John Sayers' Recording Studio Design Forum Archive

I'm in the final stages of building my studio and I'm ringing out all the wiring (96 jacks in the various tracking rooms). On my first test with a mic in the main room, imagine my surprise (and frustration) to find that in addition to hearing my "test-1-2-3" over the monitor speakers in the control room, I also very clearly heard both of the local AM stations. In fact, I'm getting better reception than I do on my real radio!

Here's the way it's all wired up:

1) In the tracking rooms, I used GEPCO snake cable (both 8 and 16 conductor) into Neutrik XLR and XLR/TRS combo jacks (one "pair" to a connector). All three wires of each pair are wired to the three terminals on the connector. 1 - Shield, 2 - Hot, 3 - Cold (Silver, Red, Black, respectively).

2) These snake cables go through the walls, over the ceiling, and then down to the control room through another hole in the wall.

3) In the control room, I've terminated the snake cables with DB25 connectors using the pin out for a DA-88 cable (I think that's the right number). The pins on the DB25s are crimp type.

4) I made a set of "pig tail" cables that are DB25 on one end and then are directly soldered to ADC jacks from a patch panel I got on eBay (these have like 7 or 8 connection points. I'm only using the tip, ring, and shield connections and the others (used for normaling and such) are left open.

5) From my mixer's XLR mic inputs, I have a PulsarTech snake (15') that is XLR-M to TRS which I then plug in to the patch panel. The particular cable run I'm trying to test is about 30' plus the 20' of the snake to the mixer.

When I was testing, I did have the gain turned up pretty high so I could hear the monitors from the main room. Adjusting the gain would turn the AM signals up and down so I suspect the RF is coming in on the cables.

I have NO flourescent lights in the facility.

The effect is "additive" in that if I disconnect some of the 8 channels, the RF is reduced accordingly.

I guess my question(s) are:

1) Should all the shields in the individual pairs be connected connected on one end, both ends, or not at all?

2) There is a BIG shield wire in each GEPCO cable. Where should these be connected? All together? Left floating? Connected at the mixer end only and then to the ground on the mixer?

3) Are my cables just too long? (The longest is probably 60' from mic to mixer).

4) How do I fix this?

Actually, if you only want to answer question #4, that's ok.

I've got my first session scheduled for this Saturday morning so I'd like to know if there is something I can do in a hurry.

Thanks!!!

len

Len,

Let's check a few more things...

2) These snake cables go through the walls, over the ceiling, and then down to the control room through another hole in the wall.

First question, does the snake run parallel to any electrical wiring? Is the snake in an attic? Are they in conduits?

Does the Gepco wire have an additional shield around all the pairs?

4) I made a set of "pig tail" cables that are DB25 on one end and then are directly soldered to ADC jacks from a patch panel I got on eBay (these have like 7 or 8 connection points. I'm only using the tip, ring, and shield connections and the others (used for normaling and such) are left open.

How is your patchbay grounded? Are all the grounds bussed together, or each wire individually grounded with shield?

5) From my mixer's XLR mic inputs, I have a PulsarTech snake (15') that is XLR-M to TRS which I then plug in to the patch panel. The particular cable run I'm trying to test is about 30' plus the 20' of the snake to the mixer.

Do you have another cable you can try on maybe just one channel?

When I was testing, I did have the gain turned up pretty high so I could hear the monitors from the main room. Adjusting the gain would turn the AM signals up and down so I suspect the RF is coming in on the cables.

Are you using outboard mic pre's or your mixers? The shorter distance you have between the mic and pre (as well as fewer connections as possible) the better.

1) Should all the shields in the individual pairs be connected connected on one end, both ends, or not at all?

Preferrably connected on both ends, but sometimes there are exceptions.

2) There is a BIG shield wire in each GEPCO cable. Where should these be connected? All together? Left floating? Connected at the mixer end only and then to the ground on the mixer?

Are you referring to a shield wire running through the entire snake, or an additional shield in each pair?

3) Are my cables just too long? (The longest is probably 60' from mic to mixer).

The shorter, the better. If you're running that long of a cable, I would probably put some mic pre's on the other end of that cable run. 60' of wire to have a pre boost up all that noise is a lot. It's not unusual for cables to be that long in general in a studio...(console to plate in tracking room, plus a 20 or 30' mic cable to the mic), but the shorter the better.

What type of mic cable are you using?

Thanks for the response Aaron! Before I get to your questions, I would like to say that the recording session Saturday went great!! It seems that most of the problem with noise/RFI was related to having way too much gain. If you remember my original post, "the radio" signal was present when I was trying to "buzz" out the various cables and connector panels. Since I was doing this myself, in order for my "testing 1-2-3" to be heard, I had to turn up the mic gain and power amp so I could hear the main control room monitors through rather well sound-proofed walls. Once I got to the point of micing the guitar with the client sitting in the room and me sitting in the CR, the gain came way down and the RFI seemed to go away. I'm sure that just about ANY wiring system no matter how well constructed would tend to pickup AM signals given enough length and gain.

Anyway, on to your questions:

Aaronw wrote:

len-morgan wrote: 2) These snake cables go through the walls, over the ceiling, and then down to the control room through another hole in the wall.

First question, does the snake run parallel to any electrical wiring? Is the snake in an attic? Are they in conduits?

Does the Gepco wire have an additional shield around all the pairs?

Yes, the cable runs through an "attic" if you want to call it that. This is a commercial buiding (as opposed to a house) so it's really just a wide open space above the inner ceiling/framing that I built. It does (I'm sure) run parrallel to electrical wires somewhere, the Gepco is NOT in conduit but ALL of the electrical wiring is (code requirement).

