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XLR/TRS Combo patchbay and phantom power Audio Interfaces
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

XLR/TRS Combo patchbay and phantom power

Hi everyone,

does anyone ever used a XLR/TRS Combo patchbay ?

Like this one -> 16 XLR to XLR 1U

I was asking to myself... almost every budget audio interface (i will upgrade to motu 896 mkII hybrid) has the option to activate phantom power on 4 chanels at the same time instead of for each channel.

So... imagine if i have a synth plugged into inputs 1-2 and a condenser mic on the 3 input. Does this will damage my synth ? The synth can be connected with tr or trs cables on the patchbay, then on the inputs of the audio interface.

Can someone explain wich scenario we should never do and wich scenario can happen but is not harmful ?

Phantom power passes through xlr oly right ? Will it pass through every in and out of this combo patchbay ? For example from the 1-2 inputs of the audio interface to the patchbay then to the front of the patchbay wich has a trs connected into it ?

I hope i'm clear enough...

Thanks !!!
Old 20th July 2014
A XLR /TRS patchbay can be potentially safe for powered mic inputs, but this particular one is not. The XLR and TRS connectors are wired in parallel on that device. That means it's safe for balanced line-level inputs on either XLR or TRS cables, but should never be used for any phantom powered mic inputs. Plugging in or removing a TRS plug to/from the patchbay will create an unbalanced short across any powered mic input which is wired in. That can easily damage a mic input from the unbalanced application of phantom power and can damage a dynamic or ribbon mic.

A safe version would have independent connections for the XLR connectors and the TRS connectors so the only mic lines wouid be on the "mic" XLRs and only line-level, non-powered inputs and outputs would be on the TRS jacks or XLRs clearly reserved for balanced XLR line level I/O. One would still have to be careful not to accidentally connect a non-transformer coupled balanced XLR line-output to a powered mic input.
That would not hurt the mic pre, but could do serious damage to a non-floating output circuit.

Applying phantom power to its output circuits can easily damage you synth, EQ, compressor, and even an interface line output, depending on if it's tolerant of DC being applied. and/or if it has output coupling caps which may experience a polarity reversal if positive DC is applied. Connecting a line output to a phantom powered input will likely void your warrantee, so the manufacturer will not be liable for the damage.

I wouldn't let that parallel wired patchbay near a mic pre input. However, it's flexible and perfectly OK for line-level ONLY I/O.

Having phantom powered mic inputs connected to TRS patchbays, or wired through TRS cables is simply bad engineering practice, although some people do get away with it. That, however, does not make it right. Soon or later it will come back to bite you when you forget to shut off the PP and pull a mic TRS patch cable, or connect a powered mic input to a active device line out that can't tolerate the application of 48 Volts DC across its output terminals.

Also remember: If you "upgrade" to a MOTU 896 you won't be able to access both the TRS line input and the XLR mic inputs at the same time. If you want to connect them to the patch bay, you will have tom choose one or the other for each input. Ther are independant inputs and are not connected in parallel like most combo interface or multi-mic pre inputs. You can't replicate the MOTU 896 inputs on that patchbay.

The problem with using this particular parallel patchbay is clearly spelled out in its specifications (see attached) wiring the H,C & G to T, R & S is perfectly OK for non-powered circuits, but is a disaster waiting to happen when you bring phantom power into the equation. Don't do it.
Attached Thumbnails
XLR/TRS Combo patchbay and phantom power-parallel-xlr-trs-patchbay.jpg  
Old 20th July 2014
Lives for gear

I wanted to add that it depends how you intend to hook this up to your interface.

I know the MOTU 896 (which I have) works differently that just a parrallel hybrid connector.

The XLR on the hybrid connector inputs connect to the mic pre where as the TRS part of the hybrid connector is simply LINE IN and does not have anything to do with the mic pre. This to me means if I engage phantom power on a channel that has a TRS plugged into it...that it shouldn't send phantom down a TRS line. (I could be wrong about that and you should verify with whatever model you have)

I would check and see if this is the same thing with your current interface...Because you wouldn't want to be plugging your synth into your interface's mic pre (directly or via this patchbay) without a DI Box anyways.

