Audio Snake Capacitance Issue

[Siegfried Meier]

Don't believe everyting you're told.

Ok...I'll believe everything I'm being told here though...    I kid I kid!!

Alright, well let me just say that all I have right now is a simple 96pt ADC punchblock patchbay - the snake ins are on the top, the pre inputs on the bottom and they are half normalled.  Very simple and normal wiring as has been done in the past, but I will change this over right now to a full normal wiring.  If this doesn't alleviate our issue, then I will definitely look into an XLR wiring system as you have done, and will make it fit into our setup.

With those XLR panels, do you suggest not having them in the same rack as other patchbays/gear for fear of the ground on the panel mounting screws interferring?  Or perhaps it doesn't matter?  I noticed in your pics that they appeared to be in a separate isolated rack.

Thanks for your help guys,
Sig

 

[Brian Roth]

In the example pix I linked, that was about the only "not prime real estate" available in the semi-modular furniture.  More accessible space (there wasn't a lot) was allocated for preamps, EQ's, limiters, etc.

The XLR's mounted onto the panels were installed so that all shields/Pin 1's did NOT come in contact with any other wiring, or any of the metal work.  I made sure, with appropriate heat shrink tubing, etc.  The idea is to keep EVERY pin 1/shield isolated from everything else, all the way from the mic to the input jack of the preamp,....just as if you were stringing mic cords down the hall and under the doors <g>.

In the case of the ADC mic bays you mentioned, how are you dealing with all the mic and preamp shields?


Bri

 

[Siegfried Meier]

Cool.

Well, I believe internally the shields are all connected together inside the ADC bay.  A continuity check confirms this.  So the snake grounds are automatically all tied to the mic preamp in grounds (pin 1).

 

[Brian Roth]

I just went back and re-read your very first post.  What isn't now clear to me is whether or not the various noise problems exist without any patching whatsoever in the bay.  Or, does the racket appear when you attempt to "over ride" the normals?

Bri

 

[Siegfried Meier]

No patching whatsoever.  The noise happens with certain dynamic mics, some worse than others it seems.  Condenser mics don't do it, and neither will a dynamic or ribbon with the use of a Fethead or Cloudlifter.  Nothing is wired incorrectly in the patchbay, as was hinted at by another post - it's a very simple and normal connection on the back of the bay.

 

[Brian Roth]

I hadn't realized it until just now, but ADC was bought out by TE (Tyco) several years ago.  The old ADC website had lots of tech stuff if you looked around, but the new TE website is pretty lame.  I did see several different sleeve/ground options on what TE has in their current literature, including bused sleeves/grounds.  Without actually seeing the setup there, I can only guess what variation you might have.  Bused sleeves would be a no-no for mic applications.

How many preamps do you have currently connected?  If not too many, try unplugging the mic inputs from them all, then measure from any shield/sleeve to any other (or for that matter, any "house" grounding point).  You should see infinity Ohms between ANY shield/sleeve to any other, and to anything in the control room that is grounded.

Bri

 

[Siegfried Meier]

32 preamps, so a bit of a pain but I will check this out when I get back to the studio.

For what it's worth, this reply just came in from Cloud Microphones:

Hello Siegfried,

Yes, the Cloudlifters are very good at canceling hum-field and other
noise from long cable runs. Certain dynamic and ribbon mics will pick
up RF/EMF in certain environments, due to the magnetic structure and
(sometimes) lack of sufficient grounding or hum-bucking wiring. The
CLs phase out the noise due to the matched dual stage JFET amps on
both the +/- side. By the way, we just started shipping a new
Cloudlifter CL-4 four channel rack unit, ideal for professional
installs.

I'm glad the Cloudlifters have helped you with this problem!

Best Regards,
Rodger Cloud

 

[Matador]

How many preamps do you have currently connected?  If not too many, try unplugging the mic inputs from them all, then measure from any shield/sleeve to any other (or for that matter, any "house" grounding point).  You should see infinity Ohms between ANY shield/sleeve to any other, and to anything in the control room that is grounded.

This seems confusing to me:  per Bill Whitlock's AES paper, pin 1 should be hard bonded to the chassis at input and output of XLR termination points.  Any powered equipment will likely have it's safety ground bonded from the IEC connector to the same chassis.  Why should we measure infinity from the chassis shield to the safety earth? 

