Ground (loop?) issues when using patch bay... could use advice.

austinm08

austinm08

the pigeon knows the way.
So I have a problem when using my patch bay with my Digimax LT.

Here's the signal path:

1) Mic
2) Female XLR to Male TRS snake
3) TRS patch bay (Neutrik NYS-SPP-L1)
4) TRS to Male XLR cable
5) Presonus Digimax LT going optical into:
6) Digi 002 control surface

This rack gear is racked in my homemade case and each unit is NOT isolated from each other, in that there is no insulator between the unit and the rackmount rails attached to the case. Is this the cause of my problem?

One thing to note, is that if I don't plug the TRS from the snake completely into the patch bay (enough for the tip and ring to make contact, but not the sleeve... I think), the signal passes through just fine. But as soon as I push it all the way in, there is an obnoxious hum and no signal seems to pass through.

One other observation, is that if I bypass the snake and patch bay and go directly into the Digimax LT, there is no hum.

What would be causing this?
 
Why are you plugging a mic into a patchbay with other gear in the patchbay?
That's asking for gound/hum issues.

The mic should be going directly to a preamp, and then the Line Out of the preamp can go to the patchbay.

If you have a lot of mics/preamps and you need a patchbay for them...it should be a seperate XLR patchbay with ONLY the inputs to the preamps on it. The outputs of the preamps go to the other patchbay where the rest of your gear is wired.
 
I'm not quite understanding this. I mean, just like you suggested (for the XLR patch bay), the only things touching the patch bay are the inputs to the preamps, consisting of my Digimax LT and a channel strip preamp I just converted to rackmount.

I thought one point of a patch bay is to allow you to more easily plug in your mics to whatever preamp you want without having to get to the rear end of your gear.

EDIT: Do you mean the patch bay should not be touching the metal rails that it's screwed to? Because I did try it without it mounted, and the problem is still there..
 
So you have no other gear in that patchbay...?

Try that same signal chain...but just remove the gear from the patchbay (skip #3).

There has to be a ground problem with one of your pieces then...or with the wiring...and when it's in the PB it passes the problem to the other grear via the common ground in the patchbay.
You just have to trace it down.
 
Nope, I just have inputs from the pres plugged into the rear of the patch bay.

I have tried that already, except I didn't use the snake (and cable from patch bay into pre) as I could just go directly into the pre from the mic. So surely that snake wouldn't be causing the hum, right?

I have even tried disconnecting my DIY converted pres from the bay, so its wiring is not the culprit (unless ground issues could still be passed through the rackmount rails)..

Another note: If I touch the Digimax LT casing, the hum is reduced.. like it's traveling through me rather than the Digimax? heh....
 
Does the patchbay have nylon/plastic jacks...and how is the signal shielded inside the PB?

If so...you may be losing the normal shiedling that a mic cable/connection provides.

TRS is not the best way to pass mic signals...IMO.

You can try running a ground wire from the PB casing to..........something else...experiment. See if you can kill the ground hum that way.
 
It's shielded by passing the sleeve to the chassis of the patch bay? I think??

Here you go:

IMAG0309.jpg


IMAG0310.jpg
 
You said this is your signal path...

1) Mic
2) Female XLR to Male TRS snake
3) TRS patch bay (Neutrik NYS-SPP-L1)
4) TRS to Male XLR cable
5) Presonus Digimax LT going optical into:
6) Digi 002 control surface

So you are, in fact, running your mics into and your preamps out of a "standard" TRS patchbay. This is a no-no. And if you've got phantom power running through there, it's a good way to blow the phantom power supply.

Miroslav mentioned an "XLR patchbay". I'll get back to that...

If you only have a few mics and preamps, and you want to avoid going behind the equipment to plug in your mics, use a female XLR to male XLR snake. Plug the male XLR's into your preamps. then you can plug your mics into the different preamps by just switching the female XLR connectors they are plugged into.

There are other solutions, such as an XLR patchbay, based on the complexity of your studio and the size of your wallet, but this is about the cheapest small-studio solution. Let me know if you want to hear about some of them.

But you definitely do not want to run mics through a TRS patchbay.
 
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I agree with the above - whats the point of adding another element to the signal chain for the sake of it? if it is a hassle to get to the rear of your pre then leave the mic cable plugged in XLR - TRS - TRS - XLR OR XLR - XLR it's a no brainer.
 
bdenton said:
So you are, in fact, running your mics into and your preamps out of a "standard" TRS patchbay. This is a no-no. And if you've got phantom power running through there, it's a good way to blow the phantom power supply.
Click to expand...
But... my preamps are not running out of the patch bay. Where are you getting this? The preamps go directly to the audio interface, never touching the patch bay on that side of things.

The only danger with the 48v and the patch bay would be leaving the 48v on and pulling out a TRS cable from the patch bay, right? If I'm careful never to do this, what's the danger?


bdenton said:
If you only have a few mics and preamps, and you want to avoid going behind the equipment to plug in your mics, use a female XLR to male XLR snake. Plug the male XLR's into your preamps. then you can plug your mics into the different preamps by just switching the female XLR connectors they are plugged into.

There are other solutions, such as an XLR patchbay, based on the complexity of your studio and the size of your wallet, but this is about the cheapest small-studio solution. Let me know if you want to hear about some of them.

