by Power and Grounding
September 7, 2013 in AV Design Tips, The Basics by Sam Davisson
This is the first of an upcoming series on “The Basics” of designing the AV systems that high end clients expect and the average client will respect.
Quality AV systems don’t happen by accident. They happen by design. Just like building a house, you’ll never reach that true quality threshold without a proper and strong foundation. With all electronic systems, especially AV, that foundation is the grounding structure.
Most designers and installers of AV systems think of grounding as black magic. How often have you or someone you know said that a cable is “picking up” noise or that the solution is “better” shielding or floating the shielding? (Note: In very high end audio systems floating the shield can be essential but in the typical AV installation, if grounding is done properly, should not be required.) Equipment manufacturers are typically of no help as they don’t have a clue as to what’s going on because they design and test their equipment in a defined, never changing environment.
The basic rules of physics are overlooked, ignored, and / or forgotten. Electrical engineering courses rarely even mention practical issues of grounding. As a result, myth and misinformation have become the normal
The preferred grounding technique for AV equipment, which contributes to this sense of mystery, is isolated grounding. The reason for this is noise interference and particularly what is known as common mode noise. In AC power systems, the difference in potential between neutral and ground is one form of common mode noise, since any change in neutral potential relative to ground also affects all of the other power circuit conductor potentials to ground. A more troublesome form of common mode noise is the differences in ground potentials throughout an electrical system. When AV devices are interconnected byway of audio, video or control cables, any difference in ground potentials between the interconnected pieces of equipment is common mode noise to the audio, video or control circuits.
Properly Conceived Isolated Grounding System Through a 3 Phase Transformer
There is much confusion over what an “isolated ground” (IG) is, how an isolated ground technique is implemented, and why it is used. Isolated ground is allowed in the U.S. by the National Electrical Code (NEC) and in Canada by the Canadian Electrical Code (CEC). But it should be noted that in both cases, isolated ground is an exception to the standard grounding requirements. NEC 250-74 and 250-75 allow IG wiring only where required for the reduction of electrical noise on the grounding circuit. In practical terms, what this means is that not all electrical contractors or electricians are versed in building isolated ground systems.
Common Myths About Earth Ground and Wires
Unfortunately, as electronic equipment developed the term “ground” became sort of a generic term. In AC power systems ground refers to a common connection point, typically earth ground. In a DC power system, such as what is powering my laptop, ground also refers to a common connection point for power return. Thus, the very meaning of the term ground has become vague, ambiguous, and often quite fanciful but make no mistake these grounds should never have thier common connection point in common.
Some electricians have a strong urge to reduce unwanted AC voltage differences by “shorting them out” with massive conductors, the results are most often disappointing. Other electricians think that system noise can be
improved experimentally by simply finding a “better” or “quieter” earth ground. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground, where they will disappear forever!
Here are some common myths about grounding:
- Earth grounds are zero volts – presumably with respect to some “mystical absolute” reference point. This leads to fanciful ideas about lots of ground rods making system noise disappear. The fact is soil resistance between ground rods is much higher, often significantly than a wire between them.
- Wires have zero impedance (is the apparent ac resistance of a circuit containing capacitance and/or inductance in addition to pure resistance) – therefore they can extend a zero-voltage reference to many locations in a system, eliminating voltage differences. In fact, wires are quite limited:
- The DC resistance of a wire applies only at very low frequencies and is directly proportional to its length.
- The inductance of a wire is nearly independent of its diameter (gauge) but is directly proportional to its length and increases at bends or loops.
- A wire resonates (becomes an antenna) when its physical length is a quarter wavelength
Something to consider, are earth grounds even necessary for low noise system operation?
Update 9/11
To answer the above question: no earth grounds are required for safety, not for low noise system operation!But what I really wanted to update here today, something I left out of my post, was the equipment rack treatment an AV integrator needs to do in order to maintain a nice clean isolated ground system.
Unless your are installing the equipment rack on a wood floor, the rack must be isolated from the floor. There are a number of ways to successfully accomplish this. While not called out, the drawing above used wooden 2×4’s underneath the rack but many equipment rack manufactures also sell ground isolation kits made from polycarbonate strips that work perfectly fine.
If your bolting the rack down you’ll need to use a nylon washer to keep the bolt itself from making the electrical path. If your simply setting the rack on the floor with no requirement to bolt it down be sure to include leveling feet with rubber covers to provide your isolation. Finally, if the rack is portable, the rubber wheels should do a good job of providing the required isolation.
