always introduce some reflections in the transmission line, and create noise – some adapters will be better than others in this regard. For most cases in your patch, that won’t be a problem. If you’re introducing noise you don’t like, revert to using an active signal path only. It’s likely that noise introduced by passive connections is more noticeable with HD formats.

Experimental use cases
Passively summing outputs together thru adapters. Could introduce issues depending on different factors in the patch. Better if the transmission lines and receiving buffers are separate from the summing amp in the circuit.

Practical advice
Consider a Sum/Dist in your system for critical signals, and for when your passive mults/adapters are too noisy.

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Thanks Matt, thanks Lars.

Passive summing does connect outputs to outputs, but it sounds like the worst thing that could happen is a noisy signal.

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That would be my primary concern. Think of it this way… if you terminate the output into an independent input jack on a summing amp, then the signal gets buffered before the sum. The cable is therefore not part of the summing circuit. So the behavior of the circuit is more deterministic.

If you passively sum, then both cables become part of the summing node’s circuit. A noisy summing node may be a byproduct of other factors, like length and quality of the cables.

Another factor to take into consideration is that a passive sum will drop amplitude according to the output resistances of the sources. With LZX modules, all output resistances are 499R.

So with 2 passively summed outputs, you will have an output gain of 1/2 of each input.
With 3 passively summed outputs, you will have an output gain of 1/3 of each input. Etc.

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And another note on passive mults:

I would personally recommend any EuroRack passive mult module using high quality 3.5mm jacks over the use of stacking cables or any sort of splitter adapters. That seems like the best solution from the perspective of signal integrity. YMMV.

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Thanks, those are helpful suggestions, as always. One takeaway from this is that if passively summing, keep the cable length to a minimum. Don’t send two long cables to one distant input.

I am, and have always been, concerned about noise issues in sending HD video through 3.5mm audio cables. The layout of my studio is such that 36" patch cables are a necessary evil. I may have to invest in some expensive shielded cables. Worst case scenario, I’m prepared to use BNCs with 3.5mm adapters, and/or a bank of video distribution amplifiers. Is this insane? Would I run into impedence issues?

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Probably not much to gain there in the 3.5mm to BNC/RCA case. Ideally I would run all long connections from only Encoder and Decoder modules, using 75 Ohm RCA cables, like any 75R video transmission line. Ultimately if you’re experimenting with setups outside the box (like AC video directly into a 3.5mm input jack) then you will want to experiment a bit, and find what works most optimally for you.

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One word of warning here – we have found that if you buy a high grade shielded audio cable, it may have higher capacitance. This will remove noise from an audio signal, but will end up filtering a video signal even more than a cheap cable will.

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I can’t help thinking by the time you buy cables and adapters, you’ll be well on your way to the price of the LZX summing module.

I was thinking of using video cable (belden 1855a) for patch cables (its ‘free’* at work). But even though its a lot more flexible than fatter versions, it is so stiff its hard to work with alongside audio cables. Its hard to solder 3.5mm connectors to it but not impossible.

*there are miles of cable for reuse. More than we’ll ever need.

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Sorry, I mutated the topic in the middle of the conversation. My thinking around shielded cables is to cut down on noise/interference in any case, not just with passive summing.

Basically, I’m out of planned rack space at this point. The Sum/Dist module just won’t fit unless I get rid of something else, or add another enclosure. And there’s no room for more enclosures, either. At some point I may need to move the whole studio into a larger room!

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I’ve got a bunch of these catwalk must modules that sit on top of modules they are pretty neat. I also have a couple 1u mults as well as some 2hp mults

this is what I use for passive panning/mixing/attenuation they are rad

I’d like to pick up a sum/dis at some point for use with a multi FKG setup

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Thanks for the recommendations. Those Catwalk mults are clever, I might do that. As for inline adapters, I’m disinclined to them. The noodle soup is already thick.

I have had great results with Intellijel and 2HP Buff mults. I used passive mults but had some issues with some DIY modules which cause me to switch.

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Thought I’d leave this here. 2HP video buffer I’ve been working on with the option to normalise several units behind the panel. Just testing a final revision of the PCB then they’ll be available to order :slight_smile:

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