Patch Panel

[buck_rogers999]

Hi Everyone,

 

I want to create an 8 channel monitor rack. I have a 2u panel with 16 XLRs in it already.

 

I want female XLR on the top as an input with male XLR on the the bottom for each channel of the monitor rack as an output. However I also want to be able to link across the rack incase I need to link two together without having to go into the back of the rack. I believe adlib audio are doing something similar.

 

Any idea on what switches are used and if there are any other components in the split or it it is just wired a Y-Split XLR?

 

Any other ideas of a way to go about achieving this are much appreciated.

 

 

Thanks

[mervaka]

Sounds like you need one of these? There's a transformer on each channel providing isolation between the two desks.

[paulears]

I have two of these

http://www.earsmedia.co.uk/splitter.jpg

an in and out for each channel, and then a Harting to go front of house. No switches or transformers - just a straight wire pin for pin across on the XLRs, and then they go to the multi pin connector.

I've never had any problems - and the best thing is that the male and females on each channel mean mixing directions is just as easy - I've found it a most useful system. If your system just does the same, having a male and female wired together, with a flying lead to the mixer input sockets, you have pretty much the same thing - a functioning mixer, 'sniffing' the signal passing through the patch. The only thing I've checked is that in this system phantom is applied to the inputs of a mixer that is not supplying it. I have found this ok on the Soundcrafts and Yamahas I've had, but you do need to make sure phantom is only supplied from one! If I get this wrong - all that happens is the channel LED meters suddenly go full. I've not found it does any harm, but it's obvious when you get it wrong.

[mervaka]

Paul: what happens when the mixer not supplying phantom power grounds the common mode? It won't be a short circuit as such, but you'd effectively set up two potential dividers of two 6k8 resistors, and only get 24v (assuming both ends use 6k8 resistors in their phantom power circuits). This would only really work at 48v if the mixer not supplying phantom power floated the common mode, which it most likely doesn't.

EDIT: at very least, you could link pins 2 and 3 with say, 10nF 63V capacitors.

[paulears]

I really don't know - in my usage, it's not happened - so can't say for certain. I guess it's a choice for simplicity - the other day in the phantom power topic there were a number of suggested circuits - and some of those were simple, others had some isolation provided.

[TimmyP1955]

Paul: what happens when the mixer not supplying phantom power grounds the common mode? It won't be a short circuit as such, but you'd effectively set up two potential dividers of two 6k8 resistors, and only get 24v (assuming both ends use 6k8 resistors in their phantom power circuits). This would only really work at 48v if the mixer not supplying phantom power floated the common mode, which it most likely doesn't.

EDIT: at very least, you could link pins 2 and 3 with say, 10nF 63V capacitors.

 

I don't get it. Please explain.

[mervaka]

Phantom power is supplied to pins 2 and 3 via two 6k8 resistors. If you connect two mixer frontends together, and set one with phantom on, and the other off, you set up two potential dividers in parallel, one on pin2, one on pin3. The XLR cable itself is getting pulled both up and down in the common mode. Assuming both mixers are using standard 6k8 resistors, it'd settle halfway between the two potentials, at 24v.

EDIT: in doing so, it would also be chewing up 169mA, before any power gets to the mic. Multiply this by the number of channels you want to use phantom on, and this stacks up to quite a waste of power.

[cedd]

Sounds like you need one of these? There's a transformer on each channel providing isolation between the two desks.

Thomann do make some really useful bits of cheap kit sometimes don't they!

[paulears]

In Merkava's example the phantom power, across the mic lines, between pins 2 and 3 is produced by applying it via two identical resistors. If a second mixer is attached, then another set of resistors is in parallel - this does not drop the phantom power voltage - which remains at 48V. It will only drop to half if the centre of the unpowered resistor pair is connected to pin 1 - completing the circuit. If there is no connection to ground - then 48V is still there. The resistor values were designed to limit the available current - the paralleling has changed this - but as long as the second pair are not grounded at the centre point - there will still be 48V.

 

Without a short down to ground (pin 1) I can't see how the voltage will be divided - a potential divider needs to be across the supply. Not the same problem we're discussing, but even if you used transformers, you're still prone to shorts - if you short a transformer split it still takes down the input - they provide electrical isolation - but they're not immune from cable faults.

[mervaka]

if the desk not supplying phantom power grounds the 6k8 resistors when not supplying phantom, then that's what would happen. if it leaves them to float, then it will have 48v across the lot.

[paulears]

Do any mixers do this then? I know Soundcraft and Yamaha don't - seems an unnecessary feature likely to cause more grief than the grounding would solve - it would also muck up the impedance wouldn't it?

[mervaka]

Well I must say, I stand corrected! While mixers do ground their 6k8 resistors, they do so through another resistor about 10x larger. I should stop relying on block diagrams for electrical accuracy..

 

I'll go back into my hole now..

[paulears]

Not to worry - I did the same thing a few weeks back and it's difficult to unremember things isn't it!