Install a copper bus bar in each rack. This bus bar is connected to the isolated ground system (at the isolated ground sub panel) and then connected to the equipment rack via a 12 gauge stranded wire. The bus bar must be isolated from the equipment rack at all other points. Additionally most AV equipment provides a grounding lug on the back of the equipment. Again use a 12 gauge stranded wire to connect equipment chassis to the copper bus bar. Note: some older equipment may strap signal ground to chassis ground. If this is the case be sure and remove the strapping mechanism.
All AV conduits coming into the rack must be isolated from the rack. Most electrical supply outlets provide isolation bushings for this. In addition your equipment rack vendor may be able to provide isolation knockout panels. Finally, be sure and use an approved IG power strip.
Following these guidelines will ensure that your IG system remains so.
As I discuss future AV basic design topics grounds and dealing with grounding issues will be a constant theme. As I said, we are simply setting the foundation at this point, not only of your AV system but for future posts as well.
Sam,
I’d love to be able to contribute to your thread which will no doubt be an extremely informative and helpful resource to every reader now matter how experienced they are.
Nice article that makes some cogent points. However, I would like to point out that it is usually unwise to make sweeping generalizations such as, “equipment manufacturers are of no help as they don’t have a clue what’s going on because they design and test their equipment in a defined, never changing environment.” Have you never heard of Rane Corporation and their famous series of technical articles titled “Rane Notes”, which deal with the whole gamut of audio issues from proper grounding and shielding techniques to the proper use of various types of signal processors? All articles exhibit an accurate and comprehensive understanding of the topics and are written in an extremely clear manner which makes it easy for persons with audio background and education levels ranging from near beginners to experienced engineers to benefit from the material. I have been working as a designer and consultant in our industry for nearly 25 years, and prior to that, I worked 8 years for the military repairing and testing advanced weapons and communications systems. And after all this time, I still find the Rane website and their technical support department to be a tremendous resource.
I should have been a bit more specific in my comment about manufactures as many of them have written some very informative information. Some of it even on grounding issues.
What I meant was that typically when your experiencing issues, many of the technical people you speak with view grounding issues like everyone else, black magic.
i haven’t dealt much with Rane. Sounds like I need to. Thanks
I am looking forward to your articles. One topic I have never seen addressed is the topic of signal to noise ratio. We care about grounding (which really is of very limited scope relating to power) first for life safety. Our concern for signal integrity is more about shielding and equipment design. While I agree that IG systems can be superior and needed in a performance grade facility, in many commercial facilties it is not needed, and if done improperly even once, defeats the benefit of the IG system wide. Hence my comment about sn ratio. You can build an electrically proper facility and still have common audio hums to to poor equipment design. We need to ask, what result do we expect?
I suspect signal to noise will come up quite a bit as this series progresses.
I’ll disagree that in many commercial facilities IG is not needed and hopefully as I go along the way will be able to show why. To many factors at this point to try to do so in a reply and it does bounce around to other things that I consider to be main issues in the basics. But, you are correct, if done improperly your almost better off without it.
Single-phase power sources (240/120V in lieu of 208/120V Y) eliminate the possibility of harmonic noises on the technical power system neutral as the signals cancel-out. One less thing to worry about. This has become more relevant since most devices have started to use switching power supplies in lieu of linear power supplies, as switching power supplies draw current in a odd manner that encourages harmonics to develop on the neutral of a 3-phase power system.
True. I have standard drawings for both cases. I just happened to grab the 3 phase one.
Great article, and quite useful for systems requiring an IG.
We have been using the AV 9000 Commissioning Checklist to certify systems for years. The tests require measurements of ambient noise, electrical signal to noise, and many other performance values.
When testing and verifying AV systems (as opposed to recording studios, performing arts centers, and quiet living rooms), we never see the need for the extra expense, additional project duration, and possible safety issues that will occur if the electrical contractor did not adhere to required practices.
To put numbers on things, unless otherwise specified, an acceptable signal to noise is 55 dB at the loudspeaker terminals. Targeted acoustic “signal” level is typically 66 dB SPL A weighted (slow) at the listeners’ ears. It would be nearly impossible to discern a noise hitting the ears at 11 dB SPL (66 minus 55), when the air diffuser over my head is developing an ambient noise of 35-40 dB SPL. These levels are typically found in conference rooms and classrooms. With today’s equipment, it is very easy to achieve S/N of 65- 70dB. When you do not, the problem is usually the wiring practices in the equipment rack, not the grounding practice.
The story is different if the project requires 80 dB S/N, in venues as mentioned above. I would expect to see a competent acoustician inspecting every item being used in the room’s construction and the way it is being installed to achieve a room NC of 10-20, and I would certainly welcome that. Unfortunately, in my opinion the typical AV venue is far noisier than that which would require an IG for the AV system.