Or is this something specific to patchbays where the intent is that the mike shields don't intermingle?

 

[Brian Roth]

I guess my explanation wasn't too clear.  I agree with the "bond pin 1 to chassis", but in my suggested ohm meter test, all the preamp inputs are to be disconnected.  THEN you measure resistance from any shield/sleeve at the patch bay to any other shield/sleeve, as well as a "house ground" (ie, a nearby rack panel, etc....something that you know is tied to electrical ground in the studio).  The idea is that, when disconnected, NONE of the mic line shields should have any connection to any other mic line shield, or any connection to the "house" ground.

The idea is to emulate the concept of "every mic line is an independent run from the mic to the preamp, just as if you were running multiple mic cables down the hall/under the doors."

I hope that makes sense....

Bri

 

[Siegfried Meier]

Question - does the snake box itself have pin 1 connected to the chassis, which in turn then connects all the grounds together?  I've never built up a snake from scratch before, but this would negate what you've just said...I'll have to test that to see as well.

 

[Brian Roth]

Anything is possible  LOL!  The main thing is that no shield connects to another, or a house ground.

You should be able to do all your measurements at the back of the ADC bay.

Bri

 

[ricardo]

There's mention of normal and half normal in this thread.  Can someone explain what this means?

 

[Brian Roth]

A patchbay is optionally normalled.  The top row of the bay connects to signal sources...microphones in this case.  The bottom row connects to destinations....mic preamp inputs in this case.

Normals add a switch mechanism to the individual jacks in the bay.  Those switches are activated when a plug is inserted intoi the jack.

In a full normal bay, the tip and ring of the top jack in the row is connected to tip and ring of the jack directly below on the bottom row if NO plugs are inserted into EITHER jack.  Inserting a plug into EITHER jack interrupts the "normal" which ties tip and ring on the upper jack to tip and ring to the bottom jack.

In half normal, tip and ring on the top row connect to tip and ring on the bottom row with no plugs inserted.  Inserting a plug into the top row doesn't change the normal connection, but inserting a plug into the bottom jack DOES break the normal, meaning tip and ring on the jack on the top row no longer connects to the tip and ring on the jack directly below.

Somewhat rare, but still existing, are jacks which have switching for tip, ring, AND sleeve.

Bri

 

[Brian Roth]

As a footnote, my description applies to commonly available/"stock" patchbays.  In more than a few recording desks, I've seen normals that were "sideways" versus one atop the other, or even "leapfrog" from one row of jacks to another..

Going from memory on a MCI JH-636 desk, there are half normals that connect the stereo/2-mix outputs into the inputs of several tape machine destinations.  All the jacks involved are on the same row of the bay, IIRC. So, the normals go sideways.

The point is, you can wire the normals any way you care to, if building a totally custom bay.

Bri

 

[ricardo]

Brian, thanks for the explanation of 'normal'

As an ex-Calrec mike designer, I need to say that the need to route P48V mikes through a patch bay requires an inordinate amount of extra complexity in a mike or mike preamp.  Practically all the protection in these is to deal with evils patch bays.

 

[abbey road d enfer]

As a footnote, my description applies to commonly available/"stock" patchbays.  In more than a few recording desks, I've seen normals that were "sideways" versus one atop the other, or even "leapfrog" from one row of jacks to another..

Going from memory on a MCI JH-636 desk, there are half normals that connect the stereo/2-mix outputs into the inputs of several tape machine destinations.  All the jacks involved are on the same row of the bay, IIRC. So, the normals go sideways.

The point is, you can wire the normals any way you care to, if building a totally custom bay.

Bri

In fact the normals are exactly the same, going vertical.  The 2-mix signal is distributed over the normalling contacts of several adjacent pairs of patch points. Without patch cables, all tape machines receive the 2-mix; inserting patch cables accesses the tape machine inputs.
Going back to the original discussion, I would be VERY surprized if the noise problem was due to the patchbay. I keep on thinking that the snake is constantly "noisy" and that low-level mics (dynamics and ribbons) requiring more gain make the noise stand out, while condenser mics being hotter by about 20dB keep noise in the background.

