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Author Topic: Parallel Spilts - maximum possible without noise?  (Read 3432 times)

peter dakin

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Parallel Spilts - maximum possible without noise?
« on: December 18, 2014, 05:29:39 PM »

So I was always told you comfortably split a microphone signal passively (no DC isolation) 3 times.

How many can you actually do safely, if using quality cabling, connections and soldering/shielding?

I'm trying to redesign our workflow to try to cover all scenarios, however I'm concerned my idea might cause noise or signal levels issues due to the amounts of splits involved.

Currently our stage box has three passive splits built into it.

This then goes to flying tails that are plugged into consoles. However with advent of digital stage boxes, its nice to have a tidy system so you want a multi-pin panel going to internal tails. Great, until you want to use the digital stage box in stand-alone mode.. you have to take the stage box along as well.

What I'd like to have is multi-pin input, but with a local input option as well, however this is introducing a 4th split to the wiring and if I added two digital consoles with same local input, that would be a 5th split.
Now not all these splits will have connectors/multi-pins attached all the time, at most we ever do is three-split shows, it'll just be the local input XLR chassis being introduced into the wiring system...

Recipe for distaster and "name that hum..."??
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Mac Kerr

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Re: Parallel Spilts - maximum possible without noise?
« Reply #1 on: December 18, 2014, 05:42:55 PM »

So I was always told you comfortably split a microphone signal passively (no DC isolation) 3 times.

How many can you actually do safely, if using quality cabling, connections and soldering/shielding?

I'm trying to redesign our workflow to try to cover all scenarios, however I'm concerned my idea might cause noise or signal levels issues due to the amounts of splits involved.

Currently our stage box has three passive splits built into it.

This then goes to flying tails that are plugged into consoles. However with advent of digital stage boxes, its nice to have a tidy system so you want a multi-pin panel going to internal tails. Great, until you want to use the digital stage box in stand-alone mode.. you have to take the stage box along as well.

What I'd like to have is multi-pin input, but with a local input option as well, however this is introducing a 4th split to the wiring and if I added two digital consoles with same local input, that would be a 5th split.
Now not all these splits will have connectors/multi-pins attached all the time, at most we ever do is three-split shows, it'll just be the local input XLR chassis being introduced into the wiring system...

Recipe for distaster and "name that hum..."??

Adding local input connectors shouldn't change anything as long as you only use one or the other. The number of splits only matters in regard to how many devices are connected to them.

Each time you add another console you are lowering the load impedance that the source sees. This lowers the level into each console a little. Most inputs will be in the 2kΩ and up range, and most sources will be in the 50Ω-200Ω range so you should be able to successfully drive 4 or 5 loads from one source electrically. Each console you add adds more unknown cable to the system, and this is where you might run into problems. Some of those possible problems can be avoided with transformer isolation, but with good cable you should also be fine without the tranformers.

Mac
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John Roberts {JR}

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Re: Parallel Spilts - maximum possible without noise?
« Reply #2 on: December 18, 2014, 05:44:06 PM »

The math is relatively straightforward. Nominal mic source impedance is 200 ohms and nominal mic preamp input termination is 10x that or 2k.

So 1 mic loses -0.8 dB
2 mic passive split -1.5 dB
3 mic passive split  -2.3 dB
4 mic passive split -2.9 dB

- 3dB is 1/2 power so a good place to stop. Note this ASSumes that all 4 consoles have identical clean ground, a very noisy console ground could corrupt all that are connected to it while in general that is not a huge issue.

Connecting one mic to 4 snakes will increase the capacitance which if too much could affect HF response.

So not a problem in theory, in practice I doubt many perform that many splits.

JR
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peter dakin

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Re: Parallel Spilts - maximum possible without noise?
« Reply #3 on: December 19, 2014, 04:05:06 AM »

Adding local input connectors shouldn't change anything as long as you only use one or the other. The number of splits only matters in regard to how many devices are connected to them.
Mac
excellent, that is exactly what I'd planned, 3 splits/connections would be the maximum, my worry was the local connections are just adding the possibility of 4-5, however they'd never all be connected at same time.

Also there would only ever be 1 x 100m copper snake attached at any-one time. Our broadcast feeds are usually converted to Fibre or into broadcast isolated splits.

Thanks for the advice.
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peter dakin

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Re: Parallel Spilts - maximum possible without noise?
« Reply #4 on: December 19, 2014, 04:07:35 AM »

The math is relatively straightforward. Nominal mic source impedance is 200 ohms and nominal mic preamp input termination is 10x that or 2k.

So 1 mic loses -0.8 dB
2 mic passive split -1.5 dB
3 mic passive split  -2.3 dB
4 mic passive split -2.9 dB
JR
Many thanks for the maths, valuable stuff.

Agreed, I'd certainly not be comfortable splitting more than 3 times.. no matter what should be possible in theory.
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Scott Holtzman

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Re: Parallel Spilts - maximum possible without noise?
« Reply #5 on: December 20, 2014, 01:17:13 AM »

Many thanks for the maths, valuable stuff.

Agreed, I'd certainly not be comfortable splitting more than 3 times.. no matter what should be possible in theory.

Yeah that last -.6db is much worse than the first -.8  ::)

If you need four then do four, as long as you have sufficient signal at the pre's to not compromise S/N ratio.

You have to look at the entire gain structure end to end, not one element.  Heck different microphones can be farther apart than these figure on the same source.

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Scott AKA "Skyking" Holtzman

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ProSoundWeb Community

Re: Parallel Spilts - maximum possible without noise?
« Reply #5 on: December 20, 2014, 01:17:13 AM »


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