I’ll concede that on the type of rooms your talking about an isolation transformer is an added expense that is unnecessary but you still can have an isolated ground scheme without an isolation transformer. The cost is minimal, separate sub-panel, three wires instead of two. Added time to project, no more than a few hours per room. Safety factors? No different than connecting high voltage in a non IG room.
The problem I have with your numbers is you seem to be confusing signal to noise with sound pressure. The signal to noise rating of the average commercial Class D amplifiers is 100dB 20Hz – 20Khz. So is what your saying is that it is normal, with no signal applied (0V), for an amplifier to output 55dB SPL? Or that it is acceptable to reduce the S/N by injecting 45db of noise?
I respectfully disagree.
Mario,
I must agree with Sam. I’ve never heard of an acceptable S/N ratio being below 100dB- Even with the crappiest equipment available, (typically commercial as Sam pointed out).
I too think you’ve either got your numbers wrong, or don’t understand that S/N, (Signal to Noise), means how much signal is to how much noise… Who in their right mind would accept 50-60% of noise within a signal.
Unfortunately a fair amount of commercial audio conference gear with sub 100dB SNR does exist. (sigh)
Not esoteric seldom heard of gear: but the common gear that most of us have experience with.
Clear One XAP.
Biamp MXP and so on.
Dont blame anyone for trying to forget those.
To be fair: Clear One and BiAmp each have some decent products as well. (Have nothing bad to say about the Audia series).
Of course a lot of consumer and low end products artificially improve their rated SNR in the factory by only measuring it a narrow frequency where their product actually sounds good instead of a the full 20hz to 20khz spectrum.
But what gear are most conference rooms using AFTER the mixer and/or conference codec?
70 volt amps and tin can ceiling speakers.
GIGO but there is nothing that will sound great coming out of tin can speaker back boxes. Everything sounds like it is coming out of a soup can: because it IS coming out of a soup can.
Couple this with the unnatural experience of looking at a display on the wall with sound coming from above.
So unless/until we are willing to do all of the sound reinforcement properly: the the gear SNR is just one more compromise that further degrades our already awful sound reinforcement. I will always offer to use better sounding gear: but if we are not going to do all of the following then we are just fooling ourselves:
Active crossovers, NOT passive
Push-pull amplifiers, directly connected to drivers NOT going through any filter, crossover or network
Asymmetric wood cabinets
Round or oval drivers and waveguides
individual speaker cabinets, not arrays or clusters, and close to single point entry (co-entrant vs co-axial wherever possible)
Any shortcut will degrade the audio reaching the listener, and each one builds upon the next until we have a typical auditory disaster where at best everyone sounds “different” (ie distorted) or at worst: we can HEAR what the person is saying, but not really UNDERSTAND what they are saying.
It is a sad reality that that the audio industry sells snake oil as if they belong in the United States Congress. The few companies who make products that actually perform languish while those who sell junk thrive and rise to the top. Nearly every consumer and pro-system is built with lousy equipment and SNR is just one of the many places where we do not trust the manufacturer’s specs if we even bothered to pay attention to them. I think that people want lousy sound, because that is all that is being bought, sold or spec’ed out. Dont buy the good sounding stuff: make it sound like my iPad.
Mario: I do not believe that your targeted acoustic level provides sufficient headroom in the specification of desiring 65dB SPL at listening positions. If we require 65dB signal: then we should design for 95dB. Why? Because people absorb sound and the SPL meter in an empty room is receiving the full signal, where a full-house audience is a completely different story.
In addition: giving equipment headroom: we prevent driving equipment to the point of damage, destruction or distortion. Sound will always be optimum with little or no distortion and minimal wear on the systems and the components that make them.
In my home: our whole house audio is capable of >124dB SPL RMS which we never run. Why have it? Because we might have a party and it would be nice to not have to worry about hurting the system to deliver the optimum level of sound. For day to day use: it never gets pushed anywhere near that level and is seldom driven above 50%. My amps and speakers are US and Italian made circa 1996. Not cheap gear but high-end with 32 high fidelity output transistors per channel, using active crossovers: one amp channel per driver without any filters between the amp and the drivers. These 18 year old amplifiers sound so transparent that I can absolutely hear the difference when the connectors begin to get dirty/oxidized and require a cleaning. I consider this no-compromises system as penance for the garbage that we get to listen to at work. This is what we SHOULD be doing at work. There is no justification for doing it wrong. Unfortunately wrong is the rule not the exception. When my 18 year old sound reinforcement system sounds better than the brand new stuff that we are installing today: we are doing things wrong today.
Engineers are supposed to build for optimum performance in worst case and that is why headroom is essential.
OK. A couple of comments here.