As far as the shielding goes, there is a light foil "shield" around the bundle of indidual pairs (actually triples), and each pair has the same foil shield around the three wires in it. Each of the pairs has a bare shield wire (the third wire in the "pair") and then there is one big shield wire in the snake cable that is outside of all the individual pairs.

Aaronw wrote:How is your patchbay grounded? Are all the grounds bussed together, or each wire individually grounded with shield?

I really don't know about the patch bays. They are Behringer PX3000's and a "home made" panel that uses ADC jacks. The PX3000s are balanced and I wired the ADC jacks balanced also. On the ADCs, each of the three wires in each "channel" are connected (+,-, and shield) and the same goes for the other end where the XLR/TRS jacks are: all three wires are connected.

It appears that the XLR connectors (Neutrik) have an extra solder tab on them that might go to the metal case of the connector. These are NOT connected together right now and the big shield wire is currently just wrapped up around the outer cable sheathing (i.e., not connected to anything). The ADC connectors are mounted to what appears to be a plastic housing (just a block of plastic with lots of holes in it) so I don't think they connect on the CR end either. The way I have it wired though, the three wires that make up a channel "pair" connect all the way to the mixer's XLR balanced inputs so it's not like I'm floating them entirely.

Aaronw wrote:Are you using outboard mic pre's or your mixers? The shorter distance you have between the mic and pre (as well as fewer connections as possible) the better.

I am currently NOT using outboard pre-amps, just what's in the mixer. In fact, that's the only reason I HAVE the mixer. I wanted the pre-amps and since this is going to be a commercial studio, well if you don't have a mixing desk, you're not a "real" studio. I do most of my mixing in the box with Cubase but the console sure LOOKS impressive. I may take an outboard preamp and stick it on a line that I'm having a real problem with and see if that makes a difference.

Aaronw wrote:Preferrably connected on both ends, but sometimes there are exceptions.

Let me see if I've got this right: ALL of the individual shield wires in each "pair" should be connected to each other on each end? Since this isn't a "shield" in the same way that the braid on a coax cable is, doesn't that mean that I could just skip the individual shields and use the big shield wire for a single connection on each end? I'm not sure what the individual shield wires would be doing (in a snake cable anyway where all the wires on each end have to go to the same place. I can see the use in a single mic cable.

Aaronw wrote:Are you referring to a shield wire running through the entire snake, or an additional shield in each pair?

Well after re-reading what I said, I guess I'm talking about both. I when I said "Where should these be connected? I was refering to the big shield wire in the snake (as a whole). I've got 7 snake cables that go to the various connector panels in the tracking rooms so my question is should these big shields be connected together in the control room where all the snakes terminate? Right now, they are floating on both ends. I was also asking if this big wire should be connected to ALL of the individual shield wires in each "pair" in the same snake cable.

Thanks for any insight you can provide.

len

I'm not sure what the individual shield wires would be doing (in a snake cable anyway where all the wires on each end have to go to the same place. I can see the use in a single mic cable.

Think of each one of those pairs in a snake as a miniature mic cable. Shields should be connected on each end.

As far as you BIG shield/drain wire in the multicore, you can try both floating or grounding. Sometimes, it doesn't do a thing, and other times it'll make a huge difference.

I'm sorry to be so dense about this but I want to be clear:

When you say "the shields should be connected on both ends" do you mean that within a single "microphone cable", the shield wire should be connected to the shield connector on both ends? If that is the case, then I've got that part right.

The other way I read what you said is that on both ends, ALL of the individual shield wires should be connected to their connector AND all of the shields in each cable connected TO EACH OTHER. Is that what you are saying? Should they ALL be connected together anywhere (except maybe in the mixer?) That's where I'm a little unclear.

len

What I mean by on both ends...Pin 1. And in the case of a patchbay (Sleeve).

The other way I read what you said is that on both ends, ALL of the individual shield wires should be connected to their connector AND all of the shields in each cable connected TO EACH OTHER. Is that what you are saying? Should they ALL be connected together anywhere (except maybe in the mixer?) That's where I'm a little unclear.

Let's see if we can clarify this. EG: You have a snake going to your outboard gear. Some connections will be XLR, others 1/4".

On the cable end of each pair (each pair has 2 conductor plus shield). For XLR: Connect Shield to Pin 1, connect colored wire to pin 2 (hot), and clear wire to pin 3 (common) (in the case of Mogami). For 1/4" TRS: Connect Shield to Sleeve, connect colored wire to Tip, connect clear wire to Ring.

The other end (patchbay side)...You have 2 options.

1: Your shield connects to the sleeve connector of the patchbay. Completely isolated from other grounds.

2: You still do the same as #1, but you tie all grounds together on the patchbay (bussing them together...before you solder on your multipair cables), making one large ground plane. (I see this quite a bit on patchbays).


If this still doesn't make sense, give me some time, and I'll try and dig something up for visual reference.

Thanks for re-re-explaining it. Except for one thing, I've wired it exactly the way you said. The only difference is that on the patch bay end, I haven't explicitly wired all of the shields together. On the Behringer patchbays, they could already be connected. I'll have to check.

On my "home made" patchbay, I'll go through and connect all the shields on all the connectors together.

Thanks again. I'll let you know how it goes.

len

How did you tie all your shields together on the patchbay side? Tied together on the patchbay?? (I hope).

Any luck on the rest?