ground chassis: what is it?

[rodrigo]

Ok, I've read the rane balanced cable wiring and seen the diagrams at their technical notes: http://www.rane.com/note110.html

It may be because I'm not a native speaker, but what exactly is tying the cable shield to the metal chassis right where it enters the chassis??

What I understand from this is the following. Let's say I want to connect an Akai sampler (which has balanced outputs) to my Metric Halo audio interface (which has switchable balanced, line, mic and unbalanced inputs).

I should build my interconnect cable using twisted pair cable (Mogami or other) and wiring the TRS connectors as per the diagram. In able to follow the directions, I should remove the cover of my sampler and wire the internal connectors' shield with a tiny cable; drill a hole in my sampler's metal encasing and attach the cable to this hole using a small screw with a toothed washer, thus completing the "chassis ground" diagram. Am I somewhere near the actual truth??

How about if my sampler has 8 balanced outputs? I should do the same and tie the 8 internal shields with 8 separate cables to a single point in the chassis ground?

Is the chassis grounding also achieved by tying the shield to, say a metal rack enclosure? I read somewhere that this may or may not be true, depending on the rack/rails type of metal and paint job.


On another subject. What is the correct way of chassis grounding (or any type of grounding) for gear that uses wall warts? I mean, there is no ground prong, and many of these decives are unbalanced, so the internal connectors don't have a shield. I have some gear that uses wall warts. i know the wall warts should be as far away from audio lines. But is that it? If I wire all my studio with the star grounding technique, should I have a separate circuit and outlet for this type of devices? should this separate outlet be tied or not to the common ground for the rest of my studio??

Thanks for any help on this matter!

Regards,

Rodrigo Montfort

[rodrigo]

I'm also shopping for a replacement for my house's electrical panel box. It uses 3 pole current. I'm trying to figure out the capacity and type of panel box I should get.

The current configuration is the following (All Square-D):

House Central Air Conditioning: 3 pole Circuit braker 70 AMP

House Power supply: Switch with a couple of 60 AMP fuses 250 V each

Studio Air Conditioning: Double Thermomagnetic Circuit braker 30 AMP

Studio Power supply: Single Thermomagnetic Circuit braker 20 AMP



As you can see it's a mess. I'd like to replace the Switch with a Thermomagnetic Circuit braker for the House. (what type? Double 60 AMP?) I'd like to beef up the studio power supply's circuit (at least 30 AMP), and add two new circuits: Studio lighting and Computer/Peripherals.

Proposed configuration:

House Central Air Conditioning: 3 pole Circuit braker 70 AMP

House Power supply: Double Thermomagnetic Circuit Braker 70 AMP

Studio Air Conditioning: Double Thermomagnetic Circuit braker 30 AMP

Studio Power supply: Double Thermomagnetic Circuit braker 30 AMP

Studio Lighting: Double Thermomagnetic Circuit braker 20 AMP

Computer/Peripherals: Double Thermomagnetic Circuit braker 30 AMP


What do you think?

I asked an electrician and he recommended a 3 phase panel box 100 AMP rated. Do you think that'd be enough?

Thanks for any help!


Rodrigo Montfort

[David Kulka]

Rodrigo, your English is excellent and I can assure you that the problem here has nothing to do with your language skills!

Gounding technique can terribly confusing and though there are some good guidelines, nearly every situation is different.

First regarding your question about "tying the cable shield to the metal chassis right where it enters the chassis". I think the Rane paper (which, though well thought out, is not the voice of God) is saying that the manufacturer should wire the panel connector this way inside the chassis. The instruction does not refer to the cable you provide, that plugs into the unit.

Usually, tying shields to a rack is not a good idea. It's very inconvenient and generally it will not help noise problems.

When the Muncy paper was written some 20 years ago, manufacturers were indeed wiring their internal grounds in strange and random ways. The Muncy paper caused a lot of gear makers to clean up their act, and the situation is much better now. Of course, with older gear, there are still some problems.

In my opinion, the best thing you can do when wiring a studio is to telescope your grounds. The rule for that is: except for mic lines, when connecting the output of one balanced piece to the input of a balanced piece, cut the shield on the input side. (You end up with a lot of male XLR's and TRS input cables with no shield connected.) This will prevent and solve a huge number of problems and it speeds up wiring, since there's one less wire to solder.

If you do this, and don't have a lot of unbalanced stuff in your room, and still have a lot of problems, it can be cleaned up, but you'll probably likely need a tech to help out.

If you do have a lot of unbalanced gear in the room, some ground noise may be inevitable and you may need to balance some of those pieces to really clean them up. Again, maybe you'd need a tech's help.

If your wiring is telescoped and just have one or two noise problems with certain pieces of gear, this is where the gear itself might be to blame. You might open the item up, see how pin 1 has been wired. If it does not have a short piece of wire to chassis, you could try changing it, but don't get your hopes up too high that this will really fix the problem.

Yes, with this scheme on a multitrack unit, you would tie the internal shields with 8 separate cables to a single point on the chassis.

