Monday, October 21, 2013

ARTICLE: Putting It All Together

All Rights Reserved © 1999 Thomas W. Day
(Originally published in Recording Magazine, June 1999 Volume 12, Number 9, P20-31, under the title "An Ounce of Cure.")

In the Beginning?

Usually, we collect a pile of equipment and, over time, figure out how to put it all together. More often than not, there isn't a way around this scenario. The only exception I can think of would be that you've just won the lottery and are out to buy everything you've ever dreamed of owning; and will never dream again when that big shopping trip is finished. So, we buy the bits as we find them, find the money, or both.

Then, we find a space in the rack and a pair of empty jacks on the patchbay and on we go. Just like we'd always planned to put things together that way. The bigger the rig gets, the more likely we are to be retroactive about how it's all assembled.

Having money or being famous doesn't seem to keep us from creating audio disaster areas. One of the biggest-buck studios I've visited had some of the worst laid-out equipment I've ever seen. And I've stumbled through dinky, one-bedroom studios with three or four pieces of equipment that were nothing but a tangled rat's nest of cables. Obviously, really pristine layouts exist in either environment, too. Planning and maintenance are what separate the mediocre from the superb.

Planning for Success

Even if you've been growing your equipment collection in the same room since the 60's, it won't hurt to wade your way through a planning exercise. It may identify a spot in your signal path that has been bugging you since you moved into your studio.

While it probably seems like a waste of time, especially if your system only includes a computer and a few MIDI pieces and a mixer, I recommend drawing a schematic of the equipment connection plan. Before you ever connect a cable or plug in a power cord, draw out the equipment you own and determine the best place in the signal path for each piece. A good example of a planned signal path is the block diagram that you'll probably find in your mixer's owners' manual.

If you aren't going to do this layout work on CAD, consider stealing an old Industrial Engineering trick. Make a small, rough drawing of each piece of equipment, piece of furniture, and rack cabinet that you'll have in the room. Cut out the drawings and put them aside for a moment. On scale with your furniture, draw your room, including the lighting and power sources. Now, using a temporary office adhesive, do a few dozen "what ifs" with the equipment and furniture cutouts on your scale studio facility. Eventually, you'll find the best place for everything and make the most use of your available space.

If you can design your rack and instrument layout to absolutely minimize cable lengths, do that too. Electrons like short paths. The shorter your cables, the less likely your wiring will pick up noise and interference from undesirable sources. Short cables also introduce fewer losses in your signal path, due to reactive cabling components (inductance, capacitance, and resistance).

Things that Go "humm" in the Night

An important reason for doing all this planning is to make your system quiet. When you start recording you can add all the noise you want, but a great recording studio has a very low noise floor. You can save yourself a few decibels in signal-to-noise by identifying the best physical locations for your equipment. If you can avoid running low level signal cabling over power transformers, do it. This is the reason that knowledgeable people isolate their power amplifiers from their low level signal equipment. Power amps have big, nasty supply transformers than can couple 60Hz harmonics into just about anything placed nearby.

While we're talking about undesirable noise sources, I should mention a few. Light dimmers, for example, are one of the most insidious things left over from the Evil Empire. There are dimmers specially made to reduce EMI, but they don't always work as well as advertised. Personally, I think it's smarter to design your lighting system so that you can illuminate what you need illuminated. Turn off lights when you don't need them.

Old style florescent lights are almost as evil. The ballasts (transformers) are terrific sources of power line harmonics and they can plague you when they get near mikes, instrument pickups, tape heads, and any other high-gain, low level signal source. Some mixers seem incapable of shielding their internal circuits from florescent fixtures that live anywhere in the vicinity. The florescent bulbs that screw into normal incandescent bulb sockets don't seem to be a problem. I understand low noise shielded florescent tube fixtures are also available.

I mentioned power supply transformers, earlier. There is an AC-to-DC power supply in nearly every piece of equipment you'll buy. Some manufacturers save themselves from power supply EMI by using wall-warts. That helps them in the specs-manship wars, but it can create all new problems for you in your studio. I downgrade my interest in any piece of equipment that uses a wall-wart. The product has to be out-of-this-world to push me past that design flaw.

Traditional power supplies end up almost anywhere in the cases of equipment and, while their location is usually optimal for that specific piece of gear, the transformer orientation can generate hum and noise in nearby equipment. Most of the time, you're going to stack a half dozen effects and other gear in a rack, piling power supply upon power supply. With some of the supply transformers at just about every location possible, relative to the other equipment's' sensitive circuitry or input connectors, it's possible that the wrong rack organization can produce a lot of noise at the output of your system.

