Multing Signal - Impedance Problems?

[Adam Smith]

I have to set up a system at my studio where I can use the direct outs of a Soundcraft 8000 series into either an MCI JH-24 or a Digidesign 192 I/O.

I had the thought to build a simple relay switching device with 16 inputs and two sets of 16 outputs, as that seems like the most transparent way to do the job. However, that would cost a couple hundred dollars, all said and done. That isn't a huge problem, but not money I want to spend if it is overkill.

My other thought was to simply split the signal coming out of the Soundcraft and send to both machines, as they will never, I swear, never be used at the same time. However, I'm worried I'm gonna create a problem because having both units hooked up at once, on or off, will effectively halve the input Z my board sees.

Here's what I'm working with, per the spec sheets of each unit...
Output Z of the Soundcraft - <75ohms
Input Z of the MCI - 10k Balanced
Input Z of the 192i/o - 12k Balanced

By my calculations, that means the soundcraft or any other mic pre I plug into the system will see in input Z of 5454.54, which I suppose would be fine with the other gear in the studio, even the 1176's, but I've never done such a thing, so I wanted to run it by greater minds than mine.

Do any of you group DIY'ers have experience doing something like this? Is it recommended?

 

[abechap024]

You could use them at the same time. Got to love high impedance line level...people do this all the time with patchbays....y cables. you just have to know what your doing and the loading losses are negligible.

Just don't try to mult 2 or more outputs into an input! that would be painful.

but mult as many inputs from an output (up to like 10 or so) is fine.

PRR had a great post about this, that cleared up a lot of stuff for me and had all the math to back it up....I'll try to find it.


Wait...and why are you talking about mic pres?? The mic pre impedance is totally independent of the output impedance and output load.

 

[ruffrecords]

As a rule, with modern equipment, you can always drive several inputs from one output. What you need avoid is connecting more than one  output to single input.

Cheers

Ian

 

[Adam Smith]

You could use them at the same time. Got to love high impedance line level...people do this all the time with patchbays....y cables. you just have to know what your doing and the loading losses are negligible.

Thanks for the responses.

That's great. The plan then is to hook up a patch bay on the back of the rack that splits one set of "multi track inputs" to the MCI and 192. The "multi track inputs" will be normalled to the output of the console.

Wait...and why are you talking about mic pres?? The mic pre impedance is totally independent of the output impedance and output load.

I don't think I explained myself well enough. I was mentioning mic pres because what I'm setting up is my main multi track input on the patch bay. The Soundcraft will be the console, but I will be using external pres a lot of the time. I have a few tube pres want to see a high Z input. I have two one bottle pres that only want to see 10k and an Ampex MX-10 that has been modified so it wants to see about 5k or higher. I wanted to be sure I don't end up with an impedance problem in either situation.

It is my understanding, from what you guys posted, that I can drive two 10k loads in parallel as long as the mic pre or comp driving it has an output that doesn't want to see a lower Z than the combined Z of the parallel loads.

So, 1176 w/ 150ohms, seeing 5454k=OK, one bottles looking for 10k, seeing 5454k=BAD.

 

[JohnRoberts]

As others have advised there is no problem with that split regarding load resistance. You mention they won't be both used at the same time. One possible issue if both destination units are not powered on, the input impedance may not be linear and well behaved , so any non-linear loading could reflect back into the source output impedance and introduce distortion. 

This will be small or not at all, just be aware of the potential mechanism.

JR

 

[pucho812]

like the others have said passively multi the  output signals won't really cause any problems. I just wanted to add a different idea how to do so. At the studios I work at we wire all our multitrack busses and direct outs to 90 pin edac connectors. This allows us to move all 24 channels of input or output of any recording device at once. We have the same kind of connections coming off our tape machines, pro tools, etc. We have a labeled panel and depending on which machine we using just hook it up, makes for easily swapping machines.. 

 

[tubestation]

Hello !!
Has  the MCI Tape Transformers in the Inputamp? If so it doesn t matter If the Machine is powered up or not,behause the Tranny is sitting  before the Elektronics ,and If plugged parallel to your Converters ,it will allways lower the input Impedanz of both Units.This may be ok by calculation,but may cause Groundpotentialdifference,a Groundloop.If both Imputs are floating (no Ground connected) you Don t have a Groundproblem.I agree that it is technical better to switch between both Targets with Multipin Plugs.But I am not a super Expert for grounding issues.But I know that it can be a kind of Vodoo and i try to avoid anything that could cause Problems,specially in Multitracsettings.
Greatings !!  Lothar.

 

[JohnRoberts]

Hello !!
Has  the MCI Tape Transformers in the Inputamp? If so it doesn t matter If the Machine is powered up or not,behause the Tranny is sitting  before the Elektronics ,

Actually no. A transformer "transforms or scales up/down any non-linear loading from a unit not powered up, but it does not buffer it.  Reflected impedance is scaled by the turns ratio squared.

JR

and If plugged parallel to your Converters ,it will allways lower the input Impedanz of both Units.This may be ok by calculation,but may cause Groundpotentialdifference,a Groundloop.If both Imputs are floating (no Ground connected) you Don t have a Groundproblem.I agree that it is technical better to switch between both Targets with Multipin Plugs.But I am not a super Expert for grounding issues.But I know that it can be a kind of Vodoo and i try to avoid anything that could cause Problems,specially in Multitracsettings.
Greatings !!  Lothar.