Audio Snake Capacitance Issue

[Siegfried Meier]

Hey guys,

We've got this issue at the studio and it's plagued me for years.  Our snake in the tracking room runs into the control room and is connected directly to a bantam TT patchbay - it's around 100 feet long.  The snake ins are normalled to the mic preamp inputs, but it also allows me to patch around in the control room as opposed to repatching at the snake.  Now, certain dynamic and ribbon mics, when connected to various preamps emit a hum/buzz with higher gains.  The buzz not only increases, but seems to change tone as the preamp level increases.

A few things kill it.  One, if I bypass the snake and patch directly into my preamps - it's obviously not ideal, because when I test I'm running through the doors and can't close them properly for tracking.  2, if I use a Cloudlifter device, which is that little 25dB gain boost box that requires phantom power - the hum/buzz is completely dead.  Also, this never happens with phantom powered mics either.

I've checked and tested all my ground points on my patchbay but cannot for the life of me figure out what might be happening.  Is it just the fact that it's a cheap shite snake?  Should I rip it out and install a Mogami or something of a higher nature?  I've been told it's capacitance problems with the inexpensive cabling.  But why is the Cloudlifter completely eliminating it then?  Maybe I should just buy 24 Cloudlifters and use them as needed...?  :-O

Any thoughts much appreciated.
Thanks!
Sig

 

[JohnRoberts]

Lets see, hanging a 100' antenna off the mic inputs picks up noise? Even though that snake is shielded it isn't perfect.

You must decide if the convenience is worth the cost.  How hard is it to break the normal at the patch bay.. perhaps figure out some way to have it only when you want it.

JR

 

[Brian Roth]

I've built many dozens of audio recording facilities (and dozens of video rooms) during the past decades.

Never had many noise problems.

Is the mic-to-desk grounding through the patchbay isolated?  IOW, infinity Ohms from the "sleeve" of the mic lines to "house ground"?

Bri

 

[Siegfried Meier]

Can you describe that process more in detail please Brian?

Currently, I only have snake wired to the ADC patchbay, and then the preamp inputs are wired to normal of those.  What are you referring to as "house ground"?

Thanks!
Sig

 

[MHanson]

Just for curiosity's sake ...

What type of normal?  Full or Half ??



Michael

 

[Siegfried Meier]

Half normal - so I can bridge out of a snake out and patch it to any preamp, but it continues the signal to the destined pre unless you patch/break.  Yes, be very careful if +48 is on here...it has come in handy many times to patch the same mic signal to several preamps for various duties.

I've seen lots of studios do this, and I can't understand why the Cloudlifter fixes this issue.  The Triton Fetheads would likely do the same, and they're a little cheaper.

 

[Brian Roth]

Siegfried, disconnect all of the mic inputs connected into the desk, so that the mic patchbay is "naked" in terms of the destination. yet still leaving all of your "incoming feeder" lines from the mics connected.

Take an Ohm meter and measure from every mic XLR "pin 1" (ie, sleeve in the patchbay) to any ground you can find.  IE, the frame of the desk, the third/round grounding  pin of an AC power outlet, the cold water pipe supplying the toilet <g>.

All should measure infinity Ohms.  Likewise. from any mic line "pin 1"/sleeve to any other should also be an open circuit.

Any "sneak path" from the mic wiring to any "house ground" (or, from "pin 1" on any given mic line to another) says you have a problem in the wiring.


Bri

 

[Brian Roth]

Never half-normal mics in a patchbay.  Yes, yes, I've seen it done and sometimes it works.

The goal for any mic patching is to emulate the idea that each mic connects to the desk via its own "home run" mic cable.

For many years, the ONLY mic patching I install is via XLR patch panels in the control room.  Incoming mic lines terminate into male XLRs on a rack panel, mic preamp inputs terminate onto female XLRs on a rack panel.  Short length male to female XLR cables link them.

Again, I've seen all sorts of normalled this way and that, grounded (pin 1) this way and that, and sometimes it works.  Often it doesn't.

Just think in terms of "Hmmmm..I gotta mic here and a preamp there.  I'll just run a cable between them!"  Any other wiring system is asking for troubles.

http://www.brianroth.com/projects/gv2010/week%204/IMG_0073.JPG

http://www.brianroth.com/projects/gv2010/summer2010/IMG_0087.JPG

http://www.brianroth.com/projects/gv2010/summer2010/IMG_0088.JPG

http://www.brianroth.com/projects/gv2010/summer2010/IMG_0073.JPG

http://www.brianroth.com/projects/gv2010/week2/IMG_0073.JPG

One bonus....the input jacks into your mic preamps are now "exposed" into the control room, so you can plug in a mic or DI easily, without having to dig around in the back of the rack where the preamps are installed.