 

[abbey road d enfer]

32 preamps, so a bit of a pain but I will check this out when I get back to the studio.

For what it's worth, this reply just came in from Cloud Microphones:

Hello Siegfried,

Yes, the Cloudlifters are very good at canceling hum-field and other
noise from long cable runs. Certain dynamic and ribbon mics will pick
up RF/EMF in certain environments, due to the magnetic structure and
(sometimes) lack of sufficient grounding or hum-bucking wiring. The
CLs phase out the noise due to the matched dual stage JFET amps on
both the +/- side. By the way, we just started shipping a new
Cloudlifter CL-4 four channel rack unit, ideal for professional
installs.

I'm glad the Cloudlifters have helped you with this problem!

Best Regards,
Rodger Cloud

Ah...manufacturers!
He forgot to mention it cures ingrown nails
But seriously.
Dynamic or ribbon mics do not pick up RFI/EMI.
It's the preamp that receives RFI/EMI and eventually rectifies it into an audible form of noise.
The Cloudlifter is probably relatively impervious to this phenomenon. It's 20 or 25dB of gain help the signal overriding whatever noise is on the line. No magic here, just an application of the well understood notion that low-level signal path must be kept as short as possible.

One thing they mention is susceptibility of said electrodynamic mics to external magnetic field.
If that was the culprit, the tell-tale sign would be that hum changes with orientation, which does not seem to be the case. Using a preamp such as the Cloudlifter would not solve this issue at all, since the noise would be amplified as much as the desired signal.
Having used a product similar to the Cloudlifter, I can vouch that it is useful at solving specific problems such as noisy lines, or providing additional gain for weak signals.
In my case, I use it in conjunction with a crappy ribbon mic that has an actual impedance of 2500 ohms, although being labelled 200!

As a conclusion, I would say that a preamp system such as the Cloudlifter is not a substitute for a clean wiring. I really suggest you check the line noise, with proper equipment and a 200 ohms dummy load.

 

[Siegfried Meier]

Going back to the original discussion, I would be VERY surprized if the noise problem was due to the patchbay. I keep on thinking that the snake is constantly "noisy" and that low-level mics (dynamics and ribbons) requiring more gain make the noise stand out, while condenser mics being hotter by about 20dB keep noise in the background.

Well, what I will do first is disconnect channel 1 from the patchbay, attach an XLR and stick it into preamp 1 directly.  If the noise is gone, then it's not the snake.

For what it's worth, I've put up a condenser at full gain on a Neve preamp and zero noise.  My instincts are telling me it might not be the snake.

 

[Brian Roth]

Going back to the original discussion, I would be VERY surprized if the noise problem was due to the patchbay. I keep on thinking that the snake is constantly "noisy" and that low-level mics (dynamics and ribbons) requiring more gain make the noise stand out, while condenser mics being hotter by about 20dB keep noise in the background.

Well, what I will do first is disconnect channel 1 from the patchbay, attach an XLR and stick it into preamp 1 directly.  If the noise is gone, then it's NOT the snake.

That should work, UNLESS there is some "funny business" going on inside of the stage box out in the tracking room.  You might pop that open and inspect the wiring.  Be particularly watchful for things like jumpers from pin 1's to the "fourth" pin on the XLR which ties the XLR to chassis, bare shields touching stuff they shouldn't, etc.  Also, disconnect all other mic cables from the stage box in case one of THEM is inadvertently connecting a pin 1 to the metalwork of the stage box.

Bri

 

[Siegfried Meier]

Ok, a couple things I just tried and discovered.

It's an ADC punchblock patchbay, with the extra blue and orange normaling posts.  I found that the bay is INDEED wired full normal - I thought for sure that I had done it half, but that was at least 8 years ago so I guess I was mistaken.  2nd, I had also tied the top point ground to the bottom point manually with a small wire - these are internally wired so they are grounded together anyway, so that was pointless.  I've pulled that for now.  I also pulled the normals and the hum is gone...however, if I use a TT cable to connect the top (snake in) to the bottom (mic pre in), the hum is back.

Next up, I wired up an XLR to channel 1 and hardwired to preamp 1.  Hum is still there...I have no other mics or preamps connected anywhere.

My guess is the snake is garbage.  Any other thoughts?

Thanks guys,
Sig