But you definitely do not want to run mics through a TRS patchbay.
Click to expand...
Good suggestions. But what is so no-no about running mics through a TRS patch bay? I worked in a studio at my school for 2 years using only TRS patch bays with mics, pres, and other gear.

Thanks for the suggestions about what I could do instead.
 
TRS patchbays & mics work fine, so long as you respect the fact that there is 48V on some of the points and you are careful not to patch anything stupid (some non-mic-preamp gear won't like phantom routed to its I/O).

Given that your problem involves the connection between the plugs on the cable and the jacks on the patchbay (that is, it works if hanging out, but not fully inserted), I'd guess that there is some incompatibility between the two. I doubt that is because the sleeve isn't grounding when hanging out; if you have no pin 1 connection on a phantom-powered mic, you have no sound. Instead, I would guess that something else is amiss.

If you are feeling adventurous, you can pull a card out of the patchbay and troubleshoot by connecting everything outside of the patchbay chassis.
 
Thanks, mshilarious... I'll try that and see what happens. The mic I used before wasn't a condenser, though -- just a 57.
 
A 57, like most dynamics, doesn't need a pin 1 connection, that's just a shield. Even that shield is relatively unnecessary because pins 2 and 3 are balanced, so long as they are both connected all should be well. It would also work if either pin 2 or 3 was grounded, it would be unbalanced but it would still work (however, that would unbalance phantom if that was on, which could cause noises in some cases).

If either pin 2 or 3 has no connection, the mic won't work. That is not necessarily true of condensers; some will work with either pin 2 or 3 lifted or grounded (and some won't), but they won't work at all with pin 1 lifted.

It might be easier to troubleshoot by simply measuring phantom power with no mic attached; at the XLR-F that should plug into the mic, measure DC voltage between pins 2 to 1 and 3 to 1. If that isn't 48V, then something is wrong, and it ain't a ground loop.
 
I good quality, all metal 1/4" TRS patchbay....like ADC quality...could work for sending mic signals, if you avoid mixing non-mic gear in the same patchbay (which you seem to be). You could even mix in other gear...BUT it's a potentially bad practice because of phantom power and maybe unbalanced gear that could disrupt things.
Thing is...a lot of pro-sumer 1/4" patchbays tend to use nylon jacks...and may not be well grounded/shielded internally...
...so IMO, it's just best to avoid those PB for mic. It's just a lot cleaner/safer to use a dedicated XLR patchbay only for mics if you have enough preamps that you really need a patchbay.
I think you will find most studios keep their mic patchbays separate from the other gear patchbays....it's just a "best practices" approach.

Those PCB style patchbays are OK for passing signals if they're good quality...but often they don't provide as good grounding schemes and shielding, IMO.

Like I said earleir...you're going to just have to experiment and see if you can isolate the cause of the hum. The fact that it goes away when you touch the PB case seems to suggest it's a ground/shield issue...now just find the real source. :)
 
I tested it out using one of the PCBs outside of the chassis and there was no hum.. and this is weird as heck, but I tested it with it back in the patch bay chassis and the hum was still gone. It's just... gone.

I wonder what has changed.... ?

Alright. I should and will probably pick me up an XLR patch bay to use if I'm going to keep using one for this purpose.
 
While it seems weird...it's actually an indication that something(?) is probably loose and was causing the ground hum.
The process of disconnecting and taking things apart obviously caused that "something" to shift/move...and the hum is gone.

It may stay like that...but then it may reappear at a later time, and you'll be right back with the same problem unless you can hunt down that actual cause and remove it permanently.

I have to say...what was suggested to you earlier might be a simpler solution than bothering with any kind of patchbay just for one preamp. Simply connect a couple of short XLR cables to the preamp, and just dropping them to side or under your gear and leaving them connected to the preamp until you need them, and then you use them as jumpers to which you connect a longer mic cable as needed......or even skip the short "jumpers" and just connect longer mic cables and tuck 'em away when you don't need them, but leave the connected to the preamp.

Don't you think?
 
But what is so no-no about running mics through a TRS patch bay? I worked in a studio at my school for 2 years using only TRS patch bays with mics, pres, and other gear.

If you always remember to turn off phantom power before plugging/unplugging anything there won't be a problem.

However, plugging/unplugging TRS connectors creates a dead short across your phantom power supply. Not a good thing, especially if the phantom power is coming from an expensive preamp.

But the way XLR connectors are constructed, the phantom power supply is not shorted during plugging/unplugging operations.

Given that the danger can be avoided at reasonable cost, I believe that "goof proofing" is a good idea...
 
Sorry for two separate posts...I'm still half asleep

Originally Posted by bdenton
So you are, in fact, running your mics into and your preamps out of a "standard" TRS patchbay. This is a no-no. And if you've got phantom power running through there, it's a good way to blow the phantom power supply.

But... my preamps are not running out of the patch bay. Where are you getting this? The preamps go directly to the audio interface, never touching the patch bay on that side of things.

Sorry, but you misread what I posted...

The input to your preamp comes out of your patchbay.
 
Thanks for the responses again, guys.

I have one other Ward Beck converted-to-rackmount preamp, but that doesn't make a lot of difference. It makes sense to do what you've suggested and just use either the short jumpers or leave longer cables plugged in. Less elegant than having a patch bay to plug into, but hey, I shouldn't be sacrificing sound quality, either, like I am with the patch bay.
 
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