Meaning if you plug your synth into this patch bay and then onto your interface via a TRS rather than an XLR then you should be fine. This is assuming of course that phantom is NOT sent down a TRS that is plugged into your interfaces line input.

I hope that made sense.
Old 20th July 2014
Lives for gear

I say this because it's not the patchbay that is generating the 48v (obviously) it's really only getting 48v if it's receiving it from your interface's inputs. You just need to be aware of what items need phantom power and which don't. Simply connect to your interface via TRS for those that don't need 48v.

The issue here of course is when you use a microphone that doesn't need phantom power as it still needs the mic pre...but the truth of the matter is that is an issue you would encounter with or without the patch's not the patch bay that is causing the problem. It's your interface. The patch bay is simply a pass through.
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

I understand both of your point of view, it answers my question perfectly.
I know how to use a patchbay Korbin, it was more about the propagation of the phantom power...

Thanks Lotus 7, so i will use XLR or a separated TRS patchbay for safety reasons in case of i don't remember to turn off the Phantom power

Like Korbin said... phantom power does not pass on trs and ts inputs of most audio interfaces i think, not sure but almost sure... So this will not affect the synth but yeah it's kind of a risk if i ever plug it with xlr cables and the fact that this particular xlr combo patchbay is wired in parallel... Not really a good idea...

Thanks !!!!
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

So if i have a hybrid patchbay like this one but without parallel wiring and with the motu 806 that send phantom power on individual inputs... it should be fine ?
Can you link me to one patchbay that i can use ?

Thanks !
Old 20th July 2014
A patch bay that connects its TRS jacks in parallel with a probably phantom powered XLR connector IS a problem. The combo inputs on that particular patchbay (the MAMBA) are not, and cannot be , wired as the typical mic pre or interface combo jack which simply uses the combo jack sections as two independent connectors. That can't happen with that patchbay which is why it never should be used with PP mic inputs.

If you find a patchbay with isolated XLR and TRS sections, it's fine as long as you're carfeul what you connect to what. Again you don't want to connect a PP input to any device output except a mic that requires PP.

Again, remember, you can only access the line OR the mic input on a combo interface or mic pre at one time. There is no way (without serious internal modifications) to connect to the XLR combo section and the TRS section at the same time. It's strictly an OR situation. You can't replicate the MOTU 896 combo jack on a patchbay, even if it has independent access to the XLR and TRS sections. You must decide ahead of time if you will be connecting the XLR (mic input) or the TRS line-level input to the patchbay Combo inputs do not lend themselves to easy patchbay connections.
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

Can you suggest me a good xlr patchbay ? I only found a art and hosa... I recently got a AD Converter and a mic pre, i don't want to get poor signal due to a poor xlr patchbay...
How you guys proceed to minimise the patching process time ?
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

Lotus, i understand the problem but i'm not sure to understand what you are saying :

"The combo inputs on that particular patchbay are not, and cannot be , wired as the typical mic pre or interface combo jack which simply uses the combo jack sections as two independent connectors. That can't happen with that patchbay which is why it never should be used with PP mic inputs."

Can you just explain in other words ? Sorry about that, i'm missing some details to visualise what you are explaining...
Old 20th July 2014
Lives for gear

Lotus I hear you but I think you are missing something.

While I agree as a safety precaution maybe don't mess around. But the patch bay being parralel or not has nothing to do with it at all.

The patch bay is a hot a cold a ground through point. Nothing more.

It all depends how the unit that is providing the phantom power functions. If the interface's TRS connection in fact doesn't send phantom power when engaged...then you simply plug the patchbay port that phantom is undesired on to the interface via TRS. Downs matter what's happening on the front of the patch bay...only the back of the interface.

And I will stress that I can't confirm that the interface doesn't send phantom down a TRS line...mostly cause I don't know what it is.

To the op. I didn't mean to imply you didn't know how to use a patch bay
Old 20th July 2014
The issue is that the OP was concerned about using a particular patch bay for phantom powered mic connections.

Phantom powered mic connections should never be carried on TRS jacks because of the well known, "unbalanced short " condition that occurs when a TRS plug is inserted or removed.

If one uses that patchbay, the PP circuit is connected to the TRS jack. That's an engineering disaster.