One simple and fairly reliable method of checking for inter-equipment interference begins with terminating the input connectors (using a balanced or unbalanced connector shunted with a 100-600 ohm resistor).

Turn off all of the pieces of equipment in the rack but the one you are testing. Crank the gain of the DUT (Device Under Test) to maximum. If you have a DMM (Digital Multi-Meter), connect its leads to an output terminal and measure the output noise. If you don't have any equipment, connect the DUT's output to your board and listen to the noise output of this piece through the board's headphone output. Once you have the base noise level, turn on the two surrounding pieces of equipment in the rack. If the noise stays constant, there is no power supply interference. If it increases, turn off each of the surrounding pieces, one at a time, and see if the interference is caused by one or both. If one of the two is the culprit, try moving it. If it's both, the DUT may be poorly shielded or you may have a ground loop problem.

With all EMI problems, distance is your friend. If you can afford to give up a rack space between each piece, you'll most likely have a quieter system than if you have to pack every space in your rack. It's a good idea to leave a couple of spaces between any piece of equipment that uses a lot of power (like power amplifiers and mixer power supplies) and small signal equipment. You should consider doing this just for the advantage you will get in heat dissipation, if not for the signal-to-noise precaution.

Be Cool

Heat is the enemy of all electrical equipment. Hot air travels upward, so it's a good idea to plan for this. If you're going to be driving power amplifiers hard enough that they will get hot (or if you have tube equipment), it's a good idea to put that kind of equipment at the top of the rack. It's a better idea to force ventilate the rack when you're producing a lot of heat in that enclosed space. Do not put high temperature equipment under equipment that has moving parts or rubber components, like tape machines. The heat will accelerate the deterioration of lubrication and flexible items like pinch rollers will quickly harden.

Making the Best Connections

Eventually, you get to start wiring stuff up, but not yet. Having spent a good portion of my life finding and repairing poor connections, I may be superstitious about wiring and connectors. While I'm not particularly fond of soldering phono (RCA) cables, I still roll my own. There are several companies who specialize in audio cabling and they do a good job at it. You can buy well assembled, high quality interconnects for not a lot more than what the cable and connectors might cost you. Still, if you are good with a soldering iron and can do a proper cable prep, do the work yourself. If you aren't, you're probably better off buying cables. I think the most significant downside to not being able to your own cables is that you need to keep plenty of spares on hand if you can't do repairs. The most significant upsides to making your own interconnects are that the cables can be exactly the right length for the job and you know who to blame for any failures.

Usually, you don't get many chances to "optimize" once you have your cables and equipment in place. But if you buy professional equipment, you may have an opportunity to use "hard" connections; screw-type barrier strips. If you have a choice between a screwed-down connection and a plug-in connector, always pick the first. Crimp a spade connector (properly sized for the wire used) onto bare cable (do not tin the wire) and torque the #6 screw to 8 in/lb. OK, I'm exaggerating about using a torque driver, but I do believe in the value of hard connections.

If you're really paranoid, which I am, go an extra mile before you haul out the screwdriver. I have used CAIG Labs products since the mid-70's. In several of my career incarnations, I've used some of their products in case lots and by the gallon. I'll be among the first to admit that product names like ProGold, DeoxIT, and CaiLube sound pretty New Agey, but the stuff works. I swear by it. I even spend my own money on it for my home gear. I spray the bare wires and the terminal connectors with DeoxIT before I crimp the two together. I spray the barrier strips before I insert the terminal spade and tighten the screws down. Unless they are plated, I spray every tin-lead connector in my system with DeoxIT. If they're plated, I use ProGold. It doesn't take much and it adds a new application to the "ounce of prevention" adage. I also use those two products on other connectors, too.

Grounds Should Not Loop

Now we've got stuff in the rack and we're ready to get connected? Not exactly. We're not through planning. The evil curse of AC-powered analog electronics are ground-loops. Once you have a ground loop problem, you can take dozens of troubleshooting hours to find it. Taking a few preventative steps in the beginning will make later troubleshooting a lot easier. And, you may have noticed that I haven't actually put a screwdriver to rack screws yet.