The 66 dB SPL A wtd comes relates to the ‘targeted’ level. Headroom is typically 15 dB or more above that. If the listening level is much greater than 66 dB SPL, then the people in the conference room next door will probably come pounding on the doors to keep the noise down. We have found this figure preferable using the empirical approach, but its the same figure arrived at by Don Eargle, and also the ANSI classroom standard.
When the loudspeakers are delivering that level,usually measured using pink noise as a signal with a Talkbox positioned at the mouth position relative to the microphone, we measure an ELECTRICAL reference for that level at the output of the amplifier. We then use that reference for the signal to noise, and for the THD test to make sure the THD is less than .5% when the system is pushed to the headroom level (80-88 dB SPL).
The signal to noise you refer to is I believe the ANSI standard for EQUIPMENT, not a SYSTEM. Wrong standard for AV, since the people on that committee were manufacturers tackling a different problem. The “signal” level is at clipping, and the inputs are replaced with a shielded connector with 150 ohm resisters between pins 2 and 3. Useless for real-life AV systems.
You will never actually measure a SYSTEM signal to noise of 124 dB. Nor will you actually measure contrast rations of 1000. These numbers are only on specification sheets, not on real systems. We have over 2000 systems where these measurements are taken, always with instrumentation calibrated to NIST traceable standards.
Best regards,
Mario
Testing the amplifier at full power can be misleading and hard to compare between different amplifiers at differently rated power levels. Also weighting will skew the results by about 10dB. Even acknowledging that, you should be able to get 80dB S/N out of most commercial amplifiers. With proper grounding and cabling techniques the AV system should not add to the noise threshold or at least be less than the S/N ratio of the lowest rated piece of equipment. In other words, the amount of noise the AV system contributes should be minimal.
[…] be next. I was thinking linearly, like a book. But in reviewing and responding to comments on the Power and Grounding post, I realized that perhaps the best approach would be to tackle misconceptions as they arose and […]
Again Sam, let me clarify, and I apologize for the confusion.
The AV 9000 Standard does not include testing of amplifiers. It is assumed the manufacturer’s Quality Management System did that before it was shipped. Instead it addresses the Quality Management of the designer and integrator, verifying that the system gain structure is set right. It has been my experience that in our culture (AV industry), it is very typical to take perfectly good equipment, and ‘set the knobs wrong’ so that the SYSTEM performance is nonconforming. AVR’s role as a third party T and V services provider identifies proper gain structure as a particularly allusive task. That is why the CQT training at AQAV requires demonstrated proficiency on real systems for that certificate.
In the AV world, the power low-Z amplifiers are rarely delivering more than a volt or so across the loudspeakers.
Mario, the crux of the issue I have is with the typical numbers you gave. 66dB SPL is a little high for natural speech at ear level but not terribly so. 35 – 40dB SPL for the ambient room noise threshold very reasonable. But the 55dB number you threw out confuses me completely. At first you say it is 55dB S/N. That would be a horrible number for an amplifier S/N.
But then you qualify (or further confuse it) by subtracting it for the 66dB SPL you quoted. Signal to noise and sound pressure levels may be both expressed in dB’s but they are completely different. You can’t subtract the two and have a meaningful answer.
Then, in the same paragraph you state that a S/N of 65 – 70dB is pretty easy to achieve with modern equipment. Then what’s the 55dB number you gave?
Now we don’t care, set the gain structure right. I’m confused!
As a consultant, from an audio perspective, here is what I am mostly going to look at when I come in to verify the conference room you just built. I’m going to ask you demonstrate that the S/N ratio at the amplifier is greater than 70dB and that the system has been adjusted for unity gain. Then I’m going to take 3 different SPL reading in 4 different locations within the room. I’m going to look at AV system de-energized ambient noise, AV system energized, no sound and AV system energized with sound playing. The three things I’m going to care about:
1. SPL is consistent throughout the room
2. Room SPL with sound is approximately 60dB at 4′ off the ground
3. The ambient sound reading with system energized is not greater than the reading without.
Getting back to system grounding, remember to use nylon shoulder washers if you are bolting your rack to the floor. If you don’t the bolt will make contact with the rack.
Also, an isolated ground for the AV gear is a great idea until you connect your control system to another sub-system with a serial cable. You now have a potential ground loop between the two systems. An optical isolator will help with this and transient surges. B and B Electronics has some that I have used.
Of course, no discussion of system grounding is complete without mentioning Bill Whitlock. His white papers and training classes have been invaluable for me. Check out this link for more info. http://jensentransformers.com/apps_wp.html
Good luck everyone. I had a client with a Doctorate in Electrical Engineering tell me ground loops are the hardest thing he has ever had to work on.
[…] still not quite sure why but when I started the basic series with "Power and Grounding" most of the comments, both public and private where that I was setting the stage to talk […]
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