Bri

 

[Siegfried Meier]

Well, there's no digging as I have TT to XLR male and female cables made up that can easily access any mic pre from in the control room.  ;-)

Brian, in your scenario if you were tracking all 24 or 32 inputs, then an equal number of XLR patch cables need to be plugged in correct?  Kinda messy and large...which is what I was trying to avoid by using the bantam patchbay.  I don't really understand how your setup is any different from using a TT bay, other than the fact that I've half normalled.  I can easily full normal my bay so the snake outs are always connected to the preamp inputs.

BTW - my console preamps are actually not connected - the pres that are on the bay are all outboard units, in the rack.

Thanks,
Sig

 

[pucho812]

Half normal on mic lines is a  big no no.  Any console I've ever wired and or used had full normal up there.  Either the mic in the lvoe room is going into the pre amp or it's not, no half jacking about it.

 

[Siegfried Meier]

Can anyone tell me why the problem goes away with condenser microphones or when using the cloudlifter?

 

[bruno2000]

We had a patchbay at a studio that I worked for that had an additional switch contact on the patch jack that lifted the shield from the normal to the next row down, and full normaled the signal.  Same switch contact was on the mic pre input.  When it was put in, I questioned the integrity of the shield connection going through a switch, but there was never any problem.
Best,
Bruno2000

 

[radardoug]

By the way, what brand of cable is the snake?
I would just wire the snake direct to the desk you are using, if you have a bunch of pres put in dedicated lines for them.
Then patch once in the studio.
Patch panels and mike lines are not a good mix.

 

[moamps]

Can anyone tell me why the problem goes away with condenser microphones or when using the cloudlifter?

That's the million-dollar question. I believe the patchbay is not wired correctly.

Could you please send some detailed pictures of the back side of your patchbay?

Could you verify that the phantom power is present on both pins (2 and 3 respectively to the 1) at the microphone end (XLRF) of the snake? Most of the condensers and active DI-boxes work with only one phantom power line. The second pin can be disconnected or shorted to the ground what can be the cause of your problems when you are using dynamic or ribbon microphones.

Connect a dynamic microphone (Shure 57 etc.) at the microphone end of one channel and verify using the DVM the impedance of cca 200-300 ohms between pins 2 and 3 at the XLRM end on the input of the preamp. Also check that there is no shorts between pins 2 or 3 to the pin 1. 

 

[abbey road d enfer]

Can anyone tell me why the problem goes away with condenser microphones or when using the cloudlifter?

Are you positive that it "goes away" or just that it is masked by the mic signal? Typically you need about 20dB less gain for a condenser mic (or a mic associated with a cloudlifter) than from a bare dynamic mic.
One possibility is that your snake runs too close to a power element ( iso transformer, dimmer, air con...).

 

[zayance]

I ran into this sometime ago, nice read, dunno if it Will help but juste sharing

audiosystemsgroup.com/AES-RFI-SF08.pdf

 

[CJ]

audio snake?

this one has about 10 pf per foot,

 

[abbey road d enfer]

I ran into this sometime ago, nice read, dunno if it Will help but juste sharing

audiosystemsgroup.com/AES-RFI-SF08.pdf

At the moment, there's no prooof that the problem is due to RFI.
This kind of presentation, although being technically correct, tends to distract from unbiased analysis of all the possible causes of the problem.
At the moment, in the absence of more tangible info from the OP, we might as well seek the help of a crystal ball.

 

[Brian Roth]

Whenever I hear mention of "mic patchbay" and "noise" in the same installation, all sorts of red flags are raised in my noggin.

Without actually being there, or seeing a detailed and accurate diagram of how the incoming/outgoing mic lines are connected into the bay, it's just guesswork.

As I have tried to stress, the bulletproof wiring method is to have a direct path for both signal wires and shield from the mic to the preamp.  A full-normal patchbay keeps the two signal wires on a "one and only one" path, but then you have the issue of "what to do with the shield?", since very few patchbays I've seen also "switch" the sleeve connection along with tip and ring.

I'm sure someone will jump in and state "Well, I've seen this and that done with normalling bays and it worked".  Fine.  Sometimes it works without problems, but often times it doesn't.  I've laid out the mic patching strategy I've used for many years, with perfect success.  I'm sorry that using XLR bays results in low density (16 vs 96 connections in 1RU), and that it requires the use of short XLR patch cords.  All I can say is that it works.

I have no clue why the Cloudlifter seems to solve noise problems in the system described, since I can only guess about what's inside.  But, I've never had to use any of that sort of device.

Bri

 

[abbey road d enfer]

  I've been told it's capacitance problems with the inexpensive cabling.

Don't believe everyting you're told. Capacitance is NOT the issue. Typical cable capacitance ranges from 20 to 100 pF/ft. But it is NOT an indicator of the cable's quality, particularly in terms of noise protection. Some excellent cables have very low capacitance and some crappy ones have high capacitance; the contrary is also true.
Vovox cables exhibit very low capacitance; they are often excellent - not always, and always overpriced.
Canare Starquad has higher capacitance than most standard cables. It is good at rejecting EMI. However I would say that the quality and construction of the shield is the dominant factor.