Ground-loops are probably the most common place where a studio's signal-to-noise ratio gets degraded. A ground-loop is what happens when there are at least two paths for current that appears on the signal or power ground. For instance, a pair of audio signal processing pieces are connected to two different power outlets but are also connected to the same patchbay, through their input and output jacks. If there is any potential voltage between the ground connections on those two power outlets, that voltage will find a common path through the signal ground wiring. That wiring is tied together at the patchbay. The current produced by the ground wiring voltage appears on the shielding of the input and output cables. The electrical components of the cables and the common-mode amplification characteristics of the signal processing equipment will allow the signal on the ground wiring into the equipment's amplifiers. If the two pieces of equipment are semi-pro and unbalanced, you have no common mode rejection and you may have lots of hum.

Obviously, one tactic for breaking this closed loop is to cut off the third prong of at least one of the power cords. That's also a great way to design up a system that may someday provide your wife with a lump sum payment from your life insurance carrier. Dumb, dumb, dumb. You do not want to set yourself up to be the return path for ground current when a power supply transformer fails and several amps of current are looking for a route back to Mother Earth.

Of course, some equipment is sold with two-prong power wiring. To get away without the safety precaution of a secure case ground path, the external case of the equipment must be "double insulated" from power ground to prevent ground current from finding its way to the case of the product. This is common in equipment designed for home use (stereo stuff). This equipment can be very difficult to integrate into a recording studio because of grounding issues.

One much better solution to power line ground loops is to use an "isolated ground" for your entire system. This means that you actually take the precaution of making sure there is a real, earth ground connected to your equipment racks and other gear. This ground will return all the way back to the main building ground at the circuit panel. This system is called "star grounding." To make the job complete, you also make sure that a big, fat solid copper wire runs from the building ground to a copper rod (your local codes will determine the length of the rod) that is driven into the earth. The rod ought to be as close to the circuit breaker box as possible. A clamp to a cold water tap is nice, but the copper rod is better. This is very practical for a home studio and ought to be one of the first things you do when you start laying out your studio design. This is especially critical in locations that see regular lightening storms.

Once you have this honest-to-NEC (National Electrical Code) ground, don't connect anything outside of your studio to the ground wire. If all of your audio gear is connected to a single, isolated ground circuit, your power ground loops ought to be minimized. It's a great theory, anyway. Connecting the pieces without ground-loops can involve magic and luck.

If everything you own is pro standard, the problem is made considerably more simple with balanced connections. Usually pro gear has balanced inputs and outputs. Balanced circuits are less susceptible than unbalanced.circuits to ground loops and other noise sources. It's possible that some ground loop current will find a way into, even, the best designed equipment. Some precautions with your interconnecting wiring will help prevent that from happening.

You will need to be prepared to disconnect occasional ground connections in the signal path to break up loops between the signal ground to the power ground. Ground exists at both ends of the cable, from the case grounds, so completing the shield ground connection is often unnecessary. I color code my XLR cables (red for single-ended ground, green for connected ground) to make ground-loop troubleshooting a little easier. I have found that I use about four times as many single-ended ground cables as I do the connected versions.

Direct boxes usually have ground-lift switches. If those switches are going to do you any good, you need to wire your XLR connectors so that pin 1 or the XLR case is not connected at one end of your cables. Again, pin 1 is already connected to circuit and case ground at the mixer or effect, so a continuous ground connection is not necessary for shielding ground.

Unbalanced connections can also benefit from wiring tactics. You can create a simulated balanced connection with a couple of lightweight tricks. If you're connecting a balanced unit to an unbalanced unit, adding a 50-100 ohm resistor to the ground terminal at the unbalanced end of the cable may provide enough resistance to the ground path to divert the ground loop current away from the connection. If the connection is unbalanced to unbalanced, the same 50-100 ohm resistor on one of the pin 3 connections may do the same trick. In both cases, use two-conductor shielded cable (like microphone cable) and only terminate the shield wire on one end of the cable.

Usually, if you are careful with the power grounding and the interconnecting shield grounding, you can avoid ground loops in your equipment. But the more equipment you have, the more likely it is that you're going to have problems. If nothing else will make you consider all-in-one multiple-effect boxes, this might. In fact, I think that analog connection hassles ought to make a lot of home studio owners think really hard about going the full digital route.