A patchbay is not simply a "a hot a cold a ground through point". If it has TRS jacks, and this one does, it's an easy way to create a serious bad condition (the unbalanced short condition) that could not exist if the patchabay was not introduced to the system. Therefore I still hold the patch bay responsible.

With very, very few exceptions, TRS line inputs do not carry phantom power. That's not the issue at all. XLR mic inputs do, and with the patchbay in question, that phantom power will be applied to a TRS jack if a XLR input is connected to the XLR patchbay section. That's the problem.

Once you introduce TRS jacks in PP mic circuits, you are way beyond "a hot a cold a ground through point", because of what happens in the dynamic state of plug insertion or removal.

TRS patchbays, combo parallel XLR and TRS patchbays are an issue, and should never be used with a phantom powered mic input which was what Modilarleadz inquiry was originally about.
Old 20th July 2014
Gear Head
ModularLeadz's Avatar

Yes Korbin, this was what i was thinking at the begining... the only problem is if the interface sends phantom power individualy or not on it's inputs...
Korbin : i said this because i thought you were going into another direction and not answering my phantom power point of view problem, sorry about the confusion.

I think i'll just test if the interface sends phantom power individualy or not and if it sends it on xlr only or trs too.
Thank you for your tips !

Can someone suggest me a good xlr patchbay that will not degrade my mic preamp and ad converter signals ?
Old 20th July 2014
If it's any common interface it does not send PP to the TRS sections of the combo jacks. They are separate inputs and are isolated from PP. There are possibly (2) very specialzed devices that provide PP on TRS inputs.
Old 20th July 2014
Lives for gear

Lotus I agree with you. My point is that there wouldn't be phantom power if he simply uses a TRS at the interface input. And there wouldn't. The patch bay would share phantom across the xlr AND TRS you are 100% right...but only if that 48v is present...which it wouldn't if the interface was connected to that patch bay port via TRS. And yes a patch bay such as these are a simple through point...there is no additional circuitry...

I agree that the shorting problem with TRS are fully correct...and yes is the operator doesn't think about it...there could Infact be a mistake made...but that doesn't change the way this are wise to warn about point is that what the op is concerned about which from my understanding was an issue with his interface sending phantom down multiple channels with one switch ( perhaps I'm wrong about that) isn't an issue if he uses the proper connections at the interface.

Anyways...OP, do me a fav and pm me if you remember when you figure out if the interface does or doesn't send phantom down its TRS line input. I highly doubt it does but have never tested it.
Old 21st July 2014
Lives for gear

Another option is simply get 2 active Di boxes and plug those into your patchbay via xlr and your synth into the Di boxes...then those active Di boxes require phantom power.

In the long run though you certainly want an interface that supplies phantom individually.
Old 21st July 2014
Gear Head
ModularLeadz's Avatar

Ok Korbin i will try to pm you when i will test it, i still have to sell my focusrite saffire pro 40 before.
Thank you for your help.
Yes the motu 896 can output PP on individual channels, that's great !

So down the line everybody uses only trs normal patchbays and xlr normal patchbays because of all this "complicated" routing that can occur so we use separate patchbays to be safe.
Do you both agree with this ?

Can someone suggest me a good xlr patchbay ? Please ? I only see some under 100$ budget hosa or art ones... Will it affect the signal ?
Old 21st July 2014
Gear Maniac


You can use XLR patch cable on back of Mamba - Male to female for mic inputs. For line input on your interface - XLR Male to TRS

As long as you don't plug any TRS on the mic input with phantom on, you are fine. You can also use rubber plugin to close the TRS hole on the combo.

If you want, we can disable the TRS input on as many input connectors. Say, if you want 8 mic input and 8 line then we can disable the TRS on first 8 combo. So, even if you plug TRS accidently nothing will happen. You can contact [email protected]

BTW, we plan to release XLR Female and combo version in 2-3 weeks.
It comes in, 8 XLR F and 8 combo or any custom option such as 12 F 6 Combo on front.
Old 22nd July 2014
Gear Head
ModularLeadz's Avatar

Thanks Netsound. I will use xlr and trs separated patchbays just in case i don't remember to turn off the phantom power.
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