A wall wart on an unbalanced piece can be a benefit, because there will be no ground loop back to the AC . You don't need to do anything special with the shield or the wall wart. For a wall wart powered box that's balanced you might find that skipping the telescoping instruction above works better. But generally, wall wart designs, though they are cheap looking and a pain in the rack, don't cause grounding problems.

I wish it were simpler. There's no one size fits all answer, but I hope this helps.

[Matt Syson]

Hi
You must get assistance from a (local) electrician with regard to the house wiring as there will be strict guidelines on what you can and cannot do, and it must be installed by someone who will ensure it is safe. It would probably be a good idea to call several electricians to see if any understand your requirements in terms of a studio. The important thing is that it is done in a safe way.
Matt S

[cornutt]

Rodrigo: Some general advice on the electrical wiring. Take all of this with a grain of salt, because I'm going from my general knowledge of the U.S. National Electrical Code. I don't know what codes apply in Mexico.

But first things first: You cannot put a breaker on a circuit that has more capacity than the type and gauge of wire that the circuit is rated for. In general, assuming all copper wiring, you can go up to 15A on a 14-gauge circuit, 20A for 12-gauge, and 30A for 10-gauge. If you Google around, you can find tables on what amperage ratings are permissible for larger gauges and different types of wire. (Do a Google on the word "ampacity"). So in most cases you can't just arbitrarily put a higher-amperage breaker on the circuit without re-wiring.

Do you know what your house's service entrance is rated for? If not, the electric company should be able to tell you. If you have a 100A service, then a 100A panel will be fine. But if your house has a 200A service, then if you only put in a 100A panel, you won't be able to use all of the power that your service is capable of supplying.

For the studio: What I'd do instead of running individual circuits back to the main panel is, instead, put in a feeder line to the studio and put a sub-panel in there. Then, you can run individual circuits off of the sub-panel inside the studio. It's more conveinent, takes less wiring, and you will have fewer ground loop problems this way (assuming that the electrician does the neutrals and grounds properly). A 3-conductor (two-pole), 50A circuit to the subpanel will probably do nicely. Leave the studio A/C circuit as is; don't run it to the sub-panel. That way, you keep electrical noise from the A/C out of your studio circuits.

One other note, not having anything to do with the studio, but it caught my eye: You mentioned your house A/C circuit having a 70A breaker. That doesn't sound right; an A/C system drawing 70 amps would be an awfully big system for a residence, even in Mexican heat. It sounds to me like someone just put in a breaker that they had laying around. Typical residential A/C systems are rated around 30-40A. If and when you get an electrician in to do the other work, ask him to have a look at that and put in a breaker of the proper rating.

[rodrigo]

Thanks everyone for your replies. I really appreciate you taking the time to read my post and write possible solutions.

David, I don't really have much unbalanced gear. Most of my equipment is balanced, but I used to get some ground noise in my audio. I've rewired some of the studio's outlets and I'm in a construction phase so I'm searching for my best options. I think I'll follow your advise on telescoping the grounds, the rane source file doesn't recommend it. But like you said, it's not the voice of God! The fellows at Metric Halo also recommend telescoping grounds, so I'll stick to this idea.


I actually brought in an electrician to see my current installation, but he didn't mention anything about the 70 amp circuit braker. In fact I didn't like his input and I'm certainly calling someone else. I assume this oversized circuit braker is there because the house used to have a very old air conditioning unit (twenty-something years old). But now it's been replaced by a new more efficient unit. So I'll need a qualified electrician to take care of all the details.

I'm aware of the circuit breaker capacity/wire gauge limitations, and the original plan has always been rewiring the new circuits. I already have a very old electrical panel box, and I plan to replace it with a new load center panel box. Reusing the wire installation and circuit brakers that I'm able to keep, and installing new circuits for the studio, and the computer. But the electrician's advice didn't convince me. If I have a 3 pole installation and I'm able to divide my power into:

1 - AC units for house and studio

2 - House power

3- Studio and Computer, peripherals;

Then I think I can achieve maximum circuit isolation and have a noise free, efficient power installation. (Obviously with star grounding for studio outlets)

So I need to find out my house's service entrance rating and hope to find a great electrician.

Thanks everyone,

Best regards,

Rodrigo Montfort

[rodrigo]

Cornutt,

Thanks for all af your input. I forgot to say that the main power is actually located in one of the studio rooms. So that's why I didn't think of a sub-panel before, the main power is right there, already available.


I already installed a separate circuit for the studio using 10 gauge wire. But the ground was wired in series (I think you call it daisy-chained). So I need to correct that and install a separate ground wire for each outlet (star grounding technique). I'm currently using less than 15 A, all gear connected to a 1800 W power conditioner. If eventually I need more than 15 A, I'll just use a different outlet and won't overload the circuit.

Since my computer and peripherals suck quite a bit of power, I thought of installing a dedicated circuit just for those things. I think 20 Amps for the studio circuit and 20 Amps for the computer circuit is enough. I'll never consume more than 20 amps for those circuits.

I read somewhere that any appliance which uses a motor or fans should be on a separate circuit and preferably a separate phase/pole. But do computer fans count? I installed about 4 external fans to my computer cabinet plus the G5 2.5 fans... that adds up to about 8-10 fans. Should I install this circuit on a different pole than the studio's circuit?

Thanks!