There are two tactics commonly used for case grounding in equipment racks. A fairly uncommon tactic is to run a buss-bar (a very large gauge wire) connected to all of the equipment cases. The other tactic is to isolate the equipment cases. This is the approach I've found to be most useful. (I have a parts bin full of #10 nylon shoulder washers for this purpose.) Put a nylon washer on each side of the rack ear and add a metal flatwasher on the outside, so the screw doesn't tear up the nylon insulator. Four screws, four flat washers, and 8 nylon washers per piece of mounted equipment. This makes the power cord ground the only source of power grounding for each piece of gear, resulting in an effective "star grounding" system.

One other item to watch out for is case to case contact. In an effort to pack 2" of circuitry into a 1 3/4" box, some manufacturers chew up all of the allowed vertical space in the rack, allowing contact with the equipment above and/or below their gear. If you have to, put some kind of insulating shield between pieces to prevent case-to-case contact in the rack.

Preventing Dead Babies

OK, now we're all wired up and screwed down. There are just a few other tips I'd offer before you get serious about using your equipment. The primary failure mode of almost all electronic equipment is "infant mortality." That usually means the first 100 hours of use. The fastest way to get that period behind you is to continuously run your gear for a few days. For most equipment, this simply means turn it on and leave it on. Doing this offers you two advantages. You eliminate a large percentage of possible failures that might, otherwise, happen at the worst possible time and you make fairly certain that, if a failure occurs, it's going to happen inside the equipment's warranty period. This kind of burn-in used to be done by manufacturers, but, in the interests of reducing manufacturing expenses and process time, now they leave it up to you. So do it. It will only cost you a few watt/hours of power and it might save you a lot of hassle when you can least afford to be hassled.

With electro-mechanical equipment, just running the gear probably isn't going to provide a decent burn-in. Switches need to be switched. Motors need to spin. Doors need to open. And so on. It won't hurt your new tape machine to cycle through a tape or two. Some equipment weak-knees will show up just by powering the equipment off and on a few times. Power amplifiers, for example, have large supply surges on power-up. Sometimes all it takes to overstress a poor connection or a weak component is a couple of cycles through the power-on cycle.

Keeping the equipment's function in mind, it's in your best interest to start using the equipment you buy as soon as possible. Don't pack your room full of gear that you intend to use . . . someday. Unless repairs costs are no problem for you, buy it when you need it and use it as soon as you buy it.

Be Your Own Power Company

This next precaution may complicate the already complicated ground loop planning we've done. However, if your power line is not as solid as the power company claims, you may want to protect some parts of your system with an Uninterruptible Power Supply (UPS). An UPS will do two very valuable things for critical components in your system: it will provide a constant, filtered source of AC power and it will give you time to safely power down your system during a blackout. If you've been around computers for any length of time, you know that power company brown-outs can wreak havoc with computer systems. A fluctuating power supply can cause random memory bit-flips that will contaminate your data or destabilize your system. The UPS transformer and filter circuitry prevents that from happening. A tripped circuit breaker or lightning strike on a nearby power pole transformer, will flip every bit that isn't already at zero. The UPS battery backup and inverter circuitry provide you with a warning and a reasonable time to save what you're doing and bail out before the battery goes dead.

The only trick in selecting an UPS is determining what size supply you need. The UPS specification sheet will help you with that. First, decide what needs to be protected and write down the volt-amperage (VA, which is roughly the input wattage rating) requirements for each component. Some equipment specs only tell you the input current requirement, not the VA rating. Simply multiply the current draw times 120V for the VA rating. Add up the VA ratings of the equipment you've decided to protect. Compare that total to the chart provided by the UPS manufacturer and decide how much protection you need and can afford.

For example, you have a computer system that requires 250VA, a computer monitor that uses 125VA, and a hard disk recording system that needs another 250VA. That adds up to 625VA required for the system. Looking at the chart for the 600VA UPS, you see that unit will provide you with about 5 minutes of backup power. The 1kVA system will keep you running for 10 minutes during a blackout. You, now, need to calculate how much time you will need to save what you're doing and power your system down during an emergency. You also have to factor in what you can afford, bigger is more expensive.

If, after all that, you still remember how to do music, I think we're ready to turn this stuff on and see what it sounds like.

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Wirebender Audio Rants

Over the dozen years I taught audio engineering at Musictech College and McNally Smith College of Music, I accumulated a lot of material that might be useful to all sorts of budding audio techs and musicians. This site will include comments and questions about professional audio standards, practices, and equipment. I will add occasional product reviews with as many objective and irrational opinions as possible.